The U.S. Federal Aviation Administration (FAA) has recommended airlines operating Boeing 737-900ER jets inspect door plugs to ensure they are properly secured after some operators reported unspecified issues with bolts upon inspections.

The recommendation issued late on Sunday follows the FAA’s grounding of 171 Boeing 737 MAX 9 planes after the Jan. 5 mid-air cabin blowout of a door plug on an eight-week-old Alaska Airlines MAX 9 jet.

Boeing’s BA.N shares fell 2.5 percent in premarket trading on Monday. They have fallen 17.5 percent since the beginning of the year.

The 737-900ER is not part of the newer MAX fleet but has the same optional door plug design that allows for the addition of an extra emergency exit door when carriers opt to install more seats.

There are 490 Boeing 737-900ER jets in service, at least 79 of which have an active door rather than the plug because they are operated by low-cost airlines with denser cabins, according to Cirium data.

In its new “Safety Alert for Operators,” the FAA said that some airlines had conducted additional inspections on the 737-900ER mid-exit door plugs and had noted “findings with bolts during the maintenance inspections.”

It recommended carriers perform key portions of a fuselage plug assembly maintenance procedure related to the four bolts used to secure the door plug to the airframe “as soon as possible.”

A Boeing spokesperson said: “We fully support the FAA and our customers in this action.”

Boeing first delivered the 737-900ER in 2007 and last one in 2019.

Alaska Airlines and United Airlines UAL.O, the only two U.S. carriers that use the MAX 9, this month said they had found loose parts on multiple grounded MAX 9 aircraft during preliminary checks. They have had to cancel thousands of flights this month because of the grounding.

The FAA on Sunday said that MAX 9 planes will remain grounded until it “is satisfied they are safe to return to service.”

United said that it was extending the cancellation of its MAX 9 flights through Jan 26. Alaska, which flies MAX 9 planes accounting for 20 percent of its fleet, previously canceled all flights through Sunday. The airline did not immediately comment on how long it planned to extend cancellations.

LITTLE 900ER DISRUPTION SEEN

In contrast to the MAX 9 that experienced the door-plug issue – a new plane with a low number of flights – Boeing 737-900ER aircraft have more than 11 million hours of operation and 3.9 million flight cycles.

The FAA said the door plug “has not been an issue with this model.”

Both United and Alaska said they had begun inspections of the door plugs on their 737-900ER fleets.

United, which has 136 737-900ER aircraft, expects the inspections “to be completed in the next few days without disruption to our customers.”

Alaska said its inspections began several days ago without any findings so far. It expects to complete the remainder of its inspections without disruption.

Delta Air Lines DAL.N, the largest operator of the 900ER, said it had opted to take “proactive measures” to inspect its fleet. It does not expect any operational impact.

A spokesperson for Korean Air, which has six 737-900ERs, said it planned to carry out additional inspections and finish them within 30 days without any schedule disruption.

Globally, the three U.S. carriers operate the vast majority of the 737-900ERs with the door plugs.

On Wednesday the FAA said inspections of an initial group of 40 Boeing 737 MAX 9 jets had been completed, a key hurdle to ending the grounding of the model. The FAA is continuing to review data from those inspections before deciding when the planes can resume flights.

FAA Administrator Mike Whitaker told Reuters this month that the FAA was “going through a process to work out how to restore confidence in the integrity of these plug doors.”

National Transportation Safety Board Chair Jennifer Homendy last week said that the investigative agency would be looking at numerous records related to the door plug. She said it is unclear if the bolts on the Alaska Airlines jet were properly secured or if they were actually installed at all.

(Reporting by David ShepardsonAdditional reporting by Lisa Barrington, Tim Hepher and Abhijith GanapavaramEditing by Edwina Gibbs and David Goodman)