Garmin GI 275 Installation Manual page 139

Aircraft must be leveled for installation of the GMU 11/44B. This is accomplished by following
procedures in the relevant aircraft maintenance manual. After the aircraft is leveled, the GMU 11/44B must
be:
1. Horizontal within 3.0° of the aircraft level reference in pitch and roll.
2. Pointing forward within ±0.5° of the aircraft longitudinal axis (butt line). Alignment within ±2.0°
is acceptable, but requires the post-installation heading compensation.
NOTE
Proper alignment of the GMU installation assures that the sensor points forward as
required.
GMU 11/44B sensitivity requires that:
• Electrical conductors within 3 feet of the GMU 44B or within 10 feet of the GMU 11 are installed
as a twisted shielded pair, not a single-wire conductor, and grounded at both ends, if possible
• Non-magnetic materials are used to mount the GMU 11/44B and that any magnetic fasteners
within 20 inches are replaced with non-magnetic equivalents (e.g., non-magnetic stainless steel or
zinc-plated steel screws). Fastener replacement is outside the scope of this STC; refer to the
aircraft structural repair manual or standard practices manual
The GMU 11/44B must not be mounted on an access panel or inspection cover. The GMU 11/44B can be
installed either in the wing or fuselage.
In general, wing mounting of the GMU is preferred. Fuselage mounting is less desirable because of
numerous potential disturbances that can interfere with the accurate operation of the GMU. To reduce the
effect of electric current in the wing skin:
1. The GMU 11/44B wiring must have a dedicated power ground return as a twisted pair with the
power source back into the fuselage.
2. The wingtip lights must not have a power ground referenced to the chassis of the light assembly
that would then be referenced back to the airframe ground via the light assembly mounting.
Electrically isolated light assembly can only be used if demonstrated to have adequate protection
against direct effects of lightning.
If installed in the fuselage, the preferred location is the fuselage tail section. However, it must maintain a
minimum of 2 feet of separation from the cabin or cargo compartments. Truss-type steel fuselage frames
are a significant source of magnetic interference and, therefore, are not recommended as a GMU 11/44B
installation location.
If the aircraft has wingtip tanks or removable fairings, the GMU 11/44B must be separated by at least a
single intermediate rib from the main fuel tank or tip tank, and the tip tank must be metallic in
construction.
For installations with dual GMU 11s or dual GMU 44Bs, co-locating the two GMUs close to each other in
the tail or in one wing is recommended, as this reduces the likelihood of heading splits or miscompares
when operating on the ground in the vicinity of local magnetic anomalies.
190-02246-10
Rev. 11
GI 275 Part 23 AML STC Installation Manual
Page 4-38