Calibration; Attitude/Heading - Garmin GI 275 Maintenance Manual

5.7 Calibration

This section provides guidance for calibrating the GI 275 system if the previous calibration has become
invalid.
5.7.1

Attitude/Heading

The interfaced AHRS will not provide valid outputs until the calibration procedures in this section are
completed. Prior to completing the Pitch/Roll Offset Compensation (Section 5.7.1.3) and Magnetometer
Calibration (Section 5.7.1.7) procedures, the annunciation "CALIBRATE AHRS/MAG" will be displayed
on the ADI, and the attitude and heading will be displayed. Once the rotorcraft is moved, the attitude and
heading display will show a red "X". This condition is normal and will automatically clear when the two
aforementioned calibration procedures are completed.
5.7.1.1 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 rotorcraft equipped with a GI 275 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 rotorcraft equipped with a GI 275 used to evaluate the site must have already completed the Pitch/Roll
Offset Compensation Procedure (Section 5.7.1.3). The completion of the Magnetometer Calibration
Procedure (Section 5.7.1.7) 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 5.7.1.7, 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.
5.7.1.2 Calibrate Yaw Offset
If required for the installation, set the yaw offset. The range is -15.0 and 15.0 and the default is 0.
190-02246-C2 GI 275 Part 27 AML STC Maintenance Manual
Rev. 3 Page 5-17