Canadian airlines quickly moved to require two crew members inside the cockpit at all times after voice recordings from the Germanwings crash revealed that the pilot was pounding on the door to get inside.

Air Canada was first out of the gate on Thursday with the change, quickly followed by other airlines including Air Transat and WestJet Airlines. Porter Airlines said it has always maintained the two-person crew rule in the flight deck at all times.

Lufthansa, the corporate parent of low-cost Germanwings, does not have a rule against a lone pilot. But U.S. carriers have required two people in the cockpit, asking a flight attendant to step in when needed.

Transport Minister Lisa Raitt announced Thursday the government was imposing an emergency directive that two members of the cabin crew be on the flight deck at all times, though they do not necessarily both have to be licensed pilots.

Here is a look at the security features meant to keep intruders out of the cockpit.

Reinforced doors

Reinforced, bullet-proof doors became standard for most of the world’s airlines after the Sept. 11, 2001, terrorist attacks as a way to prevent intruders from entering the cockpit.

But the Germanwings crash raises questions about whether the protective mechanisms have gone too far by making the cockpit near-impossible to enter.

Lufthansa, which operated the ill-fated A320 plane before it was handed to Germanwings in early 2014, said the cockpit had a fortified door with video surveillance to prevent unauthorized entry.

Fortified doors, which can cost more than $20,000, replaced flimsier ones that sometimes closed with a latch or a key.

Keypad

The door to the cockpit locks automatically, but a keypad outside allows a flight attendant to input a security code to request entry. A buzzer sounds and the pilots, who can verify the crew member through a peephole or video surveillance, must switch the door control inside the cockpit to “unlock” to release the door.

If the pilots become incapacitated, a flight attendant can trigger an emergency access entry system with a special code. It sends a 30-second warning with sounds and lights inside the cockpit that the door will be opened shortly. Then there is five seconds for entry.

Override mechanism

If they have any concerns, pilots have the capability of keeping the door in a locked position even if the emergency code is triggered. The door control toggle switch has three positions: unlock, normal and lock.

In the Germanwings case, investigators believe that the co-pilot refused to grant the pilot entry into the cockpit by keeping the switch in the lock position.

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Escape panel

Airbus models include escape hatches built into the lower part of the cockpit door, allowing a pilot to remove the panel and squeeze through in the event an emergency or the door is jammed.

However, the panel can only be removed from inside the flight deck and not from the other side.

In the event of a power failure, the door is automatically unlocked, but remains closed.

Files from Vanessa Lu, Star wire services, Airbus

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