NTSB says recovery of Alaska Airlines plane's failed door plug provides clues

The National Transportation Safety Board announced their preliminary findings late Monday night after recovering the door plug that blew out from an Alaska Airlines plane while it was mid-flight last Friday,

NTSB aeronautical engineer Clint Crookshanks explained that the door plug was used instead of an emergency exit door on some  Boeing 737 Max 9 aircraft used by Alaska Airlines and United Airlines.

Crookshanks says the door plug is usually secured by four bolts and 12 pins. The investigation so far shows those parts failed Friday.

"All 12 stops became disengaged allowing it to blow out of the fuselage. We found that both guide tracks on the plug were fractured. We'd have not yet recovered the four bolts that restrain it from his 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 DC," Crookshanks said.

At SFO and airports nationwide, passengers struggled with hundreds of canceled flights. 

"Our flight actually got delayed 8 hours, flying out to Vegas and it was one of the impacted aircraft," said Sidney Lai, a San Francisco traveler.  "So we had rush to rebook on United so we were able to stand by on the next flight but it was very stressful."

The FAA grounded 171 Max 9s operated by Alaska and United and some flown by foreign airlines for inspection after the Friday night flight. The inspections are focused on plugs used to seal an area set aside for extra emergency doors that are not required on United and Alaska Max 9s.

Monday afternoon, United Airlines said it found loose bolts and other "installation issues" on door plugs that were inspected after the Alaska Airlines incident. Alaska Airlines also says it found some issues with the door plugs during inspections.

Jennifer Homendy, chair of the National Transportation Safety Board, said Monday night that flight data shows the Alaska Airlines plane reached a maximum altitude of more than 16,000 feet before the rapid loss of cabin pressure and swift descent.

Investigators said at the news conference Monday night that the plane had three cabin pressure control systems in place and preliminary data shows so far, that those did not cause the plug problem Friday.

"At this time, we have no indications whatsoever that this correlated in any way to the expulsion of the door plug and the rapid decompression," said Homendy, "Now the NTSB is very thorough. So we will go back and look at the flight data recorder and we will get data on cabin pressure and we're also going to download the memory on the cabin pressure controllers."

The NTSB says warning lights were triggered on three flights, including each of the two days before the brand-new Boeing 737 Max 9 suffered a terrifying fuselage blowout Friday night.

Homendy said maintenance crews checked the plane and cleared it to fly — but the airline decided not to use it for the long route to Hawaii over water so that it "could return very quickly to an airport" if the warning light reappeared.

Friday’s flight was headed from Oregon to Southern California, and made it back to Portland without serious injury to any of the 171 passengers and six crew members. But the decision to allow it to fly over land in the first place struck some aviation experts as illogical.

Homendy says the NTSB interviewed flight attendants on board the Friday flight and that the crew is working with counselors.

"There's a lot of trauma that they are working through. It's going to be a long process. It was terrifying," Homendy said.

Investigators thanked the community for helping to recover the plug, which was found by a citizen early Monday morning.

The NTSB has requested that Boeing send a specialist to the scene Tuesday to assist with the investigation.