This introduces a new risk, that of sabotage by airline personnel themselves. As in the case of the Germanwings intentional downing by what's now thought to be a mentally ill co-pilot. He'd locked the captain outside the cabin.

Protocols to ensure two crew members in the cockpit at all times are now being considered.

There was an exceptionally good essay on implications of this incident, with computer/systems security implications, though I cannot locate it. James Fallows' Atlantic piece was good, though not quite what I was looking for.

http://www.theatlantic.com/international/archive/2015/03/ger...