The End Of The 4-Engine Jet Era

4-engine jest airliners are disappearing from the skies. You have probably heard last year about the retirement of the United's ageing Boeing-747 fleet. Lufthansa have been retiring their fleet of A340s. Now the first Airbus A380 is going to be broken up for parts, as no airline wants it, even though it is only 11-years old.

2-engine design has clearly won, emerging as the preferred option, offering the best setup: price points, reliability and fuel efficiency.

A single engine has never been an option - you need redundancy in the air - engine shutdowns are not that rare (e.g., the Swiss 777 on Feb 1st, 2017, the AF A380 on Sep 30th, 2017, and the LOT 787 on March 28, 2018). But they are rare enough that having two engines provides the expected reliability, considering the failures are not correlated (today it appears they may be in the case of the RR-powered Dreamliner that has just had the ETOPS downgraded, but that is considered a temporary problem).

Three engine designs went away with the DC-10, which today serves only cargo operations. The third engine adds nothing but complexity on top of a 2-engine design. With the increasing output power capability, a 2-engine aircraft can deliver the same capacity. And there is no way to accommodate the high - bypass ratio (that drives all the fuel efficiency but requires huge fans) on an engine mounted within an aircraft tail.

Four engine designs could have been accommodated on the largest aircraft (even for the large diameter high - bypass engines), but they are no longer needed. The largest new aircraft being now designed (the folding - wings 777X) is a 2-engine design. The longest - reach aircraft (the A350 ULR), capable of staying in the air for 20 hours, is also a 2-engine design.

It's an interesting exercise to watch this evolution, in some ways this is probably similar (but going much faster) to natural evolution that gave us pairs or organs - a pair of eyes, a pair of ears, a pair of hands... And if you happen to fly a 4-engine jet, you may pay a closer attention, as this may be the last such experience :)