The jet age began with four-engine aircraft, and they ushered in new possibilities for aviation. Although two (and three) engine jets followed soon, quads retained a key role thanks to both their range and capacity. This is coming to an end now, with more improvements in twin-engine efficiency and declining demand for the largest aircraft. We take a look at the story of quadjets and all they achieved over the past 70 years.
The start of the jet age – four engines
Designing the de Havilland Comet
The earliest commercial jet aircraft was the de Havilland Comet. This first flew in 1949 and entered service with launch customer BOAC in 1952.
The motivation for its development came very much from the desire for non-stop transatlantic flights between key cities. Transatlantic flights were, of course, already happening, although large commercial piston-engined aircraft required stops en route. Pan Am’s DC-4 service between London and New York in 1946, for example, took over 17 hours, with refueling stops in Gander, Newfoundland, and Shannon in Ireland.
Non-stop flights had, of course, been proposed before. The UK government had noted this as one of several proposed developments in its aviation review after the Second World War.
Jet engines had been introduced for military use during the Second World War but were considered too expensive and inefficient for commercial aircraft. It was very much due to the perseverance of de Havilland and its founder Geoffrey de Havilland that engines were developed for commercial use.
Other manufacturers were still sticking with piston engines. And although jet engines were successfully developed, these early engines were much less efficient than later ones. It was not until the development of high-bypass turbofan engines (such as the Rolls-Royce JT9D used on the 747) that efficiency really improved.
Quadjets from Boeing, Douglas, Vickers, and Illyushin
The Comet marked a major aviation milestone, allowing longer (and direct transatlantic) flights and a quieter, comfortable, pressurized cabin. This was the start of quadjet flying, but it soon ran into problems. It suffered from structural issues that lead to some catastrophic disasters, from which the Comet series never really recovered (although the issues were resolved by the later Comet models).
This may have been bad for de Havilland, but it had proven the possibilities of quadjets. Other manufacturers soon moved into the four-engine market, having learned from the problems the Comet faced. Other early quadjets that followed include:
- Boeing launched the 707 in October 1958 with Pan American World Airways (Pan Am).
- Douglas introduced the DC-8 the next year. It entered service in September 1959, with both United Airlines and Delta Air Lines.
- The Vickers VC-10. Often forgotten when discussing early quadjets, the British made Vicker VC-10 launched in April 1964 with BOAC. Only 54 aircraft were delivered, with only a few operating airlines in the UK, Middle East, and Africa.
- Illyushin Il-62. This Russian quadjet entered service in 1967 with Aeroflot. 292 were built, right up to 1995. The majority have served with Russian airlines, but many others have also used it.
The Boeing 707 vs. Douglas DC-8
With the problems faced by the Comet, the DC-8 and the 707 were the jets that sold well and really brought the jet age to the masses.
What set these two narrowbody quadjets apart? Simple Flying took a detailed look at them in a recent article. Each aircraft had several variants, but the two most popular later types (the 707-320B and the DC-8-62) were very closely matched in size, speed, range, and passenger capacity. The 707 edged slightly ahead for range (9,913 kilometers against 9,641 kilometers) and speed.
Ultimately, the 707 proved to be the most popular. Boeing went on to sell 1,010 707s (including all variants). Douglas sold 556 DC-8s. It established Boeing as a dominant civilian manufacturer, with the 7X7 series continuing, of course, to now.
Both manufacturers attracted plenty of customers, with several airlines, including Pan Am, Air France, and Lufthansa operating both types. Still, Boeing likely got ahead with its more committed adaption to customer needs.
It offered custom variants for several airlines, such as a longer range model for Qantas and larger engines for Braniff to use on South American high altitude routes. And the Boeing 720 proved a popular, shorter version more capable of operating at smaller airports.
The 707 is often credited as the real start of the jet age, although it could be argued the DC-8 was just as significant technically.
Reducing the number of engines
The challenges with four-engine aircraft were clear from the start, and it did not take long for airlines and manufacturers to consider reducing the number of engines. Aircraft such as the Boeing 720 had started to address the need for jets that could operate to smaller airports, but it maintained the fuel-hungry four-engine design.
The Boeing 727 was the next development from Boeing. It came about from joint requests from three of the largest US airlines (American Airlines, United Airlines, and Eastern Air Lines) for a smaller jet to serve shorter flights and smaller airports.
The three-engine 727 was agreed as a compromise that would deliver better efficiency (and reduced operating costs) but still allow over-water flights to the Caribbean and operation at high altitude airports. It entered service in February 1964 with Eastern Air Lines. It went on to be a success for Boeing, with 1,832 aircraft delivered.
McDonnell Douglas followed quickly with the narrowbody twin-engine DC-9 and later the three-engine widebody DC-10. The DC-10 was intended as a replacement for the quadjet DC-8. With a range of up to 9,600 kilometers (for the long-range DC -10-30 variant), it was more than capable of transatlantic flights. And Lockheed entered the large jet market with the L-1011 Tristar, a three-engine widebody.
Making quadjets larger – The Boeing 747
The early run of quadjets peaked with the launch of the Boeing 747 in 1970. Although airlines had already started introducing two and three-engined jets for shorter flights, the quadjet still had a vital role to play.
We have discussed the story of the 747 before at Simple Flying. Its story starts with a desire for size. Pan Am had worked closely with Boeing on the 707 and now saw the potential of a larger aircraft, offering a lower cost per seat. Development of the 747 began in April 1966, with Pan Am ordering 25 aircraft for $525 million.
Boeing took part of the design from previous work on a US military transport proposal, a contract it ultimately lost to Lockheed and the C5 Galaxy. The iconic shortened upper deck was originally intended to be larger, but this did not meet safety and evacuation requirements at the time.
It allowed the full use of the main deck for freight – a concept that went on to be a great success for the 747 series. And for passengers, it has offered an unrivaled space for a premium cabin or onboard lounge.
The 747 has improved through several variants. The initial 747-100 was soon improved by the 747-200, with upgraded engines and cargo conversion ability.
The 747-300 introduced a stretched upper deck but was short-lived (due to its popularity) and replaced by the improved 747-400 in 1989. This has gone on to be the best-selling variant, with 694 aircraft delivered. And it remained in production right up to 2009 (2005 for the passenger version).
It propelled the 747 to the position of best selling widebody until beaten by the twin-engine 777 in March 2018.
It has been a much-loved aircraft in use around the world. Japan Airlines has operated the most (108 including modified short-range variants for the Japanese domestic market), retiring the last in 2011. British Airways was not far behind, with 94 747s across all variants. Simple Flying took a closer look recently at the largest 747 fleets and the most popular variants.
Making quadjets faster
The story of quadjets is not just limited to large, heavy jets. Concorde has been one of the most famous commercial aircraft to date, using four engines to reach its speed of Mach 2.
Entering service in 1976, Concorde only ever flew with British Airways and Air France. There was earlier interest from several other airlines (with around 100 options from 18 airlines), but this was not to work out.
Increasing costs, concerns about noise and environmental impact, and damage to supersonic hopes from the cancelation of Boeing’s supersonic project and an early crash of the Tu-144 were all contributing factors. This was further compounded by a general change in interest to larger, high capacity aircraft (such as the 747) and the potential to offer lower fares.
Only 20 aircraft were built, and only 14 entered commercial service (seven with each of British Airways and Air France). It was, of course, an amazing achievement. Reducing the transatlantic crossing from London to New York to less than three hours has to be one of the leading milestones for quadjets.
However, it was expensive to operate, and rising fuel prices made this worse. Ticket prices had to be high as a result, limiting its market. Airlines were operating it at a profit, but governments never recovered their investment. Its fateful crash in 2000 may have marked the end, but its fate was sealed economically before that.
The Tupolev Tu-144
Concorde was not the only supersonic quadjet. The Russian-built Tupolev Tu-144 also pushed the possibilities of four engines to supersonic flight. It used four Kolesov RD-36 engines (earlier models used less efficient Kuznetsov engines).
Following two fatal crashes, the Tu-144 was withdrawn from passenger service in 1978. But even before this, it had never been very successful. It only served on one route, Moscow to Almaty, and had a very short range. It was also extremely inefficient, using afterburners at all times to maintain supersonic flights (Concorde only required this at certain points).
The American Boeing 2707 (Super Sonic Transport) would also have been a quadjet. This had well over 100 orders but ended up never leaving the design stage.
Supersonic will return, but not with four engines
And after a long gap, it is once again looking hopeful that supersonic travel will return. But these supersonic jets will not need four engines. There are several aircraft in the design and prototype stage. But even the largest of these, the Boom Overture, aims to achieve Mach 2.2 using three engines.
Among the smaller aircraft under development, the Aerion AS2 will use three engines, but the Spike S-512 is planning to use just two.
A regional quadjet – (BA)e 146
There is one more notable quadjet that served a different purpose. British Aerospace launched the (BA)e 146 regional quadjet in 1983. This remained in production until 2001, with 287 aircraft delivered (it later became the Avro RJ). As of 2021, it remains in commercial service mainly in Iran, Peru, and Australia (see the full list on Planespotters.net).
Its four turbofan engines were not for speed or heavy airframe. There were intended to aid short-field performance, as it was designed for feeder airlines operating at remote airfields. This not just provides extra power, it increases redundancy in the event of engine problems allowing safe take-off at difficult airfields. There is an interesting discussion of this on StackExchange, with input from FlightGlobal.
It has been also been popular for its much quieter operation. The hi-bypass turbofan engines are quieter than similar jet engines, and with four engines it can climb with less power. It has been popular at London City Airport for this reason.
Competition with twin-engine aircraft
It did not take long for twin-engine jets to be introduced. Boeing built on the success of the narrowbody but four-engined, 707 with the three-engine 727 as we have discussed. And the 737 was launched in 1969, ultimately going on to evolve through several improving series and variants to become the best-selling aircraft of all time.
The 727 and 737 were smaller capacity aircraft, though. The real challenges for quadjets began when widebodies starting moving to two engines.
The Airbus A300 and Boeing 767/777
Airbus launched the first twin-engine widebody aircraft, the A300, in 1974 (with launch customer Air France) as a competitor to the older Boeing 707 (and 727), filling the gap between the 737 and 747.
And it competed well. The 707 was ahead on range (9,300 kilometers for the 707-320B compared to 7,500 kilometers for the later A300-600) but had a higher capacity (247 compared to 141 typically). And of course, it came with the efficiency and lower cost of flying on two engines.
Sales were slow, but it went on to sell over 500 aircraft once its reputation improved (remember this was Airbus’ first aircraft launched).
Boeing, of course, introduced twin widebodies, but not until 1982. The 767 entered service with United Airlines in September 1982. This initially focussed on transcontinental US routes, but with ETOPS improvements, it soon could fly further afield, eating into the advantages of the quadjets.
The larger 777 followed in 1995. This has taken the capacity and range of the twinjet even further, with the 777-300ER pushing range almost up to that of the 747-400, with capacity almost there as well (365 against 416 typically).
Twin engines and ETOPS
The limitations of twin engines up to this stage were not due so much to their power (although large aircraft such as the 747 needed more engines), but to their permitted range of operation. Until the 1980s, twin-engine aircraft were limited to operating up to 60 minutes from a suitable diversion aircraft (90 minutes in some locations). This, of course, severely restricted long-distance, and in particular, over-water use.
This changed with the introduction of ETOPS (Extended-range Twin-engine Operational Performance Standards) in the early 1980s.
ETOPS came about with the realization (and evidence) that twin-engine flying was safer than first thought. Specific aircraft could be approved to extend the distance they flew from a diversion airport. The first rating of 120 minutes was given to Trans World Airlines with the 767.
Ratings have improved significantly since then. The Boeing 777 was the first aircraft to have a rating of 180 minutes. An ETOPS 180 rating already covers 95% of the earth’s surface, allowing more efficient transoceanic routes.
And ETOPS 240 was first given to the Airbus A330 in 2009, and the 787 later. The A350 now has the highest rating of ETOPS 370. With ratings as high as this, The only no-go area for twin-engine aircraft is directly over Antarctica.
The A340 – keeping choices for airlines
Despite the introduction of the twin-engine A300, 767, and 777, it was not all over for quadjets. Boeing had the 747, which was still very popular. Airbus launched a competitor with the A340. The twin-engine A330 and four-engine A340 were developed together, a smart move by Airbus that made the development of the A340 feasible, despite lower sales.
Airbus saw demand still for both twins and quads. Airbus’ vice president for strategic planning, Adam Brown, explained this (in reporting by FlightGlobal):
“There was much internal debate whether to go with the big twin or the quad… North American operators were clearly in favor of a twin, while the Asians wanted a quad. In Europe, opinion was split between the two…The majority of potential customers were in favor of a quad despite the fact, in certain conditions, it is more costly to operate than a twin -they liked that it could be ferried with one engine out, and could ‘fly anywhere’ – remember ETOPS hadn’t begun then.”
Airbus tried to persuade airlines and passengers about the advantages of four-engines once it launched the A340. Controversial advertising campaigns were even used, with slogans such as:
“If you’re over the middle of the Pacific, you want to be in the middle of four engines,” and, “4 engines 4 long-haul.”
The A340 is sometimes thought of as a failure, with lower sales than other types and early retirement of young aircraft (Iberia, for example, retired its A340s with the youngest just 10 years old). It wasn’t, though. It served a purpose at the time, brought quadjet competition for Boeing, and was efficiently developed alongside the A330.
The A380 and 747-8 – new hope for quadjets
Despite the increasing popularity and capability of twin engines from the 1980s, there was still a place for four engines with heavy airframes. The A340 program remained alive until 2011, although sales dropped with growing preference for twins. In 2005, for example, Boeing sold 155 777s compared to Airbus’ 15 A340s.
The 747 filled this role for Boeing, remaining in production still today, although only freighter versions remain to be delivered. The 747-8 launched in 2011 with Lufthansa and has gone on to sell 47 passenger models and 94 freighters. Despite the retirement of many 747-400 fleets, the -8 will keep the iconic aircraft in the sky for a few more years yet.
Airbus launched the A380 in 2007 with Singapore Airlines. This was a troubling development, with costs mounting and delays leading to the dropping of a freighter version. But despite delays, it was a great achievement. Its two decks boosted passenger capacity to around 575 (but with a maximum of 853 possible). For comparison, the 747-8 offers a typical capacity of 467 (and a maximum exit limit of 605).
14 airlines ordered the A380, with 251 aircraft sold. Almost half of these went to Emirates.
The decline of the A380
The A380 was a great achievement, pushing capacity higher than we are likely to see for some time. Unfortunately, though, it has not worked as well as hoped.
Simple Flying has looked several times at the decline of the A380. In short, it is designed for a hub and spoke operating model, carrying high volumes of passengers on key routes. It works even better if these routes fly into capacity-constrained airports, where airlines can maximize slot use.
There is now more of a preference for point to point operations with lower capacity aircraft. No US airline ordered the A380, all preferring instead to operate point to point models. China, to a certain extent, has gone the same way. Only China Southern has found a role for the A380, operating it on busy routes to Los Angeles and domestically from Beijing to Guangzhou.
Its size has also limited its use at many airports, something that the Boeing 777X has addressed with its folding wingtips.
And as a final complication, the economics of operating a small fleet has been tough. Just as with any aircraft, there are advantages in costs, maintenance, and scheduling of operating larger, similar fleets. Emirates has made this work for the A380, but many airlines have struggled with small fleets.
Quads lose out to twins
The shift to more efficient twin-engine aircraft is now well established. Not only are no new quadjets planned, but the construction of current models is coming to an end. Production of the A380 will end in 2021. The 747-8F will remain in production slightly longer, with Atlas Air ordering the final four aircraft in early 2021.
New aircraft being launched and planned for the near future are all twin-engine. The A350 is the latest offering from Airbus. The A350-1000 takes typical capacity up to 369. And Singapore Airlines A350-900ULR pushes its range to an incredible 18,000 kilometers, beating any quadjet (the standard A350-900 offers 15,000 kilometers).
The much-anticipated Boeing 777X will offer even more. Test flights began in 2020, and delivery should hopefully start in 2022. The larger 777-9 will offer a typical capacity of 426. And the smaller 777-8 will push the range to 16,170 kilometers.
And it promises significantly improved efficiency over the 777-3000ER and A350 (according to Boeing). This is due to several innovations, including the largest engines yet developed for a commercial aircraft and unique folding wingtips to improve efficiency in the air but still allowing operation at many airports.
Retiring quadjets in 2020
The events of 2020 and the slowdown in aviation have further highlighted the issues with the remaining quadjets. They have suffered the most from the drop in demand, and many have been retired early.
Simple Flying took a detailed look in late 2020 at the four-engine casualties from the pandemic. Some of the major losses include:
- Virgin Atlantic, Iberia, and South African Airways have all retired the A340 early. It is unlikely to return with Air France or Lufthansa.
- The 747-400 has seen early retirement from several airlines (including British Airways, KLM, Qantas, and Virgin Atlantic).
- And even the newer A380 has suffered, with Air France (and possibly Lufthansa) retiring it early so far. Many more remain grounded, with their futures uncertain.
Will we ever seen another quadjet?
With no more four-engine aircraft on the commercial market, you have to question whether another will ever be launched. If there were, it would most likely be to support the launch of a much larger, certainly twin-deck, aircraft.
The case of the A380, though, suggests that commercial quadjets will not be needed for the foreseeable future. It was based on the idea of high capacity operations on hub and spoke networks, but the market has not moved this way. Point to point is growing in popularity, favoring lower capacity. And new aircraft, like the 777X, already promise huge capacity with just two engines and improved airport operations.
Of course, though, much remains unknown about the aviation market post-COVID 19. While for now, it seems lower capacity, and point to point, will be the way forward, there could come a time that size once again becomes the preference. And with new larger engines, just imagine then what the possibilities of four engines could bring.
Would you like to share any thoughts on quadjets and their history? Which has been your favorite aircraft over the years? Let us know in the comments.