Discovery of loose bolts threatens wider 737 Max crisis for Boeing

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United Airlines has found loose bolts on some plugged doors of its grounded 737 Maxes, threatening to widen Boeing’s problems as it expands public scrutiny beyond the single plane that suffered a mid-air blowout on Friday.

The Chicago carrier said on Monday that inspections of its 737 Max 9s, a variant of the single-aisle jet that contains more seats than the more popular Max 8, “found instances that appear to relate to installation issues in the door plug — for example, bolts that needed additional tightening”.

The airline said its technical operations team would fix the problem “to safely return the aircraft to service”.

News of United’s discovery, which was first reported by the trade publication The Air Current, hit Boeing’s shares further. Its stock closed down 8 per cent on Monday at $229 while shares in Spirit AeroSystems, its biggest supplier, lost 11 per cent to close at $28.20.

The discovery comes after a Max 9 operated by Alaska Airlines, flying from Portland to California, lost a chunk of its fuselage at 16,000 feet. There were 171 passengers and six crew members aboard, but no one was seriously injured.

On Saturday the US Federal Aviation Administration grounded all Max 9s configured with a plugged, or permanently shut, door. Carriers with denser seating configurations use the doors, while those with fewer seats seal them off.

The National Transportation Safety Board, which is investigating the incident, located the door from the Alaska Airlines flight on Monday in a Portland suburb.

United has 79 Max 9s with this configuration, out of about 215 in operation worldwide, according to aviation data provider Cirium. That is more than Alaska’s 65, or the combined total of 52 at Copa Airlines, Aeroméxico and Icelandair.

United has cancelled 200 flights on the Max 9, a little less than 8 per cent of its flights on Monday,…