Explainer: How is the panel that blew off a plane made, and how did it break?

The fuselage plug area of Alaska Airlines Flight 1282 Boeing 737 Max 9, which was forced to make an emergency landing with a gap in the fuselage, is seen during its investigation by the National Transportation Safety Board in Portland, Oregon | NTSB/Handout via Reuters

Washington (Reuters) – Investigators have recovered a piece of fuselage that tore off the left side of an Alaska Airlines-operated Boeing 737 Max 9 jet shortly after taking off from Portland, Oregon, on Friday that forced a dramatic emergency landing.

The panel is a plug put in place on some 737 Max 9s instead of an additional emergency exit, and regulators have grounded 171 planes so airlines can conduct inspections of those crafts. At least two air carriers have already found that bolts used to secure that panel were loosened. Here is an explainer on that panel.

What happened?

As Alaska Air Flight 1282 reached just over 16,000 feet on Friday, the panel tore off from the side of the jet, leaving a refrigerator-sized, rectangular hole in the aircraft. The door plug was discovered by a Portland school teacher who found it in his backyard.

Who makes this panel?

The fuselage for the Boeing 737 is made by Kansas-based Spirit AeroSystems, which separated from Boeing in 2005. Spirit is one of two suppliers that makes the plug doors on the Max 9, but Boeing also has a key role in the plug installation process.

Why is this panel there?

The 737 MAX 9, currently Boeing’s largest single-aisle plane, can seat up to 220 people. It includes an optional extra door to allow for the approved number of evacuation paths whenever carriers opt to install the maximum number of 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.

Certain models of Airbus jetliners also feature door plugs, but the European planemaker uses a different design, people close to the company said. Details of the design were not immediately available.

How is the door plug held in place?

A diagram of the 737 MAX 9 door plug posted by the NTSB on Sunday shows the plug held down by four bolts: two in the upper corners and two lower hinge brackets. The diagram also shows two straps and two bonding jumpers near the bolts.

The plug is further secured by “stop fittings” at 12 different locations along the side of the plug and the door frame.

The stop fittings hold the door plug in place, preventing it from being pushed out of the airframe, according to US National Transportation Safety Board head Jennifer Homendy.

The plug measures 26 x 48 inches and weighs 63 pounds, Homendy said.

What broke and how did it fall off?

The NTSB has not yet recovered the four bolts that keep the door from vertical movement, and it has “not yet determined if they existed there,” NTSB engineer Clint Crookshanks told reporters at a briefing in Portland.

All 12 of the “stop fittings” became disengaged during the event, and the guide tracks on the plug were fractured as well, he said.

“The door translated up and disengaged from the stops, which then fractured the fittings,” Crookshanks said.

The NTSB said it could not yet determine whether the door plug had been properly attached but will be able to establish whether the bolts were present after examining markings on the door plug.

How complex is the production process?

The installation is a two-tier process involving both manufacturing at Spirit’s giant fuselage plant in Wichita, Kansas, and a Boeing factory outside Seattle, the sources said.

As part of the production process, Spirit builds fuselages for 737s and sends them by train with the special door assembly “semi-rigged,” according to a source familiar with production.

At its Renton, Washington, plant, Boeing typically removes the pop-out door and uses the gap to load interiors. Then, the part is put back and the installation is completed. Finally, the hull is pressurized to 150 percent to make sure everything is working correctly.

The process means that finding out where any flaw was introduced during assembly may not be clear, sources told Reuters.

How many Boeing 737 Max 9 planes are there?

Of the 200-plus 737 Max 9 planes Boeing has delivered, 171 have this panel in place of a door. In the United States, United has grounded 79 planes, and Alaska has 65. Other carriers that have grounded planes include Panama’s Copa Airlines, Aeromexico, and Turkish Airlines.

(Reporting by Valerie Insinna, David Gaffen and Tim Hepher; Editing by Mark Porter)