Histories of Flights

DHL Flight 611

DHL International Flight 611 had originated from Bahrain as a cargo flight to Brussels, Belgium with an intermediate stop at Bergamo, Italy. The accident airplane was an all-cargo configuration of the Boeing 757-200 airliner equipped with a side cargo door. The only persons aboard were the captain and first officer.

The airplane departed from Bahrain at 1330 and landed at Bergamo at 1910, where it was refueled and unloading/loading operations were conducted.

At 2106 the airplane departed for the flight to Brussels. The first officer was the pilot flying (PF) on this leg of the flight, while the captain was the pilot not flying (PNF) and was responsible for communications with air traffic control (ATC).

The flight was conducted under instrument flight rules (IFR) with a planned cruise speed of 463 knots and altitude of 36,000 feet (FL 360).

The airplane proceeded on a northwest heading, crossing the Swiss border near Lugano, Switzerland. From there, it was planned to fly north until reaching a navigational aid near Stuttgart, Germany, then turn northwest toward a waypoint near Liège, Belgium, and finally fly west to Brussels.

Bashkirian Airlines Flight 2937

Bashkirian Airlines Flight 2937 was a charter passenger flight from Moscow, Russia to Barcelona, Spain, carrying 60 passengers, a cabin crew of four, and flight crew of five.

The commander of Bashkirian Airlines Flight 2937 was sitting in the left pilot's seat, while the right seat was occupied by an instructor pilot providing airport-specific training to qualify the commander for landing in Barcelona. Behind them, facing the central instrument pedestal, was the flight navigator. Further aft, the flight engineer was in the right-hand seat and a copilot occupied the left rear seat. The copilot had no assigned functions during the flight. (Illustration of Bashkirian Airlines TU154M Cockpit).

During the investigation, a discrepancy in the documentation was noted regarding the role of pilot in command (PIC). The documents for this flight provided by the aircraft operator listed the commander as PIC, while the National Aviation Authority identified the instructor as PIC, according to their regulations. The opinion of the investigating authority, BFU is that the instructor was the PIC.

Bashkirian Airlines Flight 2937 departed Moscow at 1848 and flew on a southwest heading over Belarus, Poland, the Czech Republic, and Austria. Near Vienna, Austria, the aircraft turned west, then entered German airspace and headed towards Switzerland.

Air Traffic Control (ATC) Services

The crossing point of the planned routes of the two aircraft was located over Lake Constance, in a region where the country borders of Austria, Germany, and Switzerland meet.

In this area, the airspace boundaries do not overlap with the borders of the respective countries to efficiently manage overflying air traffic and facilitate the transfer of flights between neighboring air traffic control units.

As part of a bilateral arrangement between the German and Swiss air navigation services providers, the control of the airspace where the collision occurred is delegated to the Zurich Area Control Center (ACC) in Switzerland.

On the night of the collision, the controller at Zurich ACC was alone on position, controlling the entire Zurich airspace from a single workstation. Having one controller on duty for the night shift was normal practice, considering the low traffic volume at night. However, shortly before the accident, the controller received an unexpected flight inbound to Friedrichshafen airport. This unscheduled airplane, an Airbus A320, required the controller to physically move multiple times to a different radar monitor ten feet away from the radar monitor being used to communicate with the overflying TU154M and B757. As a result of this need to continually move between work stations, a lapse in attention to the accident aircraft developed.

Additionally, the visual Short-Term Conflict Alert (STCA) system was inoperative because of maintenance activity being performed that night on the main ATC system. The telephone system was also inoperative due to planned maintenance, and the backup system was programmed incorrectly, making it impossible for nearby ACC controllers to contact the Zurich ACC controller and relay any STCA warnings appearing on their displays.

Introduction to ACAS/TCAS

In the decades preceding this accident, the frequency of midair collisions increased as the volume of traffic in international airspace increased. In the 1950's, this resulted in the development of radar-based air traffic control systems in many areas and in many countries. This technological advancement in air traffic control reduced, but did not eliminate, the occurrence of midair collisions, especially in areas with high traffic volumes, such as near airports. In order to further mitigate the associated risks, airborne collision avoidance systems were developed.

The earliest Traffic Collision Avoidance System (TCAS) was developed in the 1980's. TCAS, also referred to as Airborne Collision Avoidance System (ACAS), is an airborne/aircraft-installed collision avoidance system based on the airplane's transponder, a radio transceiver carried on-board as part of the Air Traffic Control Radar Beacon System (ATCRBS). In its primary function, the transponder transmits an assigned code when interrogated by a ground station. This provides a means to identify a specific aircraft on a radar display. The most recent transponder design variants (Mode C and Mode S), broadcast a transponder reply that includes the pressure altitude of the aircraft. In addition to this ground-based application, the same transponder is used by TCAS as an airborne system to detect and avoid traffic by mutual transponder interrogation among nearby aircraft.

 

The TCAS computer unit transmits interrogation signals to nearby aircraft through a directional antenna that is mounted on the top of the aircraft and either an omnidirectional or a directional antenna mounted on the bottom of the aircraft. The same antennas receive signals from the transponders of other aircraft flying up to 40 nautical miles (NM) in front, up to 15 NM behind, up to 20 NM to either side, and up to approximately 9,000 feet above and below the airplane.

The TCAS computer unit analyzes signals from other aircraft to determine the bearing, range, speed, and closure rate of each nearby aircraft, and acquires its pressure altitude via Mode S (altitude and identification data) and, if available, Mode C (altitude encoder and altimeter). Having determined its own position relative to the nearby traffic, the TCAS computer unit computes the closest point of approach (CPA) for each target aircraft and calculates time remaining before reaching CPA, or tau.

The CPA and tau for each aircraft being tracked are evaluated against the thresholds programmed in the Collision Avoidance System (CAS) logic to detect any threat situation. If the vertical separation with a conflicting aircraft can no longer be ensured, a warning is generated.

The tau, at which the different levels of warnings are triggered, depends on the CAS logic, which has a variable sensitivity level according to the altitude of the aircraft.

The following thresholds are applicable for altitudes greater than 20,000 feet:

• When the tau drops below 48 seconds, the unit generates a Traffic Advisory (TA).
• At tau of 35 seconds, the unit generates a Resolution Advisory (RA).

TCAS warnings are both aural and visual. The visual representation depends on the avionic architecture of the flight deck. For example, on aircraft with a full Electronic Flight Instrument System (EFIS), also known as a "glass cockpit" layout, the traffic situation is integrated in the navigation display picture, while the additional information provided during a RA is displayed on the primary flight display.

Earlier aircraft that have been updated with TCAS at a later stage of their service life have the traffic and the RA information presented on a combined Vertical Speed Indicator/Traffic Resolution Advisory (VSI/TRA) display, which replaces the conventional VSI on the front instrument panel of the flight deck.

Both aircraft involved in the Überlingen collision had been retrofitted with a VSI/TRA display via a Supplemental Type Certificate.

A standardized symbology has been developed to display the nearby traffic according to the level of threat as determined by the CAS logic, the relative altitude, and a vertical trend arrow.

Expected crew response

Flight crew response is critical for the correct functioning of TCAS II and a number of required pilot actions were assumed when designing the system.

• In case of a TA, the white-filled diamond symbol of Proximate Traffic turns into a filled yellow/amber circle and an aural warning "TRAFFIC TRAFFIC" is annunciated. At this point, the flight crew should not initiate an avoidance maneuver based on the traffic display since the system does not provide sufficient precision, and the action would be uncoordinated with the conflicting aircraft. The flight crew is expected to attempt to establish visual contact with the conflicting aircraft and other traffic in the vicinity, and to be prepared for an RA.
• When TCAS issues an RA, corrective actions are displayed to the crew in the form of vertical maneuvers, which are coordinated between the TCAS computer units in the involved aircraft. The yellow/amber symbol of a TA traffic turns into a red-filled square. The advised avoidance action is presented as a green/red arc on the VSI/TRA indicator, showing the allowable range of vertical speeds to avoid the converging traffic. The aural warning confirms the advised action with the basic advisories "CLIMB, CLIMB" or "DESCEND, DESCEND" or more elaborated indications in specific cases. Whenever a basic CLIMB or DESCENT RA is issued, the flight crew is expected to react immediately and achieve a climb or descent of 1,500 ft/min within five seconds. The design expectation was that the RA advised maneuver occurs without clearance from, or coordination with, air traffic control. Once workload permits, the respective flight crews should advise ATC that they have performed an RA maneuver.

Two short videos, created by the FAA for this accident module, depict a TCAS encounter. The first video demonstrates a TA (FAA TA video) and the second an RA (FAA RA video), both illustrating TCAS basic functions.  Further, a training video, produced by Northwest Airlines and obtained from Delta Airlines describes TCAS and its integration into the airspace. The video is available at the following link: (Traffic Collision And Avoidance System (TCAS) Video).

During an RA encounter, the TCAS continuously evaluates the situation. If the RA does not result in a successful separation of the projected positions of the conflicting aircraft at their closest point of approach, the TCAS logic amends the original RA to achieve a better separation. The TCAS logic includes the following additional types of RA:

• Strengthening RA, requiring the flight crew to increase the vertical speed from 1,500 feet/min to 2,500 feet/min in the same vertical direction of the original RA
• Reversal RA, requiring the flight crew to achieve a vertical speed of 1,,500 feet/min in the opposite vertical direction of the original RA.

A depiction of a TCAS encounter, where the original "DESCENT" RA is momentarily amended with an "INCREASE DESCENT" RA and immediately after reversed to a "CLIMB" RA, is demonstrated in a video created by the FAA and is available at the following link: (FAA Increase/Reversal RA Video).

If both involved aircraft are TCAS II/Mode S equipped, a TCAS RA requires a coordinated action by both involved flight crews. If one of the involved aircraft is only Mode C equipped, then an RA will only be presented in the TCAS II/Mode S equipped aircraft. The effectiveness of the system as a "last resort" safety net against collision relies on operational procedures being complete, clear, unambiguous, and disseminated among all TCAS users through consistent training programs.

A more complete description of the TCAS II system is available in the FAA document, Introduction to TCAS II, and is provided at the following link: (Introduction to TCAS II).

Final sequence of events

Investigators reconstructed the sequence of events leading to the collision via analysis of the information from the aircraft recording devices. Data from the flight data recorder and cockpit voice recorder of both aircraft, the salvaged memory module of the TCAS computer unit of Bashkirian Airlines Flight 2937, and ATC communications and radar tracks recorded at the ACC Zurich and nearby ATC services were all used in the analysis.

A common timeline was created for both aircraft based on their initial contact times with ACC Zurich.

A more detailed summary of the events in the last minute before the collision is available in the following table. 

Sequence of Events

Time	Bashkirian Flight 2937	DHL Flight 611	ACC Zurich	Aero-Lloyd Flight 1135
21:21:50		While approaching the Swiss border at FL 260, DHL International Flight 611 contacts ACC Zurich. It is cleared direct to a VOR on a northern heading and to climb from FL 260 to FL 320. The crew requests a climb to FL 360.
21:26:36		The climb to FL 360 is approved.
		The aircraft reaches FL 360.
21:30:11	Initial contact of Bashkirian Airlines Flight 2937 with Zurich ACC.
21:33:00	The crew of Bashkirian Airlines Flight 2937 notices on TCAS an aircraft approaching from the left at the same altitude, still classified as proximate traffic only. A discussion followed in the cockpit and an outside visual scan was carried out to visually acquire the other aircraft.
21:34:30		The copilot, acting as pilot flying, hands over the control of the aircraft to the PIC and goes to the lavatory in the back of the cockpit. No mention of any abnormal situation. The converging traffic has not been detected yet, possibly due to the TCAS display having been set to a low display range.	The controller was busy on the radar monitor and the related radio channel for the Friedrichshafen approach sector to manage the unexpected, late arrival of Aerolloyd 1135.
21:34:36	Commander: "Here it is in sight. Look here, it indicates zero", most probably with reference to the relative altitude of the other traffic as displayed on the VSI/TRA.
21:34:42	The time-to-go to the CPA (tau) is approximately 50 seconds, triggering a TCAS traffic alert. The traffic symbol on the VSI/TRA of both aircraft turns into a solid yellow/amber circle and the aural warning "TRAFFIC, TRAFFIC" is annunciated in both flight decks.
21:34:48	The flight crew Bashkirian Airlines Flight 2937 acknowledges by calling out "Traffic, Traffic""
21:34:49			the BTC2937, ah descend flight level ah… 350, expedite, I have crossing traffic
21:34:54	The PIC in the right seat of Bashkirian Airlines Flight 2937 announces "Descend!".
21:34:56	TCAS "CLIMB" RA A short discussion between the PIC and the copilot took place. The copilot challenged the descend decision by the commander, suggesting: "It [the TCAS] says 'climb'". The commander stated: "He [the ATC controller] is guiding us down." A further question "descend?" by the copilot.	TCAS "DESCEND" RA The PIC of DHL International Flight 611 adopted the standard response to a TCAS "DESCEND, DESCEND" RA, disconnected the autopilot to initiate a descent, reaching a descent rate of 1,500 feet/min in about 12 seconds.
21:35:00			STCA (acoustic)
21:35:03			"BTC2937, descend level 350, expedite descent".
21:35:07	"Expedite descent level 350, BTC2937".
21:35:10		INCREASE DESCENT
21:35:13			"We have traffic at your… 2 o'clock now at 360."
21:35:19		"Dilmun six hundred, TCAS descend."	The controller was busy on the radar monitor for the Friedrichshafen approach sector and the related radio channel to release Aerolloyd 1135 to the tower controller.
21:35:24	INCREASE CLIMB
21:35:27	Comment by the copilot: "It says 'climb'".
21:35:32	The two aircraft collided at an altitude level of 34,890 feet. The Bashkirian Airlines Tupolev TU154M, flying on a magnetic course 274°, impacted the tail of the DHL Boeing 757, which was flying on a magnetic course of 004°.
21:36:01			The radar controller, having managed the arrival aircraft, moved back to the radar monitor for the overflying traffic and tries unsuccessfully to get in contact with Bashkirian Airlines Flight 2937.

 

 

Between 21:34:44 and the collision the controller of the adjacent German ATC service, Upper Area Control (UAC) Karlsruhe, had tried to call ACC Zurich three times through the main phone system to warn about the STCA he was receiving on his own radar display. The controller in ACC Zurich, however, did not answer.

The controller in UAC Karlsruhe could have transmitted a general warning on the international emergency frequency. However, according to regulations, controllers must coordinate with the responsible controller when undertaking any activity outside their own area of responsibility. In this case, landline communication could not be established as the ACC Zurich telephone system was not functioning.

An animation of the accident sequence is available at the following link: (TU154 / B-757 Collision Flight Path Animation).

TCAS II Safety Philosophy and Flight Crew Training

Investigators determined that differences in TCAS operational philosophies and training between the two involved air carriers were a major factor in the occurrence of this accident. Investigators cited the TCAS manufacturer’s "Pilot's Guide" as being somewhat ambiguous and not sufficiently explicit to result in adherence to TCAS RA instructions in every case.  Investigators focused on the words "TCAS 2000 is a backup to the ATC (Air Traffic Control) system and the 'see and avoid' concept."  Investigators believed that these words could be interpreted to mean that ATC instructions always took priority.  In the investigators view, it was not made sufficiently clear that TCAS was intended as a "last line of defense," and TCAS instructions must be followed, even if contrary to ATC instructions.

In another section of the manufacturer's "Pilot's Guide" ("Pilot Responsibilities"), investigators again concluded that the language used was not sufficiently direct. In one case, they noted that TCAS was characterized as being a "backup for ATC," while later in the same section emphasized the need to follow TCAS instructions even when contrary to ATC instructions.

The Flight Operations Manual for the TU154M made clear that TCAS instructions were secondary to ATC instructions, and investigators focused on a passage in the manual that provided the specific instruction: "...the correct execution of all instructions issued by ATC to be regarded as the most important tool.  TCAS is an additional instrument which ensures timely determination of oncoming traffic...and, if necessary, planning of an advice for a vertical avoidance manouevre."

Investigators further determined that the operations manual for the operator of the B757 was based on the FAA Advisory Circular, which in the investigators view was "clear and unambiguous" relative to required crew responses in the event of an RA. 

Conclusions

The accident report provided the following summary: 

 

"The RA has the highest priority, because it will only be issued if other collision avoidance mechanisms, such as vertical separation by a controller, are not sufficiently effective or are incorrect. The manual intervention in the control of the airplane by the pilot must then take place without delay. A coordination with the controller or a clarification of the situation by means of other airborne devices following an RA would question the purpose of TCAS. The time left in such a case could be too short for an avoidance manoeuvre and would increase the collision risk."

 

Investigators concluded that both flight crews were following operational procedures as developed in their individual training philosophies; however, the distinct differences, one following ATC instructions and the other following TCAS instructions, resulted in the accident.

Accident Memorials

Memorials dedicated to the victims were placed at two different locations:

• An open-air installation at the crash site near Überlingen, Germany
• A memorial area in the Southern Cemetery in Ufa, the capital of the Russian Republic of Bashkiria, which was the home of most of the passengers on Bashkirian Airlines flight

 

 

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The findings of the German Federal Bureau of Accident Investigation (BFU) covered six different areas:

• Condition of both airplanes and their TCAS equipment (6 findings)
• Circumstances of the accident (8 findings)
• Licenses and the medical qualifications of the flight crews (2 findings)
• Flight operation manuals and training programs of both operators in the parts related to TCAS (4 findings)
• ATC environment covering the organization and staffing of the Zurich ACC, the on-going maintenance activities on the radar system, the workload of the single air traffic controller on duty at the time of the accident, and a summary of the ATC-related events that preceded the collision (23 findings)
• ACAS/TCAS-generated advisories, the response of both crews and the air traffic controller, and the integration of ACAS/TCAS in the aviation system (9 findings)

The BFU identified the following immediate causes:

• Imminent separation infringement was not noticed by ATC in a timely manner. The instruction for the TU154M to descend was given at a time when the prescribed separation to the B757-200 could not be ensured.
• The TU154M crew followed the ATC instruction to descend and continued to do so even after TCAS advised them to climb. This maneuver was performed contrary to the generated TCAS RA.

Additionally, the BFU listed several systemic causes:

• The integration of ACAS/TCAS II into the system aviation was insufficient and did not correspond in all points with the system philosophy
• Regulations concerning ACAS/TCAS published by ICAO and the operational and procedural instructions of the TCAS manufacturer and the operators were unstandardized, incomplete, and partially contradictory
• Management and quality assurance of the air navigation service company did not ensure that during the nighttime operations all open workstations were continuously staffed by controllers
• Management and quality assurance of the air navigation service company accepted the practice that, during times of low traffic flow at night, only one controller worked and the other one retired to rest

An abstract of the BFU final accident report with the complete text of the findings and the causes is available at the following link: (BFU Conclusions).

The complete accident report is available at the following link: (BFU Accident Report).

 

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On October 1, 2002, the BFU issued the following Safety Recommendation to ICAO:

Safety Recommendation No. 18/2002

Change to Annex 2, Annex 6 and PANS-OPS (DOC 8168) to assign priority to TCAS Resolution Advisories (RA) over ATC instructions.

On July 21, 2003 the BFU issued the following Safety Recommendations to the Federal Office for Civil Aviation (FOCA), Switzerland:

Safety Recommendation No. 01/2003
Implementation of procedures for ATC system maintenance work.

Safety Recommendation No. 02/2003
Minimum staffing in ACC Zurich.

Safety Recommendation No. 03/2003
Initial and recurrent training for air traffic controllers covering theoretical and practical (simulator) emergency procedures.

On May19, 2004 the BFU issued the following Safety Recommendation to ICAO: Safety Recommendation No. 06/2004
Uniformity, clarity and unambiguity of rules and procedures regarding ACAS.

Safety Recommendation No. 07/2004
Education and training to ensure acceptance and confidence of flight crews in ACAS.

Safety Recommendation No. 08/2004
Downlink of TCAS RA to ATC.

Safety Recommendation No. 09/2004
Sound recorders at air traffic control workstations to support accident and incident investigation.

Safety Recommendation No. 16/2004
Adoption of measures (including audits) to ensure consistency of response of all ACAS/TCAS users.

On May 19, 2004 the BFU issued the following Safety Recommendations to the Federal Office for Civil Aviation (FOCA), Switzerland:

Safety Recommendation No. 10/2004
Minimum requirements for short term conflict alert at ATC units.

Safety Recommendation No. 11/2004
Redundancy of telephone systems at ATC units and minimum refresh rate for en-route radar displays.

Safety Recommendation No. 13/2004
Requirements for system recording/replay facilities at ATC units.

Safety Recommendation No. 17/2004
Effective operation of the Safety Management System of the air traffic service provider.

Safety Recommendation No. 18/2004
Evaluation of the staffing level required by the air traffic service provider.

Safety Recommendation No. 19/2004
Refresher and safety-related training for air traffic controllers according to the applicable Eurocontrol standards.

 

On May 19, 2004 the BFU issued following Safety Recommendation to the Civil Aviation Authority (CAA) of the Russian Federation:

Safety Recommendation No. 14/2004
Compliance of ACAS training for flight crew with Attachment B to ICAO State letter AN 11/19-2/82, including simulator or computer based training sessions and integration of ACAS in Crew Resources Management (CRM) and Line-Oriented Flight Training (LOFT) programs.

Safety Recommendation No. 21/2004
Improvement of CRM and LOFT training.

 

On May 19, 2004 the BFU issued following Safety Recommendation to the Federal Aviation Administration (FAA) of the United States of America:

Safety Recommendation No. 15/2004
Revision of the TCAS 2000 Operating Manual by the manufacturer to ensure consistency and unambiguity in stating ACAS/TCAS system philosophy, international regulations, and operating procedures.

The complete text of the recommendations is available at the following link: (BFU Recommendations)

 

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One of the key points in the Überlingen collision accident report was the investigation of the regulatory framework related to TCAS II. In the introduction to the report section regarding the legal basis, procedures, and procedural instructions for TCAS II, it is stated that:

"As TCAS II, Version 7 is designed as a semiautomatic system which shall serve as a "last line of defense" in collision avoidance, clear and unambiguous procedural instructions for the crews are an essential prerequisite. This prerequisite is so important, because the system philosophy of TCAS II, Version 7 provides only one procedure after the issuance of an RA and that is to follow the generated RA."

In the accident report, abstracts are included from the relevant regulations at international, regional, and national level, and from the handbooks issued by the involved airline operators, aircraft manufacturers, and TCAS equipment suppliers, as valid at the time of the accident. The accident report also focused on the consistency of these documents in the areas of TCAS philosophy, procedures and integration in the ATC environment, of right-of-way definitions, and of flight crew training.

The following paragraph provides the references to all the regulations cited in the accident report.

Publications of the International Civil Aviation Organization (ICAO)

Annex 2 to the Convention on International Civil Aviation - Rules of the Air

Paragraph 3.2.2, Right-of-way

Annex 10 to the Convention on International Civil Aviation - Aeronautical Telecommunications

Attachment A, paragraph 3.5.8.10.3

Annex 11 to the Convention on International Civil Aviation - Air Traffic Services

Chapter 2, General

Doc 8186 - Procedures for Air Navigation Services - Aircraft Operations (PANS-OPS)

Chapter 3, Operation of ACAS Equipment

Doc 4444 - Procedures for Air Navigation Services - Air Traffic Management (PANS-ATM)

Paragraph 15.6.3 Procedures in regard to aircraft equipped with airborne collision avoidance systems (ACAS)

Doc 7030/4 - Regional Supplementary Procedures - Edition 4 - Amendment No. 203

Section 20.2, Responsibility for separation of aircraft during maneuvers in compliance with a resolution advisory

State letter AN 11/19-02/82

3.2, ACAS Maneuvers Training

 

Publications of Eurocontrol

Aeronautical Information Circular (AIC), dated January 1996

ACAS Implementation Guidance Document, dated 1 July 1997

WP - 6-1 - ACAS II Programme, Version 1.0, dated 26 January 1999

 

Publications of the Joint Aviation Authorities (JAA)

Leaflet no. 11 "Guidance for Operators on Training Programmes for the use of Airborne Collisions Avoidance Systems (ACAS)", dated October 1998.

Paragraph 3.2, ACAS Maneuvers Training

 

Publications of the Federal Aviation Administration of the United States of America

Advisory Circular AC 120-55B "Air Carrier Operational Approval and use of TCAS II", dated 22 October 2001
Chapter 11, TCAS Operational Use

 

National Regulations of the Russian Federation

Training Programme for Cockpit Personnel of Aircompanies, Russian Civil Aviation, to use TCAS/ACAS

Aeronautical Information Publication (AIP) Russia
Right-of way

 

National Regulations of Germany

Aeronautical Information Publication (AIP) Germany
ENR 1.1 Right-of-way
ENR 1.8-22 Resolution Advisories

Air Traffic Order § 13 (Luftverkehrsordnung - LuftVO - § 13)

 

Publications of the aircraft and TCAS equipment manufacturers

Honeywell TCAS/ACAS II (Change 7) Pilots Guide
Traffic Collision and Avoidance System TCAS 2000

Paragraph 1.2, Description
Paragraph 6.1, Pilot Responsibilities

Tupolev TU154M Aircraft Operating Manual
Paragraph 8.18.3.1, dated July 2000
Paragraph 8.18.3, dated 22 March 2001
Paragraph 8.18.3.2, dated 6 December 1999
Table 8.18.3.3 (page 8.18.9), dated 22 March 2001
Paragraph 8.18.3.4, dated 6 December 1999

Boeing 757 Flight Crew Training Manual
Chapter 5, Traffic Alert and Collision Avoidance System (TCAS)

 

Publications of DHL International

Company Operations Manual (OM) Part A

Chapter 8, Procedures for the use of TCAS

Company TCAS Training

 

Publications of Bashkirian Airlines

Company TCAS Training

 

 

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Evolution of Collision Avoidance System (CAS) Philosophy

In the 1950's, airliners were separated only by procedural control during most of their flights due to the limited radar coverage offered by the national Air Traffic Control system. The "see-and-avoid" principle was still the primary means for collision avoidance. In this context, the first CAS concepts were mainly intended to supplement ground-based ATC services.

After the collision over the Grand Canyon, in Arizona, the U.S. ATC system was improved and its network of ground-based stations became the primary method of ensuring air traffic separation. However, in the following decades, a number of mid-air collisions demonstrated the need for additional back-up systems which could support the air traffic controllers and provide an additional safety net.

Ground-based back-up systems, such as the Short-Term Conflict Alert (STCA) system introduced in the late 1970s in all FAA air route traffic control centers, are by their nature integrated in the air traffic control system. Their role is to support the controller in providing separation and do not require any change in the operating procedures of the ATC system.

However, ground-based back-up systems have the disadvantage that they do not provide a complete back-up function, as they still rely on the ground-based ATC infrastructure. For this reason, even when ground-based ATC services reached maturity, there was still a need for an aircraft-based system to provide a fully independent back-up function. As aircraft-based CAS systems became technically feasible, they had to be integrated into the existing ground-based ATC infrastructure, and procedures had to be developed to avoid conflicts.

ATA's report ANTC-117, the first document to provide a technical specification for the CAS, stated in 1967 that:

"The CAS is not intended to be a substitute for a ground-based air traffic control system, nor should that system be designed to rely on the airborne CAS for separation of aircraft. The best solution to the problem of potential mid-air collisions is a positive, fail-safe air traffic control system under the jurisdiction of a central agency exercising control from ground facilities. The role of an airborne CAS is to provide safe separation of aircraft when, for any reason, the ATC system does not fulfill its intended role. The CAS must not be dependent upon the ATC system."

When the FAA focused on the Beacon Collision Avoidance System (BCAS) as the preferred solution to the CAS problem, the following statement was included in the document presenting the concept in 1978:

"2.3.1 ATC
ATC is the primary separation service provided to all controlled aircraft. BCAS is a backup to ATC and shall not interfere with the safe operation of the ATC system."
(FAA-EM-78-5, II - The FAA Concept for a Beacon Collision Avoidance System (BCAS). Volume II, Concept Description)

In 1985, ICAO published Circular 195-AN/118 "Airborne Collision Avoidance Systems," presenting the ACAS concept and technical features as agreed by the Secondary Surveillance Radar Improvements and Collision Avoidance Systems Panel (SICASP) within the ICAO Air Navigation Commission (ANC). The philosophy of ACAS was stated as follows:
"Application
1.4.1 The primary means of providing separation assurance is based upon established air traffic rules and air traffic service procedures. ACAS provides a back-up to these systems by alerting flight crews to intruding aircraft that may represent collision threats. ACAS displays traffic and resolution advisories with respect to these intruders.
Benefits
1.4.2 ACAS provides an independent back-up source of information to reduce the risk of mid-air collisions. It supports the flight crew's responsibility to see-and-avoid. Moreover, it provides an added level of safety to that maintained by ATS both in airspace where radar separation services are provided and in airspace outside radar coverage."

During flight testing of Collision Avoidance Systems with small-scale applications using industry prototypes and staged encounters, the potential for conflicts with ATC was limited and the issue of integrating the CAS concept with the existing ATC environment was not yet relevant.

When TCAS II was validated and installed on aircraft by FAA mandate, the procedures for responding to resolution advisories in actual operations were established.

The primary reference documents for training and guidance material were:

• FAA Advisory Circular 120-55, first issued on October 25, 1991
• The Supplement to the Airplane Flight Manual, to be provided by equipment manufacturers when retrofitting TCAS II equipment on existing aircraft through a Supplemental Type Certificate (STC).

The Flight Manual Supplement for the Boeing 757 when equipped with Honeywell TCAS II and the FAA Advisory Circular provide the same definition of TCAS philosophy and practically identical operating procedures.

It is important to note that the supplement to the AFM mentions that a TCAS resolution advisory would typically require "an overall altitude deviation of 300 to 500 feet," which would mean that an RA would be practically transparent to the ATC, as it would have a small impact on the vertical separation of traffic.

In the initial operational history of TCAS II, the stakeholders in the air transport industry had differing opinions about the philosophy of the system and provided mixed feedback.

On October 8, 1992, the Subcommittee on Investigations and Oversight of the Committee on Public Works and Transportation of House of Representatives of the U.S. Congress convened a hearing to review the status of the airborne traffic alert and collision avoidance system (TCAS) after the first months of actual operation. The representative from the Air Line Pilots Association (ALPA) gave positive feedback from the first months of actual TCAS II operations:

"I assure you that in spite of the problems encountered, pilots would strongly oppose any efforts to turn TCAS off or remove it from the cockpit. The situational awareness provided by the traffic display instrumentation installed by some airlines is a major asset by itself, particularly in terminal areas and especially during the hours of darkness and reduced visibility when visual approaches are being directed by ATC."
Regarding the TCAS II philosophy, a clear definition was given:
"We would like the TCAS system to be a last resort system, an emergency only system similar in nature to the ground proximity warning system. When you get a warning, you have complete confidence in it and you respond to it in a mandatory sense.
[...]

We believe that the responsibility for separation of traffic belongs on the ground in the air traffic control facility. What we are looking for from TCAS though is an emergency back-up system, a system of last resort when all else fails, it gives the pilot enough information to avoid a mid-air collision."

On a similar note, the representative from the Air Transport Association (ATA) stated:

"I think TCAS is a needed device. It should be used as a last resort in the system, kind of an emergency separation tool, an enhancement to the pilot's many tools in the cockpit. I don't think it detracts at all from a ground-based air traffic control system that seems to be of some concern to NATCA. I think it adds to that. I think during this transition period we have discovered some problems with TCAS and I think we knew we were going to have problems with TCAS, hence a transition period where appropriate changes could be made to the system to correct some of those problems is necessary. I think that the controllers believe that TCAS would be transparent to them when implemented, that they would not notice its implementation. I don't know if I would have believed that, because as soon as the first RA occurred in the system, I am sure that that probably would have been a surprise to an air traffic controller."

These two definitions, clearly mentioning a "last resort" philosophy for TCAS, appear to be misaligned with all previously cited documents, where TCAS was assigned a more general "back-up function" in relation to the ground-based separation assurance systems. Without a more specific definition, this wording can convey the perception that TCAS should be considered just as a supplement or possibly a substitute for ground-based systems, rather than an independent "last resort" safety net.

Mixed feedback was highlighted in reports about early TCAS II operations issued by the U.S. General Accounting Office and by the Aviation Safety Reporting system.

Some incidents related to TCAS operation and a reported lack of training became a matter of concern for the National Transportation Safety Board (NTSB) and on March 25, 1993, three recommendations were sent to FAA, raising the following issues:

• TCAS flight simulator training for flight crews during initial and recurrent training (Safety Recommendation A-93-46)
• Recording of TCAS RA and sensitivity setting on Flight Data Recorders (FDR) (Safety Recommendation A-93-47)
• TCAS training for air traffic controllers (Safety Recommendation A-93-48)

4/1/1994 - The board notes that the FAA has developed a training course entitled "TCAS Cadre Training" for air traffic field training officers. The first class was held on 10/26/93, at the FAA academy in Oklahoma City, OK. The training officers currently enrolled in the course will provide training to each facility training department prior to delivery of the comprehensive briefing to air traffic controllers. As an interim measure, the FAA issued a general notice (genot) to all air traffic facilities and regional offices implementing a briefing for all controllers based on a booklet entitled "Introduction to TCAS II." This booklet provides an overview of system components, target surveillance, collision avoidance logic, cockpit presentations, and how TCAS II operates within the national airspace system. The genot also required that TCAS update newsletters be accounted for on a "read and initial" basis in each facility. This newsletter is a monthly publication distributed to all air traffic facilities as a mandatory briefing item. It contains TCAS significant event scenarios, TCAS program updates, TCAS system status, and TCAS operational issues that directly affect the air traffic control system. The FAA has developed a briefing of the latest TCAS technology and changes in requirements related to the air traffic control system for the previous year and will brief all air traffic controllers on an annual basis.

In 1997, ICAO addressed at a global level the deficiencies in TCAS training for flight crews, which had been observed during TCAS implementation by the FAA, the Japanese Civil Aviation Bureau (JCAB), and the European Organization for the Safety of Air Navigation (EUROCONTROL). In Attachment E to State Letter AN 7/1.3.72-97/77, ICAO set Airborne Collision Avoidance System (ACAS) Performance-Based Training Objectives.

The apparent international differences in training programs; and, initial confusion regarding the nature of TCAS and its operational usage, led to discrepancies in training. The characterization of TCAS as a "backup to ATC" led, in some cases, to training programs that emphasized a need for ATC concurrence prior to following a TCAS RA. In other cases, training programs emphasized the need to respond immediately to an RA and notify ATC of the maneuver when flight crew workload permitted.

In this accident, investigators concluded that one carrier involved in the accident had trained crews that ATC instructions retained priority, and not to maneuver in contradiction to ATC instructions. Conversely, the other carrier involved in the accident had trained crews that TCAS instructions held priority, and to maneuver in accordance with a TCAS RA, independent of any other ATC instructions. Investigators concluded that this conflict was a major contributor in this accident, as one of the airplanes involved followed ATC instructions, counter to a TCAS RA, and the other followed the RA, resulting in both aircraft remaining on a collision course.

 

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• The regulations concerning ACAS/TCAS published by ICAO and the operational and procedural instructions of the TCAS manufacturer and the operators were not standardized, were incomplete, and partially contradictory.
• The basic principle of "always follow a TCAS RA" was not universally known, understood, accepted, or trained.
• ATC staffing was reduced during evening hours due to expected reduced traffic volume, and required controller attention to be divided between different radar displays located ten feet apart.

 

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• Flight crews would not maneuver in contradiction (opposite direction) to an RA
• TCAS II provided an adequate "last line of defense" in the event of ATC failure to provide safe separation
• ATC will provide adequate traffic separations within the airspace

 

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Japan Air Lines Flight 907, Boeing 747-400D, JA8904; Japan Air Lines Flight 958, Douglas DC-10-40, JA8546

Near Yaizu city, Shizuoka prefecture (Japan) - January 31, 2001

A Conflict Alert (CNF) was issued at Tokyo ACC warning of the proximity of Japan Air Lines Flight 907, a Boeing 747 which was making a climbing left turn, and Japan Air Lines Flight 958, a Douglas DC-10 which was cruising in level flight. While responding to this conflict alert, Tokyo ACC mistook the flight number of the DC-10 for that of the 747, and instructed the 747, which was climbing at the time, to descend.

Immediately after the 747 initiated a descent in response to this ATC instruction, its TCAS issued an RA to climb, but the 747 continued to descend in compliance with the ATC instruction. Concurrently, the DC-10 was descending in response to its own TCAS RA. Both aircraft ultimately maneuvered into very close proximity while maintaining mutual visual contact. However, before their closest point of approach, both aircraft made evasive maneuvers to avoid a collision. The 747 made an abrupt descent intending to pass under the DC-10 just before their flight paths crossed. Among the 427 persons aboard the 747 (411 passengers and 17 crew), seven passengers and two cabin attendants were seriously injured due to this sudden maneuver.

The Japanese accident report is available at the following link: (Accident Report).

Following the near-collision in Japan, the Japanese Aircraft and Railway Accidents Commission Issued several recommendations to ICAO. The primary intent of the recommendations was to emphasize the need for operators to always comply with an RA, and the potential catastrophic consequences when deviating from an RA. Coincidently, though they had been in development for several months, these recommendations were issued just a few weeks after the Überlingen accident. The recommendation letter is available at the following link: (Japanese Recommendations to ICAO)

 

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TCAS Background (Prior to Überlingen accident)

Following a 1986 collision between a DC-9 and a private aircraft over Cerritos, California, the U.S. Congress mandated that some categories of aircraft be equipped with TCAS II when operating in US airspace. By 1990 all U.S. commercial aircraft with more than 30 passenger seats were equipped with TCAS II, version 6. At the same time, EUROCONTROL and the Japanese Civil Aviation Bureau (JCAB) conducted operational evaluations of TCAS II. As use of TCAS II increased, other authorities also began conducting operational evaluations. As a result of these worldwide operational evaluations, TCAS II, version 6.04a, was released in 1993, incorporating changes suggested by results of the operational tests. The principal purpose of this revision was to reduce the occurrence of nuisance alerts at low altitudes.

In 1988 the FAA published Advisory Circular (AC) 20-131, Airworthiness Approval of Traffic Alert and Collision Avoidance Systems (TCAS II) and Mode S Transponders. The AC provided guidance relative to the certification of TCAS systems on transport category airplanes. It was subsequently revised in 1993. The 1993 version is available at the following link: AC 20-131A 

Following implementation of TCAS version 6.04a, responses to RAs were resulting in vertical deviations in excess of 300 feet and causing disruption in the air traffic control system. This led to the development of TCAS II version 7.0, intended to reduce the overall number of RAs and to minimize altitude excursions when responding to an RA. Version 7.0 was approved by the FAA in 1997, and installation began in the U.S. on a voluntary basis in 1999. At the time of the Überlingen accident, TCAS II, version 7.0 was the most current available version.

Near-Term Actions Following the Accident

Immediately following the accident, it became clear to investigators that one of the accident airplanes did not respond correctly to the RA, but instead followed air traffic instructions. As a result, European authorities took a number of actions to emphasize the need to always follow an RA. These actions included:

• July 2002: EUROCONTROL ACAS II Bulletin "Follow the RA!"
• August 2002: ARINC "TCAS Transition Program (TTP) Industry Alert Bulletin"
• August 2002: UK Civil Aviation Authority Flight Operations Department Communication (FODCOM) 19/2002 "ACAS - Action To Be Taken Following a Resolution Advisory (RA) Warning"
• August 2002: ICAO State Letter AN 11/19-02/82 "ACAS Provisions and Operational Procedures"
• November 2002: EUROCONTROL ACAS Safety Newsletter

Eurocontrol AGAS

On September 26, 2002, EUROCONTROL set up a high-level European Advisory Group on ATM Safety (AGAS) to review the safety of the European air traffic management system. The assessment of the safety nets built into the air traffic management (ATM) system was among the key areas identified as needing immediate attention, with a clear focus on ACAS. In particular, rules regarding the use of ACAS needed to be made as clear and unambiguous as possible. The AGAS action plan addressed most of the criticalities related to ACAS highlighted in the Überlingen accident report.

ICAO Air Navigation Commission

The ICAO Air Navigation Commission's safety initiatives, after reviewing the ACAS II operating procedures, were proposals to the member states in October 2002 to:

• Highlight the importance of following an RA, and of not maneuvering in a sense opposite to that of an RA by strengthening and clarifying the operating procedures in Procedures for Air Navigation Services - Aircraft Operations (PANS OPS), Volume 1 - Flight Procedures (Doc 8168), and
• Amend Annex 6 - Operations of Aircraft, Part 1 - International Commercial Aircraft Transport - Airplanes, to include a new standard in Appendix 2 concerning the content of an operations manual in regard to policy, instructions, procedures, and training requirements for the avoidance of collisions and the use of ACAS II.

These proposals were adopted by the ICAO Council on March 13, 2003 and became applicable on November 27, 2003.

Long Term Actions Following the Accident

TCAS II Version 7.1

In response to the Überlingen accident and a near mid-air that occurred in Japan in 2001, additional changes to improve the RA logic were identified. A change was made to permit additional sense reversal RAs in order to address certain vertical chase geometries. It should be noted that in each of cited events, the involved flight crews maneuvered opposite to the displayed RA. Separate from the Japan and Überlingen events, a review of other operational experience had shown that pilots occasionally maneuver in the opposite direction from that indicated by an "Adjust Vertical Speed, Adjust" (AVSA) RA. To mitigate the risk of pilots increasing their vertical rate in response to an AVSA RA, all AVSA RAs were replaced by "Level Off, Level Off" (LOLO) RAs. The Europeans and the U.S performed extensive validation of these changes, with the end result being publication of version 7.1 in June 2008.

ICAO and FAA Actions

ICAO

• Published the "Airborne Collision Avoidance System (ACAS) Manual - Doc. 9863" in 2006 to provide further guidance on ACAS technical features, operation procedures, and training requirements.

FAA

• Published the Information for Operators InFO 08039, "Responding to a TCAS resolution advisory (RA) - Fly the RA" in June 2008, that includes the Überlingen collision as the initiating event. The InFO reiterates the principle of always following a TCAS RA, in accordance with the relevant provisions of FAA AC 120-55.
• Recommended the implementation of programs that consistently underline the principle to follow the TCAS RA in all phases of pilot training, including simulator sessions where this specific accident scenario can be replicated.
• Replayed the scenario of the collision in an ATC simulator at the FAA Technical Center to demonstrate that the primary operating system of the radars at the facility would have detected the conflict and provided conflict alert warnings sufficiently in advance to prevent the loss of separation. However, the conflict alert functionality did not extend to ATC backup systems and would not have been available in circumstances similar to those leading to the Überlingen collision. 
• Published Advisory Circular (AC) 120-55 Air Carrier Operational Approval and Use of TCAS II.
• Published Introduction to TCAS II Version 7.1 Booklet describing the history and operation of TCAS II version 7.1.

 

Other Collision Avoidance Initiatives

Future Air Traffic Systems

The role of TCAS has been discussed with reference to the future air traffic management concepts, which will include the transfer of separation responsibility to the cockpit. On a regional level, projects such as these have already laid the foundations for the future ATM environment:

• NextGen Air Transportation System (NextGen) in the US,
• Single European Sky ATM Research (SESAR) in Europe, and 
• Collaborative Actions for Renovations of Air Traffic Systems (CARATS) in Japan

These initiatives will provide a three-layered concept for traffic conflict management, with a strategic conflict management function, a separation provision, and safety nets for collision avoidance.

ICAO Doc 9854 "Global Air Traffic Management Operational Concept" describes collision avoidance as:

... the third layer of conflict management, and must activate when the separation mode has been compromised. Collision avoidance is not part of separation provision, and collision avoidance systems are not included in determining the calculated level of safety required for separation provision. Collision avoidance systems will, however, be considered part of the ATM safety management. The collision avoidance functions and the applicable separation mode, although independent, must be compatible.

 

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No Airworthiness Directives were issued as a result of this accident.

 

 

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Airplane Life Cycle

• Operational

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Accident Threat Categories

• Flight Deck Layout / Avionics Confusion
• Midair / Ground Incursions

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Groupings

• Automation

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Accident Common Themes

• Organizational Lapses
• Human Error

 

 

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Organizational Lapses

• The International TCAS-related regulations and publications were incomplete, ambiguous, or inconsistent and did not reflect the philosophy of the system as a "last resort" safety net.

Human Error

• The air traffic controller did not notice the separation infringement in time.

 

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Midair collision between a Trans World Airlines Lockheed 1049A and a United Airlines Douglas DC-7

On the morning of June 30, 1956, TWA Flight 2, a Lockheed Super Constellation, collided with United Air Lines Flight 718, a Douglas DC-7, over the Grand Canyon. Both aircraft fell into the canyon. There were no survivors among the 128 persons aboard the flights.

This accident involved two of the largest commercial aircraft in service and was the deadliest aviation disaster in history at that time. It was a catalyst for changes to the air traffic control system and led to the creation of the Federal Aviation Agency.

This loss spurred public debate on the need to mitigate the risk of midair collisions in an age of unprecedented aviation growth. The accident was a major factor leading to the creation of the Federal Aviation Agency in 1958 and to major changes in the control of air traffic.

 

United Airlines DC-8-11, N8013U, and Trans World Airlines Lockheed L1049 Super Constellation, N6709C, Midair collision over Staten Island, New York

On the morning of December 16, 1960, United Airlines Flight 826, a Douglas DC-8, collided with Trans World Airlines Flight 266, a Lockheed L1049 Super Constellation. The TWA aircraft crashed at Miller Airfield on Staten Island. The United DC-8, however, continued several miles to the northeast and impacted the ground in Brooklyn. There were 128 persons killed aboard both aircraft and six additional persons on the ground. Fatalities in this accident exceeded that of the midair collision over the Grand Canyon four years earlier, making this, at the time, the worst air disaster to date.

As a result of the Grand Canyon accident, a broad reaching overhaul of the U.S. air traffic control system was underway. Although these efforts reduced the chance for additional midair collisions, the possibility was not eliminated.

This accident was a landmark event and caused a re-evaluation of the air traffic modernization effort at the highest levels of government.

 

Pacific Southwest Airlines, Inc., Flight 182, Boeing 727-214, N533PS/Gibbs Flite Center, Inc., Cessna 172, Reg. No. N7711G

On September 25, 1978, Pacific Southwest Airlines, Inc., Flight 182, a Boeing 727-214, and a Gibbs Flite Center, Inc., a Cessna 172, collided in midair about three nautical miles northeast of Lindbergh Field, San Diego, California. Both aircraft crashed in a residential area. One hundred and thirty-seven persons, including those on both aircraft, were killed; seven persons on the ground were killed; and nine persons on the ground were injured.

The NTSB determined that the probable cause of the accident was the failure of the flight crew of Flight 182 to comply with the provisions of a maintain-visual-separation clearance, including the requirement to inform the controller when they no longer had the other aircraft in sight. Contributing to the accident were the air traffic control procedures in effect which authorized the controllers to use visual separation procedures to separate two aircraft on potentially conflicting tracks when the capability was available to provide either lateral or vertical radar separation to either aircraft.

See accident module

 

Collision of Aeronaves de Mexico S.A., McDonnell Douglas DC-9-32, XA-JED and Piper PA-28-181, N4891F

On August 31, 1986, about 1152 Pacific daylight time, Aeronaves de Mexico, S.A. Flight 498, a DC-9-32 with Mexican Registration XA-JED, and a Piper PA-28-181, N4891F, collided over Cerritos, California. The NTSB determined that the probable cause of the accident was the limitations of the air traffic control system to provide collision protection through both air traffic control procedures and automated redundancy. Factors contributing to the accident were: (1) the inadvertent and unauthorized entry of the PA-28 into the Los Angeles rerminal control area, and (2) the limitations of the "see and avoid" concept to ensure traffic separation under the conditions of the conflict.

See accident module

 

Other Accidents

1996 Charkhi Dadri, India - Saudi B-747 / Kazakhstan IL-76 (349 fatalities)

1997 Namibia (off-coast) - USAF C-141 / German AF TU154 (33 fatalities)

2006 Brazil - B-737-8EH and EMB-135 BJ LEGACY

2015 Senegal - B-737-800 and HS-125

 

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Technical Related Lessons

Collision avoidance systems (such as TCAS II) are intended to provide a "last line of defense" against midair collision, requiring immediate compliance with Resolution Advisory (RA) flight path instructions by all aircraft involved, even in the presence of contradictory instructions from air traffic control. (Threat Category: Midair/Ground Incursions)

• At introduction into service, the original TCAS system was viewed largely as a backup system to air traffic control. Air traffic control instructions were assigned a higher priority, and TCAS indications were viewed largely as advisory information. As TCAS matured, and installations became more widespread, TCAS became capable of fulfilling a primary role in collision avoidance, superseding the need for air traffic instructions before performing collision avoidance maneuvers. Investigators established that the TCAS manufacturer's Pilot's Guide provided guidance that could, in some cases, be interpreted as contradictory. In one section, the system was characterized as a backup to air traffic control and in another section required an immediate flight crew response to an RA. This led to an adoption of differing operational usages by the airlines involved in this July 1, 2002 midair collision. The crew of the TU154M followed air traffic instructions to descend, contradicting the TCAS RA climb instruction they were receiving. Conversely, the B757 crew performed a TCAS RA descent without having received an air traffic control instruction. The opposite responses of the respective flight crews resulted in both aircraft descending, putting the airplanes on a collision flight path. Had the TCAS RA instructions received by both flight crews been followed, the collision would not have occurred.

Common Theme Related Lessons

The TCAS II system, in order to further reduce the risk of midair collisions, requires all flight crews involved to immediately respond to a Resolution Advisory (RA). In order to achieve this safety objective, all training programs and operational instructions must emphasize the criticality of immediate crew response to an RA. (Common Theme: Organizational Lapses)

• Following this accident, investigators performed a review of various documents produced by international regulatory authorities (JAA, FAA, ICAO, etc.), airline training program documents, and manufacturer guidance material. Of particular interest to investigators was the analysis of consistency among safety philosophies underlying the operational use of TCAS II systems. It was learned through this evaluation that differences existed between manufacturer's installation documents, and guidance provided by the regulatory authorities. The difference in safety philosophies was centered around the concept of TCAS II being a "last line of defense" versus being considered a "backup to ATC." This difference was found to have extended to the training and operational programs within the respective airlines involved in the accident.

  Investigators noted that one of the operators involved in the accident had developed their TCAS-related crew training program based on the manufacturer's TCAS 2000 Pilot's Guide, which characterized TCAS as a "back up" to ATC, including the "see and avoid" concept. Consequently, crews were trained to assign the highest priority to ATC instructions. Conversely, the other operator involved in the accident had developed crew training based upon regulatory authority guidance material that stressed immediate response to a TCAS RA, and following TCAS guidance even if contradictory to ATC instructions. Investigators cited the disparity in instructions to pilots as being a fundamental contributor to the accident.

 

 

 

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