A quadjet or four-engine jet is a jet aircraft powered by four engines. The presence of four engines offers increased power output and redundancy, allowing quadjets to be used as civilian airliners, freighters and military aircraft. Many of the first purpose-built jetliners were quadjets, among which stands the De Havilland Comet, the world's first commercial jetliner. In the past two decades, the use of quadjet airliners has gradually begun to decline due to a variety of factors, including the approval of twinjets to fly further from diversion airports and an increased emphasis on fuel efficiency.
The engines on a quadjet are most commonly mounted in pods attached to the underside of the wings, with two engines on each wing. This is known as the podded engine. Prominent examples of quadjets airliners using this configuration include the Airbus A380, Airbus A340 and Boeing 747. Many military airlifters also use this design, including the Antonov An-124, Boeing C-17 Globemaster and Lockheed C-5 Galaxy. The main advantages of this method of mounting are simplified maintenance and, in the event of an engine failure, isolation of the damage from the wing and other engines. The supersonic airliner Concorde and supersonic bomber Rockwell B-1 Lancer both have their engines mounted in rectangular pods conformal to the underside of the wing, without any pylons. This design reduces drag, an important consideration for supersonic flight. The Rockwell B-1 Lancer also has engines mounted in this configuration. Quadjets with podded engines can also have their powerplants mounted to the rear portion of the fuselage, necessitating a T-tail. Aircraft utilising this configuration are rare, with examples being the Ilyushin Il-62 and Vickers VC10.
Quadjets may also be designed with buried engines. As the name suggests, in this design the engines are contained within the aircraft structure, most commonly in the fuselage or the wings. The de Havilland Comet incorporated four turbojets that were buried within its wing roots. This design reduced drag and the risk of foreign object damage, but increased difficulty of maintenance and required a complex wing structure. With the much greater diameter of modern-day high-bypass turbofan engines, having the engines buried into the wing root is no longer possible. The Northrop Grumman B-2 Spirit stealth strategic bomber has all four of its turbofan engines buried within its wing (as a flying wing, the main structural component of the aircraft is the wing). This reduces the heat signature of the engine by concealing the fans and minimising the exhaust signature. As of 2018, the currently in development Northrop Grumman B-21 Raider is another stealth bomber proposed to use four engines.
Although rare, there are also jet aircraft with more than four engines, like the six-engined Antonov An-225 Mriya airlifter and the eight-engined Boeing B-52 Stratofortress strategic bomber. The large size of these aircraft necessitates the use of more than four engines.
Initially, the jet airliner market was dominated by quadjets. The first commercial quadjet was the De Havilland Comet, which first flew in 1949. In addition, it was also the first commercial jetliner, ushering the beginning of the Jet Age. However, due to a series of three fatal accidents that occurred in 1953 and 1954, Comets were grounded. Investigators determined that the latter two accidents, occurring in 1954, were caused by a design flaw that prevented the Comets from withstanding multiple cycles of pressurisation. In particular, the square windows had developed cracks, which eventually resulted in break-up of the aircraft in flight. The Comet was modified and eventually resumed service, but it was later quadjets that truly benefited from these advancements. In 1958, Boeing introduced the 707 and a year later, the Douglas DC-8 was introduced. Both of these quadjets were very successful commercially and are credited with advancing the Jet Age. In 1969, the Boeing 747 had its first flight. Nicknamed the "Jumbo Jet", the Boeing 747 was the first wide-bodied airliner. At the time of its introduction, it was the world's largest airliner and could carry over 350 passengers. Together, these aircraft operated on both domestic and international routes, especially on transoceanic units without diversion airports en route.
Starting from the 1970s, interests in fuel efficiency prompted the development of trijets, aircraft with three engines, and twinjets, aircraft with two engines. For trijets, the reduced number of engines meant that less fuel was consumed, while the added redundancy of having more than two engines was not sacrificed. This reduced the demand for quadjets on long haul routes. Due to limitations in engine technology, twinjets of this era were small and had relatively shorter ranges. FAA mandations also prevented twinjets from flying over 60 minutes away from diversion airports. This meant that while twinjets started to replace quadjets on shorter domestic routes, there was still a demand for quadjets on long range, transoceanic routes. Concorde also began service in the same decade (1976) as a form of luxurious supersonic travel. Due to its high operating costs, its seats were reserved for the wealthy. This was due to its engines being highly inefficient at low speeds, meaning that during taxiing, takeoff and landing, fuel consumption was extremely high.
The gradual decline of quadjets continued through the 1980s. Authorities realised that jet engines were so reliable that flying more than 60 minutes away from diversion airports with two engines was deemed to be safe. As a result, ETOPS ratings for twinjets came to prominence. This allowed them to fly on transoceanic routes that used to be serviced by quadjets, gradually marking the obsolescence of most quadjets (and trijets), as the advantage of redundancy was no longer a factor that made up for the higher fuel consumption and maintenance costs. All but the largest quadjets were deemed to be uneconomical, marking the retirement of many classic quadjets like the Boeing 707 and Douglas DC-8. Boeing launched the 747-400 in 1989, and Airbus launched its A340 in 1993. Both of these new quadjets had high capacities (over 300 passengers) and long range, a combination that was still unmatched by twinjets at the time. As a result, airlines still bought these quadjets, and the 747-400 in particular was commercially successful, in part due to its capacity of over 400 passengers. Although small twinjets had already replaced quadjets on shorter and lower capacity routes since the 1970s, a discrepancy manifested with the introduction of the four engined BAe 146 in 1983. As a short range regional airliner, it was unusual to be fitted with four engines instead of two. Ultimately, the two extra engines enabled quieter operation and better takeoff performance.
The final major advantage of quadjets, their potential for very large capacities, became much less significant when the Boeing 777 was introduced in 1995. The original 777-200 could seat upwards of 300 passengers, which was a significant increase upon existing twinjets such as the 767. Further development pushed the typical capacity of the 777 towards 400, approaching that of the 747 and superseding the A340, while being more efficient and incurring lower maintenance costs. The slightly smaller Airbus A330 twinjet underwent a similar development, and together with the 777, they have resulted in quadjets being regarded as obsolete for all but the highest capacity routes. For example, the A340, introduced alongside the A330 in the early 1990s, had its production ceased in 2011 while the A330 continues to be produced and developed further.
By the 2000s, production of the 747-400 had ceased and deliveries of the A340 dropped to 10 per year. The modern era of quadjets was invigorated in 2007, when Airbus introduced its Airbus A380, which still stands as the world's largest airliner. It was designed for routes with an ultra-high demand and with two full length decks, the A380 can typically seat 575 passengers. However, orders fell short of expectations because of a modern trend towards point to point travel (passengers travel directly from origin to destination) using smaller but highly efficient aircraft such as the Airbus A350 XWB and Boeing 787 Dreamliner with 200 - 300 seats, as opposed to a spoke-hub model (passengers are moved from smaller outlying points and concentrated at large hubs) which favours massive aircraft with huge capacities such as the aforementioned Airbus A380. The largest operator of the A380, Emirates, is able to profit from its fleet of over 100 A380s because its primary hub is situated at Dubai International Airport, where many long haul routes have their stopovers, allowing Emirates is able to fill the seats of its A380s. In response to the A380, in 2011 Boeing introduced the successor to the 747-400, the 747-8. The 747-8I passenger variant has only received 50 orders, a further indication of the conclusion of the quadjet passenger jet era.
A major advantage of quadjets is the high redundancy offered by four engines, leading to increased safety. A single engine failure is much less significant for quadjets, as the three remaining engines can usually provide sufficient power to comfortably reach a diversion airport or even continue the journey, with the decision based on factors such as altitude, terrain elevation and fuel load. With the increased reliability of jet engines, engine failures are becoming less common, reducing the significance of this advantage.
Fitting an aircraft with four engines also results in added power, allowing for more passengers, heavier payloads and increased performance. This was especially important for early jet airliners, as jet engines of that era produced far less thrust than their modern-day counterparts. The Pratt & Whitney JT3D turbofan that powered the Boeing 707 and Douglas DC-8 had a thrust output of 75,616 N (17,000 lb). For comparison, modern engines such as the General Electric GE90 can produce over 444,822 N (100,000 lb) of thrust. Hence, this is advantage is also less significant nowadays, as larger airliners no longer necessarily need four engines to operate. The largest quadjet airliners such as the Airbus A380 and Boeing 747 are so heavy that they still require four engines, and have the highest capacity of all airliners (the Airbus A380 can carry anywhere between 500 and 800 passengers, depending on configuration). This allows them to satisfy demand on extremely busy routes, and when filled, these jumbo-sized aircraft are very economical.
However, quadjets often consume more fuel, driving up operational costs and reducing range. With jet fuel costs on the rise, airlines have shifted their demand to more efficient aircraft, meaning that quadjets are much less economically attractive.
The previously mentioned advantage of the large passenger-carrying capacities of large quadjet airliners is gradually becoming a disadvantage. This is because the airline industry has transitioned from a spoke-hub model to a point to point model, which makes it more difficult to fill all the seats of the largest quadjet airliners.