Alaska Airlines and United Airlines canceled hundreds of flights on Monday due to the temporary grounding of some Boeing 737 Max 9 aircraft. 

The Federal Aviation Administration ordered the grounding after a 737 Max 9 operated by Alaska Airlines lost a door plug panel mid-flight on Friday. The aircraft, bound for Ontario, California, returned safely to Portland, Oregon. No serious injuries were reported in the incident, and no passengers were in the seats closest to the blown-out door plug.

Alaska Airlines, which operates 65 737 Max 9 aircraft, canceled 20% of its schedule, or 141 flights, as of 1 p.m. ET Monday, according to flight tracking site FlightAware.

United told FOX Business that the carrier, which has 79 Boeing 737 Max 9s in its fleet, expects to cancel about 200 flights Monday because of the grounding. 

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United has been using other aircraft types to curtail some cancelations when possible. 

"We’re continuing to work with the FAA to clarify the inspection process and requirements for returning all MAX 9 aircraft to service," a United spokesperson said. "We are working with customers to reaccommodate them on other flights and in some cases have been able to avoid cancelations by switching to other aircraft types." 

Alaska and United are the two main U.S. air carriers that operate the Boeing 737 Max 9.

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The FAA said in a statement Monday that the "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." 

The FAA also noted that operators need to complete "corrective action requirements based on findings from the inspections prior to bringing any aircraft back into service."

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Both airlines were forced to cancel upward of 600 flights throughout the weekend, so their Max 9 aircraft could be inspected.

For Alaska, roughly 25,000 guests were impacted on Sunday alone. The airline also projected that "additional significant cancelations" would occur through the first half of the week.

United canceled 180 of 265 flights it had scheduled for Sunday, with 85 saved through the use of other aircraft.

FOX Business' Eric Revell contributed to this report.