An airliner is a type of aircraft for transporting passengers and air cargo. Such aircraft are most often operated by airlines. Although the definition of an airliner can vary from country to country, an airliner is typically defined as an aircraft intended for carrying multiple passengers or cargo in commercial service.
The largest airliners are wide-body jets. These aircraft are frequently called twin-aisle aircraft because they generally have two separate aisles running from the front to the back of the passenger cabin. These aircraft are usually used for long-haul flights between airline hubs and major cities with many passengers.
A smaller, more common class of airliners is the narrow-body or single aisle aircraft. These smaller airliners are generally used for short to medium-distance flights with fewer passengers than their wide-body counterparts.
Regional airliners typically seat fewer than 100 passengers and may be powered by turbofans or turboprops. These airliners are the non-mainline counterparts to the larger aircraft operated by the major carriers, legacy carriers, and flag carriers and are used to feed traffic into the large airline hubs. These regional routes then form the spokes of a hub-and-spoke air transport model.
The lightest (light aircraft, list of light transport aircraft) of short haul regional feeder airliner type aircraft that carry 19 or fewer passenger seats are called commuter aircraft, commuterliners, feederliners, and air taxis, depending on their size, engines, how they are marketed, region of the world, and seating configurations. The Beechcraft 1900, for example, has only 19 seats.
When the Wright brothers made the worlds first sustained heavier-than-air flight, they laid the foundation for what would become a major transport industry. Their flight in 1903 was just 11 years before what is often defined as the worlds first airliner. These airliners would have a significant impact on global society, economics, and politics.
If an airliner is defined as an aircraft intended for carrying multiple passengers in commercial service, the Russian Sikorsky Ilya Muromets was the first aircraft meeting that definition. The Ilya Muromets was a luxurious aircraft with a separate passenger saloon, wicker chairs, bedroom, lounge and a toilet. The aircraft also had heating and electrical lighting. The Ilya Muromets first flew on December 10, 1913. On February 25, 1914, it took off for its first demonstration flight with 16 passengers aboard. From June 21 June 23, it made a round-trip from Saint Petersburg to Kiev in 14 hours and 38 minutes with one intermediate landing. However, it was never used as a commercial airliner due to the onset of World War I.
In 1915 the very first airliner (for commercial use) was used by Elliot Air Service. The aircraft was a Curtiss JN 4, a small biplane which was used mainly in World War I as a trainer. Later, it was also used as a tour and familiarization flight aircraft in the early 1920s.
In 1919, after World War I, the Farman F.60 Goliath, originally designed as a long-range heavy bomber, was converted for commercial use into a passenger airliner. It could seat 14 passengers from 1919, and approximately 60 were built. Initially several publicity flights were made, including one on 8 February 1919, when the Goliath flew 12 passengers from Toussus-le-Noble to RAF Kenley, near Croydon, despite having no permission from the British authorities to land.
Another important airliner built in 1919 was the Airco DH.16; a re-designed Airco DH.9A with a wider fuselage to accommodate an enclosed cabin seating four passengers, plus pilot in an open cockpit. In March 1919, the prototype first flew at Hendon Aerodrome. Nine aircraft were built, all but one being delivered to the nascent airline, Aircraft Transport & Travel Limited (AT&T). AT&T used the first aircraft for pleasure flying, and on 25 August 1919 it inaugurated the first scheduled international airline service from London to Paris. One aircraft was sold to the River Plate Aviation Company in Argentina, to operate a cross-river service between Buenos Aires and Montevideo. Meanwhile, the competing Vickers converted its successful WW1 bomber, the Vickers Vimy, into a civilian version, the Vimy Commercial. It was redesigned with a larger diameter fuselage (largely of spruce plywood), and first flew from the Joyce Green airfield in Kent on 13 April 1919.
The world's first all-metal transport aircraft was the Junkers F.13, also from 1919 with 322 built. The Dutch Fokker company produced the Fokker F.II followed by its development the F.III. These aircraft were used by the Dutch airline KLM when it re-opened an Amsterdam-London service in 1921. The Fokkers were soon flying to destinations across Europe, including Bremen, Brussels, Hamburg and Paris. They proved to be very reliable aircraft.
The Handley Page company in Britain produced the Handley Page Type W as the company's first civil transport aircraft. It housed two crew in an open cockpit and 15 passengers in an enclosed cabin. Powered by two 450 hp (340 kW) Napier Lion engines the prototype first flew on 4 December 1919, shortly after it was displayed at the 1919 Paris Air Show at Le Bourget. It was the world's first airliner to be designed with an on-board lavatory.
Meanwhile, in France the Bleriot-SPAD S.33 was a great success throughout the 1920s, initially serving the Paris-London route, and later on continental routes. The enclosed cabin could carry four passengers with an extra seat in the cockpit.
By 1921, it was becoming apparent that aircraft capacity needed to be larger for the economics to remain favourable. The English company de Havilland, therefore built the ten-passenger DH.29 monoplane, while starting work on the design of the DH.32, an eight-seater biplane with a less powerful but more economical Rolls-Royce Eagle engine. Owing to the urgent need for more capacity, however, work on the DH.32 was stopped and the DH.34 biplane was designed, accommodating ten passengers. The Fokker trimotor was an important and popular transport, manufactured under license in Europe and America.
Throughout the 1920s, companies in Britain and France were at the forefront of the civil airliner industry, often considerably aided by government subsidies.
In America, the Ford Trimotor was an important early airliner. With two engines mounted on the wings and one in the nose and a slabsided body, it carried eight passengers and was produced from 1925 to 1933. It was used by the predecessor to Trans World Airlines, and by other airlines long after production ceased. Pan Am opened up transoceanic service in the late 1920s and early '30s, based on a series of large seaplanes the Sikorsky S-38 through Sikorsky S-42.
By the 1930s, the airliner industry had matured and large consolidated national airlines were established with regular international services that spanned the globe, including Imperial Airways in Britain, Lufthansa in Germany, KLM in the Netherlands and United Airlines in America. Multi-engined aircraft were now capable of transporting dozens of passengers in comfort.
In Britain, the de Havilland Dragon was a successful aircraft during the 1930s. Its simple design used a plywood box fuselage. It could carry six passengers each with 45 pounds (20 kg) of luggage on the London-Paris route on a fuel consumption of just 13 gal (49 l) per hour. The wing panels outboard of the engines could be folded for storage. The type was attractive as a short-haul low capacity airliner and was soon in service worldwide. British production of the DH.84 ended when it was replaced on the assembly line by the more powerful and elegant de Havilland Dragon Rapide.
The first modern-looking sleek metal airliners also came into service in the 1930s. In the United States, the Boeing 247 and the 14-passenger Douglas DC-2 flew, and in 1935 the more powerful, faster, 2132 passenger Douglas DC-3 appeared. DC-3s were produced in quantity for World War II and sold as surplus afterward. The Douglas DC-3 was a particularly important airplane, because it was the first airliner to be profitable without a government subsidy.
Long-haul flights were expanded during the 1930s as both Pan American Airways and Imperial Airways competed in the provision of transatlantic travel using flying boats such as the British Short Empire and the American Boeing 314. This prefigured the dramatic growth of transatlantic travel in the post-war period.
In the United Kingdom, the Brabazon Committee was formed in 1942 under John Moore-Brabazon, 1st Baron Brabazon of Tara to investigate the future needs of the British Empire's civilian airliner market. The study was an attempt at defining in broad overview; the impact of projected advances in aviation technology and to forecast the global needs of the post war British Empire (in South Asia, Africa, the Near and Far East) and Commonwealth (Australia, Canada, New Zealand) in the area of air transport, for passengers, mail, and cargo. The crucial problem that the planners faced was that an agreement had been reached between the United States and the United Kingdom in 1942 to split responsibility for building multi-engine aircraft types for British use: the US would concentrate on transport aircraft while the UK would concentrate on their heavy bombers. This decision meant that the United Kingdom would be left at the close of the war with little experience in the design, manufacture and final assembly of transport aircraft.
The final report called for the construction of four general designs studied by the committee and members of the state-owned airlines British Overseas Airways Corporation (BOAC) and later British European Airways (BEA). The first three designs were piston-powered aircraft of varying sizes for different markets, while the Type IV design, at the urging of Geoffrey de Havilland whose company was involved in development the first jet fighters, was for a jet-powered 100-seat design.
The Type I design, after a brief contest was given to the Bristol Aeroplane Company, building on submissions they had made during the war for a "100 ton bomber". This evolved into the creation of the Bristol Brabazon.
The Type II process was complicated by the proposition of Vickers that there should be a move to the newly developed turboprop power. The specification was therefore split in two, with the conventional piston design going to the eventual de Havilland Dove and Airspeed Ambassador. The revolutionary VC.2 Viceroy, first flown in 1948 was the first turboprop design anywhere. The Type III requirement was developed as the Avro Tudor.
The Type IV for the jet-powered aircraft went to de Havilland and became, in 1949, the world's first jet airliner, the Comet. It featured an aerodynamically clean design with four de Havilland Ghost turbojet engines buried in the wings, a pressurised fuselage, and large square windows. For the era, it offered a relatively quiet, comfortable passenger cabin and showed signs of being a commercial success at its 1952 debut. However, a year after entering commercial service the Comets began suffering problems, with three of them breaking up during mid-flight in well-publicised accidents. This was later found to be due to catastrophic metal fatigue, not well understood at the time, in the airframes. The Comet was withdrawn from service and extensively tested to discover the cause. Rival manufacturers meanwhile heeded the lessons learned from the Comet while developing their own aircraft. Although sales never fully recovered, the improved Comet 2 and the prototype Comet 3 culminated in the redesigned Comet 4 series which debuted in 1958 and had a productive career of over 30 years.
This disaster, coupled with the fact that the UK's domestic market was much smaller than the US's, meant that by the 1960s it was increasingly clear that the UK had lost the airliner market to the US, and later designs like the BAC 1-11, Vickers VC10, and Hawker Siddeley Trident although successful would be unable to win a substantial part of it back again. Another committee was formed to consider supersonic designs, STAC, and worked with Bristol to create the Bristol 223 design for a 100-passenger transatlantic airliner. However, this was going to be so expensive to produce that the effort was later merged with similar efforts in France to create the first supersonic airlinerthe Concorde.
The United States, conversely, gained a huge advantage in design and production in the airline industry in the years leading up to the war, but many of the developments would be put off until after the war as the manufacturing efforts were placed on the war effort. The advancements that the United States would make in this industry were in large part due to the cooperation of the airlines discussing what they desired with the airliner manufacturers.
Soon after the war though Douglas made a large advancement with the DC-4, although this could not cross the Atlantic at every point, it was able to make a nonstop flight from New York to the United Kingdom. Due to the war going on, the first batch of these planes went to the U.S. Army and Air Forces, and was named the C-54 Skymaster. Some of these that were used in the war would later be converted for the airline industry, along with the passenger and cargo versions that were placed on the market once the war ended. Douglas would later develop a version of this plane that was pressurized and five feet longer; this redesigned plane would become the DC-6. These DC-6s would be grounded for six months to rectify a few safety issues that were causing in-flight fires.
Soon after the DC-4, Lockheed developed the distinctive triple-tail Constellation. An aviation breakthrough, it was the first commercially successful pressurized airliner, allowing it to fly higher than other airliners. Its fuselage was some 127 inches wider than the DC-4s. Drafted by the military in World War II, it experienced a similar late entry into the civilian airline industry. Safety concerns grounded it for six months soon after it entered service while problems were investigated and repaired.
In 1947 the Boeing 377 Stratocruiser entered the industry with a completely different design than Douglas and Lockheed aircraft. Based on the C-97 Stratofreighter military transport, it had a double deck and pressurized fuselage. Luxury and a 100-passenger capacity distinguished it from its rivals. While 900 C-97s were supplied to the military, only 55 were produced for civil aviation.
The American companies had done a great job of advancing the status of transcontinental travel, but there was also the aging fleet of DC-3s that had to be addressed. Convair decided that they were going to address this market, and would begin producing the Convair 240, which was a 40-person fully pressurized plane. There were 566 of these planes that would fly, including two that were equipped with jet-assisted take off units. Convair would later develop the Convair 340, which was slightly larger and could accommodate between 44 and 52 passengers, and 311 of this model plane were produced. Finally Convair would create a Convair 440, which had small modifications, including much better soundproofing than the previous models. Convair would experience a little bit of competition from the Martin 2-0-2 and Martin 4-0-4, but in general Convair was able to control this market, as the 2-0-2 had safety concerns and was unpressurized, and the 4-0-4 only sold around 100 units.
The United States was dominant in this industry for several reasons, including a large domestic market for these planes. The market would also work in the United States favor as the American companies began to build pressurized airliners. During the postwar years engines became much larger and more powerful, and safety features such as deicing, navigation, and weather were added to the planes. Lastly, the planes produced in the United States were more comfortable and had superior flight decks than those produced in Europe.
In the postwar years France developed a few significant airliners, some of these being planes that could land on water, part of the reason that the French companies were so focused on these flying boats is that in 1936 the French Air Ministry requested transatlantic flying boats that could hold at least 40 passengers. Only one model from this request would ever be put into service. The first set of these were three Latécoère 631's that Air France purchased and put into service in July 1947. However, two of these planes crashed, and the third plane was soon removed because of safety concerns. There would later be a SNCASE SE.161 Languedoc build, which was a much more successful plane, and over 100 of these were built, with 40 of them being placed into service through Air-France. The French also developed the Breguet 763 Deux Ponts, which first flew in February 1949. This was a double-decker transport airliner that would end up being used for both people and cargo. This four-engine airliner would end up being used to hold large amounts of cargo or 97 passengers. After a long silence, France then created the Caravelle, the world's first short-to-midrange jet airliner. Subsequent French efforts were part of the Airbus pan-European initiative.
Soon after the war most of the Soviet fleet of airliners consisted of DC-3s or the Lisunov Li-2. These planes were in desperate need of replacement, and in 1946 the Ilyushin Il-12 made its first flight. The Il-12 was very similar in design to American Convair 240, except was unpressurized. In 1953 the Ilyushin Il-14 would make its first flight, and this version was equipped with much more powerful engines. The main contribution that the Soviets made in regards to airliners was the Antonov An-2. This plane is a biplane, unlike most of the other airliners, and sold more units than any other transport plane.
The largest airliners are wide-body jets. These aircraft are frequently called twin-aisle aircraft because they generally have two separate aisles running from the front to the back of the passenger cabin. Aircraft in this category are the Boeing 747, Boeing 767, Boeing 777, Boeing 787, Airbus A300/A310, Airbus A330, Airbus A340, Airbus A350, Airbus A380, Lockheed L-1011 TriStar, McDonnell Douglas DC-10, McDonnell Douglas MD-11, Ilyushin Il-86, and Ilyushin Il-96. These aircraft are usually used for long-haul flights between airline hubs and major cities with many passengers.
A smaller, more common class of airliners is the narrow-body or single aisle aircraft. These smaller airliners are generally used for medium haul flights with fewer passengers than their wide-body counterparts. Currently produced narrow-body airliners are the Boeing 737, Airbus A320 family, Bombardier CSeries or Tupolev Tu-204/214.
Out-of production aircraft include the McDonnell Douglas DC-9 and its derivatives MD-80/MD-90 series and Boeing 717, using the same cabin cross section as the Boeing 737 the 757, 727 and the 707, its competitor the Douglas DC-8 or the Tupolev Tu-154. Future narrow-body airliners include the Comac C919 and Irkut MC-21.
Regional airliners typically seat fewer than 100 passengers and may be powered by turbofans or turboprops. These airliners are the non-mainline counterparts to the larger aircraft operated by the major carriers, legacy carriers, and flag carriers and are used to feed traffic into the large airline hubs or focus cities. These particular routes may need the size of a smaller aircraft to meet the frequency needs and service levels customers expect in the marketed product that is offered by larger airlines and their modern narrow and widebody aircraft. Therefore, these short-haul airliners are usually equipped with lavatories, stand up cabins, pressurization, overhead storage bins, reclining seats, and have a flight attendant to look after the in-flight needs of the passengers during Point-to-point transit or routes.
Because these aircraft are frequently operated by smaller airlines that are contracted to provide ("feed") passengers from smaller cities to hub airports (and reverse) for a "major" or "flag" carrier, regional airliners may be painted in the liveries of the major airline for whom they provide this "feeder" service so the regional airlines may offer and market a seamless transition between the larger airline to smaller airline.
The lightest (light aircraft, list of light transport aircraft) of short haul regional feeder airliner type aircraft that carry 19 or fewer passenger seats are called commuter aircraft, commuterliners, feederliners, and air taxis, depending on their size, engines, how they are marketed, region of the world, and seating configurations. The Beechcraft 1900, for example, has only 19 seats. Depending on local and national regulations, a commuter aircraft may not qualify as an airliner and may not be subject to the regulations applied to larger aircraft. Members of this class of aircraft normally lack such amenities as lavatories and galleys and typically do not carry a flight attendant as an aircrew member.
Other aircraft that may fall into this category are the Fairchild Metro, Jetstream 31, and Embraer EMB 110 Bandeirante. The Cessna Caravan and Pilatus PC-12 are single-engine turboprops, sometimes used as small airliners, although many countries stipulate a minimum requirement of two engines for aircraft to be used as airliners.
Until the beginning of the Jet Age, piston engines were common on propliners like the Douglas DC-3. Nearly all modern airliners are now powered by turbine engines, either turbofans or turboprops. Gas turbine engines operate efficiently at much higher altitudes, are more reliable than piston engines, and produce less vibration and noise. The use of a common fuel type kerosene-based jet fuel is another advantage. Prior to the Jet Age, it was common for the same or very similar engines to be used in civilian airliners as in military aircraft. In recent years, divergence has occurred so that it is now unusual for the same engine to be used on a military type and a civilian type. Those military aircraft which do share engine technology with airliners are typically transports or tanker types.
Some variants of airliners have been developed for carrying freight or for luxury corporate use. Many airliners have also been modified for government use as VIP transports and for military functions such as airborne tankers (for example, the Vickers VC10, Lockheed L-1011, Boeing 707), air ambulance (USAF/USN McDonnell Douglas DC-9), reconnaissance (Embraer ERJ 145, Saab 340, Boeing 737), as well as for troop-carrying roles.
Modern jetliners are usually low-wing designs with two engines mounted underneath the swept wings (turboprop aircraft are slow enough to use straight wings). The Boeing 747 and Airbus A380 are the only airliners in production which are too heavy (more than 400 tons maximum takeoff weight) for just two engines. Smaller airliners sometimes have their engines mounted on either side of the rear fuselage. There are numerous advantages and disadvantages to this arrangement. Perhaps the most important advantage to mounting the engines under the wings is that the total aircraft weight is more evenly distributed across the wingspan, which imposes less bending moment on the wings and allows for a lighter wing structure. This factor becomes more important as aircraft weight increases, and there are no in-production airliners with both a maximum takeoff weight of more than 50 tons and engines mounted on the fuselage. The Antonov An-148 is the only in-production jetliner with high-mounted wings (usually seen in military transport aircraft), which reduces the risk of damage from unpaved runways.
Except for a few experimental or military designs, all aircraft built to date have had all of their weight lifted off the ground by airflow across the wings. In terms of aerodynamics, the fuselage has been a mere burden. NASA and Boeing are currently developing a blended wing body design in which the entire airframe, from wingtip to wingtip, contributes lift. This promises a significant gain in fuel efficiency.
The major manufacturers with airliners currently in production include:
|Model||First flight||Net orders||Deliveries||Backlog||MTOW (t)||typ. seats||Range (nmi)|
|Airbus A320 family||22/02/1987||13160||7658||5505||68-95||107-206||3,110-4,000|
|Boeing 737 NG/Boeing 737 MAX||09/02/1997||10660||6257||4403||70.1-88.3||126-188||2,935-3,515|
|Boeing 777-200LR/300ER/Boeing 777X||24/02/2003||1369||944||425||347.5-351||301-400||7,370-8,700|
The airliner fleet went from 13,500 in 2000 to 25,700 in 2017: 16% to 30.7% in Asia/Pacific (2,158 to 7,915), 34.7% to 23.6% in USA (4,686 to 6,069) and 24% to 20.5% in Europe (3,234 to 5,272).
Since it began, the jet airliner market had a recurring pattern of seven years of growth followed by three years of deliveries falling 30-40%, except a steady growth from 2004 due to the economic rise of China going from 3% of world market in 2001 to 22% in 2015, expensive jet fuel till 2014 stimulating old jets replacement allowed by low interest rates since 2008, and strong airline passenger demand since. In 2004, 718 Airbus and Boeings were delivered, worth $39.3 billion; 1,466 are expected in 2017, worth $104.4 billion and 1,990 are forecast in 2020, worth $138.3 billion (constant 2017 dollar values): a growth by 3.5 from 2004 to 2020 is unprecedented and highly unusual for any mature market.
|Manufacturer||2016 orders and deliveries||2018-2022 projections|
|deliveries||values ($bn)||net orders||backlog||deliveries||values ($bn)|
|Boeing||726||57.8||563||5,660||4,779 (43.6%)||756.5 (51.1%)|
|Airbus||685||45.5||711||6,845||4,388 (40.0%)||648.1 (43.8%)|
|Embraer||108||2.9||39||444||498 (4.5%)||21.1 (1.4%)|
|Bombardier||81||1.9||162||437||486 (4.4%)||25.7 (1.8%)|
|ATR||73||1.5||36||236||391 (3.6%)||9.5 (0.6%)|
|Other||31||0.5||72||1,080||430 (3.9%)||19.2 (1.3%)|
In 2016, the deliveries went for 38% in Asia-Pacific, 25% in Europe, 22% in North America, 7% in Middle East, 6% in South America and 2% in Africa. 1,020 narrowbodies were delivered and their backlog reach 10891: 4,991 A320neo, 644 A320ceo; 3,593 737 Max, 835 737NG, 348 CSeries, 305 C919 and 175 MC-21; while 398 widebodies were delivered : 137 Dreamliners and 99 B777 for Boeing (65%) against 63 A330 and 49 A350 for Airbus, more than 2,400 widebodies were in backlog, led by the A350 with 753 (31%) then the Boeing 787 with 694 (28%).
|Airbus A320 family||6,838||6,516||6,041|
|Boeing 737 NG||5,968||5,556||5,115|
|Boeing 737 Classic/original||890||931||1,006|
|Embraer ERJ 145 family||454||528||606|
|Bombardier Dash 8-100/200/300||374||395||424|
|de Havilland Canada DHC-6 Twin Otter||270||266||268|
The most important driver of orders is airline profitability, itself driven mainly by world GDP growth but also supply and demand balance and oil prices, while new programmes by Airbus and Boeing help to stimulate aircraft demand. In 2016, 38% of the 25 years old airliners had been retired, 50% of the 28 years old : there will be 523 aircraft reaching 25 years old in 2017, 1,127 in 2026 and 1,628 in 2041. Deliveries rose by 80% from 2004 to 2016, they represented 4.9% of the fleet in 2004 and 5.9% in 2016, down from 8% previously.
An airliner will usually have several classes of seating: first class, business class, and/or economy class (which may be referred to as coach class or tourist class, and sometimes has a separate "premium" economy section with more legroom and amenities). The seats in more expensive classes are wider, more comfortable, and have more amenities such as "lie flat" seats for more comfortable sleeping on long flights. Generally, the more expensive the class, the better the beverage and meal service.
Domestic flights generally have a two-class configuration, usually first or business class and coach class, although many airlines instead offer all-economy seating. International flights generally have either a two-class configuration or a three-class configuration, depending on the airline, route and aircraft type. Many airliners offer movies or audio/video on demand (this is standard in first and business class on many international flights and may be available on economy). Cabins of any class are provided with lavatory facilities, reading lights and gaspers. Larger airliners may have a segregated rest compartment reserved for crew use during breaks.
The types of seats that are provided and how much legroom is given to each passenger are decisions made by the individual airlines, not the aircraft manufacturers. Seats are mounted in "tracks" on the floor of the cabin and can be moved back and forth by the maintenance staff or removed altogether. Naturally the airline tries to maximize the number of seats available in every aircraft to carry the largest possible (and therefore most profitable) number of passengers.
Passengers seated in an exit row (the row of seats adjacent to an emergency exit) usually have substantially more legroom than those seated in the remainder of the cabin, while the seats directly in front of the exit row may have less legroom and may not even recline (for evacuation safety reasons). However, passengers seated in an exit row may be required to assist cabin crew during an emergency evacuation of the aircraft opening the emergency exit and assisting fellow passengers to the exit. As a precaution, many airlines prohibit young people under the age of 15 from being seated in the exit row.
The seats are designed to withstand strong forces so as not to break or come loose from their floor tracks during turbulence or accidents. The backs of seats are often equipped with a fold-down tray for eating, writing, or as a place to set up a portable computer, or a music or video player. Seats without another row of seats in front of them have a tray that is either folded into the armrest or that clips into brackets on the underside of the armrests. However, seats in premium cabins generally have trays in the armrests or clip-on trays, regardless of whether there is another row of seats in front of them. Seatbacks now often feature small colour LCD screens for videos, television and video games. Controls for this display as well as an outlet to plug in audio headsets are normally found in the armrest of each seat.
The overhead bins are used for stowing carry-on baggage and other items. While the airliner manufacturer will normally specify a standard version of the product to supply, airlines can choose to have bins of differing size, shape, or color installed. Over time, overhead bins evolved out of what were originally overhead shelves that were used for little more than coat and briefcase storage. As concerns about falling debris during turbulence or in accidents increased, enclosed bins became[when?] the norm. Bins have increased in size to accommodate the larger carry-on baggage passengers can bring onto the aircraft. Newer bin designs have included a handrail, useful when moving through the cabin.
Above the passenger seats are Passenger Service Units (PSU). These typically contain reading lights, air vents, and a flight attendant call light. On most narrowbody aircraft (and some Airbus A300s and A310s), the flight attendant call button and the buttons to control the reading lights are located directly on the PSU, while on most widebody aircraft, the flight attendant call button and the reading light control buttons are usually part of the in-flight entertainment system. The units frequently have small "Fasten Seat Belt" and "No Smoking" illuminated signage and may also contain a speaker for the cabin public address system.
The PSU will also normally contain the drop-down oxygen masks which are activated if there is a sudden drop in cabin pressure. These are supplied with oxygen by means of a chemical oxygen generator. By using a chemical reaction rather than a connection to an oxygen tank, these devices supply breathing oxygen for long enough for the airliner to descend to thicker, more breathable air. Oxygen generators do generate considerable heat in the process. Because of this, the oxygen generators are thermally shielded and are only allowed in commercial airliners when properly installed they are not permitted to be loaded as freight on passenger-carrying flights. ValuJet Flight 592 crashed on May 11, 1996, as a result of improperly loaded chemical oxygen generators.
Airliners developed since the 1940s have had pressurized cabins (or, more accurately, pressurized hulls including baggage holds) to enable them to carry passengers safely at high altitudes where low oxygen levels and air pressure would otherwise cause sickness or death. High altitude flight enabled airliners to fly above most weather systems that cause turbulent or dangerous flying conditions, and also to fly faster and further as there is less drag due to the lower air density. Pressurization is applied using compressed air, in most cases bled from the engines, and is managed by an environmental control system which draws in clean air, and vents stale air out through a valve.
Pressurization presents design and construction challenges to maintain the structural integrity and sealing of the cabin and hull and to prevent rapid decompression. Some of the consequences include small round windows, doors that open inwards and are larger than the door hole, and an emergency oxygen system.
To maintain a pressure in the cabin equivalent to an altitude close to sea level would, at a cruising altitude around 10,000 m (33,000 ft), create a pressure difference between inside the aircraft and outside the aircraft that would require greater hull strength and weight. Most people do not suffer ill effects up to an altitude of 1,8002,500 m (5,9008,200 ft), and maintaining cabin pressure at this equivalent altitude significantly reduces the pressure difference and therefore the required hull strength and weight. A side effect is that passengers experience some discomfort as the cabin pressure changes during ascent and descent to the majority of airports, which are at low altitudes.
The air bled from the engines is hot and requires cooling by air conditioning units. It is also extremely dry at cruising altitude, and this causes sore eyes, dry skin and mucosa on long flights. Although humidification technology could raise its relative humidity to comfortable middle levels, this is not done since humidity promotes corrosion to the inside of the hull and risks condensation which could short electrical systems, so for safety reasons it is deliberately kept to a low value, around 10%. Another problem of the air coming from the ventilation (unto which the oil lubrication system of the engines is hooked up) is that fumes from components in the synthetic oils can sometimes travel along, causing passengers, pilots and crew to be intoxicated. The illness it causes is called aerotoxic syndrome.
Airliners must have space on board to store "checked" baggage that which will not safely fit in the passenger cabin.
Designed to hold baggage as well as freight, these compartments are called "cargo bins", "holds", or occasionally "pits". Occasionally baggage holds may be referred to as cargo decks on the largest of aircraft. These compartments can be accessed through doors on the outside of the aircraft.
Depending on the aircraft, baggage holds are normally inside the hull and are therefore pressurized just like the passenger cabin although they may not be heated. While lighting is normally installed for use by the loading crew, typically the compartment is unlit when the door is closed.
Baggage holds on modern airliners are equipped with fire detection equipment and larger aircraft have automated or remotely activated fire-fighting devices installed.
Most "narrow-body" airliners with more than 100 seats have space below the cabin floor, while smaller aircraft often have a special compartment separate from the passenger area but on the same level.
Baggage is normally stacked within the bin by hand, sorted by destination category. Netting that fits across the width of the bin is secured to limit movement of the bags. Airliners often carry items of freight and mail. These may be loaded separately from the baggage or mixed in if they are bound for the same destination. For securing bulky items "hold down" rings are provided to tie items into place.
"Wide-body" airliners frequently have a compartment like the ones described above, typically called a "bulk bin". It is normally used for late arriving luggage or bags which may have been checked at the gate.
However, most baggage and loose freight items are loaded into containers called Unit Load Devices (ULDs), often referred to as "cans". ULDs come in a variety of sizes and shapes, but the most common model is the LD3. This particular container has approximately the same height as the cargo compartment and fits across half of its width.
ULDs are loaded with baggage and are transported to the aircraft on dolly carts and loaded into the baggage hold by a loader designed for the task. By means of belts and rollers an operator can maneuver the ULD from the dolly cart, up to the aircraft baggage hold door, and into the aircraft. Inside the hold, the floor is also equipped with drive wheels and rollers that an operator inside can use to move the ULD properly into place. Locks in the floor are used to hold the ULD in place during flight.
For consolidated freight loads, like a pallet of boxes or an item too oddly shaped to fit into a container, flat metal pallets that resemble large baking sheets that are compatible with the loading equipment are used.