United Airlines has discovered loose bolts on the plug doors of several Boeing 737 Max 9 planes in their fleet.
The Boeing aircraft has come under fresh scrutiny after a panel fell off an Alaska Airlines jet mid-flight last Friday.
More than 170 of the planes have been grounded by the US regulator since the incident, which happened at 16,000ft.
None of the 177 passengers and crew were hurt and the jet landed safely in Portland.
In a statement, United Airlines said: ‘Since we began preliminary inspections on Saturday, we have found instances that appear to relate to installation issues in the door plug – for example, bolts that needed additional tightening.’
Aviation news site The Air Current reported that the issues affected at least five planes in the airline’s fleet.
The outlet said the locations of the loose bolts was not consistent between each of the planes.
United and Alaska are the only two airlines to operate 737 Max 9 jets in the USA.
Nine others from around the world also operate the model, including Turkish Airlines, Icelandair and Air Tanzania.
The door plug that was blown off the Alaska Airlines plane was eventually found in the back garden of a teacher named Bob.
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Investigators will examine the plug, which is 26 by 48 inches and weighs 63 pounds, for signs of how it broke free.
Jennifer Homendy, of the US National Transportation Safety Board (NTSB), said pilots had reported pressurisation warning lights on three earlier flights in the days before Friday’s incident.
The plane involved was not being used for journeys to Hawaii, after Alaska Airlines restricted the aircraft from long flights over water so that the plane ‘could return very quickly to an airport’ if the warning light reappeared.
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