The International Flight Information Manual (IFIM) |
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AC 91-70: Oceanic Operations, an Authoritative Guide to Oceanic Operations (09-06-94) |
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CHAPTER 8. LONG-RANGE NAVIGATION
1. GENERAL NAVIGATION CONCEPTS, FAA POLICIES, AND GUIDANCE.
a. General Concepts. In the early days of aviation, few aircraft
operated within any given area at the same time. The most
demanding navigational requirements were to avoid obstacles
and arrive at the intended destination with enough fuel
remaining to safely complete a landing. As aviation evolved,
the volume of air traffic grew and a corresponding need to
prevent collisions increased. Today, the most significant and
demanding navigational requirement in aviation is the need to
safely separate aircraft. There are several factors that must
be understood concerning the separation of aircraft by air
traffic control (ATC).
b. Separation of Air Traffic. In many situations, ATC does not
have an independent means such as radar to separate air
traffic, and must depend entirely on information relayed from
an aircraft to determine its actual geographic position and
altitude. A flightcrew's precision in navigating the aircraft
is critical to ATC's ability to provide safe separation. Even
when ATC has an independent means such as radar to verity the
aircraft's position, precise navigation and position reports,
when required, are still the primary means of providing safe
separation. In most situations, ATC does not have the
capability or the responsibility for navigating the aircraft
ATC relies on precise navigation by the flightcrew.
Therefore, flight safety in all instrument flight rules (IFR)
operations depends directly on the operator's ability to
achieve and maintain certain levels of navigational
performance. ATC radar is used to monitor navigational
performance, detect navigational errors, and expedite traffic
flow. Any aircraft operating in accordance with ATC
instructions must navigate to the level of accuracy required
to comply with ATC instructions. Aircraft must be navigated
with sufficient precision to avoid airspace where prior ATC
clearance or ATC instructions must be obtained. For example,
an aircraft flying adjacent to minimum navigation performance
specifications (MNPS) airspace must fly with a degree of
precision that ensures that aircraft will not inadvertently
enter MNPS airspace.
c. VFR Flight. The control of air traffic requires that a
certain level of navigational performance be achieved by
visual flight rules (VFR) flights to ensure safe separation
of aircraft and to expedite the flow of air traffic. During
cruising flight, the appropriate VFR flight altitude must be
maintained to ensure the required vertical separation between
VFR and IFR aircraft and to assist in collision prevention.
VFR aircraft must be navigated with sufficient precision to
avoid weather conditions that would prevent visual contact
with (and avoidance of) other aircraft and with sufficient
precision to locate a suitable airport and land safely. VFR
aircraft that require navigational assistance from ATC
adversely affect ATC's ability to control air traffic and
expedite its flow.
d. The Concept of an ATC Clearance. Issuance of an ATC clearance
by a controller, and the acceptance of this clearance by a
pilot, is a negotiation process that establishes conditions
for the prevention of collision hazards (in-flight and
terrain). When a controller issues an IFR clearance, a
three-dimensional block of airspace is reserved for that
aircraft along the defined route. The controller also agrees
to issue clearances to all other controlled air traffic to
ensure that all assigned flight routes will be safely
separated. When a pilot accepts an ATC IFR clearance, that
pilot is agreeing to continuously remain within the assigned
three-dimensional block of airspace and to adhere to the
flight rules for that operation. The pilot is obligated to
comply with this agreement unless an emergency is declared or
an amended clearance is received. Any deviation outside the
assigned airspace creates a flight safety hazard. In such
cases, the aircraft has failed to navigate to the degree of
accuracy required for air traffic control and has failed to
comply with Federal Aviation Regulations (FAR) and
International Civil Aviation Organization (ICAO)
requirements. In a nonradar environment, ATC has no
independent knowledge of the aircraft's actual position or
its relationship to other aircraft. Therefore, ATC's ability
to detect a navigational error and resolve collision hazards
is seriously degraded when a deviation from an agreed upon
clearance occurs.
e. Concept of Navigation Performance. The concept of navigation
performance involves the precision that must be maintained
for both the assigned route and altitude by an aircraft
operating within a particular area. Navigation performance is
measured by the deviation (for any cause) from the exact
centerline of the route and altitude specified in the ATC
clearance. This includes errors due to degraded accuracy and
reliability of the airborne and ground-based navigational
equipment and the flightcrew's competence in using the
equipment. Flightcrew competence involves both flight
technical errors and navigational errors. Flight technical
error is defined as the accuracy with which the pilot
controls the aircraft as measured by success in causing the
indicated aircraft position to match the desired position.
Standards of navigational performance vary depending on
traffic density and the complexity of the routes flown.
Variation in traffic density is reflected in the different
separation minimums applied by ATC in these two areas. For
example, the minimum lateral distance permitted between
coaltitude aircraft in Chicago Center's airspace is 8
nautical miles (NM) (3 NM when radar is used), while in North
Atlantic (NAT) MNPS airspace it is 60 NM. The airspace
assigned by ATC has lateral dimensions on both sides of the
exact centerline of the route of flight specified in the ATC
clearance equal to one-half of the lateral separation
standard (minimum). For example, the overall level of lateral
navigation performance necessary for flight safety must be
within 4 NM of the airway centerline in Chicago Center's
airspace, and within 30 NM in NAT MNPS airpace. FAR's 121.103
and 121.121 require that each aircraft must be navigated to
the degree of accuracy required for air traffic control. FAR
91.123 requirements related to compliance with ATC clearances
and instructions also reflect this fundamental concept. The
concept of navigational performance is also inherent in the
ICAO Standards and Recommended Practices (SARP). For example,
Annex 2 states that the aircraft "shall adhere to its current
flight plan" and "when on an established air traffic service
(ATS) route, operate along the defined centerline of that
route."
f. Degree of Accuracy Required. The fundamental concept for all
IFR navigation standards, practices, and procedures is that
all IFR aircraft must be navigated to the degree of accuracy
required for control of air traffic. When a flight remains
within the assigned three-dimensional block of airspace at
all times, that aircraft is considered to be navigated to the
degree of accuracy required for the control of air traffic.
If an aircraft deviates outside its assigned block of
airspace (except during a declared emergency), that aircraft
has not been navigated to the required degree of accuracy.
ATC separation minimums represent the minimum dimensions of a
three-dimensional block of airspace that can be assigned by
ATC to control flight These separation minimums have been
established for IFR operations in controlled airspace. These
standards are usually established through international
agreement and implemented through national regulations. These
minimums are established for particular categories of
navigational operation and specified areas. Examples include
navigation on airways in the national airspace of ICAO member
states and long-range navigation in oceanic or remote land
areas. Separation minimums establish the minimum lateral,
vertical, and longitudinal distances that can be used to
safely separate aircraft operating within a specified area.
Separation minimums also represent the minimum level of
overall navigation performance which can be accommodated at
any time without jeopardizing flight safety. Any aircraft
deviating greater than one-half the separation minimums
established for that operation has failed to meet the
required level of navigational performance and to navigate to
the degree of accuracy required for control of air traffic.
For example, the vertical separation minimum for airplanes
operating above flight level (FL) 290 in the United States is
2000 feet. Each aircraft's actual altitude must remain within
+ 1000 feet of the assigned altitude even when factors such
as atmospheric pressure variations and instrument or pilot
errors are considered. Where ATS's are provided by the United
States, separation minimums are established by the FAR and
ATC directives. Where ATS's are provided by contracting ICAO
member states, separation minimums are established by those
states' national regulations and in ICAO documents.
Operations in uncontrolled airspace are not provided ATS, and
separation minimums are not normally established for
uncontrolled airspace. U.S. national airspace separation
minimums can be found in FAA Order 7110.65, "Air Traffic
Control." FAA Order 7110.83, "Oceanic Air Traffic Control,"
prescribes separation minimums in international oceanic
airspace delegated to the United States by ICAO. ICAO
Document 7030/3, "Regional Supplementary Procedures,"
prescribes separation minimums in international airspace.
g. FAR Part 91 Communication Equipment Requirements. FAR 91.511
states the equipment requirements for overwater flights
operating more than 30 minutes flying time or 100 NM from the
nearest shore. The PIC is required to maintain a continuous
listening watch on the appropriate frequency when operating
under IFR in controlled airspace.
h. FAR Part 121 Communication Equipment Requirements.
Communication equipment requirements for Part 121 operations
are contained in FAR's 121.347 and 121.349. Under FAR
121.351(a), extended overwater operations may not be
conducted unless the communication requirements of FAR's
121.347 and 121.349 are met. FAR 121.99 communications
facilities requirements may be waived for Part 121 operators
for flights over certain oceanic areas with one high
frequency (HF) radio inoperative if certain conditions and
limitations are met.
i. FAR Part 135 Communication Equipment Requirements. The
communication equipment required for turbojet airplanes with
10 or more passenger seats and multiengine commuter airplanes
are contained in FAR 135.165(a). All other aircraft operated
under FAR Part 135 must meet the requirements of FAR
135.165(b). Under FAR 135.165(b)(7), aircraft are required to
have an additional communication transmitter for extended
overwater operations.
j. Communication Equipment Requirements for Ferry Flights. FAR
91.511 contains the requirements for radio equipment for
overwater operations for ferrying FAR Parts 121 or 135
aircraft under Part 91. Certain operable communications
equipment must be carried on large and turbine powered
multiengine aircraft flown overwater. If both HF and very
high frequency (VHF) equipment are required under FAR 91.511,
FAR 91.511(d) permits overwater operations with only one HF
transmitter and one HF receiver provided that the aircraft is
equipped with two independent VHF transmitters and receivers.
k. Concept of Operational Service Volume. The concept of
operational service volume is critical to understanding and
applying the principles of air navigation. Operational
service volume is the volume of airspace surrounding an ICAO
standard airways navigation facility that is available for
operational use. Within that volume of airspace a signal of
usable strength exists and that signal is not operationally
limited by cochannel interference. Within this volume of
airspace, a navigational aid (navaid) facility's
signal-in-space conforms to flight inspection signal strength
and course quality standards including frequency protection.
ICAO standard navaids are VHF omnidirectional radio range
(VOR), VOR/distance measuring equipment (DME), and
nondirectional radio beacon (NDB). The national airspace
systems of ICAO contracting member states are based on the
operational service volume of these facilities. Navigational
performance within the operational service volume and ATC
separation minimums can be predicated on the use of these
facilities. In contrast, the signal-in-space outside the
operational service volume has not been shown to meet the
flight inspection signal strength, course quality, and
frequency protection standards. Therefore, navigational
performance and ATC separation minimums cannot be predicated
on the use of these facilities alone.
l. Categories of Navigational Operations. A thorough
comprehension of the categories of navigational operations is
essential to understanding air navigation concepts and
requirements, and in evaluating an operator's ability to
navigate to the required degree of accuracy. In the broad
concept of air navigation, two major categories of
navigational operations are identified in the ensuing
paragraphs:
m. Class I Navigation. Class I navigation is defined as any en
route flight operation conducted in controlled or
uncontrolled airspace that is entirely within operational
service volumes of ICAO standard navaids (VOR, VOR/DME, NDB).
The operational service volume describes a three-dimensional
volume of airspace within which any type of en route
navigation is categorized as Class I navigation. Within this
volume of airspace, IFR navigational performance must be at
least as precise as IFR navigation is required to be using
VOR, VOR/DME (or NDB in some countries). The definition of
Class I navigation is not dependent upon the equipment
installed in the aircraft For example, an aircraft equipped
and approved to use Loran-C in the United States as the sole
means of en route navigation (no VOR, VOR/DME installed) is
conducting Class I navigation when the flight is operating
entirely within the operational service volume of federal
VOR's and VOR/DME's. In this example, the Loran-C's IFR
navigational performance must be as precise as IFR navigation
is required to be using ICAO standard navaids, if IFR
operations are to be conducted. In another example, a VFR
flight navigated by pilotage is conducting Class I navigation
when operating entirely within the operational service
volume. However, the VFR navigational performance in this
example must be only as precise as VFR pilotage operations
are required to be.
The lateral and vertical extent of the airspace where Class I
navigation is conducted is determined solely
by the operational service volumes of ICAO standard navaids.
Class I navigation cannot be conducted outside
of this airspace. Class I navigation also includes VFR or IFR
navigation operations on the following:
0 federal airways
0 published IFR direct routes in the United States
0 published IFR off-airway routes in the United States
0 airways, advisory routes (ADR), direct routes, and
off-airway routes published or approved by a foreign
government provided that these routings are continuously
within the operational service volume (or foreign
equivalent) of ICAO standard navaids
Class I navigation requirements are directly related to
separation minimums used by ATC. IFR separation minimums
applied in the U.S. national airspace system and most other
countries are based on the use of ICAO standard navaids.
These separation minimums, however, can only be applied by
ATC within areas where the navaid's signal-in-space meets
flight inspection signal strength and course quality
standards. An ICAO standard navaid's signal-in-space conforms
to flight inspection signal strength and course quality
standards (including frequency protection) within its
designated operational service volume. Therefore, air
navigation and the safe separation of aircraft within that
service volume can be predicated on the use of these
facilities.
Within areas where the safe separation of aircraft is based
on the use of ICAO standard navaids, any IFR operation must
be navigated with at least the same precision as that
specified by the appropriate national separation minimums.
Any operation or portion of an operation (VFR or IFR) in
controlled or uncontrolled airspace, with any navigation
system (VOR, VOR/DME, NDB, Loran-C, inertial navigation
system (INS), Omega) or any navigational technique (dead
reckoning (DR), pilotage), is Class I navigation for that
portion of the route that is entirely within the operational
service volume of ICAO standard en route navaids.
n. Class II Navigation. Class II navigation is any en route
operation that is not categorized as Class I navigation and
includes any operation or portion of an operation that takes
place outside the operational service volumes of ICAO
standard navaids. For example, an aircraft equipped with only
VOR conducts Class II navigation when the flight operates in
an area outside the operational service volumes of federal
VOR's/DME's. Class II navigation involves operations
conducted in areas where the signals-in-space from ICAO
standard navaids have not been shown to meet flight
inspection signal strength, course quality, and frequency
protection standards. Therefore, ATC cannot predicate
aircraft separation on the use of these facilities alone and
must apply larger separation criteria. When operating outside
the operational service volume of ICAO standard navaids,
signals from these stations cannot be relied upon as the sole
means of conducting long-range operations to the degree of
accuracy required for the control of air traffic or as the
sole means of obstacle avoidance. Therefore, when operating
outside the designated operational service volumes of ICAO
standard navaids, operators must use long-range navigation
systems (LRNS) (GPS, Loran-C, Omega, INS) or special
navigational techniques (DR, pilotage, flight navigator,
celestial) or both. These systems and/or techniques are
necessary to navigate to the degree of accuracy required for
the control of air traffic and to avoid obstacles.
The definition of Class II navigation is not dependent upon
the equipment installed in the aircraft. All airspace outside
the operational service volume of ICAO standard navaids is a
three-dimensional volume of airspace within which any type of
en route navigation is categorized as Class II navigation.
For any type of navigation within this volume of airspace,
the IFR navigational performance must be at least as precise
as the navigational performance assumed during establishment
of the ATC separation minimums for that volume of airspace.
The navigational performance for VFR operations in a Class II
navigation volume of airspace must be only as precise as VFR
navigation operations are required to be.
In many cases when ATC lateral separation minimums are large
(usually 90 NM or greater), and the Class II navigation
portion of the flight is short (less than 1 hour), it is
possible to meet required levels of navigational performance
and conduct Class II navigation using ICAO standard navaids
supplemented with special navigational techniques such as DR.
For example, it is possible in turbojet airplanes (with
proper procedures and training) to fly many routes between
the southeastern United States, Caribbean Islands, and South
America with VOR/DME and NDB equipment. In these situations,
Class II navigation requirements can be met even though
significant portions of these routes (less than 1 hour) are
outside (beyond) the operational service volumes of ICAO
standard navaids. In the domestic United States, it is not
uncommon for low altitude VFR flights in aircraft such as
helicopters to conduct Class II navigation while outside the
operational service volumes of ICAO standard navaids when
operating over routes of less than 100 NM in length.
Obviously, Class II navigation includes transoceanic
operations and operations in desolate/remote land areas such
as the Arctic.
Class II navigation does not automatically require the use of
long-range navigation systems. In many instances, Class II
navigation can be conducted with conventional navaids if
special navigational techniques are used to supplement these
navaids. Any portion of an en route operation in controlled
or uncontrolled airspace, with any navigation system or any
navigation technique, is defined as Class II navigation for
that portion of the route that is outside (beyond) the
operational service volumes of ICAO standard en route
navaids.
3. LONG-RANGE NAVIGATION PROBLEMS AND RECOMMENDED ACTIONS.
a. Background. Although the accuracy and reliability of the
newer navigation systems are excellent, malfunctions and
failures occasionally occur. When a malfunction occurs,
flightcrews should guard against jumping to conclusions since
hasty actions are seldom necessary and may further complicate
the situation. Experience has shown that successful
resolution of navigation difficulties in oceanic areas
usually requires a thorough, thoughtful process that normally
begins during preflight planning. The training program
manuals and check airman program for air carrier operations
should emphasize procedures to be followed in the event of
partial and total instrument failure. Non air carrier
operators should be prepared to demonstrate this emphasis in
their training programs if requesting an LOA for oceanic
operations in special airspace. The following guidance is
presented for consideration when navigation difficulties are
encountered or suspected.
b. Navigation Errors. Monitoring procedures used during oceanic
operations indicate the frequency and course of navigation
errors. Considering the thousands of flights that are made,
errors are actually rather infrequent. Navigation systems are
generally so reliable that there is some concern about
overconfidence; therefore, crews should guard against
complacency.
(1) Frequent causes of errors include the following:
(a) A mistake of one degree of latitude was made in
inserting a forward waypoint.
(b) The crew was recleared by ATC, but forgot to reprogram
the navigational system.
(c) The autopilot was left in the heading or decoupled
position after avoiding severe weather, or was left in
the VOR position after departing the last domestic
airspace VOR. In some cases, this occurred after
distractions by selective calling (selcal) or flight
deck warning indications.
(d) The controller and crew had different understandings
of the clearance because the pilot heard what he/she
wanted to hear rather than what was actually said.
(2) Rare causes of errors include the following:
(a) The lat/long coordinates displayed at the gate
position were incorrect.
(b) Because of a defective chip in an aircraft system,
although the correct forward latitude was inserted by
the crew, it "jumped" one degree.
(c) The aircraft was equipped with an advanced system that
included all waypoint coordinates already on tape. The
crew assumed the coordinates were correct, but one was
not correct.
(d) Although the crew had the correct coordinates
available, the information inserted into the system
was from an incorrect company flight plan.
c. Detection of System Failure. In general, system failure is
usually considered to have occurred when one of the following
situations develops:
(1) a warning indicator is activated and cannot be reset;
(2) self-diagnostic or built-in test equipment (BITE)
indicates that the system is unreliable;
(3) the position error over a known geographic location
exceeds the maximum permissible tolerance established for
a particular navigation system; or
(4) the system's operation is so abnormal that, despite the
absence of warning or malfunction indications, the
flightcrew considers the system no longer useful for
navigation.
d. Detection of System Degradation or Malfunction. While system
failures are usually straightforward, malfunctions or gradual
system degradations are usually more difficult to detect.
This is particularly true when only two systems are on board.
Navigation difficulties of this type are usually detected by
a divergence between the navigation systems, a situation that
often occurs gradually. This factor may reduce the
possibility of identifying the faulty system unless periodic
cross-checking practices are diligently used. The following
factors should be considered when attempting to identify a
faulty system.
(1) Check the BITE codes for indications of system fault.
(2) For Omega, the system receiving the most stations and the
best quality signals should generally be the most
accurate.
(3) Review the gateway gross error check for indications of
the most accurate system.
(4) If a regular record of system performance has been
maintained and is available, a review of the record may
give a clue as to which system is faulty.
(5) If possible, use VOR, automatic direction finder (ADF),
DR, airborne radar, or other navaids to obtain a position
fix.
(6) Cross-check heading, groundspeed, track, and wind
information between systems and compare this information
with the best known positive information such as position
over a fix.
(7) Attempt to contact nearby aircraft to obtain wind or
groundspeed and drift correction information that may
identify the malfunctioning system.
(8) The compass deviation check discussed in Section 2 of this
Chapter may provide a clue as to which system is faulty
for systems such as INS. Even though these steps are
taken, a divergence between systems may occur, but the
flightcrew may be unable to determine which system is at
fault. When this occurs, the practices described in the
following paragraph should be used.
e. Recommended Actions Following System Failure. After a system
malfunction or failure has been detected, ATC should be
informed that the flight is experiencing navigation
difficulties so that separation criteria can be adjusted, if
necessary. Reporting malfunctions to ATC is an ICAO
requirement and compliance is required by FAR Part 91. If the
failed system can be identified with a high degree of
confidence and the other system appears normal, the best
course of action may be to fly the normal system and
carefully monitor its performance using any additional
navaids available, including DR. In the unlikely event that a
total navigation failure occurs and other aids are
unavailable, the only action may be to fly by DR using the
flight plan headings and times. Under these circumstances,
flightcrews should continue to use all means available to
obtain as much navigational information as possible.
Flightcrews should be alert for visual sightings of other
aircraft, since a hazard may exist due to an inadvertent
deviation from the assigned track. In some cases, it may be
possible to establish and maintain visual contact with
another aircraft on the same track.
f. Recommended Action Following a Divergence Between Systems.
Since a small divergence between systems may be normal, the
significance of the divergence should be evaluated. In
general terms, if the divergence is less than 10 NM, the best
course may be to closely monitor system performance and
continue to steer the system considered most accurate. If the
divergence between systems is greater than 10 NM, one of the
systems may be degraded. Therefore, attempts should be made
to determine which system may be faulty. If the faulty system
cannot be determined using the practices described in this
section, and both systems appear normal, the action most
likely to limit gross tracking error may be to position the
aircraft so that the actual track is midway between the
crosstrack differences for as long as the position
uncertainty exists. ATC should be advised that navigation
difficulties are being experienced so that separation
criteria may be adjusted as necessary. Consideration should
be given to abandoning this "split-the-difference" practice
if the divergence exceeds the separation criteria currently
in effect on the route of flight. If a divergence of this
magnitude occurs and the faulty system cannot be isolated,
the best course may be to fly by DR using the best known wind
information. However, in some cases, the best known
information may be flight plan headings and times.
4. PROVING TESTS AND VALIDATION FLIGHTS.
a. Introduction. FAR Parts 121 and 135 require evaluation of an
operator's ability to conduct operations safely and in
accordance with the applicable regulations before issuing an
operating certificate or authorizing a certificate holder to
serve an area or route. The testing method used by the FAA to
determine an operator's capabilities are proving tests and
validation flights. FAR 121.163 and 135.145 require operators
seeking authority to operate certain types of aircraft to
conduct proving tests before being granted operating
authority. Proving tests consist of a demonstration of
ability to conduct flights and to maintain the aircraft to
the appropriate standards. Proving tests should not be
confused with aircraft certification tests, which are tests
conducted by the aircraft manufacturer to demonstrate the
airworthiness of the aircraft. FAR 121.163 requires an
operator to successfully conduct proving tests before the FAA
authorizes the operation of each aircraft type. FAR 135.145
requires proving tests before the FAA authorizes the
operation of each type of turbojet aircraft or each type of
aircraft for which two pilots are required for VFR
operations. FAR 121.93, 121.113, and 135.13(a)(2) require an
operator to demonstrate the ability to conduct operations
over proposed routes or areas in compliance with regulatory
requirements before being granted FAA authority to conduct
these operations. The FAA requires validation flights for
authorization to add any areas of operation beyond the
continent of North America and Mexico, and before issuance of
operations specifications that authorize special means of
navigation. Though proving tests and validation flights
satisfy different requirements, it is common practice for
operators to conduct both tests simultaneously. However,
validation flights are important to consideration of oceanic
operations.
b. Validation Flights. FAR 121.93, 121.113, and 135.13(a)(2)
require operators to show the capability to conduct line
operations safely and in compliance with regulatory
requirements before being authorized to conduct those
operations in revenue service. The most common method of
validating an operator's capability is to observe flight
operations. The FAA normally requires validation flights
before issuing operations specifications granting authority
to conduct operations beyond the populated areas of the North
American continent. When the FAA conducts a validation
flight, an in-depth review is conducted of the applicable
portions of the operator's proposed procedures (especially
flight following), training programs, manuals, facilities,
and maintenance programs. There are four situations that
require validation flights in association with approval of
Class II navigation: initial approval; addition of an LRNS or
a flight navigator; operations into new areas; and addition
of special or unique navigation procedures. Validation
flights are required when an operator proposes to conduct
operations that require confirmation of the ability to
operate an aircraft type within specified performance
limitations. These limitations are based on the character of
the terrain (or extended overwater areas), the type of
operation, and the performance of the aircraft. Validation
flights are also required when an operator proposes to
conduct in-flight or ground maneuvers that require special
operational authorizations.
c. Carriage of Revenue Passengers on Validation Flights. The FAR
do not forbid the carriage of revenue passengers on
validation flights. The operator may receive FAA
authorization to carry revenue passengers during the
validation flight when the proposed operation is similar to
those in the applicant's previous experience. However,
carriage of revenue passengers is normally not permitted
during validation flights in the following situations:
(1) when the operator is seeking initial approval to conduct
Class II navigation in any airspace designated as a
special area of operation;
(2) when the operator is seeking approval to conduct Class II
navigation by an LRNS or by using a flight navigator not
previously approved for that means of navigation;
(3) when the operator is seeking approval to conduct Class II
navigation by means of a long-range navigation procedure
that has not previously been approved for that operator;
and
(4) when the operator has not previously operated a specific
aircraft type in operations requiring special performance
authorization.
d. Special Areas of Operation. Certain areas of Class II
airspace are considered special operating airspace for
purposes of validation. These areas include the following:
(1) extensive areas of magnetic unreliability;
(2) NAT MNPS airspace and Canadian MNPS airspace;
(3) Central Pacific (CEPAC) composite airspace and Northern
Pacific (NOPAC) airspace;
(4) Arctic Ocean and Antarctic airspace; and
(5) politically sensitive areas of operation.
e. Special Navigation Procedures. Validation flights are
normally required when an applicant proposes to use
navigation procedures not previously demonstrated. These
procedures include the following:
(1) pilotage, including DR;
(2) flight navigator procedures;
(3) celestial navigation;
(4) pressure pattern and Bellamy drift DR;
(5) free gyro or grid procedures; and
(6) any combination of the preceding procedures.
f. Other Situations Requiring Validation Flights. Validation
flights may also be required for special operational
authorizations and special performance authorizations.
Operators who require additional information on validation
flights are encouraged to contact their local FAA flight
standards district office (FSDO).
5. DOPPLER NAVIGATION - SPECIAL PROCEDURES.
In addition to the general navigational practices and procedures
contained in this Chapter, the following information applies to
Doppler navigation systems. A Doppler system (sensor plus computer)
is a semiautomatic DR device that is less accurate than an INS or
Omega system. A means of updating the Doppler is usually required if
acceptable position accuracy is to be achieved on long-range
flights. INS, Omega or Loran-C may be used as the updating reference
for the Doppler system. The following factors should be considered
when using a Doppler navigation system.
a. Compass Accuracy. Most Doppler systems measure groundspeed to
an accuracy of about one percent and drift angle to a
fraction of a degree. Its directional reference, however, is
the aircraft's compass system. If the overall Doppler/compass
system is to be usefully accurate, the compass should be
swung and compensated so that its error does not exceed one
degree on any heading.
b. Preflight. During preflight, the flight plan course and
distances for those flight segments where Doppler navigation
is required should be verified. Normally, the courses should
be determined to the nearest one tenth of a degree and the
distances to the nearest NM. This is routinely accomplished
by using course and distance tables designed for this
purpose. Extreme care and accuracy are important
considerations during this cross-check. If the Doppler system
is to be used for navigation from takeoff, both "A" and "B"
stages should be programmed and the "auto/manual" switch
should be placed in "auto." Also, the proper position for the
"land/sea" switch should be determined since this affects the
accuracy of the groundspeed information.
c. When Approaching the Outbound Gateway. The Doppler system
performance records for recent flights over similar routes
should be reviewed to determine if a system deviation
correction should be applied. If the records indicate that a
deviation correction may be necessary, apply the correction
to the Doppler system used. Both pilots should verify that
the outbound course and distance programmed in the active
stage conforms to the currently effective ATC clearance.
Unless otherwise required by ATC, the aircraft should be
flown directly over the gateway fix to obtain the most
accurate starting position practical. When directly over the
gateway, both pilots should ensure that the Doppler computers
have been activated and that the proper stage is selected.
The aircraft should be established on the outbound track by
using the gateway navaid. Once this is accomplished, the
gross error cross-checks discussed in Section 2 above should
be accomplished. Consideration should be given to using an
additional cross-check. This is accomplished by applying
drift angle to the compass heading and comparing the result
(actual track) to the flight planned magnetic course.
d. Updating the Doppler Computer. Since Doppler systems (in a
magnetically slaved model) fly a "rhumb line" (curved track)
and most navigation charts commonly used reflect "Great
Circle" (straight tracks), certain precautions should be
observed when updating Doppler systems. Although a great
circle course and a rhumb line course begin and end at common
points, the two courses diverge between the waypoints. This
divergence normally reaches a maximum near the midpoint of
the leg, and the magnitude of the divergence increases as the
latitude and distance between waypoints increase. Under
normal circumstances, position fixes for Doppler updating
purposes should be obtained within 75 NM of a waypoint to
minimize the possibility of inducing an error into the
Doppler system due to the rhumb line effect. This practice
should be applied to both manually obtained and automatically
obtained position fixes. When Doppler systems are used in the
grid (free gyro) mode, the Doppler track will approximate a
great circle, and the rhumb line effect is not a factor.
Under these conditions, the updating restrictions detailed
above are not normally applicable.
6. INS NAVIGATION - SPECIAL PRACTICES AND PROCEDURES.
a. Preflight. Since INS is a DR device and not a position-fixing
device, any error induced during alignment will be retained
and possibly incremented throughout the flight unless it is
removed through updating procedures. Therefore, during
preflight, care should be exercised to ensure that accurate
present position information is inserted into the INS.
Although most INS will automatically detect large errors in
present position latitude during alignment, large errors in
present position longitude may exist without activating a
warning indication. When cross-checking present position
coordinates, be alert for the correct hemispheric indicator
(i.e., N, S, E, W) as well as the correct numerical values.
Since most INS cannot be realigned in flight, special
procedures such as ground realignment may be required to
correct a significant error in present position. If the INS
in use has the capability of "gang-loading" (simultaneous
loading) by use of a remote feature, care should be taken so
that any data entered by this method is cross-checked
separately on each individual INS to detect data insertion
errors. The INS software identification and modification
status codes should be verified to ensure that the proper
equipment is installed and the appropriate operating
checklist is used. The operating checklists should include a
means of ensuring that the INS is ready to navigate and that
the navigation mode is activated prior to moving the
aircraft. Any movement of the aircraft prior to activating
the navigation mode may induce very large errors that can
only be corrected by ground realignment. After the system is
placed in the navigation mode, the INS groundspeed should be
checked when the aircraft is stationary. An erroneous reading
of more than a few knots may indicate a faulty or less
reliable unit. If this occurs, a check should be made of the
malfunction codes.
b. In-Flight Updating. INS are essentially accurate and
reliable, but it is possible to introduce errors in an
attempt to improve accuracy by in-flight updating. On the
other hand, INS errors generally increase with time and are
not self-correcting. If large tracking errors are permitted
to occur, aircraft safety and separation criteria may be
significantly degraded. These factors should be considered in
any decision relative to in-flight updating. As a guide to
flightcrews, some operators consider that unless the ground
facility provides a precise check and unless the error is
fairly significant (e.g., more than 6 NM or 2 NM/hour), it is
preferable to retain the error rather than update.
7. OMEGA INFORMATION.
This section addresses only dual Omega installations. However,
operators should be aware that if an operation requires two LRNS and
one of the systems used is an Omega system, all requirements
specified for Omega as the sole means of navigation must be met.
Installations which propose to use one Omega system in combination
with one or more other types of sensors or units should be evaluated
on an individual basis, considering the performance of the
individual systems as discussed in other sections of this Chapter.
a. Background. Omega is a radio navigation system that uses a
worldwide network of VLF signals from eight ground-based
transmitters. The principal attributes of the Omega system
are the high degree of signal stability and low signal
attenuation that produce reliable position information over
great distances. Various methods of signal processing are
used by different manufacturers to develop position
information and navigation guidance (rho-rho, hyperbolic,
single frequency, 3.4 KC tracker, etc.). Because of these
variations in processing methods, each design will be
evaluated and approved individually. When Omega systems meet
the provisions described below, they may be used as the sole
means of long-range navigation for operations in oceanic
and/or remote land areas where adequate accuracy and
reliability have been demonstrated. U.S. Navy VLF
communication stations may be used to supplement Omega
navigation systems. However, the U.S. Navy VLF stations are
not dedicated to navigation and their signals may not be
available at all times. Therefore, systems approved in
accordance with this AC should be capable of operating on
Omega systems alone.
The approval process is divided into two parts. The first
part deals with approval under FAR Part 25 and the second
part deals with operational approval under FAR Part 121.
Guidance concerning compliance with FAR Part 91 regarding NAT
MNPS airspace is contained in Chapter 3, Section 1 of this
AC.
b. Airworthiness Approval. Applicants desiring airworthiness
approval of dual Omega navigation systems in accordance with
this AC should contact the appropriate FAA Regional
Engineering and Manufacturing Office at least 30 days prior
to start of the evaluation for processing a supplemental type
certificate (STC) or type certificate (TC) amendment. A dual
Omega installation includes two receiver processor units, two
control display units (CDU), and two antennas.
c. Operational Approval. FAR Part 121 requirements for en route
navigation facilities are contained in FAR 121.103 and
121.121. Air carrier applicants desiring operational approval
for use of dual Omega systems should contact the FSDO charged
with the administration of their operating certificate a
minimum of 30 days prior to the proposed start of evaluation
flights. FAR Part 91 operators desiring approval of dual
Omega systems for flights in MNPS airspace should contact the
FSDO nearest their principal base of operations to obtain an
LOA. Requests should include evidence of FAA airworthiness
approval of the system, a description of the system
installation, and the operator's experience with the system.
Prior to presenting the initial request, an operator should
have accumulated sufficient experience with the equipment to
establish a history of the accuracy and reliability of the
proposed system. The applicant may include previous or
related operational experience of other operators who have
used the same equipment on the same type aircraft, and
operational experience gained during type certification or
supplemental type certificate of the aircraft. Once a
particular system has received an equipment approval,
subsequent evaluation and approval in the same type of
aircraft installations may be adjusted to avoid duplication
of part of the accuracy and reliability data gathering
process involved in the issuance of the original approval. A
comprehensive summary of any flight experience that
establishes a history of adequate signal coverage (during day
or night operations), accuracy, lane ambiguity
detection/resolution, and in-service reliability should be
provided to show competency in the proposed operation and
maintenance of the equipment.
The applicant must present proposed revisions to the
operation manual, describing all normal and abnormal system
operating procedures and flightcrew error protection
procedures including cross-checking of data insertion,
detailed methods for continuing the navigation function with
partial or complete Omega system failure, reacquiring the
proper lane after any power outages, and procedures for
continuing operation in the event of a divergence between
systems. The applicant must also present proposed revisions
to the minimum equipment list (MEL) concerning Omega, with
appropriate justification. The applicant must present a list
of operations to be conducted using the system including an
analysis of each operation with respect to signal reception
for ground synchronization and en route operation, signal
absorption by the Greenland Icecap, sufficient redundancy of
signal coverage to permit continued operation during station
outages, procedures for operating in areas of magnetic
compass unreliability (if applicable), availability of other
en route navaids, and adequacy of gateway facilities to
support the system. (For the purpose of this AC, a gateway is
a specific navigation fix where the use of LRNS commences or
terminates.) The operator must develop a procedure for timely
dissemination of Omega NOTAM information to crewmembers. The
operator must also develop an outline of the maintenance
program for the equipment, including training of maintenance
personnel, positioning of spares and test equipment,
maintenance manual revision procedures (if applicable), and
the other means of compliance with the requirements of FAR
Part 121, Subpart L.
The Omega navigation system should be checked in-flight to
determine that the design and installation criteria are met.
All modes of operation should be functionally checked. The
airplane flight manual procedures should be evaluated
in-flight, including abnormal and emergency procedures. This
evaluation should include reinitialization, lane ambiguity
resolution, etc., during normal and adverse conditions.
Interfaced equipment should be evaluated to ensure proper
operation. Normal flight maneuvering should include 180
degree turns to verify dynamic response. An applicant for
airworthiness approval should provide data from sufficient
flights in the area of intended use to show that the Omega
navigation system can meet the accuracy requirements
stipulated for LRNS in FAR 37.205, technical standard order
(TSO) C-94, and Radio Technical Commission for Aeronautics
(RTCA) DO-164, Section III, paragraph 3.8. Consideration
should be given to time of day, season, station outages,
station geometry, and poor signal-to-noise ratio.
(1) It should be demonstrated that operation of the system
does not impose an unacceptable workload in a normal
flight environment on the flightcrew. This aspect should
receive careful scrutiny relative to crew workload during
power outages, DR operations, and detecting/resolving lane
ambiguities.
(2) The DR mode should be evaluated to determine the maximum
period for which interim use is permissible. The
information should be included in the airplane flight
manual.
d. Ground Evaluation. After installation, an
operational/functional check should be performed to
demonstrate compatibility between the Omega system and
aircraft electrical and electronic systems. This test should
be conducted with all electrical/electronic equipment
operating normally on aircraft power. A ground location
should be selected that minimizes the presence of external
electromagnetic interference. In addition, it should be
demonstrated that the Omega equipment will not adversely
affect other systems to which it may be connected; i.e., air
data, autopilot, flight director, and compass system. The
Omega velocity and heading (or track) information presented
on the control display unit (CDU) and other interfacing
instruments should have reasonable comparison to the primary
indications on other flight deck instruments. During these
tests, the primary velocity and heading inputs to the Omega
system should be slewed through their operating range to
ensure compatibility of input to interfaced equipment. This
evaluation may be conducted in-flight. Displays of all data
basic to the installed Omega systems should be demonstrated
to show no instability or discontinuity utilizing those
stations identified by the system as usable and necessary for
navigation. This evaluation may be conducted in-flight.
e. Evaluation and Final Approval. Prior to final approval for
the use of Omega as a sole means of long-range navigation, a
thorough evaluation of an operator's training program and a
flight evaluation by an FAA inspector will be required. This
flight evaluation should be requested on the operator's
application for the use of Omega as a sole means of
long-range navigation.
(1) The evaluation by an FAA inspector will include the
adequacy of operating procedures and training programs;
availability of terminal, gateway, area, and en route
ground-based navaids; operational accuracy; equipment
reliability; and acceptable maintenance procedures. Omega
equipment operations should be closely analyzed to ensure
that an unacceptable workload is not imposed upon the
flightcrew by use of the Omega equipment in normal and
abnormal operations.
(2) After the evaluation is completed, FAA approval is
indicated by issuance of operations specifications for air
carriers and by an LOA for other operators who desire to
fly in airspace where an authorization is required. The
operations specifications (or amendments thereto)
authorizing the use of dual Omega as a sole means of
long-range navigation in the areas in which operations
were demonstrated by an air carrier will limit the
operations to areas where compliance with FAR Part 121 or
FAR Part 135 requirements were demonstrated. Requirements
for LOA's are detailed in Chapter 3 of this AC.
(3) The operations specifications should contain applicable
limitations or special requirements needed for particular
routes or areas and, where necessary, list a sufficient
number of Omega ground transmitters required to be in
operation to provide the necessary amount of signal
redundancy.
f. Minimum Functions Necessary When Used for Position Fixing and
Sole Means of Navigation. Dual independent Omega navigation
systems used as a position-fixing device or position-keeping
device and sole means of navigation should meet the
performance requirements of TSO C-94, "Airborne Omega
Receiving Equipment" and Section 3 of RTCA Document No.
DO-164 titled "Minimum Performance Standards Airborne Omega
Receiving Equipment" dated March 19, 1976. When installed,
the system should provide a means of entry for at least the
following data inputs and functions:
(1) present position (initializing, reinitialization and
update);
(2) waypoints;
(3) heading, wind and true airspeed (TAS); or track and
groundspeed; or other external information required for
operation in the secondary or direct ranging mode;
(4) time;
(5) date;
(6) deselection and reselection of any station (automatic
deselection and reselection is acceptable if shown to be
effective and reliable); and
(7) lane ambiguity resolution. Automatic lane ambiguity
resolution is acceptable if shown to effective and
reliable.
g. System Displays. If the equipment is to be operated by the
pilot(s), the system controls and data display should be
visible to, and usable by, each pilot seated at a pilot duty
station. The system controls should be arranged to provide
adequate protection against inadvertent system turnoff. The
system should also provide a means of displaying the
following information:
(1) present position
(2) time
(3) date
(4) synchronization status
(5) station(s) deselected - station(s) selected
(6) time and position recall in event of power failure for up
to 7 minutes
(7) annunciation when system is not operating in the primary
Omega navigation mode
(8) a visual or aural warning of system failure, malfunctions,
power interruption, lack of synchronization, or operation
without adequate signals
(9) waypoint coordinates
(10) hearing and distance between waypoints
(11) deviation from desired course
(12) distance and time to go to selected waypoint
(13) track angle and/or error
(14) drift angle
(15) wind, TAS and heading; or track and groundspeed
(16) stations currently being installed to determine position
(17) steering information on the horizontal situation indicator
(HSI) or equivalent
(18) confirmation of data insertion
h. Failure Protection. Normal operation or probable failure of
the airborne Omega navigation system should not derogate the
normal operation of interfaced equipment. Likewise, the
failure of interfaced equipment should not render an Omega
system inoperative.
i. Environmental Conditions. The Omega equipment should be
capable of performing its intended function over the
environmental ranges expected to be encountered in actual
operations. RTCA Document No. DO-160 should be used for
appropriate guidelines.
j. Antenna Performance. The antenna design and installation
should minimize the effects of precipitation (p) static and
other noise of disturbances.
k. Dynamic Responses. The system operation should not be
adversely affected by aircraft maneuvering or changes in
attitude encountered in normal operations.
l. Preflight Test. A preflight test capability should be
provided to inform the flightcrew of system status.
m. Aircraft Electrical Power Source. One Omega system should be
installed so that it receives electrical power from a bus
that provides maximum reliability without jeopardizing
essential or emergency loads assigned to that bus. The other
Omega system should be installed so that it receives power
from a different bus that provides a high degree of
reliability. Any electrical power transient, including
in-flight selection of another source of power, should not
adversely effect the operation of either Omega system. After
power interruption of 7 + or - 2 seconds, the Omega equipment
should automatically resynchronize and resume normal
operation within 3 minutes without operator intervention.
After a power interruption of greater than 7 seconds and up
to 7 minutes, the Omega equipment should either automatically
resume normal operation (including proper lane resolution) or
retain the last "power-on" Omega equipment position and time
for display on command. A battery, if shown to be of
sufficient capacity, may be used to provide power for this
function. The Omega navigation system should not be the
source of objectionable electromagnetic interference, nor be
adversely affected by electromagnetic interference from other
equipment in the aircraft.
n. Steering Outputs. The Omega system should provide steering
outputs to the autopilot and/or HSI or equivalent so that the
equipment interface is compatible.
o. Airplane Flight Manual. The airplane flight manual should
contain the following information regarding the Omega
equipment:
(1) normal procedures for operating the equipment
(2) equipment operating limitations
(3) emergency/abnormal operating procedures (if applicable)
(4) procedures for reacquiring the proper lane after power
outages
p. Demonstration of Performance. An applicant for approval of
dual Omega navigation system installation should ensure that
the installed Omega system can demonstrate adequate
performance by a combination of ground and flight evaluations
defined in the following two paragraphs.
q. Equipment and Equipment Installation. Omega navigation
systems should be installed in accordance with the
airworthiness approved system installation requirements. If
evaluation flights are made for operations requiring an LRNS,
a navigation system already approved for the operator under
FAR Part 121 should be used as the primary means of
navigation.
r. Omega Training Programs. The training program curriculum must
include initial and recurrent training and checking for those
crewmembers who will be operating the Omega equipment.
Initial training programs should include the following:
(1) Instruction regarding responsibilities of flight
crewmembers, dispatchers and maintenance personnel.
(2) For the flightcrews who are to operate the Omega
equipment, instruction in the following:
(a) description of the Omega network, airborne system
description, limitations, and detection of
malfunctions;
(b) normal operating procedures including preflight
procedures and testing, data insertion and
cross-checking, en route procedures including periodic
cross-checking of system position display and
comparison between systems;
(c) updating procedures, if applicable;
(d) operations in areas of magnetic compass unreliability,
if applicable;
(e) abnormal and emergency procedures, including airborne
conditions, procedures for assessing and resolving
divergence between systems, and procedures for
reacquiring the proper lane in case of power outages
in excess of 7 seconds;
(f) a review of navigation, including flight planning and
applicable meteorology as necessary, if not addressed
in another approved training course; and
(g) compilation of terminal and/or gateway system errors.
(3) Procedures for operating the Omega navigation system
should be incorporated into the recurrent training program
for those crewmembers who are to operate the Omega
equipment.
(4) For flight crewmembers without previous Omega experience,
the training and qualification program should include an
in-flight qualification check based on the training
program. Accomplishment of such training during evaluation
flights is acceptable. Sufficient flightcrews considered
fully qualified by the applicant should be observed
in-flight by an FAA inspector to determine the overall
effectiveness of the training and qualification program.
Flightcrews possessing current operational experience with
the installed Omega equipment need only receive training
specifying any differences in procedures created by using
Omega as a sole means of long-range navigation, if
applicable.
(5) Annual line checks as required by FAR 121.440 should
include a check of Omega operating procedures. Required
annual checks of flight navigators, if they are to operate
the Omega equipment, should also include a check of these
procedures.
s. Accuracy and Reliability. The applicant should show the
following:
(1) That an adequate in-flight service reliability rate stated
in terms of in-flight mean time between failures (MTBF) is
in existence, with no significant unresolved problems
remaining.
(2) That in the process of proposed operation, the Omega
navigation system meets the accuracy requirements
stipulated for Omega navigation systems. If the proposed
system is to be operated in areas with special navigation
requirements (e.g., MNPS airspace), the accuracy required
for those areas must also be demonstrated. Systems that
become exceedingly inaccurate without displaying a warning
indication should be included in the accuracy accounting.
Systems that display a failure warning and are
subsequently shut down or disregarded should be included
in the accounting of failed systems but excluded from the
accuracy accounting.
(3) That Omega navigation systems which are subject to lane
ambiguity have a reliable means of reacquiring the proper
lane.
(4) That the Omega sole means system can meet navigation
separation requirements and have sufficient signal
redundancy to continue navigation during Omega station
outages. Equipment having the capability to process the
U.S. Navy VLF signals may utilize that feature to refine
Omega information to assist in meeting this stipulation.
(5) That within the proposed area of operation, navigation
capability is not predicated on the DR mode, and that any
interim operation in DR does not degrade navigation
accuracy and reliability beyond that required to comply
with ATC requirements.
t. Special Practices and Procedures. Since the CDU's of most
Omega systems are similar in appearance to those used for
INS, persons familiar with INS may have a tendency to assume
that Omega has similar performance characteristics. This
assumption could create significant problems. INS is a
precision DR device which is wholly self-contained within the
aircraft and has a nominal position degradation of about 1
mile per hour of flight. Omega, in contrast, continuously
resolves aircraft position by processing radio signals
received from a global network of transmitters. It is
therefore possible for Omega to be affected by signal
propagation disturbances and abnormally high local radio
noise levels. In normal operation, Omega provides a position
accuracy of 1 to 3 NM which, unlike INS, does not degrade
with increasing flight time. However, most Omega systems
compute position in signal "lanes," which are a function of
the signal wave-length. A disturbance of sufficient magnitude
may force the computed position into an adjacent lane and
thereby cause an error which is measured in multiples of the
basic lane width. This occurrence is termed a "lane slip."
Most Omega systems possess an auxiliary operating mode termed
"lane ambiguity resolution" (LAR). The purpose of this mode
is to correct the lane slip by returning the present position
to the correct lane. Details of lane ambiguity follow.
FIGURE 8-1.
OMEGA LANES FORMED BY HYPERBOLIC ISOPHASE CONTOURS
u. Omega Lanes Formed by Hyperbolic Isophase Contours. (Figure
8-1) The set of isophase contours between a station pair
forms a series of lanes, each corresponding to one complete
cycle of phase difference. In the direct ranging mode, lanes
are formed by concentric rings of zero phase with a constant
interval of one wavelength (16 NM at 10.2 kilohertz (kHz)).
In the hyperbolic mode, one complete cycle of phase
difference occurs every one half wavelength. Therefore, 10.2
kHz hyperbolic lanes are 8 NM wide on the baseline, and
gradually diverge as the distance from the baseline
increases. Each lane, or cycle of the phase, is divided into
hundredths of a lane called centilanes (cel). The phase
difference between station pairs, measured in hundredths of a
cycle or centicycles (cec), gives a hyperbolic line of
position (LOP) within an Omega lane. (The term cel refers to
the fraction of the charted lane. The term cec refers to the
phase measurement as a percentage of a cycle. At 102 kHz,
they are numerically equal and often used interchangeably,
with cec used most commonly.) For example, in Figure
8-2 phase differences of 20 cec and 50 cec between stations A
and B would give LOP's as shown. Twenty cec would indicate an
LOP 20 percent of a lane width from the lane boundary; 50 cec
would indicate an LOP 50 percent of a lane width from the
lane boundary. Fractional lane widths are taken from a given
lane boundary toward the direction of the station with the
letter designation occurring later in the alphabet (from the
"lower" letter to the "higher" letter). Since the same phase
difference will be observed at any point on an LOP, a second
LOP must be taken using another station pair to obtain a
position fix. In Figure 8-3, the phase difference
A-B is 50 cec, and the phase difference B-C is 80 cec. The
intersection of these LOP's gives a position fix. In actual
practice, propagation corrections (PPC) would be applied to
the observed phase difference readings before plotting.
* FIGURES HAVE BEEN DELETED ON THIS BBS FOR AC 91-70.
FIGURE 8-2.
PHASE MEASUREMENT WITHIN AN OMEGA LANE
FIGURE 8-3.
POSITION FIX BY INTERSECTION OF HYPERBOLIC LOP'S
v. Lane Ambiguity. In the preceding examples, it is assumed that
the aircraft's position is known to within a particular set
of lanes. Because of the cyclic nature of the phase
differences, the same phase difference can be observed in any
lane. This is known as lane ambiguity. On the baseline
between station pairs, there are about 600 10.2 kHz lanes.
Each lane is 8 NM wide on the baseline, and diverges to about
12-15 NM near the limits of coverage. The navigator must know
which of these lanes the aircraft is in before plotting a
fix. Lane ambiguity can be resolved by three methods. The
preferred method is to set the receiver's lane count at a
known location, such as the point of departure. As the
aircraft moves across lane boundaries, the receiver will
automatically update the lane identification numbers,
allowing the navigator to plot fixes with phase difference
measurements in a known lane. If the lane count is lost, the
lane count must be reset based on DR, celestial fix, or other
means. The third alternative is to derive a course lane using
multiple frequencies.
The preceding examples have considered only 10.2 kHz. Many
receivers are capable of using the other Omega frequencies
for various purposes. One such purpose is lane ambiguity
resolution. There is a 3:4 frequency ratio between 10.2 kHz
and 13.6 kHz. This relationship also applies to other
wavelengths. Three 10.2 kHz wavelengths are the same length
as four 13.6 wavelengths (Figure 8-4), or 24 NM on
the baseline in the hyperbolic mode (48 NM in the direct
ranging mode). A wavelength of 24 NM would correspond to a
frequency of 3.4 kHz, which is the difference between 10.2
and 13.6 kHz. The receiver can synthesize a 3.4 kHz Omega
signal by combining the 10.2 and 13.6 kHz signals. The 10.2
kHz lane numbers, which are evenly divisible by 3, form the
boundaries of 3.4 kHz course lanes (Figure 8-5). The
3.4 kHz phase differences can be plotted in these course
lanes. The resulting fix is then used to reset the 10.2 kHz
lane count.
* FIGURES HAVE BEEN DELETED ON THIS BBS FOR AC91-70.
FIGURE 8-4.
USING FREQUENCY DIFFERENCES TO DERIVE COURSE LANES
FIGURE 8-5.
COURSE LANE BOUNDARIES IN THE HYPERBOLIC MODE
w. Omega Navigation System Center. The Omega Navigation System
Center (ONSCEN) is the Coast Guard unit responsible for the
operational control of Omega. ONSCEN is staffed on weekdays
between 7:00 a.m. and 3:30 p.m., eastern time. During these
hours information on Omega, including the current system
status, scheduled off-air periods, and any navigational
warnings in effect may be obtained by calling (703) 866-3800.
At other times a command duty officer (CDO) is on watch and
can be contacted by calling the same number; a recorded
message will give the name and telephone number of the CDO.
Written inquiries may be addressed to: Commanding Officer,
Omega Navigation System Center, 7323 Telegraph Road,
Alexandria, VA 22310-3998. A recorded message giving the
current status of Omega is available at any time by calling
(703) 866-3801. This recording gives the dates and times of
scheduled off-air periods, any navigational warnings in
effect due to signal disturbances, and any other important
system information. Routine Omega status reports and
navigational warnings are also available through the
following means.
(1) Telex/mail. Omega status reports are issued weekly by
telex or mail to users of Omega equipment. Navigational
warnings are not issued by telex or mail. Write to ONSCEN
at the address given above.
(2) Radio broadcast. The U.S. Department of Commerce (DOC),
National Institutes of Standards and Technology,
broadcasts Omega status advisories on radio stations WWV,
Fort Collins, CO and WWVH, Kauai, HA on 2.5, 5, 10, and 15
megahertz (MHz). WWV also broadcasts on 20 MHz. Omega
status advisories are broadcast at 16 minutes past each
hour on WWV, and at 47 minutes past each hour on WWVH.
These advisories contain dates for scheduled off-airs and
any navigational warnings in effect. Because each
announcement is limited to 40 seconds, the specific times
for each off-air period may not be given.
(3) NOTAM. When alerted by the Coast Guard, the FAA issues
NOTAM's to warn of signal disturbances or unscheduled
off-air periods. Airmen should consult their local FAA
office for details regarding the issuance of Omega
NOTAM's.
x. Aviation Use of Omega. Whereas INS position errors normally
accrue gradually with elapsed flight time, most Omega errors
occur suddenly and are usually multiples of the basic lane
width. Effective cross-checking procedures should be
accomplished at regular intervals and LAR or in-flight
updating should be initiated when the position accuracy is in
doubt. In addition to the general practices and procedures
contained in Section 1, above, the following recommendations
apply to Omega systems.
(1) Preflight.
(a) Crews should be alert for any NOTAM's affecting the
operational status of the individual Omega
transmitters, particularly for possible abnormal
operation. Deselection of any station reported to be
in abnormal operation should be considered at the
onset of the flight. Also, crews should be alert for
any NOTAM's relating to the propagation disturbances,
such as sudden ionospheric disturbances, sudden phase
anomalies, or polar cap anomalies, which may affect
Omega positioning accuracy. Scheduled Omega status
broadcasts on station WWV should be monitored as a
means of obtaining current Omega information.
(b) The Omega software and modification status codes
should be verified by flightcrews to ensure that the
proper equipment is installed and that the appropriate
checklist is available and is used.
(c) At certain ground locations, particularly at congested
terminals, abnormally high radio noise levels may
adversely affect Omega. For example, synchronization
may take longer than normal or the inserted ramp
coordinates may drift after insertion. Synchronization
or DR warning lights usually indicate this situation.
This problem normally disappears, if the Omega
equipment is serviceable, shortly after the switch to
aircraft power or after the aircraft is moved from the
gate. Care should be exercised during taxi, since
abrupt turns may cause a momentary loss of signals
which could affect system accuracy. It is good
practice to cross-check present position coordinates
or taxi distance before takeoff to detect any errors
which may have occurred since initialization.
(2) In-Flight Updating. The same considerations basic to
updating an INS also apply to Omega due to the normal
accuracy and reliability of these systems. However, in
addition to the capability to update over a navaid, most
Omega systems are capable of performing an LAR if certain
signal strength and station geometry requirements are met.
Unless an apparent Omega error exceeds 6 NM, a lane slip
may not necessarily have occurred and LAR or updating is
not normally recommended. If an LAR appears to be
necessary, the LAR should be initiated on only one system
at a time so that the other system remains unaffected for
use as a cross-check. The LAR should be attempted first on
the system believed to be the least accurate.
y. Navigation Errors by Omega Equipped Aircraft. If a navigation
error is discovered by a crew of an Omega equipped aircraft,
or if a crew of an Omega equipped aircraft is notified of a
navigation error by ATC, a report containing the information
listed in Figure 8-6 should be submitted to the FAA. This
information should be sent by mail or facsimile (fax) to the
FSDO nearest the aircraft's base of operation or, if
applicable, to the FSDO that holds the operator's operating
certificate.
FIGURE 8-6. NAVIGATION DEVIATION REPORT FOR OMEGA EQUIPPED AIRCRAFT
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1. Details of aircraft and reported error.
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Name of operator:
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Aircraft identification:
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Date/time of observed error:
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Flight level (FL):
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Position (lat/long):
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Approximate cross-track deviation (NM):
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2. Was Omega being used as the primary means of navigation and steering
guidance?
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3. Do you consider failure of, or difficulty with, the Omega system as a
contributory cause of the deviation? (If not, do not complete items
5-10)
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4. Manufacturer of Omega equipment, type of equipment, most recent
modification date.
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5. Give details of cleared track within NAT oceanic airspace.
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6. Give details of any problems experienced with Omega, together with
the approximate geographic location.
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7. Give details of Omega/VLF signals used and received signal strength.
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8. Have there been previous difficulties with the Omega installation? If
so, give details.
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9. Have any faults been discovered during general checks/maintenance
work?
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10. What rectification work has been performed?
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11. Please provide any additional information that you feel is relevant.
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8. GLOBAL POSITIONING SYSTEMS (GPS) GENERAL INFORMATION.
a. GPS Navigation. The GPS is a satellite-based radio navigation
system that uses precise range measurements from GPS
satellites to determine a precise position anywhere in the
world. The GPS constellation consists of 24 satellites in
various orbital planes approximately 11,000 nautical miles
(NM) above the earth. The satellites broadcast a timing
signal and data message that the airborne equipment processes
to obtain satellite position and status data, and to measure
how long each satellite's radio signal takes to reach the
receiver. By knowing the precise location of each satellite
and precisely matching timing with the atomic clocks on the
satellites, the receiver can accurately measure the time the
signal takes to arrive at the receiver and thus determine the
satellite's precise position. A minimum of three satellites
must be in view to determine a two-dimensional position. Four
satellites are required to establish an accurate
three-dimensional position. GPS equipment determines its
position by precise measurement of the distance from selected
satellites in the system and the satellite's known location.
The accuracy of GPS position data can be affected by various
factors. Many of these accuracy errors can be reduced or
eliminated with mathematics and sophisticated modeling, while
other sources of errors cannot be corrected. The following
are examples of those errors which cannot be corrected:
(1) Atmospheric propagation delays can cause relatively small
measurement errors, typically less than 100 feet.
Ionospheric propagation delays can be partially corrected
by sophisticated error-correction capabilities.
(2) Slight inaccuracies in the atomic clocks on the satellites
can cause a small position error of approximately 2 feet.
(3) Receiver processing (such as mathematical rounding and
electrical interference) may cause errors that are usually
either very small (which may add a few feet of uncertainty
into each measurement) or very large (which are easy to
detect). Receiver errors are typically on the order of 4
feet.
(4) Conditions that cause signal reflections before the
satellite's transmitted signal gets to the receiver can
cause small errors in position determination or momentary
loss of the GPS signal. While advanced signal processing
techniques and sophisticated antenna design are used to
minimize this problem, some uncertainty can still be added
to a GPS measurement.
(5) A satellite's exact measured orbital parameters (ephemeris
data) can contain a small error of approximately 4 feet.
b. System Operation.
(1) The Department of Defense (DOD) is responsible for
operating the GPS satellite constellation and constantly
monitors the GPS satellites to ensure proper operation.
Every satellite's ephemeris data are sent to each
satellite for broadcast as part of the data message sent
in the GPS signal. The GPS is a system of cartesian
earth-centered, earth-fixed coordinates as specified in
the DOD World Geodetic System 1984 (WGS-84). Navigation
values, such as groundspeed and distance and bearing to a
waypoint, are computed from the aircraft's
latitude/longitude and the location of the waypoint.
Course guidance is usually provided as a linear deviation
from the desired track of a Great Circle course between
defined waypoints.
(2) GPS navigation capability from the 24 satellite
constellation is available 24 hours a day anywhere in the
world. GPS status is broadcast as part of the data message
transmitted by the satellites. Additionally, system status
is planned to be available through Notices to Airmen
(NOTAM). Status information is also available by means of
a telephone data service from the U.S. Coast Guard.
Availability of suitable navigation capability from the
satellite constellation is expected to approach 100
percent.
(3) GPS signal integrity monitoring will be provided by the
GPS navigation receiver using receiver autonomous
integrity monitoring (RAIM). For GPS sensors that provide
position data only to an integrated navigation system
(e.g., FMS, multisensor navigation system), a level of GPS
integrity equivalent to that of RAIM may be provided by
the integrated navigation system. Availability of RAIM
capability to meet nonprecision approach requirements in
the United States with the 24 satellite constellation is
expected to exceed 99 percent.
c. Selective Availability (SA). SA is essentially a method by
which DOD can artificially create a significant clock and
ephemeris error in the satellites. This feature is designed
to deny an enemy nation or terrorist organization the use of
precise GPS positioning data. SA is the largest source of
error in the GPS system. When SA is active, the DOD
guarantees horizontal position accuracy will not be degraded
beyond 100 meters 95 percent of the time, and beyond 300
meters 99.99 percent of the time.
d. Portable Units. All portable electronic systems and portable
GPS units must be handled in accordance with the provisions
of FAR 91.21. The operator of the aircraft must determine
that each portable electronic device will not cause
interference with the navigation and communications systems
of the aircraft on which it is to be used. Portable GPS units
which are attached by Velcro tape or hard yoke mount that
require an antenna (internally or externally mounted) are
considered to be portable electronic devices and are subject
to the provisions of FAR 91.21. All portable GPS equipment
attached to the aircraft by a mounting device must be
installed in an approved manner and in accordance with FAR
Part 43. Questions concerning installation should be referred
to an avionics or airworthiness inspector. A critical aspect
of any GPS installation is the installation of the antenna.
Shadowing by the aircraft structure can adversely affect the
operation of the GPS equipment. FAA approval of avionic
components, including antennas, requires an evaluation of the
applicable aircraft certification regulations prior to
approval of an installation. The regulations require that the
components perform their intended functions and be free of
hazards in and of themselves and to other systems as
installed. Pilots should be aware that a GPS signal is weak,
typically below the value of the background noise. Electrical
noise or static in the vicinity of the antenna can adversely
affect the performance of the system. It is recommended that
system installations be flight tested in conjunction with
other navigation equipment prior to using the system for
actual navigation. Unless a portable GPS receiver is TSO
C-129 approved, it is not to be used as a basis for approval
of operations in the NAT MNPS.
e. Navigation Classes. All navigation performed in flight is
either Class I or Class II navigation.
(1) Class I navigation: Any en route flight operation or
portion of a flight operation conducted in an area
entirely within the officially designated operational
service volumes of ICAO standard airways navigation
facilities (VOR, VOR/DME, NDB). The two generic types of
Class I navigation are navigation by direct reference to
ICAO standard navaids and navigation by use of area
navigation systems.
(2) Class II navigation: Any operation or portion of an en
route operation which takes place outside (beyond) the
officially designated operational service volumes of ICAO
standard navaids (VOR, VOR/DME, NDB). Any en route flight
operation or portion of a flight operation which is not
Class I navigation. There are three generic classes of
Class II navigation. These are navigation by reference to
ICAO standard navaids supplemented by dead reckoning,
navigation by use of pilot-operated electronic long-range
navigation systems (e.g. INS, Omega, GPS), and navigation
by use of a flight navigator.
f. RAIM. A technique whereby a civil GPS receiver/processor
determines the integrity of the GPS navigation signals using
only GPS signals or GPS signals augmented with altitude. This
determination is achieved by a consistency check among a
series of satellites being tracked. At least one satellite in
addition to those required for navigation must be in view for
the receiver to perform the RAM function.
g. Supplemental Air Navigation System. An FAA-approved
navigation system that can be used in addition to a required
means of air navigation. May be used as the primary
navigation system provided an operational approved alternate
means of navigation suitable for the route of flight is
installed on the aircraft.
h. System Availability. The percentage of time (specified as 98
percent) that at least 21 of the 24 satellites must be
operational and providing a usable navigation signal.
9. FAA APPROVAL OF GPS EQUIPMENT.
a. GPS Equipment Classes. GPS equipment is categorized into
classes A(), B(), and C() (ref. TSO-C129).
(1) Class A(). Equipment incorporating both the GPS sensor and
navigation capability. This equipment incorporates RAIM.
0 Class A1 equipment includes en route, terminal, and
nonprecision approach navigation capability.
0 Class A2 equipment includes en route and terminal
navigation capability only.
(2) Class B(). Equipment consisting of a GPS sensor that
provides data to an integrated navigation system (i.e.,
flight management system, multi-sensor navigation system,
etc.).
0 Class B1 equipment includes RAIM and provides en
route, terminal, and nonprecision approach capability.
0 Class B2 equipment includes RAIM and provides en route
and terminal capability only.
0 Class B3 equipment requires the integrated navigation
system to provide a level of GPS integrity equivalent
to RAIM and provides en route, terminal, and
nonprecision approach capability.
0 Class B4 equipment requires the integrated navigation
system to provide a level of GPS integrity equivalent
to RAIM and provides en route and terminal capability
only.
(3) Class C(). Equipment consisting of a GPS sensor that
provides data to an integrated navigation system (i.e.,
flight management system, multi-sensor navigation system,
etc.), which provides enhanced guidance to an autopilot or
flight director in order to reduce flight technical error.
Installation of Class C() equipment is limited to aircraft
approved under FAR Part 121 or equivalent criteria.
0 Class C1 equipment includes RAIM and provides en
route, terminal, and nonprecision approach capability.
0 Class C2 equipment includes RAIM and provides en route
and terminal capability only.
0 Class C3 equipment requires the integrated navigation
system to provide a level of GPS integrity equivalent
to RAIM and provides en route, terminal, and
nonprecision approach capability.
0 Class C4 equipment requires the integrated navigation
system to provide a level of GPS integrity equivalent
to RAIM and provides en route and terminal capability
only.
NOTE: Operators requiring additional GPS approval information
are referred to the following AC's: AC 20-130,
"Airworthiness Approval of Multi-Sensor Navigation
Systems for Use in the U.S. National Airspace System
(NAS) and Alaska," and AC 20-XXX, "Airworthiness
Approval of Global Positioning System (GPS) Navigation
Equipment for Use as a VFR and IFR Supplemental
Navigation System" (This AC was formerly FAA Notice
N8110.47).
b. Approval Criteria. A GPS installation with a TSO C-129
authorized navigation system in Class A1, A2, B1, B2, C1, or
C2 may be used in combination with other approved LRNS for
unrestricted operations in NAT MNPS airspace or may be used
as the sole means of long-range navigation on the special
routes that have been developed for aircraft equipped with
only one LRNS and on the special routes developed for
aircraft equipped with short-range navigation equipment. The
basic integrity for these operations must be provided by RAIM
or an equivalent method. A single GPS installation in Class
A1, A2, B1, B2, C1, or C2 which provides RAIM for integrity
monitoring may also be used on those short oceanic routes
which have only one required means of long-range navigation.
c. Avionics. Documentation must be provided which validates
approval of the installed GPS airborne receiver in accordance
with Notices 8110.47, 8110.48, AC 20-129 and AC 20-130A, as
appropriate, or other applicable airworthiness criteria
established for GPS installations. When it has been
established that the airborne system has been certified for
GPS IFR operations, the following criteria should be used to
determine the operational suitability of airborne systems for
GPS IFR use:
(1) Initial Installations and Continued Airworthiness. The
operator must ensure that the equipment is properly
installed and maintained. No special requirements, other
than the standard practices currently applicable to
navigation or landing systems, have been identified that
are unique to GPS, e.g., Airworthiness Directives, Service
Bulletins.
(2) Action. Aviation safety inspectors must evaluate
installation (An avionics inspector should evaluate the
avionics installation and recommend the approval prior to
the issuance of an LOA to operate in NAT MNPS airspace.),
crew capabilities, and operational responsibilities
relative to GPS oceanic operations prior to issuing an LOA
for operation in MNPS. Specific items to check are as
follows:
(a) The GPS navigation equipment used must be approved in
accordance with the requirements specified in TSO
C-129 and the installation must be made in accordance
with Notice 8110.47 or 8110.48 or the AFS/AIR joint
guidance memorandum dated July 20, 1992.
(b) The basic integrity for these operations must be
provided by RAIM or an equivalent method.
(c) The GPS operation must be conducted in accordance with
the FAA-approved flight manual or flight manual
supplement, if required.
(d) Aircraft using GPS equipment under IFR must be
equipped with an approved and operational alternate
means of navigation appropriate to the route to be
flown. This traditional navigation equipment must be
actively used by the flightcrew to monitor the
performance of the GPS system.
(e) Procedures must be established for use in the event
that significant GPS navigation outages are predicted
to occur. In situations where this is encountered, the
flight must rely on other approved equipment, delay
departure, or cancel the flight.
(f) Aircraft navigating by GPS are considered to be RNAV
aircraft. Therefore, the appropriate equipment suffix
must be included in the ATC flight plan.
10. GPS OPERATIONS SPECIFICATIONS.
Air carrier operators planning on utilizing GPS are required to have
their operations specifications amended prior to performing
operations utilizing GPS. The specific operations specifications
items that must be considered are as follows:
0 En Route authorization for class I navigation.
0 En Route authorization for class II navigation using a single
GPS.
0 En Route authorization for class II navigation using GPS and
a second Long Range Navigation System.
0 Authorization for use of GPS in Central East Pacific (CEPAC)
Airspace.
0 Authorization for use of GPS in Northern Pacific (NOPAC)
Airspace.
0 Authorization for use of GPS in North Atlantic MNPS Airspace.
0 Authorization to conduct operations in Areas of Magnetic
Unreliability with GPS.
0 Authorization for use of GPS to conduct Nonprecision
Instrument Approach Procedures in Airplanes.
0 Authorization for use of GPS to conduct Nonprecision
Instrument Approach Procedures in Rotorcraft.
Approaches using GPS equipment are subject to the following
limitations:
(1) The GPS equipment used must be approved for IFR
operations, including nonprecision approaches, and the GPS
constellation and the required airborne equipment must he
providing the levels of accuracy, continuity and integrity
required for that operation.
(2) The flightcrew must have successfully completed the
approved training program and demonstrated competency in
these operations.
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Air Traffic Organization: Operations Planning- International |