French investigators maintain that the risk of an external attack on the cockpit outweighs the risk of pilot interference, and have steered away from recommending changes in secure cockpit doors.
The armoured cockpit door was used as a barrier to lock a Germanwings Airbus A320 captain out of the flightdeck while the first officer deliberately put the jet on a collision course with terrain.
French investigation authority BEA, in its inquiry into the 24 March 2015 crash in the Alps, has acknowledged that the secure door contributed to the situation.
But it adds that the risk of illicit attacks during flight are considered a greater threat than the scenario played out in the Germanwings crash.
BEA says that, as a result, it is not issuing any safety recommendation concerning modification of the cockpit door design.
“A door cannot address a risk that could be present from both sides,” it says.
The Germanwings crash occurred some 16 months after an apparently-similar event which destroyed a LAM Embraer 190 in Namibia, and about a year after the disappearance of Malaysia Airlines flight MH370, the inquiry into which has yet to rule out pilot sabotage.
BEA notes that several potential designs could evolve to improve safety by allowing the cockpit door to be unlocked in certain circumstances, while restricting access in general.
These include the use of fingerprint scanners capable of recognising the biometric data of cabin crew and other authorised personnel, or the inclusion of keys in the cockpit which pilots can take with them if they leave the flightdeck.
An alternative possibility would be to reduce the need for pilots to cross the divide by moving the cockpit door aft of the forward lavatory.
But BEA says: “These examples are all at the detriment of security or bring additional cost with little or no additional benefit to security.”
European Aviation Safety Agency analysis last year concluded that a threat from a lone pilot locked in the cockpit could be mitigated, to an extent, by adopting a minimum two-person occupancy policy. EASA stated that it saw no need to amend cockpit-door regulations.