How a panel blew off a Boeing plane in mid-air

The incident happened as the jet climbed following takeoff from Portland, Oregon, en route to Ontario, California, on Jan. 5. As Alaska Airlines Flight 1282 reached just over 16,000 feet, the panel tore off from the side of the jet, leaving a neat, refrigerator-sized, rectangular hole in the aircraft. The pilots immediately signaled that the plane would have to land.

The aircraft landed safely with all 171 passengers and six crew on board. Several passengers suffered injuries. The plane had been in service for just eight weeks.

Why are door plugs needed?

The panel that tore off is a plug put in place on some 737 MAX 9s instead of an additional emergency exit, but not every plane is manufactured in such a way. The MAX 9, currently Boeing's largest single-aisle plane in production, can seat up to 220 people. It includes an optional extra door to allow for the approved number of evacuation paths whenever carriers opt for the ability to install more than 189 seats.

Planes that do not opt for additional seating can replace that door with a panel, or plug. Door plugs have been used to adapt aircraft and offer flexible layouts across the industry for years, including on the MAX 9’s predecessor, the Boeing 737-900ER.

According to the U.S. National Transportation Safety Board, four bolts – two in the upper corners of the plug and two lower hinge brackets – secure the plug to the fuselage. A further 12 “stop fittings” along the side of the plug prevent it from being pushed out of the airframe.

How many of these planes are there?

Of the 200-plus 737 MAX 9 planes Boeing has delivered, 189 have this panel in place of a door, according to FlightRadar24. In the United States, the carriers using that panel are United and Alaska. Other carriers that have grounded planes include Panama's Copa Airlines, Aeromexico and Turkish Airlines. The FAA grounding order relates specifically to 171 planes that are operated by U.S. airlines or fly to U.S. territory. Turkish Airlines, for example, does not fly the planes to the U.S. but it has still grounded them for inspections.

Data from FlightRadar24 shows the majority of the flights carried out by Boeing 737 MAX 9s since Dec. 15 were in the United States. Those configured to use the door plugs were also mostly U.S. based with some flying out of Panama, Mexico and Turkey. The map below gives an indication of the routes likely to be most affected by cancellations as the variants without door plugs continue to fly.