1.) All planes without a special characteristic : fly have gears down on start.
2.)Its an animation.
Something like:
bigplane animate [Door Closed]
Of course I don't know the proper syntax nor the correct animation. But I'm pretty sure that's how it works, it'll make the door be closed upon where you put the code. Soz I can't help you more on this.
3.) Use a setpos height thing code:
this setpos [getpos this select 0, getpos this select 1, (getpos this select 2)+1]
in his init field and maybe you can make him in the plane, not being under or crushed by it. Blame OFP's awesome clipping characteristics.
But this may not work cause of clipping reasons... :-\