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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. There is also 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).
On the other hand technicaly speaking NO airplane travel at an ideal cruise speed wish is around 700 kms per hour at this point the static and induced drag are at the lowest. At the moment a turboprop of 620 kms per hour (SAAB 2000) is the best around but the propellers size is limited by the speed of sound at their tips and a jet airplane HAVE to travel at the speed of the combustion in the engines well over the optimun and for both type of airplanes the maximun fuel flow is actualy at takeoff where engine failure is most common.
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