I started a thread about a related topic on the Steam forums: http://forums.steampowered.com/forums/showthread.php?t=2206529
There are sometimes unknown problems on the helicopters that cannot be found and inspected. I hold down ALT to see the inspection points, so I know I haven't missed any.
On the medium chopper that was your dad's only Engine #1 can be found an inspected and I know that helicopter has two engines. The inspection points for Engine #1 appears on both sides of the chopper when opening the left hatch, even if the right hatch is closed, and opening the right hatch has no effect what so ever.