Just Something I noticed................

[Dave Ward]

Some YouTube videos of aircraft on the ground illustrate something that I have noticed - this involves main undercarriage doors. When on the flightline, cold, the hydraulic pressure gradually falls, and the main doors will drop, sometimes the flaps, and maybe the speed brakes. As soon as they start up the hydraulic pressure builds up and the doors etc return to their correct position.
I first wondered about this on a F-86 Sabre model - the instructions would have you build with the main doors open, and it struck me that they would be very vulnerable on landing!
Sure enough a YouTube video of a Sabre start up confirmed it. Since then I've looked at quite a few videos eg Mustangs, and quite a few types of aircraft show this this feature. Of course, if a manufacturer has access to a full size plane, when taking measurements/photographs, it'll probably be cold, and the doors will be open - so that can carry through onto their instructions! Just do a bit of research to see how the doors really hang! ( it can apply to nosewheel doors, as well )
Dave

[flyjoe180]

Some aircraft also have leading edge slats which will drop aerodynamically below certain speeds and will therefore be down on the ground. The A-4 Skyhawk is one.

A 737 which hasn't been powered up for a while will drop its leading edge slats and some drooping of the trailing edge flaps. Power up the APU turn on the hydraulic switches and the slats snap up instantly. Not that it matters here but never stand near those devices when the aircraft is being brought to life, they will go right through flesh and bone.