Glossary – Measurements

More aviation terms below. Return next Friday to learn about weather and flight terms. Visit Fairway Aviation to learn more.

ABSOLUTE ALTITUDE Measurable height of an aircraft above the actual terrain.

ABSOLUTE CEILING The maximum altitude above sea level at which an aircraft can maintain level flight under Standard Air conditions.

AGL (Above Ground Level) Altitude expressed as feet above terrain or airport elevation (see MSL).

CEILING (1) The heights above the earth’s surface of the lowest layers of clouds or obscuring phenomena that is reported as “broken,” “overcast,” or “obscuration,” and not classified as “thin” or “partial.” (2) The maximum height above sea level in Standard Air attainable by an aircraft under given conditions.

GROUND SPEED Actual speed that an aircraft travels over the ground also called “shadow speed.” It combines the aircraft’s air speed and the wind speed relative to the aircraft’s direction of flight.

HORSEPOWER The motive energy required to raise 550 lbs. one foot in one second, friction disregarded.

IAS (Indicated Airspeed) A direct instrument reading obtained from an air speed indicator uncorrected for altitude, temperature, atmospheric density, or instrument error.

KNOT (Nautical Mile per Hour) Most common measure of aircraft speed equaling 6,080 feet or about 1.15 miles. (For mph, multiply knots by 1.15.)

KTAS True airspeed, in knots.

MACH (m.) A number representing the ratio of the speed of an object to the speed of sound in the surrounding air or medium in which it is moving.

MSL (Mean Sea Level) The average height of the surface of the sea for all stages of tide; used as a reference for elevations, and differentiated from AGL.

NAUTICAL MILE Historically, a minute of arc along a meridian of the Earth. Since 1929, it has been defined internationally as equivalent to 1,852 meters or 1.15 statute miles.

STATUTE MILE A unit of length equal to 5,280 feet.