ENR 1.8 Cold Temperature Barometric Altimeter Errors, Setting Procedures, and Cold Temperature Airports (CTA)

  1. Effect of Cold Temperature on Barometric Altimeters
    1. Temperature has an effect on the accuracy of barometric altimeters, indicated altitude, and true altitude. The standard temperature at sea level is 15 degrees Celsius (59 degrees Fahrenheit). The temperature gradient from sea level is minus 2 degrees Celsius (3.6 degrees Fahrenheit) per 1,000 feet. For example, at 5000 feet above sea level, the ambient temperature on a standard day would be 5 degrees Celsius. When the ambient (at altitude) temperature is colder than standard, the aircraft's true altitude is lower than the indicated barometric altitude. When the ambient temperature is warmer than the standard day, the aircraft's true altitude is higher than the indicated barometric altitude.
    2. TBL ENR 1.8-1 indicates how much error may exist when operating in non-standard cold temperatures. To use the table, find the reported temperature in the left column, read across the top row to locate the height above the airport (subtract airport elevation from the flight altitude). Find the intersection of the temperature row and height above the airport column. This number represents how far the aircraft may be below the indicated altitude due to possible cold temperature induced error.

      TBL ENR 1.8-1
      ICAO Cold Temperature Error Table

      A table depicting how much error may exist when operating in non-standard cold temperatures.
  2. Pre-Flight Planning for Cold Temperature Altimeter ErrorsFlight planning into a CTA may be accomplished prior to flight. Use the predicted coldest temperature for plus or minus 1 hour of the estimated time of arrival and compare against the CTA published temperature. If the predicted temperature is at or below CTA temperature, calculate an altitude correction using TBL ENR 1.8-1. This correction may be used at the CTA if the actual arrival temperature is the same as the temperature used to calculate the altitude correction during preflight planning.
  3. Effects of Cold Temperature on Baro-vertical Navigation (VNAV) Vertical GuidanceNon-standard temperatures can result in a change to effective vertical paths and actual descent rates when using aircraft baro-VNAV equipment for vertical guidance on final approach segments. A lower than standard temperature will result in a shallower descent angle and reduced descent rate. Conversely, a higher than standard temperature will result in a steeper angle and increased descent rate. Pilots should consider potential consequences of these effects on approach minima, power settings, sight picture, visual cues, etc., especially for high-altitude or terrain-challenged locations and during low-visibility conditions.

    REFERENCE-

    AIM Paragraph 5-4-5. Instrument Approach Procedure (IAP) Charts

    1. Uncompensated Baro-VNAV note on 14 CFR Part 97 IAPs. The area navigation (RNAV) global positioning system (GPS) and RNAV required navigation performance (RNP) notes, “For uncompensated Baro-VNAV systems, lateral navigation (LNAV)/VNAV NA below –XX°C (-XX°F) or above XX°C (XXX°F)” and “For uncompensated Baro-VNAV systems, procedure NA below –XX°C (-XX°F) or above XX°C (XXX°F)” apply to baro-VNAV equipped aircraft. These temperatures and how they are used are independent of the temperature and procedures applied for a Cold Temperature Airport.
      1. The uncompensated baro-VNAV chart note and temperature range on an RNAV (GPS) approach is applicable to the LNAV/VNAV line of minima. Baro-VNAV equipped aircraft without a temperature compensating system may not use the RNAV (GPS) approach LNAV/VNAV line of minima when the actual temperature is above or below the charted temperature range.
      2. The uncompensated baro-VNAV chart note and temperature range on an RNAV (RNP) approach applies to the entire procedure. For aircraft without a baro-VNAV and temperature compensating system, the RNAV (RNP) approach is not authorized when the actual temperature is above or below the charted uncompensated baro-VNAV temperature range.
    2. Baro-VNAV temperature range versus CTA temperature: The baro-VNAV and CTA temperatures are independent and do not follow the same correction or reporting procedures. However, there are times when both procedures, each according to its associated temperature, should be accomplished on the approach.
    3. Operating and ATC reporting procedures.
      1. Do not use the CTA operating or reporting procedure found in this section, 4.1 thru 5.5, when complying with the baro-VNAV temperature note on an RNAV (GPS) approach. Correction is not required nor expected to be applied to procedure altitudes or VNAV paths outside of the final approach segment.
      2. Operators must advise ATC when making temperature corrections on RNP authorization required (AR) approaches while adhering to baro-VNAV temperature note.
      3. Reporting altitude corrections is required when complying with CTAs in conjunction with the baro-VNAV temperature note. The CTA altitude corrections will be reported in this situation. No altitude correction reporting is required in the final segment.

        NOTE-

        When executing an approach with vertical guidance at a CTA (i.e., ILS, localizer performance with vertical guidance (LPV), LNAV/VNAV), pilots are reminded to intersect the glideslope/glidepath at the corrected intermediate altitude (if applicable) and follow the published glideslope/glidepath to the corrected minima. The ILS glideslope and WAAS generated glidepath are unaffected by cold temperatures and provide vertical guidance to the corrected DA. Begin descent on the ILS glideslope or WAAS generated glidepath when directed by aircraft instrumentation. Temperature affects the precise final approach fix (PFAF) true altitude where a baro-VNAV generated glidepath begins. The PFAF altitude must be corrected when below the CTA temperature restriction for the intermediate segment or outside of the baro-VNAV temperature restriction when using the LNAV/VNAV line of minima to the corrected DA.

  4. Cold Temperature Airports (CTA)
    1. General: The FAA has determined that operating in cold temperatures has placed some 14 CFR Part 97 instrument approach procedures in the United States National Airspace System at risk for loss of required obstacle clearance (ROC). An airport that is determined to be at risk will have an ICON and temperature published on the instrument approach procedure (IAP) in the terminal procedures publication (TPP).
    2. CTA identification in TPP: A CTA is identified by a “snowflake” icon (A graphic depicting the snowflake icon.) and temperature limit, in Celsius, on U.S. Government approach charts.
    3.  A current list of CTAs is located at: https://www.faa.gov/air_traffic/flight_info/aeronav/digital_products/dtpp/search/. Airports are listed by ICAO code, Airport Name, Temperature in Celsius, and affected segment(s).
    4. Airport Criteria. The CTA risk analysis is performed on airports that have at least one runway of 2500 ft. Pilots operating into an airport with a runway length less than 2500 ft. may make a cold temperature altitude correction in cold temperature conditions, if desired. Comply with operating and reporting procedures for CTAs.
    5. ATC Reporting Requirements. Pilots must advise ATC with the corrected altitude when applying an altitude correction on any approach segment with the exception of the final segment.
    6. Methods to apply correction: The FAA recommends operators/pilots use either the All Segments Method or the Individual Segments Method when making corrections at CTAs.
  5. Cold Temperature Airport Procedures
    1. PILOTS MUST NOT MAKE AN ALTIMETER CHANGE to accomplish an altitude correction. Pilots must ensure that the altimeter is set to the current altimeter setting provided by ATC in accordance with 14 CFR §91.121.
    2. Actions on when and where to make corrections: Pilots will make an altitude correction to the published, “at”, “at or above”, and “at or below” altitudes on all designated segment(s) to all runways for all published instrument approach procedures when the reported airport temperature is at or below the published CTA temperature on the approach plate. A pilot may request an altitude correction (if desired) on any approach at any United States airport when extreme cold temperature is encountered. Pilots making a correction must comply with ATC reporting requirements.
    3. Correctable altitudes: ATC does not apply a cold temperature correction to their Minimum Vectoring Altitude (MVA) or Minimum IFR Altitude (MIA) charts. Pilots must request approval from ATC to apply a cold temperature correction to any ATC assigned altitude. Pilots must not correct altitudes published on Standard Instrument Departures (SIDs), Obstacle Departure Procedures (ODPs), and Standard Terminal Arrivals (STARs).
    4. Use of corrected MDA/DA: Pilots will use the corrected MDA or DA as the minimum altitude for an approach. Pilots must meet the requirements in 14 CFR Part 91.175 in order to operate below the corrected MDA or DA. Pilots must see and avoid obstacles when descending below the minimum altitude on the approach.

      NOTE-

      The corrected DA or MDA does not affect the visibility minima published for the approach. With the application of a cold temperature correction to the DA or MDA, the airplane should be in a position on the glideslope/glidepath or at the published missed approach point to identify the runway environment.

    5. How to apply a Cold Temperature Altitude Correction on an Approach.
      1. All Segments Method: Pilots may correct all segment altitudes from the initial approach fix (IAF) altitude to the missed approach (MA) final holding altitude. Pilots familiar with the information in this section and the procedures for accomplishing the all segments method, only need to use the published “snowflake” icon, A graphic depicting the snowflake icon./CTA temperature limit on the approach chart for making corrections. Pilots are not required to reference the CTA list. The altitude correction is calculated as follows:
        1. Manual correction: Pilots will make a manual correction when the aircraft is not equipped with a temperature compensating system or when a compensating system is not used to make the correction. Use TBL ENR 1.8-1, ICAO Cold Temperature Error Table, to calculate the correction needed for the approach segment(s).
          1. Correct all altitudes from the final approach fix (FAF)/PFAF up to and including the IAF altitude: Calculate correction by taking FAF/PFAF altitude and subtracting the airport elevation. Use this number to enter the height above airport column in TBL ENR 1.8-1 until reaching the reported temperature from the “Reported Temperature” row. Round this number as applicable and then add to all altitudes from the FAF altitude through the IAF altitude.
          2. Correct all altitudes in final segment: Calculate the correction by taking the MDA or DA for the approach being flown and subtract the airport elevation. Use this number to enter the height above airport column in TBL ENR 1.8-1 until reaching the reported temperature from the “Reported Temperature” row. Use this number or round up to next nearest 100. Add this number to MDA or DA, as applicable, and any applicable step-down fixes in the final segment.
          3. Correct final holding altitude in the MA Segment: Calculate the correction by taking the final missed approach (MA) holding altitude and subtract the airport elevation. Use this number to enter the height above airport column in TBL ENR 1.8-1 until reaching the reported temperature from the “Reported Temperature” row. Round this number as applicable and then add to the final MA altitude only.
        2. Aircraft with temperature compensating systems: If flying an aircraft equipped with a system capable of temperature compensation, follow the instructions for applying temperature compensation provided in the airplane flight manual (AFM), AFM supplement, or system operating manual. Ensure that temperature compensation system is on and active prior to the IAF and remains active throughout the entire approach and missed approach.
          1. Pilots that have a system that is able to calculate a temperature-corrected DA or MDA may use the system for this purpose.
          2. Pilots that have a system not able to calculate a temperature corrected DA or MDA, will manually calculate an altitude correction for the MDA or DA.

            NOTE-

            Some systems apply temperature compensation only to those altitudes associated with an instrument approach procedure loaded into the active flight plan while other systems apply temperature compensation to all procedure altitudes or user entered altitudes in the active flight plan, including altitudes associated with a STAR. For those systems that apply temperature compensation to all altitudes in the active flight plan, delay activating temperature compensation until the aircraft has passed the last altitude constraint associated with the active STAR.

      2. Individual Segment(s) Method: Pilots are allowed to correct only the marked segment(s) indicated in the CTA list. https://www.faa.gov/air_traffic/flight_info/aeronav/digital_products/dtpp/search/. Pilots using the Individual Segment(s) Method will reference the CTA list to determine which segment(s) need a correction. See FIG ENR 1.8-1:

        FIG ENR 1.8-1
        Example Cold Temperature Airport List – Segments

        1. Manual correction: Pilots will make a manual correction when the aircraft is not equipped with a temperature compensating system or when a compensating system is not used to make the correction. Use TBL ENR 1.8-1, ICAO Cold Temperature Error Table, to calculate the correction needed for the approach segment(s).
          1. Intermediate Segment: All altitudes from the FAF/PFAF up to but not including the intermediate fix (IF) altitude. Calculate the correction by taking FAF/PFAF altitude and subtracting the airport elevation. Use this number to enter the height above airport column in TBL ENR 1.8-1 until reaching the reported temperature from the “Reported Temperature” row. Round this number as applicable and then add to FAF altitude and all step-down altitudes within the intermediate segment (inside of the waypoint labeled “(IF)”).
          2. Final segment: Calculate correction by taking the MDA or DA for the approach flown and subtract the airport elevation. Use this number to enter the height above airport column in TBL ENR 1.8-1 until reaching the reported temperature from the “Reported Temperature” row. Use this number or round up to next nearest 100. Add this number to MDA or DA, as applicable, and any applicable step-down fixes in the final segment.
          3. Missed Approach Segment: Calculate the correction by taking the final MA holding altitude and subtract the airport elevation. Use this number to enter the height above airport column in TBL ENR 1.8-1 until reaching the reported temperature from the “Reported Temperature” row. Round this number as applicable and then add to the final MA altitude only.
        2. Aircraft with temperature compensating system: If flying an aircraft equipped with a system capable of temperature compensation, follow the instructions for applying temperature compensation provided in the AFM, AFM supplement, or system operating manual. Ensure the temperature compensation system is on and active prior to the segment(s) being corrected. Manually calculate an altimetry correction for the MDA or DA. Determine an altimetry correction from the ICAO table based on the reported airport temperature and the height difference between the MDA or DA, as applicable, and the airport elevation, or use the compensating system to calculate a temperature corrected altitude for the published MDA or DA if able.
    6. Acceptable Use of Table for manual CTA altitude correction: (See TBL ENR 1.8-1) Pilots may calculate a correction with a visual interpolation of the chart when using reported temperature and height above airport. This calculated altitude correction may then be rounded to the nearest whole hundred or rounded up. For example, a correction of 130 ft. from the chart may be rounded to 100 ft. or 200 ft. A correction of 280 ft. will be rounded up to 300 ft. This rounded correction will be added to the appropriate altitudes for the “Individual” or “All” segment method. The correction calculated from the table for the MDA or DA may be used as is, rounded up, but never rounded down. This number will be added to the MDA, DA, and all step-down fixes inside of the FAF as applicable.
      1. No extrapolation above the 5000 ft. column is required. Pilots may use the 5000 ft. “height above airport in feet” column for calculating corrections when the calculated altitude is greater than 5000 ft. above reporting station elevation. Pilots must add the correction(s) from the table to the affected segment altitude(s) and fly at the new corrected altitude. Do not round down when using the 5000 ft. column for calculated height above airport values greater than 5000 ft. Pilots may extrapolate above the 5000 ft. column to apply a correction if desired.
      2. These techniques have been adopted to minimize pilot distraction by limiting the number of entries into the table when making corrections. Although not all altitudes on the approach will be corrected back to standard day, a safe distance above the terrain/obstacle will be maintained on the corrected approach segment(s). Pilots may calculate a correction for each fix based on the fix altitude if desired.

        NOTE-

        Pilots may use Real Time Mesoscale Analysis (RTMA): Alternate Report of Surface Temperature, for computing altitude corrections, when airport temperatures are not available via normal reporting. The RTMA website is http://nomads.ncep.noaa.gov/pub/data/nccf/com/rtma/prod/airport_temps/

    7. Communication: Pilots must request approval from ATC whenever applying a cold temperature altitude correction. Pilots do not need to inform ATC of the final approach segment correction (i.e., new MDA or DA). This request should be made on initial radio contact with the ATC facility issuing the approach clearance. ATC requires this information in order to ensure appropriate vertical separation between known traffic. Pilots should query ATC when vectored altitudes to a segment are lower than the requested corrected altitude. Pilots are encouraged to self-announce corrected altitude when flying into a non-towered airfield.
      1. The following are examples of appropriate pilot-to-ATC communication when applying cold-temperature altitude corrections:
        1. On initial check-in with ATC providing approach clearance: Missoula, MT (example below).
          1. Vectors to final approach course: Outside of IAFs: “Request 9700 ft. for cold temperature operations.
          2. Vectors to final approach course: Inside of ODIRE: “Request 7300 ft. for cold temperature operations.
          3. Missed Approach segment: “Require final holding altitude, 12500 ft. on missed approach for cold temperature operations.”
        2. Pilots cleared by ATC for an instrument approach procedure; “Cleared the RNAV (GPS) Y RWY 12 approach (from any IAF)”. Missoula, MT (example below).
          1. IAF: “Request 9700 ft. for cold temperature operations at LANNY, CHARL, or ODIRE.”
  6. Example for Calculating Altitude Corrections on CTAsAll 14 CFR Part 97 IAPs must be corrected at an airport. The following example provides the steps for correcting the different segments of an approach and will be applied to all 14 CFR Part 97 IAPs.
    1. Missoula Intl (KMSO). Reported Temperature -12°C: RNAV (GPS) Y RWY 12.
      1. All Segments Method: All segments corrected from IAF through MA holding altitude.
        1. Manual calculation:
          1. Cold Temperature Airport Temperature Limit: -12°C.
          2. Altitude at the Final Approach Fix (FAF) (SUPPY) = 6200 ft.
          3. Airport elevation = 3206 ft.
          4. Difference: 6200 ft. – 3206 ft. = 2994 ft.
          5. Use TBL ENR 1.8-1, ICAO Cold Temperature Error Table, height above airport of 2994 ft. and -12°C. Visual interpolation is approximately 300 ft. Actual interpolation is 300 ft.
          6. Add 300 ft. to the FAF and all procedure altitudes outside of the FAF up to and including IAF altitude(s):
            1. LANNY (IAF), CHARL (IAF), and ODIRE (IAF Holding-in-Lieu): 9400 + 300 = 9700 ft.
            2. CALIP (stepdown fix): 7000 + 300 = 7300 ft
            3. SUPPY (FAF): 6200 + 300 = 6500 ft.
          7. Correct altitudes within the final segment altitude based on the minima used. LNAV MDA = 4520 ft.
          8. Difference: 4520 ft. – 3206 ft. = 1314 ft
          9. TBL ENR 1.8-1: 1314 ft. at -12°C is approximately 150 ft. Use 150 ft. or round up to 200 ft.
          10. Add corrections to altitudes up to but not including the FAF:
            1. BEGPE (stepdown fix): 4840 + 150 = 4990 ft.
            2. LNAV MDA: 4520 + 150 = 4670 ft.
          11. Correct JENKI/Missed Approach Holding Altitude: MA altitude is 12000 ft.
          12. JENKI: 12000 - 3206 = 8794 ft.
          13. TBL ENR 1.8-1: 8794 ft. at -12°C. Enter table at -12°C and intersect the 5000 ft. height above airport column. The approximate value is 500 ft.
          14. Add correction to holding fix final altitude:
            1. JENKI: 12000 + 500 = 12500 ft.
        2. Temperature Compensating System: Operators using a temperature compensating RNAV system to make altitude corrections will be set to the current airport temperature (-12°C) and activated prior to passing the IAF. A manual calculation of the cold temperature altitude correction is required for the MDA/DA.
      2. Individual Segments Method: Missoula requires correction in the intermediate and final segments. However, in this example, the missed approach is also shown.
        1.  Manual Calculation: Use the appropriate steps in the All Segments Method above to apply a correction to the required segment.
          1. Intermediate. Use steps 6.1.1.1 a) thru e). Do not correct the IAF or IF when using individual segments method.
          2. Final. Use steps 6.1.1.1 f) thru i).
          3. Missed Approach. Use steps 6.1.1.1 j) thru l).
        2. Temperature Compensating System: Operators using a temperature compensating RNAV system to make altitude corrections will be set to the current airport temperature (-12°C) and activated at a point needed to correct the altitude for the segment. A manual calculation of the cold temperature altitude correction is required for the MDA/DA.

FIG ENR 1.8-2
Missoula Intl RNAV (GPS) Y RWY 12

A graohic depicting the Missoula Intl RNAV (GPS) Y RWY 12.