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The four-engined Airbus 340 jet in cruise

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, staying at the cleared flight level is the most expedient option. On long haul flights, the pilot may climb from one flight level to a higher one, as clearance is given from air traffic control. This manoeuver is called a step climb.

Commercial or passenger aircraft are usually designed for optimum performance at their cruise speed or V_C. The aircraft is designed for maximum lift to drag (L/D) ratio for that speed. Ideally, designers maximise L/D for a range of speeds, as flights face non-optimum speeds and altitudes for operational reasons. Commercial aircraft manufacturers have to tradeoff aircraft designed with higher levels of L/D over a narrow range of speeds and vice versa.

There is an optimum cruise altitude for a particular aircraft type and conditions including payload weight, center of gravity, air temperature, humidity, and speed. This altitude is usually where the combination of L/D ratio and engine efficiency are maximised.

• Takeoff
• Landing
• Step climb

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