NTSB Identifies Part That Failed on 737 Max 9

With the door plug recovered, the team is heading back to Washington, D.C.

In wrapping up the Portland, Oregon, investigation, the National Transportation Safety Board identified what part failed but doesn’t know why. [Courtesy: NTSB]

The National Transportation Safety Board (NTSB) is wrapping up its on-site investigation into the explosive decompression of a Boeing 737 Max 9 over Portland, Oregon.

On Monday, NTSB Chair Jennifer Homendy thanked the public and local law enforcement for helping find the door plug that flew off Alaska Airlines Flight 1282 shortly after takeoff from Runway 28L on Friday. The aircraft made an emergency landing at Portland International Airport (KPDX).

There was no loss of life, but the 171 passengers, two pilots, and four flight attendants were understandably shaken up, noted Homendy, adding that the flight crew acted heroically in the chaotic situation.

According to preliminary information the NTSB gathered, the explosive decompression was the result of the failure of  a door plug located in Row 26 of the Boeing 737 Max 9. The door plug is a space in the fuselage that allows for an additional emergency exit door to be installed if the customer desires. Alaska Airlines and United Airlines, the two largest U.S. customers for the Boeing 737 Max 9, have opted for fuselages with door plugs. From inside the cabin, you cannot tell there is an option for a door.

When the door plug on the left side of Alaska Airlines flight 1282 blew out, seats 26A and 26B, which were adjacent to the door plug, were empty. Parts of the seats were ripped away, and the remaining seat frames twisted by the force of the decompression. There was damage throughout the cabin but no serious injuries.

The door plug was found over the weekend in a Portland neighborhood west of the airport. Bob Sauer, a high school science teacher, discovered it in his backyard. The NTSB recovered the door Monday. Since it was dark when the incident happened, residents west of KPDX didn't know there were parts of the aircraft falling from the sky until the light of day. When Sauer realized what it was, he sent a photo of it to the NTSB at witness@ntsb.gov. One of his neighbors, meanwhile, found part of a headrest from a seat on her back patio, and two cellphones were found nearby. There were no reports of injuries or damage from the falling objects.

During the Monday NTSB briefing, Clint Crookshanks, an aerospace engineer and part of the agency’s structures team, explained the design of the door plug, stating it is held in place by 12 stop pads on the opening that interface with 12 stop pins on the plug to prevent it from blowing out of the fuselage.

The installation of the door plug requires guide tracks and roller guides to get it into position. There is a hinge on the bottom of the door plug that allows it to be moved out approximately 15 degrees for maintenance. There are four stop bolts designed to keep it from moving upward and disengaging the stops.

"The exam to date has shown that the door did in fact translate upward, [allowing] all 12 stops became disengaged allowing it to blow out of the fuselage," Crookshanks said. "We found that both guide tracks on the plug were fractured. We have not yet recovered the four bolts that restrain it from its vertical movement, and we have not yet determined if they existed there. That will be determined when we take the plug to our lab in Washington, D.C."

The search for evidence continues, said Homendy.

"We are still looking for the bottom hinge fitting and a spring,” she said. “It is a pretty large spring. The fitting is a green circular piece with a hole in it."

The NTSB will put the parts under microscopic examination in an effort to determine why the failure occurred.

Flight Telemetry Tells the Story

Although the cockpit voice recorder (CVR) was overwritten because it was not removed from the aircraft before its two-hour cycle was up, investigators do have information gleaned from the Flight Data Recorder (FDR) for the January 5 flight.

At 17:06:47 p.m., Flight 1282 took off from Runway 28L at KPDX.

At 17:12:33, the recorded cabin pressure dropped from 14.09 to 11.64 PSI. The aircraft was at approximately 14,083 feet at an airspeed of 271 knots. At this time, the cabin altitude greater than 10,000 feet warning was activated.

At 17:12:34, the master caution activated as the cabin pressure dropped to 9.08 PSI. The aircraft was at an altitude of 14,850 feet.

The aircraft reached a maximum altitude of 16,320 feet then began a descent to 10,000 feet and returned to KPDX.

Homendy noted that the agency aircraft systems team is focusing on the cabin pressure control system, as there were three occasions when the auto pressurization light illuminated. She said this computer-run system has a triple redundancy with one primary cabin pressure controller, a secondary controller, and a manual controller. If the primary and secondary computer-run systems fail, the crew activates the system manually to maintain safe cabin pressurization.

"If either one of the computer systems is inoperative, the FAA allows the operator to continue flying the aircraft," Homendy said.

The NTSB has verified the maintenance logs of the aircraft that indicate the redundant system "operated as designed on December 7, January 3, and January 4." The flight crew notified maintenance of the warning light illumination as required, but it has not been determined if the warning lights were "correlated in any way to the expulsion of the door plug and the rapid decompression."

The investigation into the blowout could take months.

In the meantime, the world's 737 Max 9 fleet is grounded, resulting in hundreds of flight cancellations and delays. The FAA is working with operators to develop an inspection process.

Alaska Airlines, the FAA, Boeing, and Spirit Aero, the makers of the fuselage of the 737 Max 9, are working together to identify the issues so that the aircraft can safely return to flight.

Meg Godlewski has been an aviation journalist for more than 24 years and a CFI for more than 20 years. If she is not flying or teaching aviation, she is writing about it. Meg is a founding member of the Pilot Proficiency Center at EAA AirVenture and excels at the application of simulation technology to flatten the learning curve. Follow Meg on Twitter @2Lewski.

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