Seven months after an almost-new Boeing Co 737 Max 9 aircraft lost a large fuselage panel during flight, US safety investigators are conducting an extensive hearing to uncover any remaining mysteries surrounding the accident that has plunged the manufacturer into crisis.

The National Transportation Safety Board (NTSB) will get an opportunity to grill executives from the planemaker and supplier Spirit AeroSystems Holdings Inc, which builds the 737 Max fuselages, for 20 hours stretched over Tuesday and Wednesday. While the main outlines are known about what led to the Jan. 5 accident, the session may shed new light on the inner workings at Boeing before the near-disaster exposed quality lapses at its factories.

The hearing is among the most extensive yet for the NTSB and follows months of investigation. Interactions between officials and the company boiled over at times, highlighting how much is at stake for both sides. NTSB chair Jennifer Homendy at one point accused Boeing of not fully cooperating with the probe and later bashed the company for providing what it called misleading information to the public.

Boeing, for its part, is only now starting to emerge from crisis mode. The company announced a new chief executive last week; it has agreed to buy back Spirit to gain greater control of its manufacturing; and the company pleaded guilty to a single conspiracy charge by US prosecutors stemming from two previous 737 Max crashes. Its stock has lost more than a third of its value this year, and the company burned through more than US$1 billion in cash each month as it slowed output.

From the beginning, the door-plug mishap was a unique case. While nobody was seriously injured, the accident attracted huge public interest. The panel was found in a yard shortly afterward, and investigators quickly identified it as missing four bolts to keep it in place. In a twist, an anonymous tipster published a lengthy online account, documenting the breakdowns that led to the Alaska Airlines-operated jet leaving the factory without the bolts.

A preliminary report released by the NTSB corroborated parts of that account. It found evidence suggesting the door plug was removed prior to leaving the facility to fix damaged rivets and then reinstalled without being properly attached. Boeing has said it’s missing formal documentation on the panel’s removal, a serious violation of its manufacturing protocols.

The jet’s fuselage arrived from Spirit with defective rivets on the mid-exit door, Elizabeth Lund, a Boeing senior vice-president of quality who is testifying at the hearing, said during a reporter briefing in June. As it wound through the factory, Boeing and Spirit representatives in the Renton, Washington, factory debated what should be done and who was responsible.

The jet was at the end of the line before the companies agreed on a plan that would require removing the door plug for the repair. But while the rivet work was entered into Boeing’s formal record system, there was no mention of the door removal. Workers who put it back thought they were doing so temporarily while the aircraft was parked outside to protect it from the elements.

In the absence of documentation, the team that prepared the plane for delivery was unaware the bolts were missing and quality inspectors wouldn’t have known to check the workmanship, Lund said at the time.

After that briefing, Boeing received a scathing rebuke from the NTSB, which said the company violated the terms of an agreement that prohibited it from sharing non-public information. As a result, the agency imposed restrictions on the company, revoking its right to ask questions during the two days of hearings or to access investigative information the NTSB produces.

The accident has spurred profound changes at Boeing. The planemaker has overhauled its management team and slowed work to a crawl in its factories as it retrained mechanics and managers, while also encouraging workers to flag quality breakdowns. It stepped up inspections, including at Spirit, in an effort to tackle defects and missing parts that lead to so-called “travelled work” where tasks are completed out of the normal sequence.

While the hearing could turn up the name of the person responsible for the almost-disastrous error, the NTSB has said it’s not focused on casting blame on any one individual. Instaed, it wants to nail down the likely cause of the accident so that it can make recommendations to avoid a repeat.

  - Bloomberg