Slitherine

This strikes me as being a bit odd really.

I guess while the aircraft were being serviced and refueled the airmen dug trenches or had their orderlies and batsmen do it while they were in the air. Then again I've never heard or read about this happening or the need for it, nobody puts aircraft in the front or second line if they can help it anyway.

Aircraft don't benefit from any entrenchment bonuses. The reason is so that it is possible to have a air unit behind your line without your reserve entrenchments deteriorating. Think of it as the unit 'maintaining' the trenches rather than using them.