How so? If you don't trust the pilots to the point where you want, who, the FAs? taking over the plane, you've got far bigger issues. An override system should be a remote-control thing requiring keys from multiple people.

There's no failsafe way to allow entry from the cabin, even if a system of multiple secure coding for override is instituted in the cabin. And you can't depend upon ground based authentication for a number of reasons as things currently stand.

A dead mans switch, where a member of the flight crew must override a timed flight deck door release, would only work if the crew themselves were willing and capable.

And an Air Marshall who has overriding authority and perhaps access code is yet another security issue.

There are three scenarios with a dead mans switch.

Firstly, the flight deck crew may be unable to code against entry. This works as intended.

Secondly, the flight deck crew is friendly, and codes against entry. This works as intended.

Thirdly, the flight deck crew is hostile, and codes against entry. This last possibility needs a solution.

A solution might be that all flights ping ground stations at timed intervals, and that all flight deck dead man switch override code entries are transmitted to ground, instituting cross reference queries against flight deck and flight cabin personnel. All flights of a certain class would therefore require satellite communications capability.

Any dead man switch override event would be classed as a possible emergency.

[edit] Forgot to say that turning off transponders and or phone homes turns dead man switch into a coded entry switch. This of course can allow entry for hostiles into an already hostile flight deck.