The engines aren’t flat, their housing ( cowling ) is.
But why are those flat?
When the 737 first got designed in the sixties, it was envisioned to be a plane for small airports without much infrastructure. This meant it needed to be low on the ground so it could be accessed with only a small stair. Engines in those times were of the low bypass kind which are very small in diameter, for the 737 the designers hung them on very short pylons again to be so close as possible to the ground.
One of the reasons Ryanair loves the 737. Less need of airport infrastructure means lower costs.
But when it came to update the 737 in the eighties, Boeing wanted to install higher bypass engines, which burn a lot less fuel. This would keep the 737 revelant against the brand new Airbus A320 One problem though, high bypass engines are jet engines with a big fan in front of them. This makes them have a much bigger diameter than low bypass engines, and the space just wasn’t there on the 737. Look at the height difference between the A320 and the 737:
The landing gear couldn’t be enlarged°. So Boeing asked for some changes to the CFM56 engines ( smaller diameter front fan and gearbox on the side rather than on the bottom of the engine ), with this result:
The 737 Max got even bigger engines, and because of the still too short landing gear meant the engines needed to moved forwards so it could hang a bit before the wings rather than completly under it. This changed the balance of the aircraft, together with bad software and a lack of sensors this caused the 737 max crashes which kept them grounded for so long.
° Interestingly, for the 737 max 10 Boeing did redesign the landing gear. Not for the engines, but to allow for the longer fuselage. Had Boeing implemented this for all 737 max models, it could have avoided a lot of problems.
Author – Matthew Bekaert