Data from 40 Boeing 737-9 door plug assembly inspections conducted by Alaska Airlines and United Airlines is in the hands of an FAA team of subject-matter experts tasked with determining whether the work is sufficient to clear grounded aircraft for revenue service.

The 40 checks, done in recent days based on a preliminary set of Boeing instructions, will be reviewed by FAA experts with specific knowledge of the plug door, its potential hazards, and broader maintenance practices, sources with knowledge of the process told Aviation Week.

Neither the FAA nor Boeing will discuss the process or speculate on how long the inspection review will take.

Alaska and United have 65 and 79 door plug-equipped 737-9s on the ground following a Jan. 5 inflight loss of a plug from an Alaska 737-9. Alaska pulled its aircraft immediately after the occurrence, while the FAA ordered the affected fleet grounded via a Jan. 6 emergency airworthiness directive (AD).

Complying with the AD’s order to inspect and correct any issues requires FAA-approved instructions from Boeing. The 40 checks are FAA’s first step in a deliberate process to determine what operators must do to ensure affected 737-9s are safe.

The NTSB is investigating the Jan. 5 occurrence onboard Alaska Flight 1282 in which the left-side mid cabin exit door plug tore free as the 737-9 was climbing out of Portland, Oregon. The crew immediately declared an emergency and returned to Portland. None of the 176 occupants reported serious injuries.

The plug’s separation caused an immediate depressurization of the cabin and significant damage to interior parts, such as seats and seat frames, the NTSB said. No structural damage was reported.

Investigators have not officially linked the Alaska occurrence to problems subsequently found on other aircraft, including loose hardware, during preliminary post-grounding inspections. But statements by the FAA and Boeing suggest supply chain missteps linked to preliminary inspection findings, including non-conforming work at fuselage manufacturer Spirit AeroSystems and gaps in Boeing’s quality assurance process, likely contributed to Flight 1282’s emergency.

The FAA’s review will cover immediate actions needed to ensure the plug assemblies—which cover exit door gaps cut into every 737-9 fuselage that are not needed for lower-capacity seating configurations—meet Boeing’s approved design standards. The agency is also expected to review the door plug design to determine if any changes are needed to mitigate risks spotlighted by Flight 1282.

Broader reviews of 737-9 production and Boeing’s supply chain oversight are also underway. Boeing said it is working to identify any problems linked to the door plug issue or other production quality shortcomings.