United Airlines has found bolts in need of “additional tightening” during inspections of Boeing 737 Max 9s.

It comes after the Federal Aviation Administration (FAA) grounded 171 Boeing 737 Max 9 aircraft following an incident on an Alaska Airlines flight Friday evening, where a section of the fuselage flew off mid-flight.

“We’re working to return our Boeing 737 MAX 9s to service in the days ahead. As of Monday, service on that aircraft remains suspended and we have canceled 200 MAX 9 flights. We expect significant cancellations on Tuesday as well. We have been able to operate some planned flights by switching to other aircraft types, avoiding about 30 cancellations each on Monday and Tuesday,” United said in a statement on Monday.

“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. These findings will be remedied by our Tech Ops team to safely return the aircraft to service,” the United spokesperson added.

United Airlines has 79 737-9 aircraft in its fleet and is awaiting final approval on the full inspection process of its aircraft before they return to active service.

READ: FAA grounds 171 Boeing 737 Max 9s

“As operators conduct the required inspections, we are staying in close contact with them and will help address any and all findings. We are committed to ensuring every Boeing airplane meets design specifications and the highest safety and quality standards. We regret the impact this has had on our customers and their passengers,” Boeing said after United’s findings.

“The FAA’s priority is always keeping Americans safe. In that spirit, Boeing 737-9 aircraft will remain grounded until operators complete enhanced inspections which include both left and right cabin door exit plugs, door components, and fasteners. Operators must also complete corrective action requirements based on findings from the inspections prior to bringing any aircraft back into service,” the FAA stated on Monday.

Alaska Airlines Flight 1282 was forced to return to Portland Oregon shortly after takeoff when the door plug flew off, resulting in the rapid depressurisation of the cabin.

The aircraft reached 16,000ft before beginning its emergency descent. Footage online showed the gap in the side of the aircraft, with oxygen masks deployed.