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Alaska Airlines and United Airlines have grounded their Boeing 737 Max 9 aircraft following a safety alert related to an inflight fuselage blowout.

Alaska Airlines had briefly returned 18 of its 737 Max 9 jets to service before receiving a notice from the Federal Aviation Administration (FAA) indicating that “additional work might be needed on those 18 planes,” leading to the grounding. United Airlines also canceled about 180 flights on Sunday, resorting to using alternative aircraft not affected by the grounding.

This precautionary measure stems from an incident involving an Alaska Airlines flight where a panel used for an exit door blew out, causing the aircraft to return to Portland International Airport. “These aircraft have now also been pulled from service until details about possible additional maintenance work are confirmed with the FAA,” said a statement from Alaska Airlines.

Following this incident, the FAA ordered the grounding of 171 Max 9s, including all those operated by Alaska and United, until they could be inspected.

The current situation brings back concerns about the Safety of Boeing’s 737 Max series. Senator Maria Cantwell, chair of the Senate’s Commerce, Science and Transportation Committee, commented on the grounding, stating, “Safety is paramount. Aviation production has to meet a gold standard, including quality control inspections and strong FAA oversight.”