Frequently asked questions

Updated: June 26, 2019

Q. 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?

A. Following the release of OPSB 0166-17 Rev 4 in December 2017, Rockwell Collins has continued to discuss this issue with the FAA. These discussions have resulted in the release of the following publications. The FAA performed a review of these documents and provided feedback on their content before release.

  • 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 5 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). 

Entering the part numbers (number beginning with 523) for the documents listed above in ourTechnical Publications index will allow for access to these documents.

Q. Am I affected by this issue?

A. The applicable part numbers are listed in SIL FMC-XX00-18-1. The affected versions are Pro Line 4 and Proline 21 3.3.X through 4.X. For assistance in finding the version of FMS installed in an aircraft, see SIL FMC-3000/4200/5000/6000-17-1 (Technical Publication document number523-0824752).

Q. When will there be a permanent solution?

A. Rockwell Collins, along with the FAA, determined this to be the best long-term solution with the least overall impact to operators. Affected procedures will remain intact and no software upgrade will be required.

Q. How do I determine if my aircraft has automatic temperature compensation enabled?

A. Go to the FMS INDEX. If TEMP COMP line is visible, the feature is enabled. If it is not, then the feature is not enabled. See SIL CSU-XX00-18-1 (523-0825521) section B.(6) for more information.

Q. Where can I go to have automatic temperature compensation disabled?

A. As per SIL CSU-XX00-18-1 B (1), the change is to be made by Rockwell Collins authorized dealers only. A list of dealer can be found here.

Q. 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 approach procedures (including missed approach procedures)?

A. The flight crew may use the altitude preselect knob (sometimes called “preselector” or “altitude alerter”) on the Flight Control/Guidance Panel and/or using basic pilot procedures and techniques to ensure compliance with a changed altitude restriction. The crew may need to deselect VNAV to prevent leveling at flight plan altitudes that differ from the altitude alerter.

The PFD has a baro-Min set that can be used for the temperature compensated MDA or DA.

Q. What is temperature compensation?

A. 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 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.

Q. When do I need to use temperature compensation?

A. The pilot is responsible to apply cold temperature corrections whenever corrections are needed. There is not usually explicit guidance for when corrections are needed, and some states clearly state the flight crew is responsible for temperature corrections while still en route. Most states require or recommend that the flight crew advise Air Traffic Control when the flight crew is using temperature compensation. See EUR OPS BULLETIN Serial Number 2015_001 Subject: Interim guidelines to airspace users in order to ensure a harmonised application of cold temperature correction to minimum flight altitudes for more information.

Some states provide additional guidance for specific conditions when the flight crew is explicitly required to use temperature compensation. For example, each year the FAA publishes a list of Cold Temperature Restricted Airports along with guidance for performing temperature compensation manually.

Refer to current guidance for the local authority. Examples, which may not be current, include:

Q. How can manual Cold Temperature Compensation be accomplished?

A. 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 differs in different states. Some states allow simplified methods for applying manual corrections. Some do not.

For FAA guidance, look at the current FAA AIM 7-2-3 Altimeter Errors and at the current FAA NOTAMS. See also FAA Notice To Airmen Publication Cold Temperature Restricted Airports, available from theNTAP, Part 3, Section 1.

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

A. 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 use 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, “There may be occasions where it is operationally advantageous to use the LNAV line of minima rather than the LNAV/VNAV minima during an instrument approach procedure. 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.

Q. Will I be unable to use any approaches due to this issue?

A. In general no, if the limitations provided in OPSB 0166-17 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.

Specific to this issue, there is an additional limitation LNAV/VNAV minimum is not authorized outside the charted temperature limitations. 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.

Q. 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?

A. Yes, only operators using FMS versions 3.3.X through 4.X are prohibited from these functions.

For any other questions you may have, please call Rockwell Collins Customer Support at 319.295.5000.