The increasingly likely scenario, according to some of these people, is based partly on an apparent absence of markings on the Alaska door plug itself that would suggest bolts were in place when it blew off the jet around 16,000 feet over Oregon on Jan. 5.
They also pointed to paperwork and process lapses at Boeing’s Renton, Wash., factory related to the company’s work on the plug door.
The National Transportation Safety Board has been conducting metallurgical analysis of the plug door but hasn’t released the results of the testing. Laboratory tests might show whether the bolts were in place or not there at all. An update in the NTSB probe is expected as soon as this week.
New evidence could later emerge before accident investigators reach final conclusions. It couldn’t be determined how many people were involved with work on the plug door at Boeing’s 737 factory.
Supplier Spirit AeroSystems delivered the 737 fuselage to Boeing’s factory with the door plug installed. The plug door itself was constructed in Spirit’s Malaysian factory, while the fuselage was assembled in Wichita, Kan.
Boeing opened or removed the door plug after the 737 MAX 9 jet’s fuselage arrived at the plane maker’s Renton, Wash., factory for final assembly, The Wall Street Journal and other news outlets have reported.
Other 737 MAX 9s could have the same problem if the theory pans out.
The FAA said last week that it will not grant Boeing any production expansion of the MAX, including the 737-9 MAX. The agency also approved a thorough inspection and maintenance process that must be performed on each of the grounded 171 MAX 9 jets before being eligible to return to service.
Alaska Airlines resumed service of its 737 MAX 9 fleet on Friday with a flight from Seattle to San Diego after completing the final inspections on its first group of 737 Max 9 aircraft. Inspections on its remaining aircraft are expected to be completed by the end of this week.
The FAA is still investigating Boeing as well as its manufacturing practices and production lines, including those involving subcontractor Spirit AeroSystems, which makes fuselages for Boeing.