AD of the Week: When the Gauge Says You Have Fuel and You Don't

The fuel gauge indicated enough to make it to the airport, but the tank was almost dry

That’s the exact design vulnerability that prompted AD 2022-03-15. A Piper production line first flagged the problem: after fueling to a known level, the flight crew noticed that the fuel quantity indicator displayed more fuel than was actually in the tanks. It wasn't a calibration error, it was built into the system.

Field Notes: The Garmin G3X Touch and GI 275 MFD use a 1K ohm inline resistor for fuel tank lightning protection. The issue? The Garmin GEA 24 engine adapter is very sensitive to the ambient temperature. When it operates at a different temperature than the ambient air during initial calibration, it introduces a systemic error.

The Risk: Systems with fuel sending units having an operating range below 100 ohms are highly vulnerable. The displayed fuel quantity disparity can be as much as four gallons per tank.

The Fix: A physical modification of the resistive fuel probe interface, required within 100 hours TIS or 12 months.  

The Hardware Update: Garmin's updated GEA 24B eliminates the inline resistor in new installations entirely, removing the source of the temperature-dependent error. However, the modification remains mandatory for existing GEA 24 installations under the affected STCs.

Why It Matters: A fuel gauge reading falsely high removes the primary tool a pilot relies on to check their fuel state. In a single-engine aircraft at cruise, unexpected fuel starvation means your engine shuts down. You're landing immediately, and you don't have many options. A four-gallon error isn't a rounding issue; it’s a potential emergency.

Affected Fleet: An estimated 920 U.S.-registered retrofitted aircraft modified with a Garmin G3X Touch (STC SA01899WI) or GI 275 MFD (STC SA02658SE) using Master Drawing List Revision 9 or earlier with a resistive probe interface.

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