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VNAV Guiding to Temperature Compensated Altitudes

Feb 25, 2019
Stacy Honda, Marketing Manager

VNAV Guiding to Temperature Compensated Altitudes

We have identified an inaccuracy when flying Area Navigation (RNAV) (GPS) and RNAV (GNSS) approaches with SBAS-FMS in the following conditions:

  • Lateral Navigation (LNAV)/Vertical Navigation (VNAV) Level of Service (LOS)
  • No Localizer Performance with Vertical Guidance (LPV) LOS for that runway/approach (this can be identified by the absence of LPV minima at the bottom of the approach chart and no published Channel ID in the upper left corner of the approach chart)
  • Temperature compensation is active

Affected SBAS-FMS models include the UNS-1Ew, -1Espw, -1Fw, and -1Lw with Software Control Number (SCN) 1000/1100.8, 1001/1101.0/1/2/3, and 1002/1102.0/1.

Beginning at the Final Approach Fix (FAF) and continuing to the End of Approach (EOA), the FMS approach VNAV incorrectly guides to the temperature compensated vertical approach path with the system altitude set equal to the GNSS altitude (as opposed to barometric altitude). This results in the aircraft flying above the procedural glidepath by an offset altitude equal to the temperature compensated altitude, and thereby flying a path steeper than the designed glidepath.

The FMS is incorrectly using temperature compensated barometric altitude to calculate the glidepath instead of using uncompensated GNSS altitude.


We recommend operators who have a requirement to Temperature Compensate RNAV (GPS) or RNAV (GNSS) procedures with LNAV/VNAS LOS as described above to mitigate this error by manually selecting the LNAV LOS, if available, for the approach. In this case, the FMS approach VNAV will be based on temperature compensated barometric altitude.

Additional Information

For more information, please reference UA Service Letter (SL) No. 2894 on UniNet or contact Customer Support.

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