You know, I think I have the answer....
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Rather than insist that when your buddy goes for a break he/she is replaced by another crewmember (who lets face it could have joined the company 5 minutes ago and may have heaven knows what history) simply make it mandatory that if one pilot leaves the flight deck then so must the other one...a sort of "no go alone zone"
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As I see it then there's then absolutely no risk of a lone occupant either taking control and diverting the flight or slumping over the controls whilst eating that delicious sandwich..................
What does the team think? Think I'd get it past the TSA (States) or Transec (UK)........any obvious flaws??????