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Cruise is the level portion of aircraft travel where flight is most fuel efficient. It occurs between ascent and descent phases and is usually the majority of a journey. Technically, cruising consists of heading (direction of flight) changes only at a constant airspeed and altitude. It ends as the aircraft approaches the destination where the descent phase of flight commences in preparation for landing.
For most commercial passenger aircraft, the cruise phase of flight consumes the majority of fuel. As this lightens the aircraft considerably, higher altitudes are more efficient for additional fuel economy. However, for operational and air traffic control reasons it is necessary to stay at the cleared flight level. On long haul flights, the pilot may climb from one flight level to a higher one as clearance is requested and given from air traffic control. This maneuver is called a step climb.
Commercial or passenger aircraft are usually designed for optimum performance at their cruise speed or VC. The cruising speed depends to a great extent on the engine itself, as any (combustion) engine has an optimum work level, where it will consume the least fuel vs the greatest mechanical output. This is by the way true for any vehicle, and is not just limited to airial vehicles. Generally, a piston engine will run the most efficient somewhere between idle speed and 25% away from full throttle.
With aerial vehicles, there are however also other factors to consider: there is for example an optimum cruising altitude for a particular aircraft type and conditions including payload weight, Center of gravity of an aircraft, 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 air speed for long-distance commercial passenger flights is 475–500 knots (878–926 km/h; 546–575 mph).
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