Tried the individual key per aircraft, and I'm here to tell you it doesn't work.
Well it does lock the door, but if nobody's got the right key it can bring up some really interesting situations and I'm only talking about a fleet of 2 dozen aircraft.
Wound up with one key fits all and how do you then control the issue of keys and copying?
Bring on electronic or key card locks.
But the answer still lies on the ground BEFORE they get on board.