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Input source for a ground proximity warning system

Aviation Mechanic Airframe (AMA) practice question 19 of 26 on Communications, Light Signals, and Runway Lighting Systems. ACS element AM.II.I.K15 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONOn a large aircraft, a ground proximity warning system (GPWS) aurally alerts the crew to dangerous terrain closure using information from which system?

  1. AThe pressure altimeter
  2. BThe weather radar transceiver
  3. CThe radio altimeter
Show the answer and explanation

Correct answer: C. The radio altimeter

  • A — incorrect A pressure altimeter reports altitude above a barometric datum, not height above the terrain directly beneath the aircraft.
  • B — incorrect Weather radar returns come from precipitation rather than from terrain height, so it does not feed the GPWS.
  • C — correct The handbook states that large aircraft may incorporate radio altimeter information into a GPWS that aurally alerts the crew to dangerous proximity to the terrain below.

BACKGROUND

A radio altimeter, also called a radar altimeter, measures the distance from the aircraft to the terrain directly beneath it by broadcasting a carrier wave toward the ground and timing the return. Because it reports height above ground level rather than above a barometric datum, it is more accurate and responsive than an air pressure altimeter at low altitude. Large aircraft may incorporate that radio altimeter information into a ground proximity warning system, which aurally alerts the crew to potentially dangerous proximity to the terrain below the aircraft. Glass flight deck displays commonly show decision height awareness from the radar altimeter as a digital number with a bug, light, or color change.

STUDY TIP

Anchor the GPWS to the radio altimeter, and remember that the radio altimeter is the only altitude source that measures actual height above the ground.

KEY POINTS

  • GPWS alerts are built on radio altimeter information.
  • The radio altimeter measures height above ground level directly beneath the aircraft.
  • A pressure altimeter cannot supply terrain clearance information.

SOURCES

  • FAA-H-8083-31B ch11 "Radio Altimeter"
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-31B Aviation Maintenance Technician Handbook – Airframe, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.