The US Federal Aviation Administration (FAA) has issued a new recommendation for airlines to check the door plugs on Boeing 737-900ER models after some operators reported issues with bolts during inspections.

The Boeing 737-900ER is separate to the 737 MAX-9 model that was grounded after a door blew out during an Alaska Airlines flight in January, but its fuselage features the same door plugs where an emergency exit would be if a plane had more seats.

The FAA issued the new "safety alert for operators" late on Sunday after some airlines conducted their own checks "and have noted findings with bolts during the maintenance inspections."

It recommended that airlines perform key maintenance procedures related to the four bolts "as soon as possible."

Boeing said in an email to Reuters that "we fully support the FAA and our customers in this action."

Airlines expect no disruptions

According to Boeing, 505 of the 737-900ER models have been delivered worldwide.

Three US airlines operate the vast majority of the fleet: Delta Air Lines, United Airlines and Alaska Airlines.

Delta said it "elected to take proactive measures to inspect our 737-900ER fleet" and does not anticipate any operational impacts.

United also said its inspections are due "to be completed in the next few days without disruption to our customers."

Alaska Airlines said it began inspections several days ago and has no findings to date. It expects "to complete the remainder of our 900ER fleet without disruption to our operations."

zc/wmr (Reuters, dpa)