5 questions about the investigation of the Alaska Airlines plane and the previous alert that anticipated a problem on the Boeing 737 MAX 9

Examining the damage to the door will be crucial to the investigation. (National Transportation Safety Board via AP)

Investigators said Sunday that they had found the Piece of fuselage that flew from a Boeing plane over Oregon Friday, and they expected him to provide physical evidence of what went wrong.

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

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

WHY IS THE PLUG THERE

Alaska Airlines - Accident - Door
Alaska Airlines configures its 737 Max 9s to have fewer than 180 seats, so the planes do not need the two emergency exits in the center of the cabin to comply with US evacuation rules. (National Transportation Safety Board via AP)

Some large Boeing 737s have emergency exits on the fuselages behind the wings to meet a federal requirement that planes be designed to passengers can evacuate in 90 secondseven if half of the exits are blocked.

The more seats a plane has, the more departures are needed.

Some companies, such as the Indonesian Lion Air and Corendon Dutch Airlinesput more than 200 seats in their Max 9, so they must have additional emergency exits. However, Alaska Airlines and United Airlines configure their 737 Max 9 to have less than 180 seatsso airplanes do not need the two exits in the center of the cabin to comply with US evacuation regulations.

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

ARE THEY ONLY FOR THE MAX 9?

Alaska Airlines - Accident - Door
An Alaska Airlines Boeing 737-9 Max flies over Paine Field, near the Boeing manufacturing plant in Everett. (AP Photo/Ted S. Warren, File) (Ted S. Warren/)

Boeing also manufactures larger versions of their 737-900 -a predecessor to the Max- and the Max 8 with space for additional outlets at the rear. Buyers of these aircraft may also choose to install exit doors or plugs.

WHO INSTALLS THE PLUGS?

A spokesperson for Spirit AeroSystems -which is not related to Spirit Airlines- confirmed to The New York Times that the company was installing plugs in the doors of Max 9s, including the plug on the Alaska Airlines plane involved in Friday’s incident. The spokesperson told The Associated Press that the plugs are assembled on 737 fuselages at Spirit’s factory in Wichita, Kansas, but declined to comment further.

Boeing declined to comment on the matter.

THE BOEING SUPPLIER

A worker walks past Boeing's 737 MAX-9 under construction at its production facility in Renton, Washington, USA.  February 13, 2017. (REUTERS/Jason Redmond)
A worker walks past Boeing’s 737 MAX-9 under construction at its production facility in Renton, Washington, USA. February 13, 2017. (REUTERS/Jason Redmond) (JASON REDMOND/)

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

THE INVESTIGATION

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National Transportation Safety Board Chairwoman Jennifer Homendy (AP Photo/Claire Rush) (Claire Rush/)

Officials of the National Transportation Safety Board, led by its president, Jennifer Homendyarrived in Portland (Oregon) on Saturday to begin a investigation that will probably last a year or more. Homendy declined to discuss possible causes when he briefed reporters Saturday night.

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

Alaska Airlines - Accident - Door
A resident of Beaverton, Oregon, shows off the 14 miles he biked through his neighborhood and around southwest Portland in search of fuselage parts from a Boeing 737 Max 9. (AP Photo/Claire Rush) (Claire Rush/)

Examination of the damage to the door will be crucial to the investigationaccording to independent experts.

“The good thing about metal is that it paints a picture, it tells a story”said Anthony Brickhouseprofessor of accident investigation at Embry-Riddle Aeronautical University in Daytona Beach, Florida.

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

WERE THERE WARNINGS?

The Boeing plane was not used for flights to Hawaii after a Warning light which could have indicated a pressurization problem came on on three different flightsHomendy said Sunday.

Alaska Airlines decided to restrict use of the plane on long flights over water so that if the warning light appeared again, the plane “could return very quickly to an airport”he said at a press conference.

Homendy warned that the Pressurization light may not be related to Friday incident and that it was necessary to continue investigating.

OTHER FUSELAGE BURST

On rare occasions, holes have occurred in the fuselages of passenger aircraft. In most cases, they have been the result of metal fatigue in the aircraft’s aluminum skin.

In the most terrible case, An Aloha Airlines flight attendant was ejected from the cockpit of a Boeing 737 over the Pacific Ocean in 1988 after a 5.4-meter-long piece of the roof fell off. Her body was never found. The tragedy led to a tightening of rules requiring airlines to inspect and repair microscopic cracks in the fuselage before they open during flight.

In 2009, a hole opened in the roof of a Boeing 737 southwest which was flying at 35,000 feet over West Virginia.

In 2011, another Southwest Boeing 737 suffered a 5-foot gash that forced pilots to make an emergency landing at a military base in Arizona. No one was injured in either of those cases, both attributed to metal fatigue.

Source-www.infobae.com