Site Evaluation Of Magnetic Disturbances - Garmin GI 275 Installation Manual

7. Touch Done when the test sequence is completed. The GI 275 will display a test Pass or Fail, the
worst-case percentage of magnetic interference, and the time stamp at which it occurred.
8. Repeat the procedure for each installed AHRS unit if they were not completed simultaneously.
9. If the check passes, no further action is required.
If the check fails, the installation is considered unreliable until the source of magnetic interference is
identified and fixed. The Magnetometer Interference Test must be repeated until passed. When the
Magnetometer Interference Test fails, record the three magnetometer maximum deviation values and their
corresponding timestamps. A maximum deviation value greater than 5.0 mGauss in either the X or Y axes,
or greater than 8.0 mGauss in the Z axis, indicates a problem that must be resolved. Compare the
corresponding timestamps with the prepared test sequence to identify which action produced the failure.
Contact Garmin Support for assistance.
Three common reasons for a failed Magnetometer Interference Test are:
• Equipment, wiring, or ferro-magnetic items are installed too close to the magnetometer
• An electronic device has become grounded through the aircraft structure instead of the proper
ground wire in a twisted shielded pair, especially if the ground return path through the aircraft
structure passes near the GMU 11/44B
• The Interference Test was not run for a long enough duration. At least 25 seconds are required for
the test to properly complete
6.4.5

Site Evaluation of Magnetic Disturbances

Typically, a compass rose is an acceptable location to perform the Magnetometer Calibration Procedure.
However, even an existing compass rose can be evaluated to determine if it is free of magnetic
disturbances. If the evaluation of an existing compass rose indicates that magnetic disturbances are present,
an alternative location must be found to perform the Magnetometer Calibration Procedure.
A GI 275-equipped aircraft can be used to evaluate a candidate site for magnetic disturbances and
determine whether it is a suitable location to perform the Magnetometer Calibration Procedure. The
Magnetometer Calibration Procedure itself contains the logic to simultaneously survey the location for
magnetic cleanliness while it is computing the magnetometer calibration parameters.
The GI 275-equipped aircraft used to evaluate the site must have already completed the Pitch/Roll Offset
Compensation Procedure (Section 5.7.1.2). The completion of the Magnetometer Calibration Procedure
(Section 6.4.1) is not required. In order to evaluate a site, the Magnetometer Calibration Procedure must be
performed twice: once turning clockwise around the site and once turning counterclockwise. Both times,
the procedure should be conducted as described in Section 6.4.1, with the exception of the direction of
turns around the site.
If, upon completion of the Magnetometer Calibration Procedure in each clockwise and counterclockwise
direction, the GI 275 displays the "CALIBRATION SUCCESSFUL/SITE IS CLEAN" message, then the
candidate site is sufficiently free of magnetic disturbances and is acceptable for performing the
Magnetometer Calibration Procedure. It is important to obtain successful results in both the clockwise and
counterclockwise directions.
If, upon completion of the Magnetometer Calibration Procedure in either of the two directions, the GI 275
displays either the "MAG FIELD AT SITE NOT UNIFORM" or "MAG FIELD AT SITE DIFFERS
FROM IGRF MODEL" message, then the site contains magnetic disturbances that are too large, and an
alternate site should be used for the AHRS magnetic calibration.
190-02246-10
Rev. 11
GI 275 Part 23 AML STC Installation Manual
Page 6-27