The FAA has once again expanded its oversight into Boeing’s (BA) use of door plugs that may be defective, recommending new inspections of a 737 class jet.

“As an added layer of safety, the Federal Aviation Administration (FAA) is recommending that operators of Boeing 737-900ER aircraft visually inspect mid-exit door plugs to ensure the door is properly secured. The Boeing 737-900ER is not part of the newer MAX fleet but has the same door plug design,” the FAA said on its website late Sunday night.

The FAA is recommending airlines conduct the inspection to ensure the door plugs remain secured via two “upper guide track bolts” and two “lower arrestor bolts.” Failure of the plug and its restraining bolts led to the mid-flight door plug blowout of an Alaska Airlines 737-9 Max flight in early January.

The Boeing 737-900ER, the plane referred to in the latest FAA safety memo posted Sunday night, was the last variant of the 737-NG jet, which was the predecessor to the updated 737 Max. Per Cirium (an aviation analytics firm) data via Reuters, there are 490 737-900ER jets in service, with Delta Air Lines (160), United Airlines (136), and Alaska Airlines (79) operating the vast majority of them.

While the 737-900ER will remain in service as the inspections take place, the latest FAA SAFO memo issued late Sunday is another reminder of the safety and reliability issues that affect Boeing and its door plug provider Spirit AeroSystems (SPR). Shares of both companies are trading lower today, continuing their slide since the start of the year.

Following the Jan. 5 incident on the Alaska Airlines flight and subsequent grounding of all 737-9 Max aircraft in the US on the following day, Boeing and the FAA have engaged in a regulatory back-and-forth as Boeing tries to get the Max in service.

On Jan. 9, Boeing’s efforts to get its grounded 737–9 Max back in service hit a snag after the FAA said the company's instructions to airlines on how to inspect the planes were inadequate; Boeing was seen as rushing the process while airlines were finding loose bolts in panels during inspections.

“The safety of the flying public, not speed, will determine the timeline for returning the Boeing 737-9 Max to service,” the FAA said at the time.

The FAA further clarified on Jan. 12 that the 737-9 Max will stay grounded until “extensive inspection” and additional data is provided from Boeing for returning the Max to service. The FAA also increased its oversight of Boeing production and manufacturing and launched an investigation to determine if Boeing failed to ensure completed products conformed to its approved design and in compliance with FAA safety regulations.

On Jan. 16, Boeing named retired Admiral Kirkland Donald of the US Navy as independent adviser to lead a “quality review” of Boeing’s commercial quality management system, including practices at Boeing plants and Boeing’s oversight of “supplier quality.”

The addition of supplier quality foreshadowed more movement by the FAA on that front. A day later, the FAA on Jan. 17 expanded its investigation into the manufacturing practices at Boeing, as well as Spirit AeroSystems. This came after reports emerged that Spirit AeroSystems was allegedly cutting corners in its production process, leading to an “excessive amount of defects.”

After attending a town hall and touring Spirit AeroSystems’ facility in Wichita, Kan., later that day, Boeing CEO Dave Calhoun vowed things were going to change.

“We're going to get better, not because the two of us are talking, but because the engineers at Boeing, the mechanics at Boeing, the inspectors at Boeing, the engineers at Spirit, the mechanics at Spirit, the inspectors at Spirit. They're going to speak the same language on this in every way, shape, or form. We're going to learn from it, and then we're going to apply it to literally everything else we do together,” Calhoun said.

Pras Subramanian is a reporter for Yahoo Finance. You can follow him on Twitter and on Instagram.

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