Computed stab TRIM value review following aft fuel tank transfer during long taxi
Effectivity: Global 7500 aircraft
ATA: 34-60
By: Maxime Boyer-Beaucage – Technical Services During a long taxi in the Global 7500 aircraft, per the normal fuel burn sequence under a light fuel load configuration, once the wings reach 42.5%, the aft tank fuel transfer will be activated on ground with fuel AFX XFER selected to AUTO.
The fuel transfer from the aft tank to the inner wing tanks does impact the aircraft’s centre of gravity (CG) and gross weight (GWT) values used by the Flight Management System (FMS) for the horizontal stabilizer TRIM calculation shown on the DEP PERF 1/2 page.
However, per current FMS design, the CG and GWT displayed on DEP PERF page are not refreshed once takeoff FMS Vspeeds are posted on the Primary Flight Display (PFD), or when the FMS DEP PERF page stays open. As fuel continues to transfer, the actual CG and GWT indicated on the WT & BAL page may differ from the displayed value on DEP PERF page. As a result, the computed TRIM displayed on DEP PERF may no longer be accurate, as it remains unchanged instead of being recalculated.
As the FMS does not currently monitor the impact of background CG and GWT updates due to auto fuel transfer, the crew does not get any indication of a computed TRIM value mismatch with the values provided in the aircraft flight manual (AFM) for actual CG and GWT. Unless a specific action forces the refresh of the FMS DEP PERF page, the displayed CG and GWT on DEP PERF page will remain unchanged and the computed TRIM value will not be updated to reflect new takeoff parameters.
An AFM takeoff check was added to require the flight crew to manually do a check on CG and GWT, to potentially reset the computed TRIM prior to takeoff.
AFM 04-02-4:
The associated FCOM 1 note was also revised to better explain the possible need to refresh the FMS DEP PERF page to get an accurate computed TRIM value, using the actual CG and GWT, to set the stabilizer trim for takeoff.
FCOM 1 04-04-4:
Bombardier and Collins Aerospace are evaluating future software upgrade solutions to define alerting systems and improve flight crew awareness for this condition. This would allow the AFM check against STAB trim to be removed and reduce the crew workload during the taxi phase.