Cruise (aeronautics)
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Cruise is the phase of aircraft flight that starts when the aircraft levels off after a climb, until it begins to descend for landing. Cruising usually comprises the majority of a flight, and may include small changes in heading, airspeed, and altitude.
A Qantas four-engined Boeing 747-400 at cruise altitude
Altitude
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Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context. Although the term altitude is commonly used to mean the height above sea level of a location, in geography the term elevation is often preferred for this usage.
A Boeing 737-800 cruising in the stratosphere, where airliners typically cruise to avoid turbulence rampant in the troposphere. The blue layer is the ozone layer, fading further to the mesosphere. The ozone heats the stratosphere, making conditions stable. The stratosphere is also the altitude limit of jet aircraft and weather balloons, as the air density there is roughly 1⁄1000 of that in the troposphere.