Definition
Definition 1 — Resolution Advisory: A TCAS (Traffic Alert and Collision Avoidance System) command issued to the flight crew that directs a specific vertical maneuver — such as climb, descend, or maintain vertical speed — to resolve a predicted collision threat with another aircraft. An RA is a higher-level alert than a Traffic Advisory (TA), which only informs the crew of nearby traffic without commanding action. RAs must be followed promptly unless doing so would jeopardize the safety of the flight. Definition 2 — Radio Altitude: The aircraft's height above the terrain directly below it, measured by a radio (radar) altimeter that times the return of a radio signal reflected off the surface. Unlike pressure altitude or indicated altitude, radio altitude is independent of barometric settings and reflects the true vertical distance to the ground at that moment, typically used at low altitudes during approach, landing, and certain low-level operations.
Plain English
Plain-English Meaning 1 (Resolution Advisory): A clear instruction from the aircraft's anti-collision system telling the pilots exactly what to do — climb, descend, or hold a vertical speed — to avoid hitting another aircraft. Plain-English Meaning 2 (Radio Altitude): The aircraft's actual height above the ground right below it, measured by bouncing a radio signal off the surface and timing how long it takes to come back.
Context Anchor
Seen in instrument procedures, cockpit alerting systems, approach discussions, and avionics displays where abbreviations are used.
Derivation
Resolution Advisory: "resolution" comes from Latin resolvere, meaning to loosen or settle — here, to settle or resolve a conflict. "Advisory" sounds soft, but in TCAS usage an RA is in fact a command, not a suggestion. Radio Altitude: "radio" because the measurement is made by a radio signal reflected off the ground, distinguishing it from altitudes derived from air pressure.
Why Pilots Care
Following the RA immediately can prevent a mid-air collision when visual contact is lost or time is critical.
Intuition Check
Do not assume RA always means one thing. In traffic-alert context, RA means Resolution Advisory; in height-measuring context, RA means Radio Altitude.
Example Sentence 1
When TCAS issued an RA to descend, the captain disconnected the autopilot and followed the commanded vertical speed immediately.
Example Sentence 2
The pilot smoothly followed the RA while advising ATC of the maneuver.