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Avionics

Altitude Encoders: More Than Just Mode C

For legacy analog panels, the blind encoder lives on. Choose for a simple installation and data output.

Larry Anglisano
Verified

Edited By:

Omar Filipovic

That’s the rear chassis of a Garmin GTX-series transponder. The arrow points to the GAE 12 pressure sensor that also functions as a configuration module for the transponder.

One of the many reasons to keep up with FAR 91.411 and 91.413 inspections is to check on the health of the altitude encoder/digitizer. An unappreciated remote box, it may be the main source of altitude for the ADS-B, GPS and autopilot. If it hiccups in controlled airspace, as aging ones do, you’ll hear about it from controllers and maybe from the FAA with a follow-up letter from the FAA looking for proof the system has been repaired.

While the tech has gotten better because even entry-level EFIS can source high-resolution pressure altitude through the air data computer, there are plenty of legacy analog panels still flying with aging standalone altitude encoders. That means replacement is imminent. Here are some tech tips for dealing with the chore.

Larry Anglisano

Editor in Chief Larry Anglisano has been a staple at Aviation Consumer since 1995. An active land, sea and glider pilot, Larry has over 30 years’ experience as an avionics repairman and flight test pilot. He’s the editorial director overseeing sister publications Aviation Safety magazine, IFR magazine and is a regular contributor to KITPLANES magazine with his Avionics Bootcamp column.