Loose exit panel bolts found in other 737 MAX 9 planes in United's fleet

United Airlines has found loose bolts within the paneled-over exit doors of some of its Boeing 737 MAX 9 planes — the same aircraft model on which a "plugged" exit door panel blew off during an Alaska Airlines flight late Friday.

Chicago-based United, which took its MAX 9 fleet of planes out of service pending inspections required by the FAA, on Monday said that it had found instances “that appear to relate to installation issues in the door plug” involving bolts that “needed additional tightening.”

On Saturday, following the Alaska incident, the FAA grounded the MAX 9, which means about 171 airplanes operating within the U.S. or over U.S. territory have been taken out of service until they can be inspected.

In the United States, Alaska and United operate MAX 9 planes. Alaska grounded all 65 in its fleet. United Airlines now has 79 of the aircraft in its inventory.

The National Transportation Safety Board is leading the investigation into what caused the Alaska door to fly off at 16,000 feet. (Investigators found the door Sunday in a backyard in Portland, Oregon.) The agency's final report on what caused the accident is likely at least a year away, but early attention has focused on what's called a "plug door" — or a paneled-over exit door that is not in operation.

On Sunday night, NTSB Chair Jennifer Homendy again called for airplanes to be equipped with cockpit voice recorders that are capable of retaining more than a few hours at a time. At present, the recorders work on a two-hour loop. Homendy said that in the Alaska flight, the recorder was "completely overwritten."

In November, the FAA announced it would propose a rule that would require new aircraft to be equipped with cockpit voice recorders capable of capturing 25 hours of information. Homendy said it wasn't enough.

“I’m calling on the FAA to change the rulemaking," she said.

The FAA on Monday said it had approved Boeing’s method to comply with its MAX 9 inspection directive, which has been provided to the affected operators of the aircraft.

MAX 9 planes will remain grounded until inspections are completed — which includes extra attention to both the left and right cabin door exit plugs, door components and fasteners, the FAA said. Operators must also complete “corrective action requirements based on findings from the inspections prior to bringing any aircraft back into service,” the agency said.

Meanwhile, United said it canceled 200 of its flights as of Monday that were originally scheduled aboard the MAX 9, adding it “expects significant cancellations” on Tuesday, too.