Reflexions on “plug doors” and “door plugs” By now it is clear to anyone interested in aviation accidents that the “door plug” installed on some 737-9 MAX airliners replaces the Mid-Cabin Emergency Exit Door (MED) when the aircraft is configured to carry less than 200 passengers. What is not so obvious is that neither the MED, nor the “door plug” that replaces it, is in fact a true “plug door”. A plug door is larger than the door frame and when closed it is wedged into the frame, which requires that the door must first be pulled inward before the door can be angled to pass through the opening and open outward. This type of door cannot be opened once the cabin is pressurised and this “fail-safe” design has been used for the main passenger doors on all Boeing and many other types of pressurised airliners.1 The same principle was applied to traditional overwing emergency exits. The Boeing MED has twelve stop fittings that, in the closed position, rest against twelve stop pads on the door frame. If the door is in the correct closed position and once the cabin is pressurised, the differential pressure holds the door in place. If there is no (or a very small) pressure difference and once unlatched, the emergency exit is opened by moving the door up about 4 cm in order to clear the stops and the two upper sliding guides, an action that is aided by springs on the bottom hinges. When the door is clear of the stops and guides, it pivots on the bottom hinges, falls and the escape slide is automatically activated. The Boeing “door plug” is lighter than the MED, has a full-size window and requires no maintenance. Inside the cabin, the door plug is covered with insulating blankets and plastic panels in such a way that that is no indication that it exists. The door plug uses the same twelve stops, lower hinges with springs and sliding guides as the MED, but four bolts with safety nuts, two in the upper guide fittings and two in the lower hinge guides, prevent the door plug from sliding up. It can be argued the operating principle of the Boeing MED door is more suitable for an emergency exit than a traditional plug door because it facilitates opening of the door, especially by someone other than the cabin crew. However, it is definitely not as safe, as evidenced by the Alaska Airlines Flight 1282 accident. In this event, the bolts required to prevent the door plug from moving up were not installed and the door-plug slowly slid up on previous flights until the remaining contact area between the stops was insufficient to hold the door against the inside cabin pressure. Therefore, despite its name, the door plug is not a “plug door”. Boeing offers different types of MED on the larger 737 MAX variants: 737-8200, 737-9 and also on the -10 which has yet been certified. The door plug option is only available on the 737-9. The Next Generation 737-900ER, introduced in 2007 with a capacity of up to 220 passengers, also had a MED with the door plug option. As for Airbus, the passenger access doors are not true plug doors and they can be opened with residual pressure inside the cabin. An Airbus safety brochure states that “several events are reported to Airbus each year where residual cabin pressure caused a door to open violently, leading to serious injuries or aircraft damage.”2 According to Airbus All the doors on the first jet airliner, the de Havilland Comet 1, were plug doors, including the baggage and cargo hatches. 2 Preventing Violent Door Opening due to Residual Cabin Pressure. Safety first, #27 October, 2018. 1 training videos, moving the door control handle up causes the door to move slightly in and then up, after which the door can be pushed out, in some cases assisted by a pressurised nitrogen cylinder. Presumably, a high differential pressure makes it physically impossible to move the door control handle, as is the case of the Boeing MED, but if the pressure difference is low and sufficient force is applied, the door will open violently. All Airbus airliners from the A320 series on, have a Residual Pressure Warning Light on each door and emergency exit, which flashes to warn anyone who wants to open it that the aircraft is still pressurized. On the A350 and A380, there is also an aural warning, which will sound if the door control handle is moved a few degrees up. The A321, introduced in 1994, has “Type C” MEDs which can also be used as passenger access doors. The operation of these doors is similar to that of the main passenger doors except that the speed at which they can be opened is controlled by a damper in order to allow the escape slide, installed in the fuselage bellow the door, to fully extend. The A321neo, introduced in 2017, offers a door plug option for the MED if the number of passengers is limited by a lower density seating arrangement. RRS 2024-03-08
