cruise

Cruise is the phase of aircraft flight that starts when the aircraft levels off after a climb, until it begins to descend for landing. Cruising usually comprises the majority of a flight, and may include changes in heading (direction of flight), airspeed, and altitude. Airliner cruise 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, air temperature, and humidity. Cruise altitude is usually where the higher ground speed is balanced against the decrease in engine thrust and efficiency at higher altitudes. Common narrowbodies cruise at like the Airbus A320, or the Boeing 737NG; While modern widebodies cruise at like the Airbus A350 or the Boeing 787. 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 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 .