A generally cylindrical security door module is designed for retrofit into the passageway of an aircraft leading to the cockpit. Two doors, forward and aft, are provided which are mounted for opening and closing upon commands emanating from the cockpit. Both doors are normally closed and only one normally opens at a time. A candidate for entrance to the cockpit is permitted to enter the module when the cockpit personnel open the aft door, which is then closed. Sensing apparatus may be employed to establish the identity and clearance of the candidate. Upon approval, the cockpit personnel open the forward door to admit the candidate. Emergency release apparatus is also described.