Following examination and downloading of the Cockpit Voice Recorder (CVR), French prosecutor Brice Robin concluded that co-pilot Andreas Lubitz had locked pilot Patrick Sondheimer out of the cockpit and then deliberately put the aircraft into a downwards glide path which caused the crash.
The CVR also revealed the way that Sondheimer had attempted to override to door lock, only to be over-ridden by Lubitz. The recording included Sondheimer shouting through the locked door before using an axe, unsuccessfully, to batter down the hardened door.
However, pilots have said the rush to enforce the four-eyes rule — whereby a member of the cabin crew swaps places with either pilot leaving the cockpit for a break — is not a panacea.
They cite a previous example of one pilot having to lock another pilot out of the cockpit because he had 'gone mad'. Others say the cabin-swap highlights the opening and closing of the cockpit door, which could be exploited by terrorists. Worse still, many US pilots carry handguns and would be able to shoot the cabin crew.
John Walton, writing in Airways News said: "While horrifying and shocking, pilot murder-suicide is infinitesimally rare. An impenetrable flight deck has been a feature, not a problem, in the past. Almost exactly two years before Germanwings 9525, JetBlue pilot Clayton Osbon had a significant mental health event during the flight and was locked out of the flight deck by his first officer.
Germanwings Rogue Pilot — Nothing New
Aviation sources say there have been at least four other crashes since 1994 that investigators believe may have been caused by deliberate pilot action: A Royal Air Maroc ATR 42 in 1994, a Silk Air Boeing 737 in 1997, an Egyptair Boeing 767 in 1999 and a Linhas Aereas de Mocambique (LAM) Embraer 190 in November 2013.
Many pilots — blogging anonymously — have also pointed out that cabin crew called off normal duties to go to the cockpit door and swap with one of the flight crew will draw attention to the opening and closing of the door, which could be exploited by terrorists in the front seats rushing onto the flight deck.
John Walton said:
"A pilot determined to carry out an atrocity like Germanwings 9525 is likely to be able to — ironically, especially in the United States where pilots are routinely permitted to carry firearms."
In any case, "a truly determined pilot, acting for whatever reasons, is unlikely to be able to be prevented from taking action by a flight attendant sitting in the jumpseat," he said.