Cruise (aeronautics)

Boeing 747 in cruise at roughly 35000 feet, showing contrails from the four engines.

The cruise portion of a flight usually consists of heading (direction of flight) changes only. Typically a constant airspeed(in Mach Number) and altitude will be maintained until the aircraft arrives near the destination and then enters the descent phase of flight. For most commercial passenger aircraft, the cruise phase of flight is where most of it's fuel is burned. This lightens the aircraft considerably and for minimum fuel consumption, the aircraft needs to cruise-climb to higher altitudes. For operational and Air Traffic Control reasons, staying at the Flight Level it has been allocated is the most expedient option. However, on Long Haul flights, the aircraft may climb from one Flight Level to a higher one periodically, to minimise it's fuel consumption.

Commercial or Passenger Aircraft are usually designed, in principle, for optimum performance at their cruising speed or V_C. The designer will try to maximize the Lift / Drag (L/D) ratio of the airframe for that speed. Ideally, designers will look to maximise the (L/D) for a range of speeds, as commercial aircraft have to fly at non optimum speeds and altitudes for operational reasons. Commercial aircraft manufacturers have to form a view on whether it is better to design aircraft with high levels of L/D but over a narrow range of speeds or vice versa.

There will also be an optimum cruise altitude for a particular aircraft type and condition (it's payload and speed). This altitude is usually where the combination of L/D ratio and engine efficiency will be maximum.

See also