Originally Posted by LSM
Or just inaction by one mind persuaded so by the other.
Now you're clutching at straws.
So your hypothesis is that a CC, trained and competent to deal with difficult passengers, will be readily persuaded by a rogue pilot to deny re-entry to the other pilot on the grounds of mental incapacity and that the first pilot will then be completely free to take unimpeded action to fly the aircraft into the ground; and all the while the CC is ignoring the pleas of the second pilot to allow re-entry and remains completely accepting of the first pilot's instructions without question or hesitation. Yep, that's really plausible.
