Any speculation regarding a decompression, either catastrophic or gradual, is NOT supported by the evidence provided by the CVR (assuming the reports are correct). A catastrophic decompression makes a heck of a racket and so do the cockpit alarms when a gradual decompression occurs. None of these noises have been reported.
Also, if a pressure differential is detected between the cockpit and the cabin, the electrical door locks disengage allowing the door to open freely to equalise the pressure differential. If this didn't happen, the door and or bulkhead structure could easily be damaged by the 8.5psi pressure. That force could total several dozens to hundred tonnes over the area of the door/bulkhead.
Relevant info directly quoted from the A320 flight crew manual:
A forward-opening hinge door separates the cockpit from the passenger compartment. It has three
electric locking strikes, controlled by the flight crew. In normal conditions, when the door is closed,
they remain locked. When there is a request to enter the cockpit, the flight crew can authorize entry
by unlocking the door, that remains closed until it is pushed open.
When the flight crew does not respond to requests for entry, the door can also be unlocked by
the cabin crew, by entering a two to seven-digit code (programmed by the airline) on the keypad,
installed on the lateral side of the Forward Attendant Panel (FAP).
The door is bulletproof and fully compliant with rapid decompression requirements.
A mechanical override enables the flight crew to open the door from the cockpit side.
A deadbolt is installed at the level of the center latch area of the cockpit door. This deadbolt bolts the
door from the cockpit side, in the event that more than one locking latch strike fails, or in the case of
a total CLS failure.
*** (Note - this deadbolt is NOT on ALL A320 aircraft, just a specified serial number block. I have not been able to ascertain if the Germanwings aircraft was so equipped)
The Cockpit Door Locking System (CDLS) provides a means of electrically locking and unlocking the
cockpit door. This system is mainly composed of :
- A keypad, located in the forward cabin, near the cockpit door,
- A toggle switch, located in the center pedestal’s Cockpit Door panel,
- A control unit and its CKPT DOOR CONT normal panel, located on the overhead panel,
- A buzzer.
The keypad enables the cabin crew to request access to the cockpit. There are two different access
request types : “Routine” and “Emergency” access request
(The routine code is known by all crew in the airline, the emergency code is known by all flight crew and senior cabin attendants.)
The toggle switch enables the flight crew to lock or unlock the cockpit door, following an access
request, thereby allowing or denying the entry to the cockpit.
The cockpit door control unit is the system controller, in charge of :
- Locking or unlocking the door latches, upon flight crew action.
- Unlocking the door, in case of cockpit decompression (the door then opens towards the cockpit
under differential pressure).
- Indicating system failures of electrical latches and pressure sensors.
- Activating the access request buzzer and turning on the keypad LEDs.
The buzzer sounds in the cockpit for 1 to 9 s to indicate that a routine access request has been
made, or sounds continuously if an emergency access procedure has been initiated.
CENTRAL PEDESTAL COCKPIT DOOR PANEL
The secured cockpit door opening is controlled by a toggle switch, located on the central pedestal.
COCKPIT DOOR toggle switch:
UNLOCK position : This position is used to enable the cabin crewmember to open the door. The switch must be pulled and maintained in the unlock position until the door is pushed open.
NORM positon : All latches are locked, and EMERGENCY access is possible for the cabin crew.
LOCK position : Once the button has been moved to this position, the door is locked ; emergency access, the buzzer, and the keypad are inhibited for a preselected time (5 to 20 min - as defined by the airline).
(Use of the emergency code will unlock the door unless the LOCK position is selected on the control panel.)
I hope this information is useful.