this video from Airbus shows that if the cockpit is video monitored by the crew or airlines or traffic controller, a lot of attacks could be better dealt with.There is an override outside, but the pilot in the cockpit can delay it five minutes, this for anti-hijacking reasons. Having an immediate override would just increase hijackings again, some like 9/11 with suicidal intent.
I'm afraid this is a "between devil and deep blue sea" situation, there is no foolproof answer.
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All Airbus planes have fly by wire capability. It can't be too difficult to disable access to the computer from the airlines HQ or gas the nutter who locks himself in.