Definition
The pressure altitude information transmitted automatically by an aircraft's transponder to ATC's radar system in response to a Mode C or Mode S interrogation. The altitude is referenced to a standard pressure setting of 29.92 inches of mercury, regardless of the local altimeter setting in use.
Plain English
When ATC's radar pings the aircraft, the transponder sends back the aircraft's altitude on its own, without the pilot needing to do anything. The altitude it sends is always based on a fixed pressure setting, not the one the pilot has dialed in.
Context Anchor
You will see this term in discussions of transponders, altitude reporting, radar services, and airspace where air traffic control needs to see an aircraft’s altitude.
Derivation
Automatic comes from words meaning “self-acting.” Altitude means height. Report means to send information back. Together, the phrase points to height information being sent back by the aircraft’s equipment on its own.
Why Pilots Care
ATC uses this reported altitude to maintain vertical separation between aircraft and to verify pilots are at their assigned altitudes. If the transponder isn't transmitting altitude correctly, ATC may ask the pilot to verify altitude or recycle the transponder. In most controlled airspace, altitude reporting is required.
Intuition Check
Do not read “report” here as only a spoken radio call from the pilot. In this term, the report is an electronic altitude message sent automatically by the aircraft’s equipment.
Example Sentence 1
After takeoff, the controller confirmed the automatic altitude report by saying, "Radar contact, altitude readout indicates 2,500."
Example Sentence 2
ATC used the automatic altitude report to confirm the aircraft was level at 4,500 feet.