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I have noticed for a while the A340 has four engines and the Boeing 777 only has two engines and they are about the same size. So why can't an A340 have two engines instead of four?

A340 A340

.Boeing 777 B777

B747-8

On a jumbo jet like the B747-8 four engines are a requirement.

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The A340 was designed at a time when ETOPS (Extended-range Twin-engine Operational Performance Standards) had not been developed. Some airlines preferred two engines which reduced operational costs, while others preferred four engines with increased reliability at an additional cost.

Airbus decided to split the development into distinct aircraft having the same wing and airframe - A330 with two engines and A340 with four engines.

However, as time has passed, ETOPS has become the norm with improved engine reliability, and A340 production has been stopped. Almost all the civil airliners under development now have two engines.

The Boeing 777, on the other hand, was developed with twin engine operation in mind.

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  • $\begingroup$ Very informative. I am concerned flying over the Pacific in two engine planes. Volcanic ash, etc... Even birds at 37,900 feet! (Nov 29,1975: Ruppell's griffon vulture, Ivory Coast). I guess remote monitoring of engine state works well. $\endgroup$ – Stein Åsmul Mar 9 at 21:22
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    $\begingroup$ This is not quite true, ETOPS predates the A330/340 by quite a bit. The issue is, when the A340 was introduced, the norm was ETOPS-120 (i.e. you had to fly routes with a diversion airfield within 120 minutes of flight time), which limits the routes you can choose. These days, ETOPS-180 is the norm and some aircraft are certified for ETOPS-240, which makes more routes available. en.wikipedia.org/wiki/ETOPS $\endgroup$ – Hobbes Jun 28 at 7:31
  • $\begingroup$ @Hobbes I'd suggest editing the original post to correct that, it is indeed incorrect that ETOPS didn't exist when the A340 was being developed. $\endgroup$ – zymhan Jun 28 at 15:40
  • $\begingroup$ I don't mind making minor changes to someone else's post, but a change this big would change the intent, and I'm not comfortable doing that. $\endgroup$ – Hobbes Jun 28 at 15:56
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If it had only 2 engines, it would be an A330. The main difference between the two models is that the A340 has four engines, and it was designed and produced specifically for airlines that needed that.

At the time the A330/A340 series was designed, having more than two engines was an operational benefit because a two-engined plane needs to stay within a certain range of diversion airfields, which made the A330 unsuitable for some routes. This benefit has since become smaller due to improved reliability of engines (or improved trust in the reliability), since the maximal distance from nearest landing opportunity is determined mainly by how long time you dare to need a plane to stay aloft after an engine failure enroute, before the risk that the remaining engine will randomly fail too becomes unacceptable.

The A340s that are already built keep flying until they reach the end of their operational life, of course.

The only large quadjets still in production are the 747 and the A380. The A380 needs more than two engines because of its size, in order to produce enough takeoff thrust. The 747 might conceivably have ended up with two engines if it was designed from scratch today.

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    $\begingroup$ Regarding your last statement about designing the 747 as a twin jet... that's debatable, those would have to be some seriously big engines. But you could also argue that the new 777X will come pretty close in capacity. $\endgroup$ – fooot Sep 1 '15 at 14:50
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    $\begingroup$ @fooot: That's why I wrote "might conceivably" instead of "would probably". $\endgroup$ – Henning Makholm Sep 1 '15 at 14:53
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    $\begingroup$ Inconceivable!! $\endgroup$ – fooot Sep 1 '15 at 14:56
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I have a model of an Airbus A340-600 and a Boeing 777-200 and it is easy to notice by size that the 777-200 uses more powerful engines: A340 and 777 engine size comparison (left: 777, right: A340)

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