I think they are under more of a load a cruising speed than you'd expect. At speed both the car and plane are primarily overcoming wind resistance, which is mor

I think they are under more of a load a cruising speed than you'd expect. At speed both the car and plane are primarily overcoming wind resistance, which is more about speed and elevation than anything. If I'm understanding right the plane would be going more than highway speed within a few thousand feet of the ground for most of it's (flying) life so it's probably putting more stress on the engine than a car on the highway would. More importantly: if your car engine dies you gradually slow down and pull over. If your plane engine dies calmly pulling over isn't an option. It's much easier to justify spending the money ensuring a plane's engine is reliable.