This glossary is written for virtual airline and flight simulation use within Virtual Air Canada Airline. It is not intended as a real-world aviation reference.
RA
Resolution Advisory / Radio Altimeter
RA has two distinct meanings in aviation context. As a Resolution Advisory, it is a command issued by the Traffic Collision Avoidance System (TCAS II) directing the crew to climb or descend to avoid a collision threat. TCAS continuously interrogates nearby transponder-equipped aircraft and, when a collision threat is detected, coordinates maneuvers between the two aircraft - one receiving a “Climb” RA and the other a “Descend” RA. Crew response to an RA is mandatory and immediate: all other instructions, including ATC clearances, are secondary until the RA is resolved.
As a Radio Altimeter (also called a Radar Altimeter), RA refers to the instrument that measures the aircraft’s actual height above the terrain or water surface directly below the aircraft using radio waves. Unlike the barometric altimeter, which measures pressure altitude above MSL, the radio altimeter provides a precise ground clearance reading from 0 to typically 2,500 ft AGL. Radio altimeter readouts trigger automated callouts during approach (including minimums callouts and GPWS alerts), arm autoland systems, and provide inputs to ground proximity warning systems. Both meanings of RA are in common crew use; context makes the distinction clear.