(Bloomberg) -- Security regulations introduced after the Sept. 11 terror attacks mean that even a .44 Magnum would have been useless in penetrating the cockpit door of the Germanwings jet that crashed in the Alps last week killing 150 people.

An aircraft’s bulkhead, door and other boundaries separating the flight-deck from areas occupied by passengers must be able to resist “forcible intrusion,” including small-arms fire, according to European Aviation Safety Agency rules, rendering them impenetrable even to armed sky marshals.

“While sky marshals do carry guns, the door to the cockpit is bulletproof, so they would likely not be able to make a difference,” said Joerg Handwerg, a spokesman for German pilot union Vereinigung Cockpit. The first officer of the Germanwings Airbus Group NV A320 is said by investigators to have locked the captain out of the flight-deck before crashing the plane.

Germany’s federal police, which run sky marshal squads to protect aircraft registered in the country, wouldn’t confirm that those marshals carry firearms. In any event, a flight from Barcelona to Dusseldorf by a low-cost carrier might not fit the profile of a service to be staffed with such security personnel.

Cockpit doors must be able to withstand 9 millimeter (0.35 inch) full-metal-jacket, round-nose bullets as well as Magnum jacketed hollow-point bullets, and fragmentation devices including grenades, Easa stipulates. Guns of a size suitable to be carried concealed by marshals are generally small-caliber.

Adams Rite Aerospace, a closely-held manufacturer from Fullerton, California, was chosen by Airbus in 2002 to install cockpit door-locking systems on all its aircraft, including door-bolting systems, electric strikes, keypads and controllers.

In the U.S., the Federal Aviation Administration in 2002 set new standards to protect cockpits from intrusion, requiring operators of more than 6,000 aircraft to install reinforced doors. The purchase and installation of such a barrier was estimated to cost from $12,000 to $17,000 at the time.

--With assistance from Andrea Rothman in Toulouse.

Special Reports /

Germanwings aircraft's cockpit door would have locked out armed sky marshals

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