airshows are one time I shoot in TV, that way you can play with the shutter for different planes, since they look way better with a slightly blurred prop. and DOF doesn't really matter when the background is the sky.

I shoot in raw and spot meter on the plane and that usually helps with the underexposing issue. also if your shooting at 400mm at an airshow, you can usually fill a frame with a close flying plane, so the bright background has less influence.

I go about doing it the same way as Bird in flight shots, so AI servo, center autofocus point.