A chunk of fuselage that blew out of a Boeing 737 Max 9 aircraft during an Alaska Airlines flight has been found in a teacher’s back yard, the US’s transportation safety board (NTSB) has said. The plug door tore off the left side of the jet on Friday after takeoff from Portland, Oregon, en route to Ontario, California, depressurising the plane and forcing the pilots to turn back and land safely with all 171 passengers and six crew onboard. US regulators ordered the temporary grounding of 171 Boeing 737 Max 9 aircraft after Friday’s incident. The missing plug door was recovered on Sunday by a Portland school teacher identified only as “Bob” in the Cedar Hills neighbourhood, who found it in his back yard, the NTSB chair, Jennifer Homendy, said, adding that 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 incident had 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. 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 of 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 7 December, 3 January and 4 January, but it was unclear if there was any connection between those incidents and the the most recent one. Alaska Airlines made a decision after the warnings to restrict the aircraft from making long flights over water to Hawaii so that 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”. Alaska Airlines added it had an internal policy to restrict aircraft with multiple maintenance write-ups on some systems from long flights over water that was not required by the FAA. The FAA said on Sunday the affected fleet of Boeing Max 9 planes, including those operators by other carriers including United Airlines, would remain grounded until the regulator was satisfied they were safe. The FAA initially said on Saturday the required inspections would take four to eight hours, leading many in the industry to assume the planes could very quickly return to service. But criteria for the checks have yet to be agreed between the FAA and Boeing, meaning airlines have yet to receive detailed instructions, people familiar with the matter said. The FAA must approve Boeing’s inspection criteria before the checks can be completed and planes can resume flights. Alaska said late on Sunday it had still not received instructions from Boeing. Alaska Airlines canceled 170 flights on Sunday and a further 60 on Monday and said travel disruptions were expected to continue at least until the middle of this week. United, which has grounded its 79 Max 9s, cancelled 230 flights on Sunday, 8% of scheduled departures. The accident has put Boeing back under scrutiny as it awaits certification of its smaller Max 7 as well as the larger Max 10, which is needed to compete with a key Airbus model. In 2019, global authorities subjected all Max planes to a wider grounding that lasted 20 months after crashes in Ethiopia and Indonesia linked to poorly designed cockpit software killed a total of 346 people.