FAA Instrument Flight Instructor (FII)Instruments and SystemsMedium
A pilot's aircraft is equipped with an encoding altimeter that feeds Mode C data to the transponder. Which statement correctly describes what this equipment transmits to ATC?
- AThe pilot's indicated altitude corrected to the current local altimeter setting
- BDensity altitude computed from OAT and pressure altitude
- CTrue altitude corrected for nonstandard temperature
- DPressure altitude referenced to 29.92 inHg, which ATC's computer corrects for display
Show answer & explanationAnswer & explanation
Correct answer: D. Pressure altitude referenced to 29.92 inHg, which ATC's computer corrects for display
An encoding altimeter always transmits pressure altitude referenced to a standard 29.92 inHg setting, regardless of the local altimeter setting the pilot has dialed in. The ATC ground computer then applies the correct local altimeter setting to display corrected altitude to controllers.
Why the other options are wrong
- A. The transmitted value is unaffected by the pilot's Kollsman setting; only pressure altitude is sent.
- B. Density altitude is not part of the transponder's altitude reporting function.
- C. True altitude requires temperature correction, which encoding altimeters do not compute.
Encoding Altimeter (Mode C)
An altimeter that transmits pressure altitude (referenced to 29.92 inHg) to the transponder for ATC altitude reporting, regardless of the pilot's altimeter setting.
- Always references 29.92 inHg internally
- ATC ground computer applies local altimeter correction
- Verified against pilot's altimeter every 24 months per 91.411/413
Memory trick: Encoders always speak '29.92' — ATC translates it for you.