United Airlines says inspections found loose bolts on its 737 MAX planes

United Airlines said Monday it had discovered loose bolts on Boeing 737 MAX 9 planes in its fleet, following preliminary inspections after an Alaska Airlines mid-flight incident. 

United has "found instances that appear to relate to installation issues in the door plug, for example, bolts that needed additional tightening," the carrier said. 

On Friday, an Alaska Airlines passenger plane made an emergency landing after the door plug component blew out.

"We're working to return our Boeing 737 MAX 9s to service in the days ahead," United said in a statement.

United has canceled 200 MAX 9 flights since the incident and expects "significant cancelations" on Tuesday, the carrier said.

A door plug is a cover panel used to fill an unneeded emergency exit in planes with smaller seat configurations.

Following the Alaska Airlines incident, the Federal Aviation Administration (FAA) ordered operators to ground 171 jets with the same configuration.

U.S. investigators continue to probe Friday's incident, but analysts have posited that the fault may stem from a manufacturing or quality control defect.

With 79 MAX 9 planes, United has the largest fleet of the aircraft in question.

On Monday, the FAA announced that it approved a roadmap for carriers to complete inspections that include both left and right door plugs, components and fasteners.

However, United said it was still awaiting final approval on the full inspection process.

In December, Boeing urged airlines to undertake additional inspections to check for loose hardware on plane rudder control system after an international operator discovered a bolt with a missing nut while performing routine maintenance.