Frequently asked questions

What is temperature compensation?

Temperature compensation is a method to adjust procedure altitudes to compensate for the effects of non-standard temperature conditions. Barometric altimeters are calibrated to indicate true altitude only for International Standard Atmosphere (ISA) conditions of temperature and sea level pressure. When the temperature is higher (hotter) than ISA, the true altitude will be higher than the altitude indicated by the altimeter. Conversely, when the temperature is lower (colder) than ISA, the true altitude will be lower than indicated.

The flight crew is responsible to adjust procedure altitudes in accordance with regulatory guidance in order to compensate for this phenomena. Procedure altitudes can be raised to compensate for colder than standard temperature conditions. Procedure altitudes can be lowered to compensate for hotter than standard temperature conditions.

See also SkyLibrary AltimeterTemperature Error Correction

When do I need to use temperature compensation?

The pilot is responsible to apply temperature corrections in accordance with regulatory guidance whenever corrections are needed. There is not usually explicit guidance for when corrections are needed

  • Most states require or recommend that the flight crew advise Air Traffic Control when the flight crew is using temperature compensation.
  • Some states clearly state the flight crew is responsible for temperature corrections while still en route.
  • Some states provide guidance for specific conditions when the flight crew is explicitly required to use temperature compensation.

For example, for FAA procedures:

  • The LNAV/VNAV minimum has charted temperature limitations, both hot and cold:
    • For uncompensated barometric VNAV, LNAV/VNAV minimum can only be used when the OAT is within the charted temperature limits
    • For systems impacted by OPSB 0166-17 (523-0824828), the LNAV/VNAV minimum cannot be used when OAT exceeds the charted limits. (See separate FAQ below.)
  • Each year the FAA publishes a list of Cold Temperature Restricted Airports along with guidance for performing temperature compensation manually. See FAA Notice To Airmen Publication Cold Temperature Restricted Airports (August 15, 2019)

Other examples of guidance, which may not be current, include

Am I affected by this issue?

The affected FMS versions are Pro Line 4 and Proline 21 FMS 3.3.X through FMS 4.X. The applicable part numbers are listed in SIL FMC-XX00-18-1 (523-0825523). For assistance in finding the version of FMS installed in an aircraft, see SIL FMC-3000/4200/5000/6000-17-1 (523-0824752).

How do I know if Automatic Temperature Compensation Feature is activated on my aircraft?

This can be determined using SIL CSU-XX00-18-1, sections B.(5) through B.(6). If Automatic Temperature Compensation Feature has been previously deactivated or is not active, make sure the airplane flight manual has been amended by adding the information IAW AD 2020-10-5 instructions.

If the Automatic Temperature Compensation Feature in the CSU needs to be deactivated, accomplish the task within the 12 months after June 24, 2020 (the effective date of the AD)

NOTE: Check ALL CDU INDEX pages to confirm the Temp Comp function is removed.

For further detail on accomplishing the SIL see Where can I go to have automatic temperature compensation feature deactivated?

Where can I find more information on the FMS wrong turn issue precipitated by sequencing a “Climb To” that is manually edited or temperature compensated?

[Docket No. FAA-2018-0977; Product Identifier 2018-CE-041-AD; Amendment 39-21123; AD 2020-10-05].

Collins Aerospace documents are:

  • OPSB 0166-17 (523-0824828): THE FMS MAY TURN IN THE WRONG DIRECTION AFTER SEQUENCING A “CLIMB TO” ALTITUDE THAT WAS MANUALLY EDITED OR TEMPERATURE COMPENSATED
    • Revision 6 has been released to clarify the Limitations for the aircraft, and the error conditions.
  • Service Information Letter FMC-XX00-18-1 (523-0825523) : INSTRUCTIONS FOR AIRCRAFT FLIGHT MANUAL LIMITATIONS FOR ALTITUDE EDITS ON SPECIFIC PRO LINE 4 AND PRO LINE 21 FLIGHT MANAGEMENT SYSTEMS.
    • This document provides the list of applicable FMS part numbers which are affected by the issue and the FAA approved wording for the AFM Limitation.
  • Service Information Letter CSU-XX00-18-1 (523-0825521) : INSTRUCTIONS FOR DISABLING OF AUTOMATIC TEMPERATURE COMPENSATION OPTION IN PRO LINE 4 AND PRO LINE 21 SYSTEMS.
    • This document provides a reference for those aircraft type which have previously enabled the Temperature Compensation option (in those which have the affected FMS part number).

To access the documents listed above, enter the part numbers (number beginning with 523) in our Technical Publications website.

When will there be a permanent solution?

[Docket No. FAA-2018-0977; Product Identifier 2018-CE-041-AD; Amendment 39-21123; AD 2020-10-05] is the solution developed in collaboration with the FAA. The FAA AD actions must be undertaken to permanently deactivate the Temperature Compensation Function to comply with the directive.

Where can I go to have automatic temperature compensation feature deactivated?

As per SIL CSU-XX00-18-1, the change can be made by a Collins Authorized Dealer, your local OEM Service Center or your own Maintenance Department, if qualified. A list of dealers can be found here.

NOTE: CDU configuration straps are only read at Power Up. After corrections to the CSU configuration, a full aircraft power reset must be accomplished for strapping to be read correctly. Aircraft must also show WoW and not simulated “In Air”.

How do I work around not being able to edit procedural barometric altitudes on the FMS CDU ACT/MOD/SEC LEGS page for departure procedures and missed approach procedures?

This is described in OPSB 0166-17 (523-0824828); however, these methods cannot be used to operate the LNAV/VNAV minimum. The methods are:

  • For approach operations (prior to the missed approach), the flight crew may apply manual temperature compensation without using the FMS’s automatic temperature compensation feature, as follows
    • On the CDU, manually entering altitudes on waypoints that precede the missed approach point in order to correct the approach altitudes and the vertical path in the initial and intermediate segments. (Manually editing altitudes does not correct the vertical path in the final segment.)
    • Setting temperature-corrected barometric altitudes with the altitude preselect knob on the Flight Control/Guidance Panel and/or using basic pilot procedures and techniques ensuring compliance with the compensated, procedural barometric altitudes.
  • For missed approach operations, the flight crew may apply manual temperature compensation without using the FMS as follows
    • Requesting Air Traffic Control for an alternate missed approach
    • Setting temperature-corrected barometric altitudes with the altitude preselect knob on the Flight Control/Guidance Panel and/or using basic pilot procedures and techniques ensuring compliance with the compensated, procedural barometric altitudes.

When using Baro-VNAV vertical guidance, the primary barometric altimeter(s) must be the flight crew’s primary reference to ensure compliance with all ATC clearances for all flight operations and all phases of flight, including compliance with all procedural barometric altitude constraints. This is applicable to all methods of using Baro-VNAV vertical guidance, such as raw data display, coupled to flight director, and/or coupled to autopilot.

NOTE: Manually editing altitudes does not correct the vertical path in the final segment (after FAF), and for this reason the LNAV/VNAV minima is prohibited when temperature compensation is required.

NOTE: The crew should use the chart to identify all altitudes that require temperature compensation. The FMS flight plan may not include all procedural altitudes, such as procedural altitudes at stepdown fixes and the MDA. The PFD has a baro-Min set that can be used for the temperature compensated MDA or DA.

How can Cold Temperature Compensation values be determined manually?

ICAO published a Cold Temperature Correction Table where the flight crew can look up the cold temperature correction to add to a given altitude. The correction is a function of airport-reported temperature and height above the airport elevation. This is determined by subtracting the airport elevation from the altitude to be corrected. Regulatory guidance may differ in each state. Some states allow simplified methods for applying manual corrections, while others may not.

For FAA guidance, review the current FAA AIM 7-2-3 Altimeter Errors and at the current FAA NOTAMS.

For further detail see When do I need to use temperature compensation?

Self Help Instruction Video is available at: https://portal.rockwellcollins.com/web/support-self-service/self-help-videos

If I can’t use the LNAV/VNAV minimum, can I still use VNAV?

Yes, VNAV can still be used as approved in the AFM, subject to charted limitations and other state-specific limitations. Review the chart to determine if other minimums are available, approved, and authorized, such as LPV minimum. If the LPV feature is installed, VNAV can be used for the LPV minimum.

  • LNAV minimum: VNAV may be used to operate to the LNAV minimum. FAA AC 20-138D and AC 90-105A Appendices A and B provide operational guidance.
  • The database may have an advisory path angle or may be to a “V-MDA”. (V-MDA is essentially “dive and drive” after FAF.)
  • VNAV may be used as an advisory aid on conventional approaches, subject to the AFM and the standard limitation that the flight crew must use the primary barometric altimeter to comply with all altitude restrictions, including all associated step-down fixes.
  • If no other minimum is available, approved and authorized, it will be necessary to request a different approach.
  • Reference: FAA AC 20-138D Change 2 section 4-2 states, “... It is acceptable for approved baro-VNAV installations to provide advisory vertical guidance when using the LNAV line of minima. However, during these operations, the flight crew must use the primary barometric altimeter as the primary reference for compliance with all altitude restrictions associated with the instrument approach procedure; including compliance with all associated step-down fixes (see paragraphs 12-8 and 18-2.b).
  • Note 1: Baro-VNAV integrations may use non-GPS RNAV position sources to generate lateral path deviations for approaches that do not require GPS.
  • Note 2: Baro-VNAV is subject to performance limitations that could potentially cause advisory vertical path guidance to fall below step-down fixes on LNAV approaches.

Will I be able to use approaches impacted by this issue?

Yes. If the limitations provided in OPSB 0166-17 (most recent version) are followed, this issue will not limit approach availability. However, it may be required to use a different minimum. Some minimums are not authorized if temperature compensation is not applied. RNAV(GPS) charts contain a limitation for uncompensated baro-VNAV systems for the LNAV/VNAV Decision Altitude. The manual methods described in these FAQs and in OPSB 0166-17(most recent version) cannot be used to operate to the LNAV/VNAV minimum.

For FMSs impacted by OPSB 0166-17, there is an additional limitation LNAV/VNAV minimum is not authorized outside the charted temperature limits. Even when manual temperature compensation is applied, the LNAV/VNAV minimum is not authorized. This limitation is because the final segment VNAV path is not corrected when manually compensated.

My FMS version was not listed as affected in the OPSB, can I manually edit an altitude and use the FMS Automatic Temperature Compensation Feature?

Yes, only operators using FMS versions 3.3.X through 4.X are prohibited from these functions. For assistance in finding the version of FMS installed in an aircraft, see SIL FMC-3000/4200/5000/6000-17-1 (523-0824752). For any other questions you may have, please call Collins Customer Support at 319.295.5000.

What is the compliance period?

Per the Airworthiness Directive (AD), the compliance period is 12 months and compliance must be achieved by June 24th, 2021