Two large engines generally are far more fuel efficient than four smaller engines
However, historically, aviation regulators were initially unwilling to allow twin engine aircraft to fly extended distances far away from a diversion airport, fearing that a single engine failure could progress to a double engine failure. That’s part of the reason why many of the earlier transoceanic flights relied on large 3-4 engine aircraft running between large airline hubs. (A hub and spoke network).
As time went on, aviation regulators became convinced that jet engines, maintained under stringent procedures, are reliable enough that flights that might travel 2-5 hours away (at single engine speed) from a diversion airport don’t pose a notable safety risk. These ETOPS rules paved the way for “long and skinny” point to point routes using smaller twin engine jets.
so, three engined planes were A Thing for a while, but its mostly to do with legal restrictions why they were, and why they went away.
So, in general, designers want as few engines as they can get away with, as one big engine is lighter then two smaller engines for a given output, as the two engines need duplicate fuel lines, control runs, structural hardpoints, etc, that all increase the weight, and engines, being full of high precision engineering using exotic alloys, are very expensive. one big engine on the rear of the plane would be ideal for this.
however, theirs a bunch of factors that work against this: one factor is redundancy, in case of engine failure, another is the need to keep the main fuselage free for payload space, and so on. So, the next best set-up is two engines. three engines with the third in the rear fuselage are possible, and three engine designs are cheaper and slightly more fuel efficient, but the desire to maximise payload often pushes towards four wing mounted engines.
however, the early jet engines had a less than stellar reliability rating. Engine failures were not common, but not exactly unknown either. Like i said, high precision engineering.
Their was a standard applied by the FAA, and copied widely, that specified that no twin engine commercial plane could fly a route more than 120 minutes form the nearest airport that could land it. This 120 minute rule was so that in case of a engine failure, the plane would be close enough to a runway it could safely land. this placed limits on what routes a plane could fly, and specifically limited trans-oceanic flights.
three and four engine planes were not bound by the 120 minute rule, as it was felt not needed with the extra redundancy form three and four engines. These bIg, heavy planes affected airline route selection, as they could only run on specific, high volume routes and turn a profit. This, in turn, fed into the “hub and spoke” system of route selection, where air travel was routed though major airports to create the passenger numbers for 4 engine aircraft. for example, you’d fly form Manchester, England, to London Heathrow on a 2 engine, then over the Atlantic to JFK in New York on a 3 or 4 engine, then onwards from JFK to Baltimore on a 2 engine.
As technology improved and engine failures became less and less common, they were able to convince the FAA and equivalent bodies to allow type and airline specific relaxations for the 120 minute rule, called ETOPS (Extended-range Twin-engine Operations Performance Standards, aka “Engine Turns Or Passengers Swim”). This are a combination of plane performance standards and airline operating procedures that rate, for example, British Airways to fly the 787-200 direct form Manchester to Baltimore on a route that takes them 240 minutes flying time form the nearest suitable runway.
TLDR: early planes had reliability issues that mandated four engines for long haul, newer tech has allowed twin engine planes to take over a lot of these routes.
Cost, efficiency, improved technology and some rule changes (in the US).
It costs less to maintain 2 engines rather than 4 other things being equal. Also fewer but larger engines are more fuel efficient. Technology has also improved such that engines now rarely fail.
In the past, there was a rule (in the US) that flights that had to go a certain distance from an airfield or over water had to have more than 2 engines (in case of failure of an engine, the plane must be able to remain flying). Once the rule changed, airlines and plane manufacturers made the switch to 2 engine planes. (nowadays a 2 engine plane must still be able to fly if one engine fails)
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.
It’s a number of factors
For context, long-distance flight paths will always put the plane within a certain distance of an airport in case of emergency. These emergency diversion airports can even be located on tiny remote islands in the middle of nowhere
In the early days of jet travel, engines were less reliable. Four-engined planes had the power/thrust needed, and if 1 or 2 engines go out, there would still be enough engine power to get the plane landed
Modern jet engines are incredibly reliable, powerful and fuel efficient. They proved reliable enough to perform long distance flights.
Now let’s talk about ETOPS. It stands for **E**xtended-range **T**win-engine **O**perational **P**erformance **S**tandards, though aviation nerds sometimes call it “Engines Turn Or People Swim.”
You’ll sometimes see the letters ETOPS on the outside of a airplane or on the safety placard inside. The number after the ETOPS rating is the number of minutes the plane can fly on one engine.
The Boeing 777-300ER, a very ubiquitous and popular twin-engine long distance plane, is ETOPS 180, so it can fly for about 3 hours on one engine. When flights are being planned, they’ll make sure the plane is always within that amount of flight time from an emergency airport. The highest ETOPS rating is the Airbus A350, which is certified for 370 minutes on one engine
First, ETOPS. It’s been explained elsewhere. But as engines have gotten stupidly powerful at a given rate of fuel consumption, and engines are ridiculously expensive, less is more baby!
Also, PART 25. Let’s just look at US regs for now. 14 CFR part 25 is the standard for certification of a transport category airplane, which your airliners are. It says things like, there must be a separate fuel cell for each installed engine, each engine should be able to get fuel from any cell if its own runs dry, and any one cell should be able to run multiple engines at once. So imagine a fuel system to meet those criteria for a 2-engine airplane. Now for a 4-engine airplane. See how much more complex the latter is, how many more valves and pumps and lines are required? Twin-engine planes are simpler, and therefore cheaper, to build.
Answer: Planes were getting larger and larger by the time 4 engines rolled around for dominant use, but eventually they reached a peak efficiency for routes versus passenger fares. Deregulation had made the market flooded with cheaper alternatives after the mail carrier industry was upset with the likes of UPS/Fedex flying mail forcing the airlines to drop fairs to increase ridership. This has led to decades of statistics and modifications to see exactly how many humans can be squeezed in a given seating arrangement for a given utility payload of the aircraft in question. Unlike the other top comment, the aircraft design was an answer to the industry only needing so much thrust and the engines being far more capable and efficient than in the past. To one commenter’s point, there is a bit of stigma around single engine operation, but it’s a layperson/flying-public’s perspective, since every good pilot knows if one of your two engines goes, the good one will take you safely to the crash site. To that point, somewhere between an abundance of precautions and peak capitalism, we have settled on maximum capacity in an airframe powered by two over-powered engines that could singularly power the flight, all governed by seating (unless you’re smart like Southwest and have min-maxed into a single airframe for synergies in maintenance, parts, and training savings.
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