Operators of Boeing 737-900ER planes should "visually inspect mid-exit door plugs to ensure the door is properly secured," the U.S. Federal Aviation Administration advised late Sunday.

Why it matters: The Boeing 737-900ER is not part of the newer 737 MAX 9 fleet that the FAA grounded in U.S. territory earlier this month following a mid-air incident, but the agency noted it "has the same door plug design." It's recommending the inspections as "an added layer of safety."

Between the lines: The 737 MAX-9 aircraft had a low number of flights when a "plugged" emergency exit door flew off mid-air on Alaska Airlines Flight 1282 earlier this month, per industry officials.

• However, there has never been an issue with the older-generation Boeing 737-900ER jets.
• That's after more than 11 million hours of operation and 3.9 million flight cycles, according to the latest information on the 900ERs. The planes that are up to 17 years old typically undergo safety checks every two years.

What they're saying: An Alaska Airlines spokesperson said in an emailed statement just after midnight Monday that the company had begun inspections of 900ERs days ago and had "no findings" to report to date "and expect to complete the remainder of our -900ER fleet without disruption to our operations."

• A United Airlines spokesperson said in an emailed statement early Monday that the company "started proactive inspections of our Boeing 737-900ER aircraft earlier this week and expect them to be completed in the next few days without disruption to our customers."
• A spokesperson for Boeing said in an emailed statement Sunday night: "We fully support the FAA and our customers in this action."

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Editor's note: This article has been updated with new details throughout.