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GRS 77 mounting racks are loosened for any reason, a new post-installation calibration procedure, A-1, B
and D (plus C if required initially) must be carried out before the aircraft can be returned to service.
Any GMU 44 removal and replacement requires repeating the magnetometer calibration, and if
applicable, the heading offset compensation.
The addition, removal or modification of components that are ferrous, or otherwise magnetic, within 10.0
feet of the GMU 44 magnetometer location after the magnetometer interference test or magnetometer
calibration procedure were completed requires a repeat of both procedures.
Furthermore, electrical changes to the installation that affect components within 10.0 feet of the GMU 44
magnetometer after the magnetometer calibration and magnetometer interference procedures were
completed will require a repeat of the magnetometer interference test. If new magnetic interference is
detected, it must be resolved and then the magnetometer calibration procedure must be repeated. Wiring
or grounding changes associated with a device located in the same wing as the GMU 44 is a good
example of such a change.
GRS 77/GMU 44 Installation Manual
Page 5-3
190-00303-10
Revision L
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