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VNAV Pitch Command on RNAV (GPS) Approach with Temperature Compensation Active

Feb 25, 2019
Stacy Honda, Marketing Manager

VNAV Guiding to Temperature Compensated Altitudes

We have identified an inaccuracy when flying Localizer Performance with Vertical Guidance (LPV) and Lateral Navigation (LNAV)/Vertical Navigation (VNAV) Levels of Service (LOS) on a Satellite-Based Augmentation System (SBAS)-based Area Navigation (RNAV) (GPS) or RNAV (GNSS) approaches with Temperature Compensation active. This applies to UNS-1Ew, -1Espw, -1Fw, and -1Lw SBAS-Flight Management Systems (FMS) with Software Control Number (SCN) 1000/1100.8, 1001/1101.0/1/2/3, and 1002/1102.0/1, integrated in aircraft that are pitch steered on approach. The following aircraft with the IMS or Primus Bus interface are affected:

  • Bombardier Dash 8 Q-400
  • Cessna Ultra
  • Cessna Ultra II
  • Cessna Bravo
  • Cessna Bravo II
  • Cessna Excel
  • Cessna XLS
  • Cessna Encore
  • Embraer 145
  • Learjet 40/45
  • Falcon 900
  • CL 601-3A

Beginning at the Final Approach Fix (FAF) and continuing to the End of Approach (EOA), when flight guidance is coupled to the FMS, the VNAV pitch command incorrectly steers the aircraft to a vertical offset below the designed procedure glidepath. Depending on the temperature compensation value entered, the offset below the procedural glidepath can range from 0 to -70 feet.

The vertical deviation output to the displays on the flight deck is incorrect and will display that the aircraft is below glidepath. The flight director receives pitch commands corresponding to flight guidance and should be disregarded.


We recommend operators who have a requirement to Temperature Compensate RNAV (GPS) or RNAV (GNSS) procedures with LPV or LNAV/VNAV as described above to mitigate this error in one of two ways:

  • Manually select the LNAV LOS, if available, for the approach. In this case, the FMS approach VNAV will be based on temperature compensated barometric altitude.
  • Do not allow flight guidance to be coupled for the approach. Fly the approach manually using the displayed raw deviations on the flight deck instruments.

Additional Information

For more information, please reference UA Service Letter (SL) No. 2895 on UniNet or contact Customer Support.

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