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

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

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

Germany’s federal police, which run sky marshal squads toprotect aircraft registered in the country, wouldn’t confirm thatthose marshals carry firearms. In any event, a flight fromBarcelona to Dusseldorf by a low-cost carrier might not fit theprofile of a service to be staffed with such securitypersonnel.

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

Adams Rite Aerospace, a closely-held manufacturer fromFullerton, California, was chosen by Airbus in 2002 to installcockpit door-locking systems on all its aircraft, includingdoor-bolting systems, electric strikes, keypads andcontrollers.

In the U.S., the Federal Aviation Administration in 2002 set newstandards to protect cockpits from intrusion, requiring operatorsof more than 6,000 aircraft to install reinforced doors. Thepurchase and installation of such a barrier was estimated to costfrom $12,000 to $17,000 at the time.

--With assistance from Andrea Rothman in Toulouse.

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