Installation and Trouble Shooting Guidance with the GMU

What is a GMU?

• GMU stands for Garmin Magnetometer Unit.
• A magnetometer senses magnetic field and provides data to the ADAHRS to determine aircraft magnetic heading.
• Garmin offers two variants for the GMU, including:
  • GMU 44/44B
    • Applicable for Class I through IV aircraft.
    • Communicates with the ADAHRS via RS-485 and RS-232.
  • GMU 11
    • Applicable for Class I and II aircraft only.
    • Limited to single and multi-engine aircraft with a gross takeoff weight of 6,000lbs or less (Class I and II aircraft).
    • Communicates with the ADAHRS via RS-232.
• The GMU 11 and 44/44B are very sensitive magnetic sensors.  Before they are installed, their location must be surveyed to verify that they'll pass a Magnetic Interference Test.

How Does a GMU work?

• The GMU is a microprocessor-based magnetometer.  It can be used to accurately sense the Earth's magnetic field alignment and provide this data to a compatible ADAHRS processor for use in referencing aircraft magnetic heading.
• The magnetometer works as an electronic compass, showing the ADAHRS which direction the aircraft is facing in relation to Magnetic North.

What are the common reasons for a failed magnetometer?

• Equipment, wiring, or ferro-magnetic items are installed too close to the magnetometer.
  • Examples:
    • Ferrous (magnetic metal) bolts were used to install the GMU.
    • Magnetic screwdriver left in the wing.
• An electronic device has become grounded through the aircraft structure instead of the proper ground wire in a twisted shielded pair, especially if the ground return path through the aircraft structure passes near the GMU.
  • Examples:
    • A strobe light that is grounded at the wing tip instead of the ground ran back with a specific wire in a twisted pair.
• Control surface cables become magnetic.

What does it mean when I see yellow on the display of my magnetic heading?

• If the display senses the GMU's magnetic heading measurement is valid, but possibly outside of the internal accuracy limits, the numeric heading value will be displayed in yellow.
• If the magnetometer input fails, the heading output is flagged as invalid with a red "X" but track information is displayed instead if valid GPS data is available.

Example of a heading failure from the TXi HSI Display:

Example of a heading failure from the G500 PFD Display: