Definition
An aircraft's altitude, transmitted automatically by the Mode C or Mode S transponder and displayed on the air traffic controller's radar screen in 100-foot increments. The altitude shown is referenced to 29.92 inches of mercury (the standard altimeter setting), regardless of the local altimeter setting in use.
Plain English
The altitude number that shows up next to your aircraft on the controller's radar screen. Your transponder sends it down automatically — you don't read it out over the radio.
Context Anchor
You will encounter this term in ATC radar, transponder, and altitude reporting discussions.
Derivation
Readout' simply means a value displayed by an instrument or system. Here, it's the altitude that 'reads out' on the controller's radar display, sent automatically by the transponder.
Why Pilots Care
Confirms the aircraft is at the assigned altitude and maintains safe vertical separation from other traffic.
Intuition Check
Do not read this as simply “the altitude you read on your cockpit altimeter.” In this FAA context, it means the altitude information ATC receives and sees from your aircraft’s reporting equipment.
Example Sentence 1
Center advised, 'Cessna 24 Bravo, your altitude readout indicates 6,500; verify altitude.'
Example Sentence 2
An incorrect altitude readout was traced to a faulty transponder encoder.