The US Federal Aviation Administration has recommended that airlines inspect door plugs on Boeing 737-900ER jets after a blowout on another type of aircraft with a similar mid-cabin exit earlier this month.
The move comes as regulators heighten scrutiny on Boeing following a scare on an Alaska Airlines plane this month when a panel came off mid-flight and forced an emergency landing.
While there were no fatalities or serious injuries in the January 5 incident, US regulators grounded 171 jets from the 737 MAX 9 fleet with the same configuration as the plane involved in the case.
The aviation regulator said in a statement late Sunday that operators "are encouraged to conduct a visual inspection to ensure the door plug is restrained from any movements" when it comes to the 737-900ER as well.
Boeing's 737-900ER model is an older generation than the MAX family, but according to the FAA it has an "identical door plug design."
"As part of their Safety Management Systems, some operators have conducted additional inspections on the 737-900ER mid-exit door plugs and have noted findings with bolts during the maintenance inspections," the FAA said.
The latest recommendation is for "an added layer of safety," the FAA added.
In its own statement, Boeing said: "We fully support the FAA and our customers in this action."
Last week, Boeing named an independent advisor to lead a "comprehensive quality review" into its system.
The affected door plug panel in the Alaska Airlines flight is used to fill an unneeded emergency exit in planes.
National Transportation Safety Board investigators have suggested that the part was not affixed adequately.
The FAA said it would return the 737-9 MAX to service once safety was established.
Delta and Alaska Airlines did not immediately respond when contacted by AFP on Monday.
United Airlines said: "We started proactive inspections of our Boeing 737-900ER aircraft last week and expect them to be completed in the next few days without disruption to our customers."
The latest 737 MAX 9 incident was the first major in-flight safety issue on a Boeing plane since two fatal 737 MAX crashes -- one in 2018 and one in 2019 -- led to a nearly two-year grounding of the aircraft.
The FAA has since said its probe would extend to Boeing's manufacturing practices and production lines.
Boeing shares ended the day nearly flat while United Airlines slid nearly 1.0 percent before climbing around 7.0 percent in after hours trading, following its report of an encouraging full-year outlook.