Explainer: Missing door ‘plug’ may hold vital clues to how gaping hole opened in passenger plane

Investigators on Sunday were searching for the piece of fuselage that a Boeing plane exploded over Oregon on Friday, hoping to obtain physical evidence of what went wrong.

The gaping hole in the side of the Alaska Airlines plane opened up where plane maker Boeing placed a “plug” to cover an emergency exit the airline does not use.

The plugs are found on most Boeing 737 Max 9 planes. The Federal Aviation Administration has temporarily grounded those planes until they undergo inspections of the area around the door plug.

WHY THE PLUG IS THERE

Some larger Boeing 737s have emergency exits in the fuselages behind the wings to meet a federal requirement that planes be designed so that passengers can evacuate within 90 seconds even if half the exits are blocked.

The more passenger seats there are on a plane, the more exits are needed.

Some airlines, including Indonesia’s Lion Air and Corendon Dutch Airlines, cram more than 200 seats into their Max 9s, so they must have extra emergency exits. However, Alaska Airlines and United Airlines configure their 737 Max 9s to have fewer than 180 seats, so the planes do not need the two middle cabin exits to comply with US evacuation rules.

On Alaska and United, the only two U.S. airlines that use the Max 9, those side exits near the rear of the plane are replaced with a permanent plug the size of an exit door.

ARE THEY ONLY IN MAX 9?

No. Boeing also makes larger versions of its 737-900 (a predecessor to the Max) and Max 8 with space for additional exits in the rear. Buyers of those aircraft may also choose to install exit doors or outlets.

WHO INSTALLS THE OUTLETS?

A spokesperson for Spirit AeroSystems, which has no relationship with Spirit Airlines, confirmed to The New York Times that the company installed plugs in the doors of Max 9s, including the plug on the Alaska Airlines plane involved in Friday’s incident. The Seattle Times reported that the door plugs are assembled on 737 fuselages at Spirit’s factory in Wichita, Kansas.

A Spirit AeroSystems spokesman did not respond to questions from The Associated Press. Boeing declined to comment on the matter.

THE BOEING SUPPLIER

Spirit is Boeing’s largest commercial aircraft supplier and makes fuselages and other parts for Boeing Max aircraft. The company has been at the center of several recent problems with the manufacturing quality of both the Max and a larger plane, the Boeing 787 Dreamliner. Last year, Boeing and Spirit AeroSystems discovered improperly drilled retaining holes in a bulkhead that keeps 737 Max planes pressurized at cruising altitude.

THE INVESTIGATION

National Transportation Safety Board officials, led by board Chairwoman Jennifer Homendy, arrived in Portland, Oregon, on Saturday to begin an investigation that will likely last a year or more. Homendy declined to discuss possible causes when she briefed reporters Saturday night.

The NTSB team includes a metallurgist, and Homendy said investigators will examine the exit door plug if they can find it, as well as its hinges and other parts.

According to independent experts, examination of the damage to the door will be crucial to the investigation.

“The nice thing about metal is that it paints a picture, it tells a story,” said Anthony Brickhouse, a professor of accident investigation at Embry-Riddle Aeronautical University in Daytona Beach, Florida. “I’m pretty sure they’ll find the piece that came off and be able to talk scientifically about what happened to cause this failure.”

Brickhouse said exit doors, whether covered or not, are not necessarily a weak point in the fuselage. He had never heard of an exit door plug falling off a plane before Alaska Airlines Flight 1282.

WERE THERE WARNINGS?

Aerospace analysts at investment bank Jefferies wrote that the plane involved in Friday’s incident experienced pressurization problems on two previous flights. The NTSB has not commented on the plane’s history, but Homendy said investigators would examine maintenance records even on such a new plane.

OTHER FUSELAGE EXPLOSIONS

There have been rare cases of holes opening in the fuselages of airliners. In most cases, they have been the result of metal fatigue in the aircraft’s aluminum skin.

In the most horrific case, an Aloha Airlines flight attendant was ejected from the cabin of a Boeing 737 over the Pacific Ocean in 1988 after an 18-foot-long chunk of the roof fell off. Her body was never found. The tragedy led to stricter rules for airlines to inspect and repair microscopic cracks in the fuselage before they opened during flight.

In 2009, a hole opened in the roof of a Southwest Boeing 737 flying at 35,000 feet over West Virginia. And in 2011, another Southwest Boeing 737 suffered a five-foot-long gash, forcing pilots to make an emergency landing at a military base in Arizona. No one was injured in either of those cases, and both were attributed to metal fatigue.

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