PURPOSE
A GTN 725 or GTN 750 connected to a synchro heading source incorrectly interprets the synchro heading data as aligned to true north rather than magnetic north. This issue results in the following behavior:
- The ownship is aligned to a heading that is skewed by the magnetic variation angle, so on the map display, the ownship will appear to be crabbing.
- Traffic targets and lightning strikes on the map are depicted correctly in relation to the ownship (e.g., traffic at 3 o’clock in relation to the aircraft is shown at 3 o’clock in relation to the ownship icon), but in relation to the underlying geographical features, their placement is skewed by the magnetic variation angle.
- When the GTN is providing GPS roll steering to the autopilot on heading legs, the aircraft will follow a flight path that is skewed by the magnetic variation angle.
- Winds are calculated incorrectly.
Note: This issue does not affect the aircraft’s primary display of heading.
PRODUCTS AFFECTED
All GTN 725/750 Units with Main Software Version 2.00 installed with a synchro heading input.
DESCRIPTION
The synchro heading to the GTN 7XX Unit must be disabled until a future GTN software release correcting this issue is available. This Service Bulletin provides instructions for disabling the synchro heading to the GTN 7XX.
COMPLIANCE
Mandatory: This Service Bulletin is mandatory for affected products.