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Cruise is the phase of
aircraft An aircraft is a vehicle that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines ...
flight that starts when the aircraft levels off after a climb, until it begins to descend for landing. Cruising usually consumes the majority of a flight, and it may include changes in
heading Heading can refer to: * Heading (metalworking), a process which incorporates the extruding and upsetting processes * Headline, text at the top of a newspaper article * Heading (navigation), the direction a person or vehicle is facing, usually s ...
(direction of flight),
airspeed In aviation, airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are: * Indicated airspeed ("IAS"), what is read on an airspeed gauge connected to a Pitot-static system; * Calibrated a ...
and
altitude Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...
. Commercial or passenger aircraft are usually designed for optimum performance around their cruise speed ( VC) and cruise altitude. Factors affecting optimum cruise speed and altitude include payload,
center of gravity In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weight function, weighted relative position (vector), position of the distributed mass sums to zero. Thi ...
, air temperature, and humidity. Cruise altitude is usually where the higher
ground speed Ground speed is the horizontal speed of an aircraft relative to the Earth’s surface. It is vital for accurate navigation that the pilot has an estimate of the ground speed that will be achieved during each leg of a flight. An aircraft diving ve ...
is balanced against the decrease in engine thrust and efficiency at higher altitudes. A typical cruising airspeed for a long-distance commercial passenger aircraft is approximately . The typical cruising altitude for commercial airliners is . The speed which covers the greatest distance for a given amount of fuel is known as the maximum range speed. This is the speed at which drag is minimised. For jet aircraft, "long-range cruise" speed (LRC) is defined as the speed which gives 99% of the maximum range, for a given weight. This results in a 3-5% increase in speed. It is also a more
stable A stable is a building in which livestock, especially horses, are kept. It most commonly means a building that is divided into separate stalls for individual animals and livestock. There are many different types of stables in use today; the ...
speed than Maximum Range Speed, so gives less autothrottle movement. However, LRC speed does not take account of winds, or time-related costs other than fuel, so it has little practical value. Instead, the speed for most economical operation (ECON) is adjusted for wind and the cost index (CI), which is the ratio of time cost to fuel cost. A higher cost index results in a higher ECON speed. Cost index can be given in "Boeing" or "English" units as , equivalent to . A typical cost index in these units might be anywhere from 5 to 150. Alternatively cost index can be given in metric or "Airbus" units of . In the presence of a tailwind, ECON airspeed will be reduced to take advantage of the tailwind, whereas in a headwind, ECON speed will be increased to avoid the penalty of the headwind. In a strong tailwind, the aircraft can fly at a low speed to conserve fuel, while the tailwind does most of the work getting the aircraft to its destination. In an area of strong headwind, the aircraft should fly faster to pass through that region sooner. In the presence of a tailwind, LRC speed may give a higher fuel burn than ECON. As the aircraft consumes fuel, its weight decreases and the ECON speed decreases. This is because a heavier aircraft should fly faster to complete the trip more quickly. ECON speed will also be higher at higher altitudes. For propeller aircraft, drag is minimised when the
lift-to-drag ratio In aerodynamics, the lift-to-drag ratio (or L/D ratio) is the lift generated by an aerodynamic body such as an aerofoil or aircraft, divided by the aerodynamic drag caused by moving through air. It describes the aerodynamic efficiency under give ...
is maximised. However, the speed for this is typically regarded as too slow, so propeller aircraft typically cruise at a significantly faster speed. Combustion engines have an optimum efficiency level for fuel consumption and power output.Cruising speed definition
/ref> Generally, gasoline piston engines are most efficient between idle speed and 30% short of full throttle. Diesels are most efficient at around 90% of full throttle. As the aircraft consumes fuel, its weight decreases and the optimum altitude for fuel economy increases. For traffic control reasons it is usually necessary for an aircraft to stay at a cleared
flight level In aviation and aviation meteorology, a flight level (FL) is an aircraft's altitude at standard air pressure, expressed in hundreds of feet. The air pressure is computed assuming an International Standard Atmosphere pressure of 1013.25 hPa ...
. On long-haul flights, the pilot may ask
air traffic control Air traffic control (ATC) is a service provided by ground-based air traffic controllers who direct aircraft on the ground and through a given section of controlled airspace, and can provide advisory services to aircraft in non-controlled airs ...
to climb from one flight level to a higher one, in a manoeuvre known as a
step climb A step climb in aviation is a series of altitude gains that improve fuel economy by moving into thinner air as an aircraft becomes lighter and becomes capable of flying in the thinner air at a higher altitude. Description Since the early days of ...
.


See also

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Fuel economy in aircraft The fuel economy in aircraft is the measure of the transport energy efficiency of aircraft. Efficiency is increased with better aerodynamics and by reducing weight, and with improved engine BSFC and propulsive efficiency or TSFC. Enduran ...
*
Range (aeronautics) The maximal total range is the maximum distance an aircraft can fly between takeoff and landing. Powered aircraft range is limited by the aviation fuel energy storage capacity (chemical or electrical) considering both weight and volume limits. Un ...
*
Lift-to-drag ratio In aerodynamics, the lift-to-drag ratio (or L/D ratio) is the lift generated by an aerodynamic body such as an aerofoil or aircraft, divided by the aerodynamic drag caused by moving through air. It describes the aerodynamic efficiency under give ...
*
Supercruise Supercruise is sustained supersonic flight of a supersonic aircraft with a useful cargo, passenger, or weapons load without using afterburner (also known as "reheat"). Many supersonic military aircraft are not capable of supercruise and can only m ...
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Takeoff Takeoff is the phase of flight in which an aerospace vehicle leaves the ground and becomes airborne. For aircraft traveling vertically, this is known as liftoff. For aircraft that take off horizontally, this usually involves starting with a t ...
*
Visual flight rules In aviation, visual flight rules (VFR) are a set of regulations under which a pilot operates an aircraft in weather conditions generally clear enough to allow the pilot to see where the aircraft is going. Specifically, the weather must be better ...


References

Flight phases {{Aviation-stub