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In this terrific question we see the advantages of four engines over two. Namely:

  • Engine out capability: Losing an engine means losing only one quarter of the thrust, rather than half.
  • Ground clearance.
  • Drag: Increased yaw control with four engines in an engine-out scenario means that the vertical stabilizer can be smaller, thus reducing drag.
  • The wing is loaded more evenly.

If that is so, then why don't new commercial airliner designs use four engines?

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Because there is no need for more engines.

  • Though one can argue that losing one engine in a twin results in loss of half the thrust, the question is how often does that happen. Engine reliability has improved significantly through the years and engine failures are quite rare today.

  • I'm not sure how ground clearance is an advantage for quads. For quad engined aircraft using today's high bypass turbofans, the inboard engines would have to be mounted higher for suffecient ground clearance- this is actually a disadvantage as it results in a longer landing gear.

  • In aircraft, more engines means more (cost and) maintenance and in a way, more things to fail, as there are more subsystems. In effect, there is no great reliability advantage of quads over twins.

  • One main reason for having more than two engines, ETOPS certification, is no longer relevant. In fact most of the longest routes in the world are flown by the 777, a twin.

  • The best way is to look at the market- there are no quad engined aircraft under development and most of the aircraft in order are twins (A380 and 747-8s are struggling to get orders). This clearly shows that the customers prefer twin engines over quads.

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    $\begingroup$ I'm the same design, choosing four engines instead of two would allow smaller engines, which may be why they are saying that it helps with ground clearance. $\endgroup$ – Lnafziger Feb 8 '16 at 14:22
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    $\begingroup$ 4 RR Trent1000s would give significantly more thrust than would 2, and in the same design would be overkill. $\endgroup$ – FreeMan Feb 8 '16 at 17:05
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    $\begingroup$ How is ETOPS no longer relevant? ETOPS was exactly what allowed twins to start flying those long-haul routes. Before ETOPS, they had to be at least tri-engine. $\endgroup$ – reirab Feb 8 '16 at 19:15
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    $\begingroup$ @reirab I meant that due to ETOPS we have twins in long routes but looks like ended up saying the opposite. $\endgroup$ – aeroalias Feb 8 '16 at 23:06
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Engines are expensive, maintenance-intensive and heavy. You only need 2 to keep a large plane in the air.

In the days of the 747 regulations required at least 3 engines if you ventured more than 60 minutes of flying time away from a suitable diversion airport (in case the second one also failed). However as reliability of engines increased this got extended with ETOPS certificates.

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