Could someone use an axe to break through a locked cockpit door?
Axes are not carried in the cabin and the door is designed to resist attack using tools available in the cabin


  Fire/crash axe/crowbar. Fire axes were provided to obtain emergency access to areas and parts of the airplane which are not easily accessible (e.g. behind sidewall, electrical or ceiling panels). The handle is insulated to protect against electric shock. In the past, fire axes might be found in the flight deck and in the passenger cabin but on most carriers, in compliance with anti-terrorism regulations and procedures, axes are no longer carried and have been replaced by insulated crowbars in the passenger/cargo compartment.


From http://www.skybrary.aero/index.php/Cabin_Fire