Flight Level (FL)

Flight Level (FL) is a unit of altitude expressed in hundreds of feet, with a standardized altimeter setting of 29.92 inHg (1013.2 hPa). In Canada, flight levels are used at and above 18,000 ft ASL. FL180 = 18,000 ft with 29.92 inHg set. FL350 = 35,000 ft.

Why Flight Levels Use Standard Pressure

By having all aircraft at high altitude set the same altimeter setting (29.92 inHg), aircraft flying at the same “flight level” are at the same pressure altitude regardless of the actual QNH. This provides a uniform reference for separation. Below the transition altitude, each aircraft may have a different QNH set, but at cruise all high-altitude traffic is on the same pressure reference.

Transition Altitude and Transition Level

The transition altitude in Canada is 18,000 ft ASL. Pilots climbing through 18,000 ft set 29.92 inHg and switch from altitudes to flight levels. When descending, the transition level (typically FL180 in Canada) is the level at which pilots switch back to QNH. ATC coordinates the transition.

RVSM

Reduced Vertical Separation Minima (RVSM) applies in Canadian RVSM airspace from FL290 to FL410 (inclusive). RVSM reduces the vertical separation between aircraft from 2,000 ft to 1,000 ft. RVSM-equipped aircraft must meet specific equipment and training standards.

Related Terms

See also: Altitude, Altimeter Setting, Pressure Altitude.