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Page 9-2 G3X Installation Manual –Troubleshooting
Revision A 190-01115-01
9.2 GMU 44
Figure 9-1 – Heading Failure Indication (Full-Screen PFD)
9.2.1 Red-X Failures
If a Red-X (steady or intermittent) is displayed on the heading (Figure 9-1), check the following while the
aircraft is on the ground:
1. When taxiing without reliable GPS information, heading performance is susceptible to the
presence of magnetic anomalies (metal buildings, underground steel culverts, steel grates in
the ramp, rebar). Localized sources of interference on the ground may consistently cause a
Red-X to be displayed on the heading in the same spot while taxiing, this is not caused by a
failure of the GMU 44 or its calibration.
2. When the aircraft is taxiing on the ground with a yaw rate of less than 1.5 degrees/second
(i.e., taxiing essentially in a straight line) GPS track information is used to update heading
information. This logic is applied regardless of magnetic anomaly detection.
3. The GDU may display Red-X on the heading if the GSU 73 does not have GPS information,
or if it senses a magnetic anomaly.
4. While a magnetic anomaly is detected and the aircraft is determined to be stationary, the
value of the heading output is frozen. When either the aircraft is determined to be no longer
stationary or the magnetic anomaly ceases, heading will be unfrozen and determined as
useable. In this context, the aircraft is considered to be stationary when its yaw rate is less
than 1.0 degrees/second and all other angular rate and acceleration values are sufficiently
small. (moving or shaking the wings or tail for example can cause a Red-X to be displayed).
5. Check to see if any new equipment has been installed on the aircraft, and reference Table 3-5
for minimum distances for installed equipment from the GMU 44 to prevent interference.
If the GMU 44 heading is not present on the GDU 37X, there may be a problem with the RS-232 line
between the GSU 73 and GMU 44. Troubleshoot any possible wiring/connector issues before replacing
either unit.