A TRAINING video prepared by Airbus demonstrates how pilots could secure the cockpit to keep others out.

The five-and-a-half minute video shows the mechanisms in place to ensure the safe operation of an aircraft, and how crew can override them if the pilots lose consciousness.

But there is scope for the pilot or pilots to block colleagues from entering, if they are alert and willing.

The security measures employed by Airbus and other aircraft manufacturers, are now believed to have contributed to the crash of Germanwings’ flight 4U 9525 in the French Alps on Tuesday.

The A320 had just reached cruising altitude of 38,000 feet when it suddenly began descending before briefly levelling out to 6800 feet and crashing into a mountain.

There were no mayday calls issued by the pilot and all 150 people on board were killed including two Australians.

An unnamed military source has told the New York Times the plane’s cockpit voice recorder reveals one of the pilots was locked out of the cockpit, and unable to get back in.

Australian aviation expert Ron Bartsch said the scenario reportedly outlined by the cockpit voice recorder presented a plausible explanation for the crash and lack of mayday calls from the pilot.

Mr Bartsch said it was not unusual for one of the pilots to take a toilet break once an aircraft reached cruising altitude, and briefly leave the cockpit.

“It is a scenario that is consistent with what we do know and what we do know is that the aircraft descended to a pre-assigned level of 6800 feet at more or less a constant rate of descent,” said Mr Bartsch, the chairman of AvLaw International.

“We also know there was no mayday call made from the aircraft, which could be explained by this scenario which suggests a deliberate act by one of the pilots.”

He said the cockpit of an A320 was designed so that only a conscious person inside could keep others locked out.

“If the pilot is unconscious, an emergency code can be used by flight crew to access the cockpit,” Mr Bartsch said.

“If the pilot wants to keep the cockpit door locked, he or she can override this code, but they have to be awake in order to do that.”

There is no mechanism for flight crews to make a distress call outside of the flight deck, said US aviation expert Captain John Cox.

“A mayday call could only come from the cockpit,” he said.

Safety mechanisms in the flight deck of aircraft were upgraded after the September 11 terrorist attacks.

In the US, another member of the flight crew must take a pilot’s place when he or she leaves the cockpit to go to the toilet.

But that is not the case in most other countries, including Europe.

Mr Bartsch said it was possible that policy may be adopted by other airlines in the wake of the Germanwings’ crash, if the pilot lockout story is confirmed.

French authorities investigating the incident have not responded to the reports.

The pilots’ names are yet to be released by Germanwings.