United Airlines conducted checks on all its Boeing 737 Max 9 aircraft (Picture: AFP via Getty Images)

United Airlines has discovered loose bolts on the plug doors of several 737 Max 9 planes in their fleet.

The Boeing aircraft has come under particular 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.