4 missing key bolts from Boeing 737 MAX 9 led to Alaska Airlines jet mid-air door blowout

Four key bolts securing the door panel of a Boeing 737 MAX 9 Alaska Airlines jet that blew off mid-flight on 5 January were missing. The bombshell new finding by the US investigators comes after a month of the incident that triggered a 19-day emergency grounding of all Max 9s.In a 19-page preliminary report released on Tuesday, the National Transportation Safety Board (NTSB) investigators included observations from a laboratory disassembly of the Alaska Airlines 1282's door plug which fell 16,000 feet into an Oregon backyard.Revelation made in initial probe reportThe initial report into the incident said the lack of damage or deformation around the holes "indicate that the four bolts that prevent upward movement of the (door) plug were missing before the plug moved upward off the stop pads."“Overall, the observed damage patterns and absence of contact damage or deformation around holes associated with the vertical movement arrestor bolts and upper guide track bolts in the upper guide fittings, hinge fittings, and recovered aft lower hinge guide fitting indicate that the four bolts that prevent upward movement of the MED plug were missing before the MED [mid exit door] plug moved upward off the stop pads,” the report said referring to the mid exit door.The report by NTSB also included photos as well as written documents, showing that Boeing employees removed four bolts from these locations during an inspection at the Renton plant in Washington State before the delivery of the aircraft in October last year.

​NTSB investigators examine the door plug from Alaska Airlines flight 1282, a Boeing 737-9 MAX.​ Source: ntsb.gov It is being said the operation was carried out to replace five damaged rivets in the cabin of the aircraft.Boeing 737 MAX 9 Alaska Airlines jet flew with bolts missingThe above findings show the Boeing 737 MAX 9 Alaska Airlines jet flew for a couple of months before the 5 January blowout with the bolts missing.Meanwhile, the investigators continue to determine what documentation was used to authorize the opening and closing of the plug during the rivet repair.The report raises questions about who initially installed the bolts and why the door's opening at Renton to correct the rivets was not properly documented, US aviation safety expert John Cox said."When was the last time those bolts were installed? Did Spirit not install them and then when Boeing opened it the guys didn't realize that they didn't have the bolts? Or did Boeing not install them? That is something that I don't think we have an answer for yet," Cox said.The other photos taken by the investigators after the rivets were replaced show that at least three of the bolts had not been reinstalled.This door plug was used to block an exit that was not intended to be used, as the model already has enough emergency exits in this configuration.On 5 January, the panel flew off mid-flight after the Alaska Airlines MAX 9 with six crew and 171 passengers on board took off from Portland, Oregon bound for Ontario in California.The aircraft returned safely to Portland International Airport and the incident caused only minor injuries.Alaska Airlines reported finding "loose equipment" following its preliminary inspections.United Airlines, which owns the largest fleet of 737 MAX 9s at 79 aircraft, said it had discovered "bolts that needed additional tightening" during checks.Boeing has slowed deliveries after several of its aircraft have experienced a series of problems over the recent past.The NTSB report is based only on the preliminary finding and has not assessed blame or cause of the incident which may come in a final report that could be more than a year away.With inputs from agencies