(through a button or lever) for some amount of time, such as 20 seconds. It is also common for these exits to remain completely locked until somebody tests them. Some have alarms activated when they are opened, to alert staff of unauthorized use during non-emergencies. On many exits, the user may have to hold down a crash bar or other door opening device for a period of time to unlock the door. Many exits have a sign reading, 'emergency exit only, alarm will sound if opened', to warn of the fact that it is an emergency exit only. Aircraft In aircraft terms, an "exit" is any one of the main doors (entry doors on the port side of the aircraft and service doors on the starboard side) and an "emergency exit" is defined as a door that is only ever used in an emergency (such as overwing exits and permanently armed exits). Passengers seated in exit rows may be called upon to assist and open exits in the event of an emergency. The number and type of exits on an aircraft is regulated through strict rules within the industry, and is based on whether the aircraft is single or twin-aisled; the maximum passenger load; and the maximum distance from a seat to an exit. The goal of these regulations is to make possible the evacuation of an airliner's designed maximum occupancy of passengers and crew within 90 seconds even if half of the available exits are blocked. Any aircraft where the emergency exit door sill height is above that which would make unaided escape possible is fitted with an automatic inflatable evacuation slide, which allows occupants to slide to the ground safely. † 9 passenger aircraft only Ventral exits must allow the same rate of egress as a Type I exit, tailcone exit are aft of the fuselage. Aircraft for less than 19 passenger must have one sufficient exit in each side of the fuselage, two per side for more, no more than apart from each other. In November 2019, the EASA allowed 'Type-A+' exits with a dual-lane evacuation slide to increase maximum accommodation increased to 480 seats up from 440 with four pairs of doors on the A350-1000, and up to 460 on the A330-900. Gallery References External links Information on fire exit signs in Britain Architectural elements Construction law Safety