Cruise speed
Cruise is level flight after an aircraft climbs to a set altitude and before it begins to descend. It is usually the majority of a flight. Cruising may include heading (direction of flight) changes at a constant airspeed and altitude.
For most passenger aircraft, the cruise phase consumes most of the aircraft's fuel. This lightens the aircraft and raises the optimum altitude for fuel economy. For traffic control reasons it is usually necessary for an aircraft to stay at the cleared flight level. On long-haul flights, the pilot may ask air traffic control to climb from one flight level to a higher one. This maneuver is called a step climb.
Commercial or passenger aircraft are usually designed for optimum performance at their cruise speed or VC. Combustion engines have an optimum efficiency level for fuel consumption and power output. Generally, piston engines are most efficient between idle speed and 25% short of full throttle.
With aircraft, other factors affecting optimum cruise altitude include payload, center of gravity, air temperature, humidity, and speed. This altitude is usually where the higher ground speeds, the increase in aerodynamic drag power, and the decrease in engine thrust and efficiency at higher altitudes are balanced.
Typical cruising airspeed for a long-distance commercial passenger aircraft is 475–500 knots (878–926 km/h; 546–575 mph).
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