Definition
The aircraft's height directly above the terrain or surface immediately below it, measured by a radio (radar) altimeter that transmits a signal downward and times its reflection back. RA is displayed in feet AGL and is typically active and reliable from about 2,500 feet down to the surface, depending on the equipment.
Plain English
How high the aircraft is above whatever is right beneath it at that moment, found by bouncing a radio signal off the ground and measuring how long it takes to come back.
Context Anchor
Seen in instrument flying, radio altimeter discussions, approach procedures, and cockpit callouts near the ground.
Derivation
Radio' here points to how the height is measured — a radio signal sent down and reflected back — not to communication. 'Altitude' comes from the Latin altus, meaning high. So radio altitude literally means a height figured out by radio.
Why Pilots Care
It gives accurate terrain clearance information essential for safe approaches and avoiding controlled flight into terrain.
Analogy
Think of RA like a measuring tape hanging straight down from the aircraft to the ground below. If the ground rises, the number can get smaller even if the aircraft has not changed its height above sea level.
Grounding Statement
Radio altitude is the aircraft’s real-time height above the surface directly underneath it.
Intuition Check
Do not read “altitude” here as height above sea level. In RA, it means height above the ground or surface directly below the aircraft.
Example Sentence 1
Passing 1,000 feet RA on the approach, the crew confirmed the runway environment was in sight.
Example Sentence 2
The radio altitude read 150 feet as the aircraft crossed the runway threshold.