Inspections of grounded Boeing 737 MAX 9 aircraft are already underway at some airlines. But it seems the Alaska Airlines jet that lost a door plug Friday night was just the canary in the coal mine for issues with the plane’s finishes.

United Airlines, the largest operator of the MAX 9 in the U.S., with 79 of the aircraft in its fleet, confirmed in a statement to USA TODAY that its preliminary inspections have identified problems on multiple jets.

“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 statement said.

According to United, it’s still waiting on final approval from the Federal Aviation Administration for its official inspection protocols and canceled about 200 flights as of Monday due to the MAX 9 grounding. The airline said it expects significant cancellations on Tuesday as well and has a flexible rebooking policy in place for affected passengers.

United said it expects each inspection to be performed by a team of five technicians and will take several hours per aircraft. The process will include removing two rows of seats near the door plug, inspecting and verifying that the plug was installed properly, opening the door to check the area and its seal, and then resecuring the plug.

Zach Wichter is a travel reporter for USA TODAY based in New York. You can reach him at zwichter@usatoday.com

This article originally appeared on USA TODAY: United Airlines confirms finding defects in multiple 737 MAX 9 jets