NTSB Report: Alaska 737-9 Door Plug Bolts Left Behind At Boeing
Analysis by investigators revealed that damage to the mid-exit door (MED) plug and related hardware “indicate that the four bolts that prevent upward movement of the MED plug were missing before the MED plug moved upward off the stop pads,” the report said. This means the bolts did not break during the Jan. 5 flight that included a rapid decompression and required an emergency landing.
More digging determined the MED, which stays bolted in place except during maintenance or non-routine repairs, was not opened from the airplane’s Oct. 31, 2023, delivery to Alaska and the accident flight. This period includes time spent at AAR Corp’s Oklahoma City, Oklahoma, facility where a Wi-Fi antenna was installed.
“The manufacturing/human performance group has done a complete records review from the time the event airplane left the Boeing factory to the time of the accident and found no evidence that the left MED plug was opened after leaving Boeing’s facility,” the NTSB said.
Investigators are focusing on a repair done on Boeing’s factory floor as the period when the bolts were forgotten.
Fuselage and door plug manufacturer Spirit AeroSystems shipped the affected fuselage with several damaged rivets just in front of the left side MED plug that blew out, the NTSB found. The fuselage arrived at Boeing’s Renton, Washington, 737 production facility on Aug. 31. A day later, Boeing flagged the rivet problem and ordered it repaired.
Spirit workers assigned to the 737 factory completed the work on Sept. 19, the NTSB said. But the bolts were apparently never replaced, setting the stage for the Alaska accident.
A Boeing-supplied photo taken before the work started shows the retaining bolts in place. Photos pulled from communications between Boeing “team members” sent just after the rivet fixes were done and included in NTSB’s report show a photo of the plug in the closed position without the bolts.
Investigators don’t know exactly what happened in between or in the weeks leading up to the aircraft’s delivery.
“The investigation continues to determine what manufacturing documents were used to authorize the opening and closing of the left MED plug during the rivet rework,” the NTSB wrote.
The NTSB preliminary report does not analyze the investigators’ findings. It is not clear whether Boeing or Spirit personnel were ultimately responsible for putting the bolts back.
Boeing’s quality assurance process and its FAA-approved safety management system (SMS)—effective enough to detect the original rivet non-conformances—did not flag the missing bolts.
“Whatever final conclusions are reached, Boeing is accountable for what happened,” company CEO Dave Calhoun said in a statement. “An event like this must not happen on an airplane that leaves our factory.”
Investigators are still gathering facts that will help them understand what happened.
“Interviews of Boeing and Spirit AeroSystems’ personnel will be scheduled at a future date,” the NTSB said. “The group will also be looking at Boeing’s SMS and Spirit AeroSystems’ ongoing development of its voluntary SMS program. The group will also assess the FAA’s involvement in the manufacturers’ development of their respective SMS programs and the level of oversight applied to each.”
Fallout from the accident and related quality problems at Boeing and Spirit have both companies under intense scrutiny. The FAA has sent a team to Renton to inspect aircraft and records as part of a wave of new surveillanceand review of 737 MAX production. It also is limiting deliveries of newly built 737s to 38 per month as part of voluntary production-rate freeze.
Boeing has added internal inspections as well as more oversight in Spirit’s Wichita factory as it struggles to get its arms around chronic issues within its walls and those of its most important supplier.
Reported on 6 February 2024 by AviationWeek.