U.S. DEPARTMENT OF TRANSPORTATION
FEDERAL AVIATION ADMINISTRATION
Air Traffic Organization Policy

Section 10. Wind/Altimeter Information

2-10-1. WIND INSTRUMENT SENSORS

Air traffic managers must designate in a facility directive which wind sources must be used for operational purposes.

a. Towers equipped with LLWAS may use direct dial or LLWAS wind information for weather observations, except where automated wind information is available.

b. Approach control facilities may use direct dial, LLWAS, or automated display wind information for operational purposes.

c. FSSs must use direct dial or automated display wind information for operational purposes.

d. Other exceptions must be referred to the Manager of System Safety and Procedures for approval.

2-10-2. WIND INDICATOR CROSS CHECK

All FAA facilities having an associated NWS office or military weather station using the same sensing equipment must compare wind direction and speed indicator readings at the beginning of each work day with those of the NWS or military weather station, keeping in mind that the NWS wind direction equipment are oriented to true north. Apply the magnetic variation to ensure a correct reading. Coordinate the time of the cross-check and the associated procedures with the meteorologist-in-charge or other appropriate officer. Wind instrument errors must be handled as follows:

a. If an FAA wind direction indicator is out of tolerance with other indicators on the same sensor by 5 degrees, or if the wind speed indicator reveals a disparity of plus or minus 5 knots, notify the appropriate maintenance personnel immediately for corrective action.

b. If the indicators show an error of over 10 degrees or 10 knots, the equipment must be considered inoperative. In this case, obtain further wind information from other properly functioning wind instruments in the tower, local FSS, the NWS, or military weather office. Notify the appropriate maintenance personnel of all outages.

2-10-3. ALTIMETER REQUIREMENTS

a. At least two aneroid altimeter setting indicators (ASI) or one ASI and a traceable pressure standard are required in a TRACON, radar approach control (RAPCON), terminal radar approach control in tower cab (TRACAB), combined center/RAPCON (CERAP), radar ATC facility (USN) (RATCF), tower cab, and a FSS that takes weather observations and/or provides LAA. When two or more facilities (or a NWS commissioned/certified automated weather observing system) are located on the same airport, the requirement may be reduced to one aneroid ASI per facility. Aircraft altimeters must not be used in reporting altimeter settings.

NOTE-
1. Stand alone RADAR approach control facilities (TRACON, RAPCON, RATCF, CERAP) not associated with a control tower are only required to maintain altimeter settings for those airports under their jurisdiction.

2. A digital ASI (DASI) system is considered as one aneroid ASI instrument for the purpose of this paragraph.

b. At locations with commissioned ASOS or commissioned dual transducer AWOS units, the ASOS/AWOS becomes the pressure standard. If the ASOS/AWOS is inoperative, a Stand Alone Weather System (SAWS) or DASI may be considered as the pressure standard.

2-10-4. COMPARISON CHECKS

a. Facilities equipped only with aneroid instruments:

1. Compare the reading of each aneroid instrument (ASI) daily and each nonpressure standard digital instrument (DASI) monthly with the altimeter setting issued by an associated facility having a traceable pressure standard located either on the airport or within the distances set forth in subparas d and e.

2. When the differences between the two altimeter settings exceeds 0.05 in. Hg. at nonprecision approach locations or 0.02 in. Hg. at precision approach locations, remove the instrument from service and notify Technical Operations personnel. When all ASI instruments in the facility are found to exceed the tolerances, report the altimeter setting as missing.

3. When the difference is less than the tolerances specified in subpara 2 above, the value (+ or -) is applied as the correction factor to determine the operational altimeter setting.

(a) On dial-type display ASIs, post the correction factor directly on the face of the instrument. Use the same comparison procedures and determine the correction factor for each instrument in the facility.

(b) On digital ASI (DASI) systems, post the correction factor on or near the display/s. Local facility procedures may be developed in coordination with the associated airway facilities office to adjust the DASI to display the corrected altimeter setting.

b. Facilities equipped with aneroid instruments and a traceable pressure standard:

1. Make two comparisons at least 6 hours apart, but not more than 8 hours, on the same day of the week. Enter all comparison data on the appropriate form. Every week, determine the mean of the 10 last comparisons, and use this figure as the posted correction to apply to the reading of the ASI.

2. Additional comparison procedures are described in handbooks applicable to the facility.

c. At locations with commissioned ASOS or commissioned dual transducer AWOS units, the ASOS/AWOS becomes the pressure standard. Compare the reading of each aneroid ASI to the pressure standard daily and each digital ASI (SAWS/DASI) monthly. In the event of a failure of the pressure standard instruments, a comparison must be made within 36 hours. Tolerances and posting procedures are contained in subparas a2 and a3.

d. At locations not served by a weather reporting station, make a comparison against an adjacent weather service office, commissioned dual transducer AWOS or ASOS systems, a FSS or a LAWRS facility having a traceable pressure standard.

1. At locations where precision approaches are conducted, the weather reporting station is not more than 10 NM away, and at both locations the wind speed is 12 knots or less with no gusts above 15 knots.

2. At all other locations the distance must not exceed 25 NM, and at both locations the wind speed must be 15 knots or less with no gusts above 20 knots.

3. The difference in elevation does not exceed 100 feet at precision approach locations and 200 feet at all other locations.

4. The station's temperature at both locations must be within 30 degrees Fahrenheit of the standard atmosphere temperature for the station's elevation.

NOTE-
The following formula may be used to determine the standard atmosphere temperature for station elevation:

EXAMPLE-
1. Tower A field elevation 600 feet: 600 x 0.0036 = 2.16°F of change, is rounded to 2°F.
59°F - 2°F = 57°F standard temperature for Tower A adjusted for elevation.

2. Tower B field elevation 700 feet: 700 x 0.0036 = 2.52°F of change, is rounded to 3°F.
59°F - 3°F = 56°F standard temperature for Tower B adjusted for elevation.

If both sites are between ± 30°F {87°F and 27°F for Tower A and 86°F and 26°F for Tower B} a comparison check is appropriate for temperature.

5. Do not use altimeter setting values from aneroid instruments when the difference exceeds 0.02 in. Hg. at precision approach locations or 0.05 in. Hg. at all other locations.

e. A traceable pressure standard is required for routine altimeter setting comparison checks at all facilities that exceed the requirements of subpara d

2-10-5. DELIVERY OF ALTIMETER SETTING TO ARTCC

ARTCCs having a requirement for interphone delivery of altimeter settings, or changes of report, must make arrangements with FSS/terminals for delivery to associated sector/s.

2-10-6. BROADCAST DENSITY ALTITUDE ADVISORY

Terminal and FSS facilities at airports with field elevations of 2,000 feet MSL or higher must broadcast a density altitude advisory to departing general aviation (GA) aircraft whenever the temperature reaches a certain level. These broadcasts must be made on ground control (GC), clearance delivery (CD), airport advisory, transcribed weather broadcast (TWEB, Alaska only), or automatic terminal information service (ATIS) as appropriate. Use the following table to determine broadcast applicability: (See TBL 2-10-1.)

TBL 2-10-1
BROADCAST APPLICABILITY