Encoding altimeter is used to convey to air traffic control what information?

• A. A standard pressure altimeter
• B. A special type of pressure altimeter used to send a signal to the air traffic controller on the ground, showing the pressure altitude the aircraft is flying
• C. A radar altimeter
• D. A GPS altitude display

Answer: (B)

Encoding altimeter works by feeding the aircraft’s altimeter data into the transponder so air traffic control can receive the altitude automatically. The altimeter is set to the standard pressure, so the altitude transmitted is the pressure altitude—the altitude the aircraft would have if the sea-level pressure were 29.92 inHg. This provides a consistent, machine-readable altitude for separation on ATC radar screens. It isn’t simply a standard pressure altimeter that displays numbers to the pilot, nor is it a radar altimeter (which measures height above the ground) or a GPS altitude display.