Loose bolts found on Boeing planes after door plug falls off mid-flight

United Airlines has discovered loose bolts on multiple Boeing 737 MAX 9 aircraft, raising new concerns over the jet model.

US regulators grounded some 171 Max 9s on Saturday after a panel blew off an Alaska Airlines plane mid-flight from Portland, Oregon, forcing pilots to scramble to land the plane safely.

United, one of the country’s largest airlines, is the biggest operator of the model with 79 Max 9s in its fleet.

The airline announced on Monday night that its assessments of the jets had revealed loose bolts in panels that are similar to the part that blew out on the Alaska Airlines flight.