Since the September 11, 2001, terrorist attacks on the US, using hijacked planes, reinforced security has crucially included the cockpit — with provisions that are usually redundant. Yet these provisions also created deadly circumstances that abetted Tuesday’s air crash in the southern French Alps.

The revelations about Germanwings suggest that an unbreakable door, button code locking and ignored procedures contributed to its doom. One flight recorder has yielded this conclusion.

Germanwings owner Lufthansa is not one of the companies that require a cabin crew member to temporarily enter the cockpit if one of the pilots steps out — a two-is-better-than-one principle applied by other carriers.

Cockpit locks are designed to be controlled principally from the inside, electronically.

Some critics say keypad entry could endanger cabin crew pressured to reveal them.

An unopposed outsider can get in, but can also be denied entry.

The Germanwings pilot was locked out by the cop