Airlines have canceled hundreds of flights since the Federal Aviation Administration ordered carriers to take 737 Max 9 planes out of service for urgent inspections.
Here's what travelers should know:
The FAA grounded more than 170 Boeing 737 Max 9 planes so they can be inspected after a door plug panel blew out on Flight 1282 on Friday. No one was seriously injured on the Ontario, California-bound flight, which returned to Portland, Oregon, when the incident occurred minutes into the flight at about 16,000 feet.
and Alaska Airlines are the biggest operators of the Boeing 737 Max 9, with 79 and 65 of the planes in their fleets, respectively.
United canceled 229 mainline flights on Monday, representing about 8% of its schedule, according to FlightAware. The carrier has been using other types of aircraft, where possible, to avoid cancellations.
Alaska Airlines, which has fewer types of planes in its fleet compared to United, canceled 143 flights, about one-fifth of the day's schedule.
United scrubbed 385 flights over the weekend, while Alaska canceled 328 flights.
Other carriers including Panama's Copa and Aeromexico are affected by the inspection order. Copa has canceled more than 150 flights since Saturday, and Aeromexico has canceled 100 during that time, FlightAware shows.
The more common Boeing 737 Max 8 plane is not affected.
and have put travel waivers in place that allow customers to rebook their flights without paying change fees, or to cancel their flights altogether.
The FAA approved Boeing's inspection instructions for airlines on Monday, a key step in getting the planes flying again. Still, it's not immediately clear how long the inspections will take.
United said on Monday its inspections had on several Max 9 aircraft.
The airline said that the inspections require "a team of five United technicians working for several hours on each aircraft."
Carriers will likely be able to return the planes to service once they comply with the inspections if they meet the standards.
It is not clear what caused the door plug to blow out during the flight. The piece of the aircraft sits where an emergency exit would be on planes that carry more travelers. On planes that are configured for fewer passengers, like United's and Alaska's, a traveler sitting in the cabin wouldn't know that a door-shaped piece is cut in the fuselage.
The National Transportation Safety Board has recovered the door, which was spotted by a schoolteacher in Oregon. Examining the door, its fasteners and other details will be key to the NTSB's investigation into the rare accident, but results of that could take weeks, if not months, to piece through.
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