I read about 9/11 and read somewhere that after 9/11 all aircraft cockpit doors were replaced with bulletproof ones.
Is this true?
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For all air transport operating under 14 CFR 121, 121.313(f) requires a door to be installed between passenger compartment and cockpit.
14 CFR 25.795(a) outlines the relevant design requirements for such a door:
(a) Protection of flightcrew compartment. If a flightdeck door is required by operating rules:
(1) The bulkhead, door, and any other accessible boundary separating the flightcrew compartment from occupied areas must be designed to resist forcible intrusion by unauthorized persons and be capable of withstanding impacts of 300 joules (221.3 foot pounds).
(2) The bulkhead, door, and any other accessible boundary separating the flightcrew compartment from occupied areas must be designed to resist a constant 250 pound (1,113 Newtons) tensile load on accessible handholds, including the doorknob or handle.
(3) The bulkhead, door, and any other boundary separating the flightcrew compartment from any occupied areas must be designed to resist penetration by small arms fire and fragmentation devices to a level equivalent to level IIIa of the National Institute of Justice (NIJ) Standard 0101.04.
So to summarize, any airplane operating under 121 rules (i.e. scheduled air carrier operations) must have bullet-resistant cockpit door.
Bulletproof cockpit doors are required by ICAO Annex 6, however only for larger passenger aircraft (above 45.5 t or above 60 passengers):
13.2.1 In all aeroplanes which are equipped with a flight crew compartment door, this door shall be capable of being locked, and means shall be provided by which cabin crew can discreetly notify the flight crew in the event of suspicious activity or security breaches in the cabin.
13.2.2. From 1 November 2003, all passenger-carrying aeroplanes of a maximum certificated take-off mass in excess of 45 500 kg or with a passenger seating capacity greater than 60 shall be equipped with an approved flight crew compartment door that is designed to resist penetration by small arms fire and grenade shrapnel, and to resist forcible intrusions by unauthorized persons. This door shall be capable of being locked and unlocked from either pilot's station.
(ICAO Annex 6 - Chapter 13.2 Security of the flight crew compartment, emphasis mine)
The ICAO can only provide recommendations. The implementation of actual rules lies with the aviation authorities of the respective countries. E.g. in Europe, EASA requires compliance with this rule:
My aircraft is not fitted with a reinforced cockpit door. Can I use it for flights to the EU?
Chapter 13 “Security” of Annex 6 to the Chicago Convention is fully applicable in the EU.
If your aircraft falls under the criteria for a reinforced cockpit door listed therein, but it is not equipped with one, it cannot be used for commercial air transport operations to the EU.