A "key missing component" from an Alaska Airlines jet involved in an emergency landing has been recovered from a suburban backyard, the US National Transportation Safety Board says.
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The plug door tore off the left side of the Boeing 737 jet on Friday following takeoff from Portland, Oregon, en route to Ontario, California.
The incident depressurised the plane and forced pilots to turn back and land safely with all 171 passengers and six crew on board.
The Federal Aviation Administration (FAA) on Saturday ordered the temporary grounding of 171 Boeing MAX 9 jets installed with the same panel, which weighs about 27kg and covers an optional exit door mainly used by low-cost airlines.
The missing plug door was recovered on Sunday by a Portland schoolteacher identified only as "Bob" who found it in his backyard, National Transportation Safety Board (NTSB) chair Jennifer Homendy said, saying she was "very relieved" it had been found.
She had earlier told reporters the aircraft part was a "key missing component" to determine why the accident occurred.
"Our structures team will want to look at everything on the door - all of the components on the door to see to look at witness marks, to look at any paint transfer, what shape the door was in when found. That can tell them a lot about what occurred," she said on Sunday.
The force from the loss of the plug door was strong enough to blow open the cockpit door during flight, said Homendy, who said it must have been a "terrifying event" to experience.
"They heard a bang," Homendy said of the pilots, who were interviewed by investigators.
A quick reference laminated checklist flew out the door, while the first officer lost her headset, she said. "Communication was a serious issue ... It was described as chaos."
Homendy said the cockpit voice recorder did not capture any data because it had been overwritten and again called on regulators to mandate retrofitting existing planes with recorders that capture 25 hours of data, up from the two hours required at present.
Homendy said the auto pressurisation fail light illuminated on the same Alaska Airlines aircraft on December 7, January 3 and January 4, but it was unclear if there was any connection between those incidents and the accident.
Alaska Airlines made a decision after the warnings to restrict the aircraft from making long flights over water to Hawaii so it could return quickly to an airport if needed, Homendy said.
The Seattle-based carrier said earlier in a response to questions about the warning lights that aircraft pressurisation system write-ups were typical in commercial aviation operations with large planes.
The airline said "in every case, the write up was fully evaluated and resolved per approved maintenance procedures and in full compliance with all applicable FAA regulations."
Australian Associated Press
