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One seemingly plausible theory about the MH370 disappearance is that one or other of the pilots locked the other one out of the cockpit and then depressurised the cabin.

However, this answer mentions a code that can be entered to unlock the cockpit door. Could this apply in this case (on a 777)?

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Yes, see the Ethiopian Airlines hijacking in February 2014.

From this BBC article about MH370:

The security of the cockpit door offers protection against intruders, but it also prevents action being taken if something does go wrong. Last month the co-pilot of an Ethiopian Airlines flight waited for the pilot to go to the toilet before hijacking the aircraft and flying it to Switzerland.

As for higher authority codes, from the same answer you linked to:

Even if the stronger code is entered that will open the door the pilots still have some time to cancel the unlock request.

Meaning that if a pilot is willfully locking the other out of the cockpit, there is no way for the other to regain access to the cockpit.

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They must balance the need for the pilot "outside" to get back in to assist (or overpower) the other pilot, and the need to protect against someone taking the key or otherwise forcing the "outside" pilot to unlock the door for them. It's not an easy choice for the designer to make, and so far they seem to be more concerned with the latter case. If there are more cases of rogue pilots, the choice may swing back to making it easier to enter from the outside. As soon as there are a number of hijackings, the pendulum will again swing... – Phil Perry Mar 27 '14 at 19:22
Well-I think as of this moment-the pendulum has swung the other way! Incredulous that a pilot can be lovked out of his own cabin for any reason! – user7811 Mar 26 '15 at 0:57
To clarify: Keeping the door locked if the code is entered requires action by the crew, correct? That is, if the code is entered and the flight crew does nothing, after a delay the door will open? @user7811 Unless you're worried about someone learning or watching the code, and having someone who shouldn't have cockpit access enter the code. You can't magically tell "should this person have access," so you have to decide whether you're more worried about someone breaking into the cockpit or more worried that the person inside will be actively doing something bad. – cpast Mar 26 '15 at 5:43
@cpast, context of user7811's comment (I suspect): bbc.com/news/world-europe-32062278 – Benjol Mar 26 '15 at 6:51
@user7811: I'm not incredulous that a pilot in the cockpit can prevent a hijacker with a knife forcing the other pilot, returning from a break, to open the cockpit door for them. Phil Perry's comment seems appropriate to me. Difficult dilemma. – RedGrittyBrick Mar 26 '15 at 10:11

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