The FAA has recommended that Boeing 737-900ER operators inspect the planes' exit door plugs to make sure they are properly secured.
The FAA made the recommendation "as an added safety layer," while 171 Boeing 737 Max 9 aircraft remain grounded in the wake of the Jan. 5 door plug blowout on an Alaska Airlines flight.
Delta, United and Alaska are the primary U.S. operators of the 737-900ER. The aircraft is not part of Boeing's current generation 737 Max fleet but has the same door plug design. Delta has 163 737-900ERs, according to Planespotters.net. United has 136, the carrier said. Alaska has 79, according to its website.
Specifically, the FAA has asked airlines to make sure mid-exit door plugs are restrained from movement by two upper bolts and two lower bolts.
The recovered Max 9 door plug that blew out of Alaska Flight 1282 over the Portland area was missing all four of its bolts. The National Transportation Safety Board is investigating what happened to the bolts.
In statements, United and Alaska said that they proactively began inspections of their 737-900ERs last week. Neither airline expects operational disruptions.
Travel Weekly has also reached out to Delta for comment.