Who, What, Why: How are cockpit doors locked?

A prosecutor in Marseille has said the Germanwings plane may have been deliberately crashed by the co-pilot, with the pilot locked out of the cockpit. How could this situation arise?

After 9/11, changes were made to the security of cockpits in an effort to make hijackings more difficult. According to the US Federal Aviation Administration, doors should typically be tough enough to withstand a grenade blast. They are usually left locked throughout the flight.

Cockpit security systems are supposed to allow a pilot the ability to access the cockpit. But access can be deliberately denied from within the cockpit.

Cockpit doors on an Airbus - according to this video understood to have been produced by the company - have three modes that are operated from the pilots' seats: unlock, normal, lock.

In the event of whoever is in the cockpit being incapacitated there is a touchpad that will allow cabin crew who know the code to enter.

In "normal" mode the cockpit is locked but can be accessed - after a 30-second delay - by touchpad should the cabin crew get no response from inside.

"Unlocked" mode is what a pilot would use to open the door for a colleague returning from the toilet.

"Locked" means the locking mechanism ignores the touchpad entry code and remains locked for five minutes (it can be repeated). It's easy to see how this would be used to prevent hijackers who have managed to get hold of the code from cabin crew from entering the cockpit.

Some planes may have a screen to tell the pilots who is outside the cockpit door.

If a pilot is unable to access the cockpit, it suggests that his colleague has denied entry.

There is also the question of how many people must be in the cockpit at any one time. Flight attendant Heather Poole tweeted: "In the US a flight attendant always goes into the cockpit whenever a pilot takes a break/leaves."

In contrast, Germanwings cockpit protocols are in line with rules established by the German aviation safety authority, the Luftfahrt Bundesamt. This dictates that when there are two crew, one can leave the cockpit - but only for the absolute minimum time.

Other airlines have a "rule of two", where if a pilot leaves the cockpit for any length of time, another crew member must replace them. However, this is not the case with Lufthansa or other major airlines.

The pilot had been with the airline group for 10 years and has more than 6,000 flying hours. The co-pilot joined the airline in 2013 and had 650 flying hours.

The answer

• After 9/11 cockpit doors were made stronger to prevent terrorists gaining access

• Locking system means that the door cannot be entered unless the pilot inside allows someone to enter

• A touchpad will allow cabin crew who know the code to enter if the pilot is incapacitated

• Pilot in cockpit can deny entry even if someone enters touchpad code

Reporting by Tom de Castella

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