Shares in Boeing, and the parts supplier Spirit AeroSystems, have fallen amid renewed fears over the safety of the aircraft manufacturer’s 737 Max planes after an Alaska Airlines flight was forced into an emergency landing on Friday. Part of the jet’s fuselage that was ripped out in midair was found on Sunday in the back yard of a house in Portland, Oregon. The plug door – a panel fitted to replace an emergency door in some versions of the 737 Max 9 – tore away from the plane soon after it took off from Portland airport, bound for Ontario, California. US regulators ordered the temporary grounding of all Max 9 models with door plugs pending inspections after Friday’s incident, whose consequences could have been much more severe but for the fact the nearest seats were vacant and passengers were still wearing seatbelts. The hole in the fuselage was described as being “as wide as a refrigerator” and the wind blowing out of it was so powerful it sucked the shirt off a teenage boy sitting nearby, but the plane managed to land safely with no casualties. The US National Transportation Safety Board (NTSB) has launched an investigation and said the discovery of the missing door plug, which fell near a schoolteacher’s house, would be a “key component”. The chair of the NTSB, Jennifer Homendy, said the incident must have been a “terrifying experience”. Investigations will proceed without evidence from the cockpit voice recorder, because it had been overwritten after capturing only two hours of data. Homendy also said there had been three previous automatic alerts in the cockpit on the same Alaska Airlines aircraft over cabin pressure, in its two months in service. The auto pressurisation fail light was triggered on 7 December, 3 January and 4 January, but she said it was unclear if there was any connection between those events and Friday’s near-disaster. The airline had, however, decided to restrict the plane from long flights over water in case the warning reappeared, Homendy said. Indonesia said it was grounding the three Max 9 models operated by Lion Air, despite having a functioning exit door, until further notice. Lion Air suffered the first of two major air crashes that prompted the 737 Max to be withdrawn from service worldwide in 2019, which were eventually blamed on issues with its flight control software. Shares in Boeing dropped 9% when Wall Street opened, before regaining some losses to be down 7% by Monday afternoon. Shares in Spirit AeroSystems were down 14%, though eased later to 8% down. Other aerospace manufacturers also appeared affected by the negative market sentiment on Monday, with Senior, a Boeing supplier, down by 3%. Spirit manufactures the door part at its plant in Wichita, Kansas, but fits them semi-completed, according to a report for Reuters. Boeing will normally remove the plug in the next part of the production process at its factory in Renton, Washington, as it fits out interiors. The process means that investigators will be looking for flaws in assembly at both firms. Boeing said it plans to hold a company-wide virtual meeting on Tuesday to address safety after the incident. The chief executive, Dave Calhoun, told employees in a message on Sunday that its response “is and must be the focus”, adding: “When serious accidents like this occur, it is critical for us to work transparently with our customers and regulators to understand and address the causes of the event, and to ensure they don’t happen again.” Alaska Airlines cancelled a further 60 flights on Monday and said disruption was expected to continue at least until the middle of this week. The biggest operator of the model, United Airlines, has grounded its 79 Max 9s, and cancelled 230 flights on Sunday, 8% of scheduled departures. No European airlines operate the 737 Max 9 in the relevant configuration, according to the EU safety regulator, EASA.