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Inside the FMS: Aircraft Positioning

Inside the FMS

FMS Position Drift with Gulfstream G350/450/550 ASC-912B Certification

An issue has been discovered that impacts the latest software load, ASC-912B. This load allows performance-based communication and surveillance (PBCS) authorization to the Gulfstream G-350/450 and 550 fleets. However, an unrelated software change has introduced a different issue.

After ASC-912B (latency timer update) was installed, operators have reported that the aircraft position on the airport chart diagram and Synthetic Vision System (SVS) can drift somewhat.

Airport Chart Diagram

During taxi, the aircraft position may not be displayed accurately. As the figure below shows, the aircraft is physically on the center of Taxiway Alpha at KBUR, but it is depicted on the airport diagram as being left of the taxiway.

Aircraft on Taxiway Alpha at KBUR

SVS Position

In the following figure, the aircraft is physically on the centerline of Runway 8 at KBUR, but the SVS and airport diagram show the aircraft position to be left of its actual position.

KBUR RWY 8 Takeoff Position

These issues may be observed at any airport, though the observed position difference will vary. Once the aircraft is airborne, the FMS position offset is diminished. Note that all CDI scaling and RNP/EPU alerting are correct, and if the FMS position exceeds the current required RNP value, the appropriate alert will be triggered. This is an FMS software issue specific to ASC-912B and is a result of how the FMS integrates the IRU position into the final FMS position.

Potential workarounds are being analyzed and a fix will be determined shortly.

Gulfstream has released MOL-19-0004 regarding this issue.

Program Pilot David Rogers supports Gulfstream, Cessna, and NG FMS. He can be reached via email at