The activity that aviation nerds call planespotting is a great way to pass the time while waiting for your flight to board. It's always exciting to see unusual aircraft like the double-decker Airbus A380, which is something of a white elephant nowadays. On the other hand, one of the most common jets you're likely to see is Boeing's 737. Although the updated 737 Max has been a major headache for Boeing, there's no denying the overall success of this design, which has been in service for almost 60 years.
The Boeing 737 also happens to be easy to identity due to the unique flat section on the bottom of its two engine casings. To be clear, unlike most jet engines, the units on the 737 are not totally round and don't form a perfect circle. Rather like the racing steering wheels that have filtered into high-performance passenger cars, there's a prominent flat spot on the bottom of the 737's engine cowls. One of the main reasons for flat bottom steering wheels, besides a sporty appearance, is extra clearance under the wheel for the driver's legs in cramped quarters. As it turns out, that's exactly the same reason for the flat-bottom jet engine casings: extra clearance underneath.
When the Boeing 737 entered the jet set in the late 1960s, flying was a considerably different experience than today. This was an age before the elevated jet bridge connecting plane and terminal was commonplace. Passenger exited the aircraft via stairs onto the tarmac. Their luggage was frequently unloaded by hand and without the modern conveniences of today, like motorized conveyor belts down to tarmac level.
All of this is to say that the aircraft of this era needed to be quite low to the ground for loading and unloading, not to mention maintenance. It's the same reason why the 737's landing gear is so short and, some enthusiasts theorize, why it flies with its wheels exposed. However, as technology progressed and jet engines became more powerful and fuel efficient, they also grew larger. The 737's low ground clearance was a serious obstacle to fitting larger engines and for a period of time, it looked like a complete redesign of the aircraft was in the cards à la Boeing's 727, which had its engines mounted high up in the tail section.
Eventually, engineers realized that the size of the larger engines wasn't the problem. Instead it was the casings, also called nacelles, that were too large. These casings are actually considered part of the aircraft's frame, not of the engine. And they're manufactured by the company that makes the plane, while the engines are outsourced.
By bringing the nacelles tighter to the turbofan assembly on the bottom, creating the 737's unique flat spot, larger engines could be fitted to it without other significant modifications or pilot retraining. The solution wasn't totally without compromise, because the irregular-shaped nacelles are less aerodynamic than a totally round counterpart, but it was viewed as a negligible sacrifice compared to the alternatives.
While many airports in the United States have modernized so the 737's low ground clearance is no longer necessary, that's not the case everywhere. Airports in some other parts of the world still lack the infrastructure to accommodate larger or taller jets. As well, some low-cost carriers like Ryanair — a major operator of the Boeing 737 — frequently eschew available jet bridges in favor of stairs to save money. And after this jet bridge in Ireland collapsed and ripped the door off a Boeing 787, using stairs might not be a bad strategy after all.