Bolts are now a key focus of the National Transportation Safety Board (NTSB) investigation into what caused an exit door plug to blow out of an Alaska Airlines Boeing 737 Max plane on Jan. 5.
In a Monday night press conference in Portland, Ore., NTSB chairwoman Jennifer Homendy and accident investigator Clint Crookshanks explained that four bolts -- two on the top of the door plug and two on the bottom -- which could have prevented the door plug from moving vertically into a position where it could blow out of the aircraft, are missing.
"We don't know if they were there or if they came out during the violent, explosive decompression event," Homendy said.
Her remarks came in the hours after United announced that preliminary inspections of grounded Max 9 aircraft had turned up what appear to be loose bolts on some emergency exit door plugs. The plugs are used by some Max 9 operators in place of an exit door that would be required if the plane were configured with a larger number of seats.
Alaska also revealed Monday evening that its technicians had noted visible "loose hardware" on some of its Max 9 planes.
Emergency at 16,000 feet
The FAA grounded approximately 171 Max 9 aircraft, most of them flown by Alaska and United, in the aftermath of the Jan. 5 incident. Alaska flight 1282 had ascended to 16,000 feet several minutes after departure from Portland when the door plug disengaged. The plane was able to return to Portland and land without casualties.
NTSB investigators retrieved the door plug from the garden of Portland-area schoolteacher on Monday morning and began analyzing it. Further analysis will be conducted after the plug is transported to the board's laboratory in Washington.
Crookshanks explained that the plug is held in place by 12 fittings that connect it to the air frame. In addition, the plug is locked in place for operations by sliding it downward and inward, where it is bolted to two bottom hinges and two top roller guides.
In its investigative work Monday, the NTSB determined that the plug was able to disengage from the hinges and move upward before blowing out of the aircraft.
NTSB knows the what but not the why
Homendy said that while the NTSB now knows what happened to the door plug, it doesn't know how or why.
"We never jump to conclusions, and we don't speculate. We deal only with facts, and we will have to look at this aircraft," she said.
Still, Homendy didn't rule out the possibility that Boeing Max aircraft could have broader issues with loose bolts. Late last month, Boeing, in consultation with the FAA, urged 737 Max operators to look for a loose bolt in the rudder control system after an unnamed airline discovered a bolt with a missing nut while performing routine maintenance of a 737.
"Right now, we're focused on this, but we can go broader at any time," Homendy said of the Flight 1282 investigation. "We don't have to finish the investigation to issue an urgent safety recommendation."
She emphasized, however, that fleet grounding decisions are the responsibility of the FAA, not the NTSB.
Boeing has delivered a total of 1,179 737 Max aircraft, but of those, 1,179 are of the Max 8 variant. Only 215 of the newer Max 9 variants has been delivered, a situation that suggests the potential bolt issue could be contained to the Max 9, Bloomberg Intelligence analysts George Ferguson and Melissa Balzano wrote in a Tuesday analysis.