FAR 91.217 — Altitude Reporting Equipment
FAR 91.217 explains when automatic pressure altitude reporting equipment must be accurate, calibrated, and aligned between Mode C/S and ADS-B Out sources.
FAR 91.217 governs how your automatic pressure altitude reporting equipment (the Mode C/S transponder altitude encoder, and ADS-B Out altitude source) must work in relation to the altimeter you actually fly with.
You may not operate the automatic altitude reporting equipment if:
- ATC has directed you to deactivate it (for example, when your reported altitude is bad and controllers ask you to turn off altitude reporting).
- The equipment, as installed, was not tested and calibrated to transmit altitude data within 125 feet (95% probability) of the indicated or calibrated datum of the altimeter normally used to maintain flight altitude, with that altimeter set to 29.92 inches of mercury, from sea level to the aircraft's maximum operating altitude.
- The altimeters and digitizers do not meet TSO-C10b (altimeters) and TSO-C88 (digitizers), respectively.
If the aircraft has both a transponder and ADS-B Out, the pressure altitude reported by Mode C/S and ADS-B Out must come from the same source.
Why it matters: ATC separation, TCAS, and ADS-B traffic services rely on accurate, consistent altitude data. Mismatched or uncalibrated encoders create real collision risk and will get you a phone number to call.