Four-engined jet aircraft

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The de Havilland Comet, the first commercial jetliner, used four jet engines. Mexicana de Havilland Comet 4 APM.jpg
The de Havilland Comet, the first commercial jetliner, used four jet engines.

A four-engined jet, sometimes called a quadjet, is a jet aircraft powered by four engines. The presence of four engines offers increased power, allowing such aircraft to be used as airliners, freighters, and military aircraft. Many of the first purpose-built jet airliners had four engines, among which stands the De 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 of twin-engine jets to fly farther from diversion airports as reliability increased, and an increased emphasis on fuel efficiency. [2]

Contents

Design

Podded engines mounted under the wings of a Boeing 747-8I. 747-8i Lufthansa on final approach at SBGR.jpg
Podded engines mounted under the wings of a Boeing 747-8I.

Podded engines

The engines of a four-engined aircraft are most commonly found in pods hanging from pylons underneath the wings. [3] This can be observed in the Airbus A340, Airbus A380, and Boeing 747. Many military airlifters also feature this design, including the Antonov An-124, Boeing C-17 Globemaster, and Lockheed 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 larger yawing moment during an engine failure. [4] The supersonic airliner Concorde 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]

Rear view of a Vickers VC10, showing its four podded engines mounted on the rear fuselage Vickers VC10 from the rear arp.jpg
Rear view of a Vickers VC10, showing its four podded engines mounted on the rear fuselage

The four podded engines can also be attached to the rear fuselage, necessitating a T-tail. [3] This reduces cabin noise and frees up more space on the wings for high-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 the centre of gravity aft, and are located further from the fuel supply. [4] The Ilyushin Il-62 and Vickers VC10 both have their four engines mounted in this configuration. [6] [7]

Buried engines

Intakes leading into the buried engines of a de Havilland Comet 4 De Havilland Comet pic 1 REJS.jpg
Intakes leading into the buried engines of a de Havilland Comet 4

Jet aircraft can also be designed with engines buried within the aircraft structure. [3] The de Havilland Comet incorporated four turbojets buried inside its wing 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] The Northrop Grumman B-2 Spirit stealth strategic bomber has all four turbofans buried within its wing (as a flying 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]

Other

The Hawker Siddeley Trident 3B not only has two engines in rear fuselage external nacelles, but also has other two engines mounted vertically in the tail. The aircraft was initially designed as trijet, but the Trident 3B added a fourth engine as additional power was required by the stretched fuselage, increased wing chord and raised gross weight.

Advantages and drawbacks

Advantages

A major advantage of having four engines is the redundancy offered, leading to increased safety. [2] A single engine 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 than three or two engines. This is because three of the four engines will still be functioning, constituting a 25% reduction in thrust, compared to 33% for trijets and 50% for twinjets. This can be observed in the following example involving the Boeing 747-400 quadjet, McDonnell Douglas MD-11 trijet, and Boeing 767-300ER twinjet. With all engines operative at maximum 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] The Boeing 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 jet engines at the time produced less thrust. The Pratt & Whitney JT3D from 1958 had a thrust output of 76 kilonewtons (17,000  lbf ), [13] while modern engines like the General 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—the Airbus 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]

Drawbacks

Four smaller engines consume more fuel than two larger ones, driving up operational costs. Specifically, the Boeing 747 quadjet consumes[ clarification needed ] 2.5 litres (0.66 US gallons) more fuel per kilogram (2.2 pounds) of payload compared to the Boeing 787 twinjet. [16] With jet 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 a spoke-hub model to a point-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 of low-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, the wide-body fleets of these airlines are dominated by lower capacity, long range twinjets such as the A330 and 787. [20]

History

A Boeing 707-120B, the earliest production variant of the 707 Turkish Airlines Boeing 707 at Zurich - April 1976.jpg
A Boeing 707-120B, the earliest production variant of the 707

Early history

Prior to the Jet Age, airliners were powered by piston 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 early jet engines, it was most practical to design large jet airliners with four engines. [4] [22] The first commercial jet aircraft was the four-engined De Havilland Comet, which first flew in 1949. [1] However, due to a series of fatal metal 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 the 707 and a year later, Douglas rolled out its DC-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]

Later decades and gradual decline

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 large trijets and twinjets. Due to limitations in engine technology, twinjets of this era were small and had relatively short range. The FAA'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 the 747. Nicknamed the "Jumbo Jet", it was the first wide-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 the McDonnell Douglas DC-10 and Lockheed L-1011 TriStar.

Concorde on landing, showing its four underwing mounted engines. Concorde.planview.arp.jpg
Concorde on landing, showing its four underwing mounted engines.

Within its own category in commercial aviation, the supersonic airliner Concorde began service in 1976. [30] Its four Rolls-Royce/Snecma Olympus 593 turbojets 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 the BAe 146 was introduced in 1983, it was unusual because it was a four-engined short range regional airliner. [32] Its design ultimately enabled quieter operation and short take off and landing capabilities. [33]

In the 1980s, the increased reliability and available power of jet engines enabled twinjets to safely fly on one engine. This prompted the introduction of ETOPS 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 ageing 707 and DC-8 fleets for passenger service. Nonetheless, Boeing rolled out the 747-400 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 with McDonnell Douglas who went on to produce the MD-11 long-range trijet, instead launched the A340 quadjet in 1993 as a long-range derivative of the A330 twinjet as their initial variants shared the same fuselage and wing. [36]

Large twins such as this Boeing 777-300ER can almost match the capacity of the largest four-engined types. Boeing 777-31H-ER Emirates A6-EGK - 01.jpg
Large twins such as this Boeing 777-300ER can almost match the capacity of the largest four-engined types.

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 widespread General 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-engined Boeing 777 was introduced in 1995, equipped with the purpose-designed General Electric GE90 engine developed from further advancements in high-bypass turbofan technology. [37] The original 777-200 could seat upwards of 300 passengers, a significant increase upon existing twinjets such as the 767, which could typically only seat 200-300 passengers. [38] The subsequent development of the 777-300ER pushed the passenger capacity to just under 400, approaching the 747 and superseding the A340, while being more efficient and incurring lower engine maintenance costs. Airbus, not seeing much success with updated A340-500/600 variants powered by the Rolls-Royce Trent 500, went for the all-new A350XWB to compete against the 777 and upcoming 787 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 the September 11 attacks, the increase in fuel prices and decline in the aviation industry heightened the need to minimise operating costs and expenditures. Production of the 747-400 passenger variant ceased by 2005 and deliveries of the A340 dropped to 11 per year, as they faced competition from more efficient and comparably capable twinjets. [39] [40]

Current status

The Airbus A380 is currently the world's largest airliner Asiana Airlines, A380-800, HL7634 (17765412761).jpg
The Airbus A380 is currently the world's largest airliner

The use of four engines was invigorated in 2005 when Airbus introduced the A380, 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 towards point-to-point travel using smaller but highly efficient twinjets such as the Airbus A350 and Boeing 787, as opposed to a spoke-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 at Dubai 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.

Some airlines, such as Lufthansa, have opted to use both the Airbus A380 and Boeing 747. Lufthansa Airbus A380 and Boeing 747 (16431502906).jpg
Some airlines, such as Lufthansa, have opted to use both the Airbus A380 and Boeing 747.

As engine power continued to grow and capacity demands decreased, twinjet aircraft such as the Boeing 787 Dreamliner and Airbus A350 XWB ate into the markets that traditionally demanded four engines.

In response to the A380, Boeing introduced the 747-8 in 2011 as a successor to the 747-400. [43] The 747-8I passenger variant has only received 50 orders as of 2018, while the 747-8F freighter variant has been more successful with over 100 orders. [44] As of 2018 the 747-8F is unmatched in range and payload, [45] making it an option for cargo carriers.

After the Airbus A380 ended production, Boeing 747 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]

Types currently in production

Airliners

Military

Related Research Articles

<span class="mw-page-title-main">Boeing 767</span> Wide-body twin-engine jet airliner family

The Boeing 767 is an American wide-body aircraft developed and manufactured by Boeing Commercial Airplanes. The aircraft was launched as the 7X7 program on July 14, 1978, the prototype first flew on September 26, 1981, and it was certified on July 30, 1982. The initial 767-200 variant entered service on September 8, 1982, with United Airlines, and the extended-range 767-200ER in 1984. It was stretched into the 767-300 in October 1986, followed by the extended-range 767-300ER in 1988, the most popular variant. The 767-300F, a production freighter version, debuted in October 1995. It was stretched again into the 767-400ER from September 2000.

<span class="mw-page-title-main">Boeing 777</span> Wide-body, long-range, twin-engine jet airliner family

The Boeing 777, commonly referred to as the Triple Seven, is an American long-range wide-body airliner developed and manufactured by Boeing Commercial Airplanes. The 777 is the world's largest twinjet and the most-built wide-body airliner. The jetliner was designed to bridge the gap between Boeing's other wide body airplanes, the twin-engined 767 and quad-engined 747, and to replace aging DC-10 and L-1011 trijets. Developed in consultation with eight major airlines, the 777 program was launched in October 1990, with an order from United Airlines. The prototype was rolled out in April 1994, and first flew in June. The 777 entered service with the launch operator United Airlines in June 1995. Longer-range variants were launched in 2000, and first delivered in 2004.

<span class="mw-page-title-main">Airbus A340</span> Type of aircraft

The Airbus A340 is a long-range, wide-body passenger airliner that was developed and produced by Airbus. In the mid-1970s, Airbus conceived several derivatives of the A300, its first airliner, and developed the A340 quadjet in parallel with the A330 twinjet. In June 1987, Airbus launched both designs with their first orders and the A340-300 took its maiden flight on 25 October 1991. It was certified along with the A340-200 on 22 December 1992 and both versions entered service in March 1993 with launch customers Lufthansa and Air France. The larger A340-500/600 were launched on 8 December 1997; the A340-600 flew for the first time on 23 April 2001 and entered service on 1 August 2002.

<span class="mw-page-title-main">Airbus A330</span> Wide-body twin-engine jet airliner

The Airbus A330 is a wide-body aircraft developed and produced by Airbus. Airbus conceived several derivatives of the A300, its first airliner from the mid-1970s. Then the company began development on the A330 twinjet in parallel with the A340 quadjet and launched both designs with their first orders in June 1987. The A330-300, the first variant, took its maiden flight in November 1992 and entered service with Air Inter in January 1994. The slightly shorter A330-200 variant followed in 1998.

<span class="mw-page-title-main">Airbus A380</span> Wide-body double deck aircraft

The Airbus A380 is a very large wide-body airliner that was developed and produced by Airbus. It is the world's largest passenger airliner and the only full-length double-deck jet airliner. Airbus studies started in 1988, and the project was announced in 1990 to challenge the dominance of the Boeing 747 in the long-haul market. The then-designated A3XX project was presented in 1994; Airbus launched the €9.5–billion ($10.7–billion) A380 programme on 19 December 2000. The first prototype was unveiled in Toulouse on 18 January 2005, with its first flight on 27 April 2005. It then obtained its type certificate from the European Aviation Safety Agency (EASA) and the US Federal Aviation Administration (FAA) on 12 December 2006.

<span class="mw-page-title-main">Wide-body aircraft</span> Airliner with two aisles

A wide-body aircraft, also known as a twin-aisle aircraft and in the largest cases as a jumbo jet, is an airliner with a fuselage wide enough to accommodate two passenger aisles with seven or more seats abreast. The typical fuselage diameter is 5 to 6 m. In the typical wide-body economy cabin, passengers are seated seven to ten abreast, allowing a total capacity of 200 to 850 passengers. Seven-abreast aircraft typically seat 160 to 260 passengers, eight-abreast 250 to 380, nine- and ten-abreast 350 to 480. The largest wide-body aircraft are over 6 m (20 ft) wide, and can accommodate up to eleven passengers abreast in high-density configurations.

<span class="mw-page-title-main">Jet airliner</span> Passenger aircraft powered by jet engines

A jet airliner or jetliner is an airliner powered by jet engines. Airliners usually have two or four jet engines; three-engined designs were popular in the 1970s but are less common today. Airliners are commonly classified as either the large wide-body aircraft, medium narrow-body aircraft and smaller regional jet.

<span class="mw-page-title-main">ETOPS</span> Rules for aircraft with a failed engine

ETOPS is an acronym for Extended-range Twin-engine Operations Performance Standards—a special part of flight rules for one-engine-inoperative flight conditions. The International Civil Aviation Organization (ICAO) coined the acronym for twin-engine aircraft operation in airspace further than one hour from a diversion airport at the one-engine-inoperative cruise speed, over water or remote lands, or on routes previously restricted to three- and four-engine aircraft.

<span class="mw-page-title-main">Rolls-Royce Trent</span> Family of turbofan aircraft engines

The Rolls-Royce Trent is a family of high-bypass turbofans produced by Rolls-Royce. It continues the three spool architecture of the RB211 with a maximum thrust ranging from 61,900 to 97,000 lbf . Launched as the RB-211-524L in June 1988, the prototype first ran in August 1990. Its first variant is the Trent 700 introduced on the Airbus A330 in March 1995, then the Trent 800 for the Boeing 777 (1996), the Trent 500 for the A340 (2002), the Trent 900 for the A380 (2007), the Trent 1000 for the Boeing 787 (2011), the Trent XWB for the A350 (2015), and the Trent 7000 for the A330neo (2018). It has also marine and industrial variants like the RR MT30.

<span class="mw-page-title-main">Boeing Commercial Airplanes</span> Division of the Boeing Company that builds commercial jet airplanes

Boeing Commercial Airplanes (BCA) is a division of The Boeing Company. It designs assembles, markets, and sells commercial aircraft, including the 737, 767, 777, and 787, along with freighter and business jet variants of most. The division employs nearly 35,000 people, many working at the company's division headquarters in Renton, Washington or at more than a dozen engineering, manufacturing, and assembly facilities, notably the Everett Factory and Renton Factory, and the South Carolina Factory.

The Boeing NLA, or New Large Airplane, was a 1990s concept for an all-new quadjet airliner in the 500+ seat market. Somewhat larger than the 747, this aircraft was similar in concept to the McDonnell Douglas MD-12 and later Airbus A380. In 1993, Boeing chose not to pursue development of this concept, focusing instead on the Boeing 747-500X and -600X, and then on the 747X and 747X Stretch, and subsequently on the Boeing 747-8. The project names for this aircraft were NLA and Boeing 763-246C.

<span class="mw-page-title-main">Boeing 747-8</span> Wide-body airliner, last production series of the 747

The Boeing 747-8 is the final series of the large, long-range wide-body airliners in the Boeing 747 family from Boeing Commercial Airplanes. The 747-8 is the largest variant of the 747 and Boeing's largest aircraft. After introducing the 747-400, Boeing considered larger 747 versions as alternatives to the proposed double-deck Airbus A3XX, later developed as the Airbus A380. The stretched 747 Advanced was launched as the 747-8 on November 14, 2005, for a market forecast of 300 aircraft. The first 747-8F Freighter performed its maiden flight on February 8, 2010, and the passenger 747-8I Intercontinental followed suit on March 20, 2011. The cargo version was first delivered in October 2011 and the airliner began commercial service in June 2012.

<span class="mw-page-title-main">Trijet</span> Aircraft propelled by three jet engines

A trijet is a jet aircraft powered by three jet engines. In general, passenger airline trijets are considered to be second-generation jet airliners, due to their innovative engine locations, in addition to the advancement of turbofan technology. Trijets are more efficient than quadjets, but not as efficient as twinjets, which replaced trijets as larger and more reliable turbofan engines became available.

<span class="mw-page-title-main">Twinjet</span> Jet aircraft powered by two engines

A twinjet or twin-engine jet is a jet aircraft powered by two engines. A twinjet is able to fly well enough to land with a single working engine, making it safer than a single-engine aircraft in the event of failure of an engine. Fuel efficiency of a twinjet is better than that of aircraft with more engines. These considerations have led to the widespread use of aircraft of all types with twin engines, including airliners, fixed-wing military aircraft, and others.

<span class="mw-page-title-main">Airbus Corporate Jets</span> Business unit of Airbus that sells corporate jet variants of parents airliner range

Airbus Corporate Jets (ACJ) is a business unit of Airbus which markets and completes business jet variants of the company’s airliners. Following the entry of the 737-based Boeing Business Jet into the market, Airbus introduced the A319-based Airbus Corporate Jet in 1997. Although the term Airbus Corporate Jet was initially used only for the A319CJ, it is now used for all models in a VIP configuration. As of June 2019, 213 corporate and private jets are operating; 222 aircraft have been ordered, including 128 A320 family jets.

The Emirates fleet is composed of two wide-bodied aircraft families, the Airbus A380 and Boeing 777. The airline also has the Airbus A350-900, Boeing 777X and Boeing 787 aircraft on order.

British Airways operates a fleet of Airbus and Boeing aircraft. It operates a single-aisle fleet of Airbus aircraft, including the Airbus A320-200 and the Airbus A320neo. It also operates a twin-aisle aircraft fleet of the Airbus A350-1000, Airbus A380, Boeing 777 and 787.

Virgin Atlantic operates a fleet consisting exclusively of wide-body twinjet aircraft from both Airbus and Boeing.

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  45. ↑ Aboulafia, Richard. "Last Call For Quadjets: Will Boeing 747 Outlive Airbus A380?". Forbes. Retrieved 2018-10-26.
  46. ↑ "Jumbo jet era ends as Boeing delivers last 747 plane – DW – 01/31/2023". Deutsche Welle .
  47. ↑ https://www.aviation24.be/manufacturers/boeing/747/747-production-to-end-as-boeing-adapts-to-post-covid-19-reality-says-globaldata/
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