Why did big airplanes go from four engines to two engines?
In: Engineering
There was a regulation requiring flights over water and + x amount of hours from their diversion to have more than two engines. Hence designs like the DC-10/MD-11 where you had two engines mounted on pylons and one in the tail. That was a popular translantic option for airlines because it was cheaper than the 747 which has 4 engines.
As the next generation of high bypass turbofans were developed by GE/CFM/Rolls Royce, largely based on the design used to power the C5 Galaxy Heavy Lifter, it became apparent that the failure rate was so low that the chances of losing 2 of them on the same flight was worse than lottery odds. After successful lobbying the regulation was changed and the idea of ETOPS (extended twin operations performance standards), also known as ‘engines turn or passengers swim’ was introduced. The first commercial jetliner to get ETOPs was the Boeing 767. Now almost every 737 can be ETOPS and the last flight I took to Kona from SFO was on a 737.
ETOPS is more than having a certain engine, it is the airframe and a maintenance schedule and a bunch of other things. One of them is a plan for if a plane is diverted. For example, when the 787 was first launched one of them had to divert to Alaska and United had a replacement plane in the air ready to pickup the passengers in very little time.
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