The altitude of aircraft in flight can vary by the type of aircraft. Generally, commercial aircraft fly at higher cruising altitudes than smaller general aviation aircraft. The flight level of an aircraft can also vary based on weather, air traffic control instruction and the purpose of the flight. Each type of aircraft has its optimal cruise altitude.
Aircraft with jet engines (generally medium- to long-range cargo planes, passenger planes and private jets) are capable of much higher altitudes than those with propellers. In most cases, these aircraft are assigned cruising altitudes ranging from 25,000 to 37,000 feet depending on the distance of the flight and air traffic control guidance. As a rule, the longer the flight, the higher the altitude. The average cruising altitudes of aircraft are much less than their maximum sustainable altitudes, also known as "absolute ceilings," as well as their "service ceilings," the altitude at which the aircraft no longer climbs at an effective rate. Most commercial jetliners have service ceilings around 42,000 feet and business jets are around 51,000 feet.
General aviation aircraft tend to be propeller aircraft, therefore their cruising heights are almost always well below those aircraft with jet engines. To give an example of the difference, a Cessna Skyhawk single-engine plane has a service ceiling of 13,000 to 15,000 feet, while the service ceilings for commercial jetliners are almost always tens of thousands of feet higher. This is true with most general aviation aircraft.
Almost all aircraft in the world are under the control of air traffic controllers, who assign headings and altitudes to aircraft in flight. When assigning altitudes for aircraft, controllers take into account the flying (atmospheric) conditions, the flight levels of other aircraft in the area, flight distance, the stage of the flight the aircraft is in (ascent, cruise, descent) and the aircraft's optimal flight level.
An aircraft's ceiling is a function of aerodynamics. Smaller planes are not able to fly as high because their engines lack the power that is needed to produce enough lift after a certain flight level is reached. The concept of lift is the central point of flight. The higher the altitude, the thinner the air; therefore more force (called thrust) from the aircraft's engine is needed to compensate for the aircraft's drag to create lift and adequate airflow around the aircraft's wings to climb (service ceiling) or maintain altitude.
An altitude too low for the conditions can present many dangers, including the risk of striking mountains, buildings and towers. The result of flying at an aircraft's maximum ceiling are aircraft stalls (loss of lift) and engine failure, as was the case in 2004, when the pilots of an empty Northwest passenger plane tried to push their aircraft to its maximum ceiling, which resulted in engine failure, causing the aircraft to crash into a neighborhood near Jefferson City, Missouri.
