Rack Considerations; Cabling And Wiring; Cooling Air - Garmin GPS 155XL Installation Manual

2.3 RACK CONSIDERATIONS

2.3.1 Accessibility
Plan a location that gives the pilot comfortable access to the keypad and provides good visibility from the
pilot's perspective. Check that there is adequate depth for the rack in the instrument panel. A location
away from heating vents or other sources of heat generation is optimal. Figure 2-1 illustrates a typical
aviation rack installation.

2.4 CABLING AND WIRING

The recommended antenna cable type for both antennas is M17/155-0001 (RG-58A/U) per MIL-C-17.
Maximum allowable length for the GPS antenna using this cable type is 40 feet. Other cable types with
50 ohms nominal impedance and longer lengths can be used for the GPS antenna, provided the installer
insures that the attenuation does not exceed 10 dB at 1.5 GHz for the specific installation.
Check that there is ample space for the cabling and mating connectors. Avoid sharp bends in cabling,
particularly the GNC 300XL COMM antenna cable, and routing near aircraft control cables. Cabling for
the GPS 155XL/GNC 300XL should not be routed near components or cabling which are sources of
electrical noise. Do not route the GNC 300XL COMM antenna cable near any ADF antenna cables.
Route the GPS antenna cable as far as possible away from all COMMs and COMM antenna cable.

2.5 COOLING AIR

The GPS 155XL/GNC 300XL units meet all TSO requirements without external cooling. However, as
with all electronic equipment, lower operating temperatures extend equipment life. On the average,
reducing the operating temperature by 15-20 °C (25 to 35 °F) doubles the mean time between failure
(MTBF). Recommended airflow rating is 1 CFM (cubic foot per minute) at a pressure equivalent to 0.1
inches of water.
Units tightly packed in the avionics stack heat each other through radiation, convection, and sometimes
by direct conduction. Even a single unit operates at a much higher temperature in still air than in moving
air. Fans or some other means of moving the air around electronic equipment are usually a worthwhile
investment. A 5/8" diameter air fitting is provided on the rear of the mounting rack for the purpose of
admitting cooling air under such conditions. If a form of forced air cooling is installed, make certain that
rainwater cannot enter and be sprayed on the equipment.
GPS 155XL/GNC 300XL Installation Manual Page 2-3
190-00067-22 Revision F