Hi FPT
Seems to me that all the FD crew need is a means to override the door locking system whatever the setting on the FD. That's to say a system that makes it impossible for FD crew to be prevented from reentry
As has been said here a number of times, there are valid reasons to
prevent Flt Crew entering the Flt Deck e.g. the JetBlue case, and Hijack where the Flt Crew is being "persuaded" to open the door.
The system worked as designed - it just was not designed for this scenario
