Specifically, the engine casing's bottom flattens out, rather than continuing around with a perfect circular shape.

具体来说，发动机外壳的底部变平了，而不是继续围绕着一个完美的圆形。

Instead, the redesigned engine casings meant that pilots could work interchangeably between older and newer 737s at the time.

相反，重新设计的发动机外壳意味着飞行员可以在当时的新旧 737 之间交替工作。

However, engineers looking at the aircraft realized that the engines' casing, rather than the size of the powerplants as a whole, was the problem.

然而，查看飞机的工程师意识到问题出在发动机外壳上，而不是整个动力装置的尺寸上。