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Ditching Four in favor of Two Engines

Why Airlines are ditching Four Engines in favor of Two Engines? What does this mean for air travel?

With the end of production of the Airbus A380 and the ongoing retirement of Boeing 747s, are we seeing the end of four engine aircraft? In this article, we take a look at the story of four versus two engines, and why it seems airlines are switching to two, just accelerated by the Covid-19 pandemic, but nothing more.

The four engine jet – where it all started

Looking back to the first jet aircraft, four engines certainly used to more common than two. The first jet aircraft to be introduced in 1952 was the de Havilland Comet. This was a four engine aircraft (also referred to as a quadjet), as was the very successful Boeing 707 that followed it in 1958. Others quadjets included the Douglas DC8 and the joint British / French built Concorde. Perhaps the most famous though was the Boeing 747, introduced in 1970 and the most successful wide body jet to date. January 22, 2020 marks the 50th anniversary of the first commercial flight by a Boeing 747. On that day, the first jumbo jet took off with fare-paying passengers on a flight from New York to London. It was a day late – the Pan Am flight was scheduled to depart on the evening of 21 January but engine overheating grounded the aircraft. Another 747 was called into service instead with a delay of several hours. Despite that initial hiccup, over the past half century the Boeing 747 has proven its worth, and then some.

It was a different world back then. In 1970 Richard Nixon sat in the White House. World's population was half what it is today, you watched television in black-and-white, for the first time you could buy a digital electronic watch and your parents might have been tootling around town in a Volkswagen Type 1 'Beetle' or BMW 2002Tii. All have passed into the pages of history, yet after several permutations the Boeing 747 is still alive and well. Since the first 747 took off Boeing has delivered more than 1500 of the aircraft and over 500 were still in service in December 2019, but that's coming to an end. Boeing is no longer producing the 747 as a passenger aircraft. Its superjumbo competitor, the Airbus A380, made its commercial debut in 2007. The A380, as the largest passenger aircraft to date, re-invigorated the possibilities of four engines. Airbus hoped this high capacity aircraft would suit high demand long haul routes. This was true, but unfortunately many airlines have since moved away from the ‘hub and spoke’ operating model this favors, yet Airbus will cease production of the aircraft in 2021, a casualty of falling orders. The era of big double-decker four-engine aircraft is grinding to a halt. Traditionally the 747 and A380 have been the workhorses of long-haul routes – although Japan Airlines used a 747 on some of its Tokyo-Osaka flights, with a flight time of just 1 hour 20 minutes. These days, airlines looking for aircraft that can transport large numbers of passengers at competitive cost and capable of flying long distances are signing up for either Airbus' A350 or Boeing's 787 Dreamliner, with Boeing's 777X expected to enter service in 2021.

Opening up the possibilities for two engines in the 1980s

Safety was one of the main reasons for the choice of four engines during these early years. Four engines of course gave better redundancy, and were seen as safer in the event of engine failure. A twin engine aircraft (under FAA rules) could not fly more than 60 minutes away from a diversion airport – limiting the possibilities for trans-oceanic flights. This changed during the 1980s, when ETOPS (Extended-range Twin-engine Operational Performance Standards) was introduced. ETOPS came about with the realization (and evidence) that twin engine flying was safer than first estimated. Specific aircraft could be approved to extend the distance they flew from a diversion airport. ETOPS certainly opened up the market for twinjets, and the first rating was given to Trans World Airlines flying a 767, with a rating of 120 minutes. Power was also a consideration. Jet engines in the 1950s and 1960s were of course not as powerful as we see today. A 1958 Pratt & Whitney JT3D engine for example (used on the Boeing 707) had a thrust of 17,000 pounds, versus the new a high-bypass turbofan General Electric GE9X for the Boeing B777X with around 110,000 pounds of thrust. Over time, two engines became enough for most airframes.

With this history in mind, the following are a few of the reasons why newer aircraft being developed now twin engine.

Less safety and regulatory concerns established the Two Engines

A major reason for the switch to two engine aircraft is improving safety. Whilst four engines was traditionally seen as safer, this is not necessarily the case. Jet aircraft have proved very reliable – with very few cases of dual engine failure. There is a fascinating list on Wikipedia of all known cases where a jet has been forced to glide without power. The relaxation of regulatory limitations for twin engine aircraft, through the introduction of ETOPS, has really opened up the options for twin engine operations. It was not just the original extensions of operating distance in the 1980s though. ETOPS limits continue to expand along with aircraft improvements. The Airbus A350-900 for example is now rated to fly 370 minutes from a diversion airport. In some way, two engines are also safer! The possibility of a single engine failure is of course higher when you have four engines rather than two. And with this comes the risk of fire, or damage to the air frame structure.

Changes in airline operating models

The four engine B747 and A380 best suit the hub and spoke operating model, where an airline operates high capacity flights between major hub airports. Recent times though have seen more of a switch to the point to point model, best suited to lower capacity aircraft such as the A330, A350 or Boeing 777 and 787. An A380 full to capacity can be very economically efficient, but this is not the case if you can’t fill it on a less busy route. Compounding this are operating limitations. Large aircraft such as the A380 can operate at airports with the required facilities, gates and runway length, seriously limiting airlines routing and scheduling flexibility.

Changing economic times and a focus on efficiency

Rising fuel prices have had a major effect on the airline industry over recent years. Four engine aircraft have significantly higher fuel consumption, and this has become much more important to airlines. Some interesting analysis compares the total cost for Emirates to operate a Boeing 777 from Dubai to Sydney (around $190,000) with the cost for an A380 on the same route (around $305,000). On this 12'042 km route, Fuel is the most significant difference here.

Newer aircraft too are designed to be much more fuel efficient. Airbus for example report the A350 as up to 25 per cent more fuel efficient than other wide body aircraft.

Boeing from his side claims that the Boeing 787-9 is the most fuel-efficient aircraft on transpacific flights at 39 passenger kilometers per liter of fuel, or 60% better than the A380.

Another cost consideration for airlines is Maintenance. This may not seem as obvious as fuel costs, but almost half of typical maintenance costs is engine related (IATA have published a detailed guide to maintenance costs here) – so costs rise quickly with four engines.

Last but not least, the Range : as airlines contemplate ever more ambitious long-distance routes you can expect those to happen aboard twin-engine aircraft. Although there's not a great difference between the range of the four-engine jumbos and the leading twin-engine widebody aircraft, when airlines are looking for the longest legged aircraft to service their furthest routes the twin-engine aircraft ace the deck. Singapore Airlines uses an Airbus A350-900ULR for its Singapore-New York service, currently the world's longest non-stop flight. Qantas has chosen the Airbus A350-1000 for its proposed non-stop flights from Australia's east coast to New York and London, which will take the longest-flight trophy from Singapore Airlines if it happens. Sydney to New York is 16,200 kilometres, to London is a shade over 17,000 kilometres. That will require the aircraft to be fitted with extra fuel tanks. Airbus developed the A350-900ULR specifically to suit the needs of Singapore Airlines in order to operate its Singapore-New York flights, so they could do something similar for Qantas. Qantas also uses a Boeing 787 Dreamliner for its Perth-London flights, which sits in third on the table of world's longest flight routes. Not too far behind, Emirates currently uses an A380-800 for its 14,200-kilometre Dubai-Auckland flight, the longest scheduled flight aboard a four-engine aircraft, but Qatar Airways uses a twin-engine Boeing 777-200LR for its slightly longer flights from Doha to Auckland.

Current wide body development is focused on twin engines

The A340, A380 and to a certain extent the 747 will remain in the skies for some time, serving both passenger and cargo operations, (as will military aircraft such as the Boeing C17). But current development trends for passenger jets generally involve two engines. The recently developed Boeing 787 and Airbus A350 of course are two engine as is the upcoming 777X. There may be a time when four engines or even maybe more become popular again of course, thinking on the future aircraft propulsion. But at the moment with more powerful engines, safety improvements and a shift away from superjumbo operations two seems the way to go...

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