A commonly used cruising altitude for jet powered airliners is 30,000 feet. Therefore, one often hears this altitude referred to in casual conversation or on television. However, airliners actually cruise at a variety of altitudes. There are several factors that determine the cruising altitudes at which an airliner flies.
A cruising altitude is the altitude at which an airplane spends most of its flight. This is the altitude at which it levels out after completing its long climb after takeoff. This altitude allows the aircraft to fly more efficiently, to avoid bad weather and to stay clear of other aircraft. Depending upon the length of the flight and the type of aircraft, cruising altitudes typically vary between approximately 25,000 feet and 40,000 feet. For longer flights, it is not uncommon for an airliner to change cruising altitudes several times, gradually working its way to higher levels as it burns off fuel and becomes lighter in weight. This process is known as a step climb. For every flight, there is an optimum cruising altitude that is calculated based on weight. The step climb process is used to keep the aircraft at the optimum altitude as the plane's weight decreases over time. Altitudes in this range are measured in feet above sea level with a barometric altimeter set at 29.92 inches of mercury.
The air in the Earth's atmosphere becomes consistently thinner as altitude increases. This is why a barometer can be used to measure changes in altitude. As the air becomes thinner, it creates less resistance to objects flying through it. Therefore, less thrust is needed to move the aircraft forward at a given speed. As a result, airplanes can fly more efficiently at higher altitudes. However, there are limits to how high an airplane can fly. If the aircraft is very heavy, more fuel is required to lift the aircraft higher. Secondly, because the air gets thinner with altitude, the speed of sound decreases with increases in altitude. Since most airliners are not designed to fly at or above the speed of sound, they eventually reach a point at which their speed is limited by the speed of sound. At the same time, an aircraft must maintain a certain airspeed in order for the wings to create lift. This is called the stall speed. In the thinner air at high altitudes, the plane must fly faster and faster in order to remain above the stall speed. Therefore, if an airplane climbs too high, it can become trapped between its increasing stall speed and the decreasing speed of sound. Pilots refer to this trap as "coffin corner." As a result of these dangers, cruising altitudes on conventional airliners are strictly limited and rarely exceed 40,000 feet.
Cruising altitudes are referred to as in aviation parlance as "flight levels" and are identified by dividing the altitude by 100. Thus, an altitude of 30,000 feet is referred to as "flight level 300," and an altitude of 35,000 feet is referred to as "flight level 350." In order to reduce the danger of mid-air collisions, cruising altitude assignments are based on the compass heading that the aircraft is flying. If an airplane is flying a heading between 360 degrees and 179 degrees, then it will fly at an odd-numbered flight level. If an airplane is flying a heading between 180 degrees and 359 degrees, it will fly at an even-numbered flight level. Thus, an airliner flying due east (090 degrees) would fly at an odd numbered flight level such as flight level 310 or flight level 330, while an airplane flying in the opposite direction (270 degrees) would choose an even-numbered flight level such as 300 or 320. Thus, even if the two planes were flying directly toward each other along the same course, they would be vertically separated by at least 1,000 feet.
In aviation parlance, altitudes are normally defined in relation to feet above sea level. Since the ground rises and dips constantly, it would be impractical to use altitude above the ground as a general reference. However, pilots must nevertheless be constantly aware of the terrain height where they are flying. Twenty thousand feet offers plenty of ground clearance when flying over southern Florida, but the same altitude would put a plane in a very precarious position in the Himalayas. Terrain heights are marked on aeronautical charts that pilots use. In addition, many aircraft are equipped with radar altimeters, which measure vertical separation with the ground.
Weather is another consideration that can affect the cruising altitude of an airliner. During flight, pilots receive weather reports from the ground as well as from other pilots in the area. They will often request a change in cruising altitude from air traffic control to avoid building thunderstorms or to avoid reports of clear air turbulence. Another consideration is the length of the flight. The cruising altitudes for short flights will typically be quite low. For a flight that only lasts an hour, it would not make practical sense to climb all the way up to flight level 350, since the plane would likely have to begin descending to its destination the moment it reached cruising altitude. Therefore, such flights often cruise as low as flight level 250 even though fuel efficiency is poorer at this altitude.