Why Airlines Are Ditching Four Engines In Favour Of Two Engines

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.

Airbus A380
The Airbus A380 – the newest four engine aircraft developed, but will it be the last? Photo: Łukasz Golowanow via Wikimedia

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.

Boeing 707
The Boeing 707 was one of the first jetliners, and featured four engines. Photo: Altair78 via Wikimedia

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. Over 1,500 have been produced, and are still being rolled out – although well reduced.

Concorde, a quadjet with its four engines mounted in rectangular pods under the wings. Photo: Spaceaero2 via Wikimedia

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 realisation (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. The first rating was given to Trans World Airlines flying a 767, with a rating of 120 minutes.

British Airways 747
The 747 came to dominate the wide body long haul market – at a time when twinjets could not operate trans-oceanic routes. Photo: Simple Flying

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 (according to Wikipedia data), versus a Rolls-Royce Trent 900 engine (in use on the A380 today) with around 80,000 pounds of thrust. Over time, two engines became enough for most airframes.

Keeping it going with the A380

ETOPS certainly opened up the market for twinjets, but four engines has remained for a number of larger, heavier aircraft. Boeing dominated the civilian four engine market with the 747 for a long time. It was not until 1993 that Airbus launched the four engine A340, and the A380 in 2005.

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 favours.

Emirates A380
The Airbus A380 – four engines again became useful for heavy aircraft rather than a routing necessity. Photo: Simple Flying

So, with this history in mind, why are the newer aircraft being developed now twin engine? The following are a few of what we see as the main reasons.

Less safety and regulatory concerns

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 for example is now rated to fly 370 minutes from a diversion airport.

Qatar Airways A350
ETOPS ratings with the A350 has reached 340 minutes. Photo: Juke Schweizer via Wikimedia

In some ways, 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 747 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.

A380s at Heathrow
The A380 works for high density hub routes, but airlines are experiencing limitations with this. Photo: Simon Boddy via Wikimedia

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 for example on opshots.net compares the total cost for Qantas to operate a Boeing 777 from Sydney to Los Angeles (around $190,000) with the cost for an A380 on the same route (around $305,000). 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.

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.

Trent 900 engine
Engine maintenance is a significant part of operating expenditure. Photo: Dr Brains via Wikimedia

Will we see any more four engine aircraft?

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.

Boeing 777x
The Boeing 777X. Current wide body development is focused on twin engines. Photo: Boeing

There may be a time when four engines become popular again of course. But at the moment with more powerful engines, safety improvements and a shift away from superjumbo operations two seems the way to go