OK I'll start by saying that I haven't read the full regs - just those posted here.
But on the basis of those, I suggest there is a strong argument in law that a person can be in the crew compartment whilst the aircraft is in flight provided they entered prior to take-off.
The basis of my reasoning is the language:
A person must not be allowed to enter the cockpit after the aircraft has taken off unless etc.
The operative words/phrases are
enter,
after, and
taken off.
Parliament has chosen those exact stipulations. If they'd have wanted to ban access or presence at all during flight, alternative words were available such that the provision could have read:
A person must not be allowed to enter or be present in the cockpit at any stage of the aircraft's operation unless etc.
Whilst there is a strong body of legal interpretation which takes a purposive stance, there is an equally strong body which takes a more literal view, especially where alternative language is easily available. I would suggest that, in this instance, Parliament had words available to it and by choosing exact words, has given effect to what it wanted to achieve - ie. making unauthorised entry into the flight deck during flight, illegal.
However, these regulations represent a minimum standard and, as has been argued by others, if an operator then chooses to put reasonable extensions on them, then that's their call. The difference is this: if a PIC lets his 9-y-o niece up front for departure (in contravention of
company policy), then the PIC in question will probably not, on this analysis, have committed an offence. But they may fall foul of company regs and so face company disciplinary procedures.
