Page 7-10
G900X/G950 Installation and Maintenance Manual – GRS 77 and GMU 44
Revision D
190-00719-00
7.8.1 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 7-2. 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.
Possible causes for a failure of this check include the following:
a)
The rack is fastened down to a surface that is not sufficiently flat
b)
The rack is warped or damaged
c)
The GRS 77 has a center baseplate external shim that is damaged or has been removed
d)
The GRS 77 baseplate has been warped or damaged
In the event of a failed test (gap on unfastened end of unit not within the range of 0.010 inch to 0.070
inch), these possibilities must be examined, and any deficiencies corrected to pass this check before the
installation is acceptable.
MEASURE GAP
MEASURE GAP
AT UNFASTENED END
AT UNFASTENED END
Figure 7-2. Measuring GRS 77 to Mounting Rack with Feeler Gauge
7.9 Continued
Airworthiness
The GRS 77 utilizes an Earth magnetic field model which is updated once every five years. The update is
expected to be available from Garmin by July 1 of each of the following years: 2010, 2015, and every
five years thereafter, so long as the GRS 77 remains a Garmin-supported product. Otherwise
maintenance of the GRS 77 is ‘on condition’ only.
Maintenance of the GMU 44 is ‘on condition’ only.