Table of Contents
Advertisement
3.5
GRS 77 and GMU 44 Mounting Instructions
When mounting the GRS 77 rack to the airframe, and the unit to the rack, it is
important to ensure that fastening hardware is tight for proper unit operation.
Use a #2 Phillips screwdriver to tighten the GRS 77 to the rack, rather than
hand tightening the knurled screws. The recommended torque is 22-25 inch
pounds.
After ensuring that requirements are met, assemble the GRS 77 and GMU 44 mounting plate kits
according to the dimensions given in Appendix B. Install the unit assemblies. While installing the
GRS 77 on its rack, perform the flatness check described in Section 3.6. After completion, tighten the
four mounting screws securing the GRS 77 unit to the rack.
Mount the GMU 44 to its mounting plate, taking care to tighten the mounting screws firmly.
The metal components in the GMU 44's connector may slightly affect the magnetic field sensed by the
GMU 44. Place the connector at least 2 inches from the body of the GMU 44 to minimize this effect.
After attaching the GMU 44's connector to its mate in the aircraft wiring, secure the connector in place
using good installation practices. This will ensure that any remaining magnetic effect can be
compensated for using Calibration Procedure B: Magnetometer Calibration.
If the GMU 44 is ever removed, the anti-rotation properties of the mounting screws must be restored.
This may be done by replacing the screws with new Garmin PN 211-60037-08. If original screws must
be re-used, coat screw threads with Loctite 242 (blue) thread-locking compound, Garmin PN
291-00023-02, or equivalent. Important: Mounting screws must be brass.
3.6
GRS 77 Rack to Unit Flatness Check
While installing the GRS 77 unit on its rack, a flatness check is required to ensure that the unit's base is
properly preloaded after installation.
Place the unit on its rack, and tighten the screw fasteners on one end of the unit to the rack (recommended
torque is 22-25 inch pounds), but leave the screw fasteners on the other end of the unit unfastened.
At the unfastened end of the unit, there should now be a gap between the unit baseplate and the rails of
the mounting rack. Measure the gap to determine if it is within tolerances. See Figure 3-1. Using feeler
gauges, check to ensure that the gap between the unit and each rack rail is at least 0.010 inch, but less
than 0.070 inch.
If the gaps between the unit and each rack rail are within tolerance (0.010 inch, but less than 0.070 inch)
tighten the remaining two screw fasteners to hold the GRS 77 unit firmly to its rack (recommended torque
is 22-25 inch pounds).
If the gap is less than 0.010 inch, or greater than 0.070 inch, then the proper amount of preload will not be
exerted on the unit baseplate when the unit is fastened down, and the installation is not acceptable.
Page 3-4
Revision L
NOTE
GRS 77/GMU 44 Installation Manual
190-00303-10
Advertisement
Table of Contents
