United (UA) and Alaska Airlines most affected by 737 MAX’s latest grounding

The US Federal Aviation Administration ordered the temporary grounding of certain Boeing (NYSE:) 737 MAX 9 jets flown by US airlines, after one such aircraft belonging to Alaska Airlines (ALK) lost an emergency exit door midair on Friday.

The agency’s directive affects 171 airframes in total, with United Airlines (UAL) and Alaska Airlines impacted most by the regulation – the companies operate 79 and 65 of the jet types respectively.

Late Saturday, both airlines confirmed that they grounded their fleets of Boeing 737 MAX 9s. United had ~8,000 flights scheduled with the aircraft for January, and Alaska had ~5,000.

While the order only seeks to ground the aircraft type for an inspection that reportedly takes 4-8 hours per plane, Alaska Airlines said it would keep the planes temporarily grounded out of an abundance of caution.

Southwest Airlines (NYSE:), America and world’s largest 737 MAX operator, only flies the smaller version of the jet, the MAX 8, and is not affected by the FAA’s decision.

The incident on Friday saw an Alaskan 737 MAX 9 lose an exit ‘plug’ shortly after takeoff – carriers that don’t have high-density seating configurations, are not required to use the additional emergency exit and choose to shut it down by installing a ‘plug’ instead of an actual emergency exit door.

On Saturday, Reuters reported that the plugs for the jet type are manufactured and installed by Spirit Aerosystems (SPR). The company has not issued a comment on the situation as of the time of this writing.