Afour-engined jet, sometimes called aquadjet, is ajet aircraft powered by fourengines. The presence of four engines offers increased power and redundancy, allowing such aircraft to be used asairliners,freighters, andmilitary aircraft. Many of the first purpose-builtjet airliners had four engines, among which stands thede Havilland Comet, the world's first commercial jetliner.[1] In the decades following their introduction, their use has gradually declined due to a variety of factors, including the approval oftwin-engine jets to fly farther from diversion airports as reliability increased, and an increased emphasis onfuel efficiency.[2]
The engines of a 4-engined aircraft are most commonly found inpods hanging from pylons underneath the wings.[3] This can be observed in theAirbus A340,Airbus A380, andBoeing 747. Many militaryairlifters also feature this design, including theAntonov An-124,Boeing C-17 Globemaster, andLockheed C-5 Galaxy. In this location, the engines can act as a relieving load and reduce the structural weight of the wing by 15%. They are also in a more accessible location for maintenance or replacement. However, disadvantages include a higher risk of the engines ingesting foreign objects as they have a lower ground clearance, and a largeryawing moment during anengine failure.[4] Thesupersonic airlinerConcorde had its engines mounted in rectangular pods conformal to the underside of the wing, without any pylons. The omission of pylons reduces drag and eliminates the risk of them being overstressed.[5]
The four podded engines can also be attached to the rearfuselage, necessitating aT-tail.[3] This reduces cabin noise and frees up more space on the wings forhigh-lift devices and fuel storage. The airflow over the wings is also undisturbed due to the absence of pylons. However, the rear-mounted engines shift thecentre of gravity aft, and are located further from the fuel supply.[4] TheIlyushin Il-62 andVickers VC10 both have their four engines mounted in this configuration.[6][7]
Jet aircraft can also be designed with engines buried within the aircraft structure.[3] Thede Havilland Comet incorporated fourturbojets buried inside itswing roots, the most common location for buried engines. This design reduces both drag and the risk of ingesting foreign objects, but increases the difficulty of maintenance and complicates the wing structure.[4] TheNorthrop Grumman B-2 Spirit stealth strategic bomber has all fourturbofans buried within its wing (as aflying wing, the wing is the main structural component). This reduces the heat signature of the engines by concealing the fans and minimizing the exhaust signature.[8]
TheHawker Siddeley Trident 3B not only has two engines in rear fuselage externalnacelles, but also has other two engines mounted vertically in the tail. The aircraft was initially designed astrijet, but the Trident 3B added a fourth engine as additional power was required by the stretched fuselage, increasedwing chord and raised gross weight.
A major advantage of having four engines is the redundancy offered, leading to increased safety.[2] A singleengine failure is much less significant as the three remaining engines can usually provide sufficient power to comfortably reach a diversion airport or continue the journey, depending on factors such as the severity of the malfunction, altitude, fuel load, and weather conditions.[9] With the increased reliability of jet engines,engine failures rates can be as low as 1 in-flight shutdown per 100,000 engine-hours,[10] reducing the significance of this advantage.
During a single-engine failure, the amount of power lost with four engines is proportionately lower thanthree ortwo engines. This is because three of the four engines will still be functioning, constituting a 25% reduction in thrust, compared to 33% fortrijets and 50% fortwinjets. This can be observed in the following example involving theBoeing 747-400 quadjet,McDonnell Douglas MD-11 trijet, andBoeing 767-300ER twinjet. With all engines operative atmaximum takeoff weight, all three aircraft have the power to weight ratios of approximately 1 to 3.4. Following the failure of one engine, the power to weight ratio drops to 1 to 4.7 (747-400), 1 to 5.5 (MD-11), and 1 to 6.6 (767-300ER).[11] TheBoeing 747-400 experiences the least degradation in performance, making it safer during an engine failure.
Fitting an aircraft with four engines also increases power, enabling more passengers, heavier payloads, and increased performance.[12] This was especially important for early jet airliners, as thelow-bypass turbofans andturbojets of the time were much weaker compared to modernhigh-bypass turbofans. ThePratt & Whitney JT3D from 1958 had a thrust output of 76kilonewtons (17,000 lbf),[13] while modern engines like theGeneral Electric GE90 can produce over 440 kilonewtons (100,000 lbf) of thrust,[14] making this advantage less significant nowadays as larger airliners no longer necessarily need four engines.
The largest four-engined jet airliners are distinguished with having the highest passenger capacities—theAirbus A380 can carry up to 853 passengers in a single class layout.[15] This allows them to satisfy demand on extremely busy routes, and when filled with passengers to distribute the cost, they can be very profitable.[12]
Four smaller engines consume more fuel than two larger ones, driving up operational costs. Specifically, theBoeing 747 quadjet consumes[clarification needed] 2.5 litres (0.66 US gallons) more fuel per kilogram (2.2 pounds) of payload compared to theBoeing 787 twinjet.[16] Withjet fuel a large part of total costs, this makes quadjets less attractive to airlines and many are shifting their attention towards more efficient aircraft types.[17]
Four engines also incur higher maintenance costs, as each one needs to undergo regular inspections and servicing. Approximately half of the airliner maintenance costs are derived from routine engine maintenance, so the additional expense in maintaining four engines is significant.[18]
The ability of a very large aircraft to carry a large number of passengers can be a drawback when the seats are not filled. This is an emerging trend, particularly because the airline industry has been transitioning from aspoke–hub model to apoint-to-point model.[12] In the spoke-hub model, passengers are moved from smaller outlying points and concentrated at large hubs. This introduces a need for high-capacity aircraft.[19] Conversely, the point-to-point model transports passengers directly from origin to destination, spreading them out across different routes and requiring fewer seats on the servicing aircraft.[19] Especially with the recent[when?] appearance oflow-cost carriers which operate many point-to-point flights, it is more difficult to fill the seats of the largest airliners.[12] For this reason, thewide-body fleets of these airlines are dominated by lower capacity, long rangetwinjets such as theA330 and787.[20]
Prior to theJet Age, airliners were powered bypiston engines. Engine failures were relatively common, so providing redundancy with four engines was important for long range flights.[21] This need extended into the beginning of the Jet Age, and combined with the limited thrust available from earlyjet engines, it was most practical to design large jet airliners with four engines.[4][22] The first commercial jet aircraft was the four-enginedDe Havilland Comet, which first flew in 1949.[1] However, due to a series of fatalmetal fatigue accidents between 1953 and 1954, the Comet was grounded.[23] This greatly tarnished its reputation and it was the later airliners that truly benefited from the subsequent improvements. In 1958,Boeing introduced the707 and a year later,Douglas rolled out itsDC-8, both types also with four engines.[24][25] Both were very successful and the 707 in particular is credited with advancing the Jet Age.[26] The large airliners flourished during this period, frequently operating on both domestic and international routes.[27]
By the 1960s it became apparent that having four engines reduced fuel efficiency. This was not an issue for long-haul routes which carried 300 or more passengers for 8 to 12 hours, allowing for a high cost-to-passenger-mile ratio. On the other hand, the large four-engined types were less suited for frequent short-haul services, which demanded multiple take-offs and landings daily, costing more fuel while also typically carrying fewer passengers per flight. This prompted the development of largetrijets andtwinjets. Due to limitations in engine technology, twinjets of this era were small and had relatively short range. TheFAA's 60-Minute Rule also prevented them from flying farther than 60 minutes away from diversion airports due to their lower engine redundancy.[28] Trijets represented a compromise between fuel efficiency and redundancy. In 1969,Boeing launched the747. Nicknamed the "Jumbo Jet", it was the firstwide-body airliner, able to carry significantly more passengers than any other aircraft.[29] Its capacity and performance were unmatched, even after the launch of wide-body trijet competition in the form of theMcDonnell Douglas DC-10 andLockheed L-1011 TriStar.
Within its own category in commercial aviation, thesupersonic airlinerConcorde began service in 1976.[30] Its fourRolls-Royce/Snecma Olympus 593turbojets allowed it to cruise at twice the speed of sound.[5] At the time of inception it was regarded as the future of air transportation. However, in large part due to high operating costs and noise issues, Concorde never achieved the predicted level of success.[31]
When theBAe 146 was introduced in 1983, it was unusual because it was a four-engined short rangeregional airliner.[32] Its design ultimately enabled quieter operation andshort take off and landing capabilities.[33]
In the 1980s, the increased reliability and available power ofjet engines enabled twinjets to safely fly on one engine. This prompted the introduction ofETOPS ratings for twinjets, allowing them to circumvent the 60-Minute Rule and fly on transoceanic routes previously serviced by four-engined types.[28][34] The advantage of redundancy brought by four engines was no longer necessary and they could no longer compete with the lower fuel consumption and maintenance costs of twinjets with higher-powered engines. All but the largest four-engined types, such as the Boeing 747, became uneconomical and this led to the retirement of the ageing707 andDC-8 fleets for passenger service. Nonetheless, Boeing rolled out the747-400 in 1989, which enjoyed high capacities (over 300 passengers) and long range, a combination still unmatched by twinjets at the time, making this the most commercially successful 747 variant.[35] Airbus, after ending collaboration talks withMcDonnell Douglas who went on to produce theMD-11 long-range trijet, instead launched theA340 quadjet in 1993 as a long-range derivative of theA330 twinjet as their initial variants shared the same fuselage and wing.[36]
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Between the 1970 and the 1990s, twinjets, trijets, and quadjets shared engines of similar output, such as when the DC-10, MD-11, Boeing 767, and Airbus A300, A310, and A330, and Boeing 747 all had variants powered by the widespreadGeneral Electric CF6, so at the time additional engines were needed for larger capacities and longer range. The major advantages of three and four engines became much less significant when the twin-enginedBoeing 777 was introduced in 1995, equipped with the purpose-designedGeneral Electric GE90 engine developed from further advancements in high-bypass turbofan technology.[37] The original777-200 could seat upwards of 300 passengers, a significant increase upon existing twinjets such as the767, which could typically only seat 200-300 passengers.[38] The subsequent development of the777-300ER pushed the passenger capacity to just under 400, approaching the747 and superseding theA340, while being more efficient and incurring lower engine maintenance costs. Airbus, not seeing much success with updated A340-500/600 variants powered by theRolls-Royce Trent 500, went for the all-newA350XWB to compete against the 777 and787 Dreamliner.
By the early 2000s, the only remaining advantage of the largest types was their ability to carry more passengers than the largest twinjets. In the years following theSeptember 11 attacks, the increase in fuel prices and decline in the aviation industry heightened the need to minimise operating costs and expenditures. Production of the747-400 passenger variant ceased by 2005 and deliveries of theA340 dropped to 11 per year, as they faced competition from more efficient and comparably capable twinjets.[39][40]
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The use of four engines was invigorated in 2005 when Airbus introduced theA380, currently the world's largest airliner.[41] It was designed for routes with ultra-high demand, typically seating 575 passengers in two full-length decks. However, as of 2018, Airbus has only fulfilled a quarter of its initial projected figure of 1,200 sales over two decades.[42] This can be attributed to a modern trend towardspoint-to-point travel using smaller but highly efficient twinjets such as theAirbus A350 andBoeing 787, as opposed to aspoke–hub model which favours massive aircraft such as the A380.[12] The largest operator of the A380,Emirates, profits from its fleet because its primary hub is situated atDubai International Airport, where many long-haul routes have their stopovers. This makes it easier for Emirates to fill the seats of its A380s.[12] After Emirates reduced its last order in February 2019, Airbus announced that A380 production would end in 2021.
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As engine power continued to grow and capacity demands decreased, twinjet aircraft such as theBoeing 787 Dreamliner andAirbus A350 XWB ate into the markets that traditionally demanded four engines.
In response to the A380, Boeing introduced the747-8 in 2011 as a successor to the 747-400.[43] The 747-8I passenger variant has only received 50 orders as of 2018[update], while the 747-8F freighter variant has been more successful with over 100 orders.[44] As of 2018[update], the 747-8F is unmatched in range and payload,[45] making it an option for cargo carriers.
After the Airbus A380 ended production, the Boeing 747-8 also stopped production with the last delivery taking place on January 31, 2023,[46] meaning no double-deck passenger jets were any longer in production. Boeing attributed the retirement of their 747 fleets to the effect of the COVID-19 pandemic.[47]