A United Airlines Boeing 737 Max 9 aircraft lands at San Francisco International Airport.Justin Sullivan | Getty ImagesUnited Airlines on Friday joined other U.S. carriers in scrapping Boeing 737 Max plans to return to service until January, cancelling hundreds more flights as the worldwide grounding from two fatal crashes enters its eighth month, forcing airlines to cancel hundreds of flights.United removed the Max from its schedule until Jan. 6. On Wednesday, American Airlines extended cancellations from the plane’s grounding until Jan. 16. Southwest in July said it would remove the plane from its schedules until Jan. 5.Boeing is scrambling to finalize fixes for the planes, which have been grounded worldwide since mid-March after the second of two fatal crashes. The Chicago-based manufacturer has said it expects to gain approval in the fourth quarter but aviation officials haven’t said when they expect to allow the planes to fly again. The prolonged grounding has dented airlines’ revenue and creating a headache for planners and travelers alike.United had expected the planes to return Dec. 19 but the new change means cancellations will continue during the busy Christmas holiday period.The airline expects to cancel 93 flights a day in November and 75 a day in December and plans to swap out planes or use larger aircraft to limit the number of passengers affected.”If we are unable to place them on a different flight, we will proactively reach out to try and offer other options,” United said.Even if the Federal Aviation Administration clears the plane to fly, airlines have said they would need at least a month to retrain their pilots and prepare the planes to resume service.Crash investigators implicated flight-control software that misfired, repeatedly pushing the nose of the planes down in both disasters, which together killed 346 people. Boeing has developed fixes for the system, but regulators haven’t yet approved them.After the first crash, a Lion Air flight that went down shortly after take off on Oct. 29, 2018 with 189 aboard, pilots complained they didn’t know the system even existed.An international panel of air safety regulators are set to criticize the FAA in a report on Friday, saying the agency didn’t adequately review the system, Reuters reported.