An airliner is a type of aircraft used for air transport of passengers or cargo, usually in commercial service. An airliner is also referred to as a jet aircraft or a wide-body aircraft. The largest and most commonly used class of airliner is the wide-body jet, often called twin-aisle because it has two separate aisles running from the front to the back of the passenger cabin.
Most airliners are powered by gas turbines, either turbofans or turboprops. They are more reliable than piston engines and operate efficiently at higher altitudes. They do not produce vibration and noise, have a common fuel (kerosene-based jet fuel), and are more compact than the earlier piston engines.
Until the introduction of the jet engine in the 1940s, many airlines operated aircraft with piston engines. This changed as jets were developed, and most airlines now use jet engines for all of their long-haul flights, although some still operate piston-engined aircraft on short-haul routes.
In the 1940s and early 1950s, many airlines flew aircraft with pressurized cabins. This allowed them to fly at high altitudes where low oxygen levels and air pressure would have caused sickness or death for their passengers. It also allowed them to fly faster and further, as the higher pressures allowed the aircraft to have a lower drag.
Some airlines still fly aircraft with pressurized cabins, but they are rare on domestic flights. They are more common on international flights where the risk of flying at high altitudes is less.
The air inside an aircraft cabin is supplied to the cabin from diffusers located in the ceiling of the aisle, above windows and in the overhead baggage compartments. The air is matched at each inlet location as much as possible to minimize net flows and to ensure that the incoming air mixes evenly throughout the cabin. The exhaust is typically vented from the cabin at floor level or through a side wall, while some of it may be recirculated back into the cabin to mix with the incoming air.
Passenger-side air circulation is controlled by air inlet and outflow valves. The inlet diffusers distribute the incoming air into the cabin while the outflow valves drain the excess from the cabin and discharge it overboard through a pipe that connects to the tail of the aircraft or to the engine nacelles.
Larger airliners have multiple inlet and outflow diffusers along the length of the cabin. This enables a more even distribution of the air to the cabin, and also reduces the pressure at the front of the aircraft, which may cause discomfort for some passengers.
Typical inlet and outflow rates are controlled by a control panel or by switches in the cockpit. The control panels can be set to provide a constant supply or a variable supply. The inlet and outflow rate can be changed to suit the flight requirements of the aircraft.
The inlet diffusers and outflow valves must be able to provide a uniform flow of air across the whole length of the aircraft. The incoming air is distributed in a circular pattern over the entire area of the cabin, while the outflow is directed around and through the cargo hold or ducted to the outside of the aircraft.
