Regulation Text
(a) No person may operate any automatic pressure altitude reporting equipment associated with a radar beacon transponder—
(1) When deactivation of that equipment is directed by ATC;
(2) Unless, as installed, that equipment was tested and calibrated to transmit altitude data corresponding within 125 feet (on a 95 percent probability basis) of the indicated or calibrated datum of the altimeter normally used to maintain flight altitude, with that altimeter referenced to 29.92 inches of mercury for altitudes from sea level to the maximum operating altitude of the aircraft; or
(3) Unless the altimeters and digitizers in that equipment meet the standards of TSO-C10b and TSO-C88, respectively.
(b) No person may operate any automatic pressure altitude reporting equipment associated with a radar beacon transponder or with ADS-B Out equipment unless the pressure altitude reported for ADS-B Out and Mode C/S is derived from the same source for aircraft equipped with both a transponder and ADS-B Out.
[Docket 18334, 54 FR 34304, Aug. 18, 1989, as amended by Amdt. 91-314, 75 FR 30193, May 28, 2010]
Research Notes
Section 91.217 — Data correspondence between automatically reported pressure altitude data and the pilot's altitude reference — requires the transponder's reported altitude to match the pilot's altimeter within 125 feet.
Paragraph (a) — The matching rule: No person may operate any automatic pressure altitude reporting equipment if the equipment displays an altitude that differs by more than 125 feet from the altitude reference being used by the pilot to maintain flight altitude.
Why this matters: ATC separates IFR aircraft based on Mode C reported altitudes. If the Mode C reports 5,300 feet when the pilot is actually at 5,000 feet (a 300-foot disparity), ATC's separation assumptions are off — potentially conflicting with another aircraft.
Practical implication: When the pilot's altimeter says 5,000 feet on the local altimeter setting, the Mode C should be reporting between 4,875 and 5,125 feet on its 29.92 reference. The transponder shop verifies this at the periodic § 91.413 check.
Reference: FAA Order 8900.1 Volume 4, Chapter 6 on transponder/altimeter calibration.
Amendment History
Amendment History Coming Soon
Every time this regulation changes, we'll record it here — the date, what was amended, and a plain-English summary of what shifted.