(aviation) The altitude (which varies by country) above which an aircraft's barometric altimeters are set as if the sea-level atmospheric pressure were equal to 1013.25 hectopascals (regardless of the actual atmospheric conditions) and altitudes are reported as flight levels.

plural of transition altitude

(aviation) The lowest altitude at which an aircraft is permitted to fly during a non-precision approach without the runway being in clear sight.

plural of minimum descent altitude

(aviation) The lowest altitude which will provide a minimum clearance of 1000 ft (300 m) above all objects located in an area within a circular sector having a radius of 25 nautical miles (46 km) around a radio navigational aid.

plural of minimum sector altitude

(military, defence) A United States Army system designed to shoot down ballistic missiles in their terminal phase.