What went wrong with Boeing’s 737 Max jets?

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The dramatic blowout of a section of the fuselage on an Alaska Airlines flight has put the focus back on Boeing’s 737 Max — the US company’s most popular plane and also its biggest source of revenue in its commercial aircraft business. 

The latest crisis involves the 737 Max 9, a longer version of the most common model, the Max 8 which was at the centre of a controversy after two crashes in 2018 and 2019 killed a combined 346 people.

About 217 Max 9 aircraft are in operation, according to aviation consultancy Cirium. The Federal Aviation Administration, the US regulator, has grounded 171 of these, including United Airlines’ 79-plane Max 9 fleet. 

United and Alaska Airlines on Monday found loose parts on some of the grounded aircraft, threatening to widen the problems for Boeing.

The blowout wiped $12bn off Boeing’s market value on Monday. Whatever the outcome of the investigation, Boeing will have a massive battle to restore passenger confidence in the Max family.

What is the Max?

The Max is the updated version of Boeing’s best-seller, the 737 which for decades was the workhorse of short-haul air travel. The US plane maker launched the model in a rush to catch up with European arch-rival Airbus whose new, more fuel-efficient A320neo, was snaring orders. In its haste to get the aircraft certified and launched by 2017, Boeing cut corners, leading to the Max 8 accidents.

What was behind the Max 8 crashes?

The Max 8 was the biggest crisis in Boeing’s history. Investigations following the crashes revealed the company had concealed design flaws in flight control software from pilots and regulators in a race to get the aircraft certified.

The crisis led to the grounding of the Max fleet in late 2019 for 20 months, costing Boeing billions of dollars. It led to the ousting of its then chief executive and called into question its credibility as a manufacturer. The Max was modified and then comprehensively retested before being recertified and allowed back into the skies. Airlines, including Ryanair, have since placed large new orders for the Max 8.

What happened this time with the Max 9?

A panel, known as a door plug, installed in the fuselage to cover an unused emergency exit blew out on the aircraft less than 10 minutes into the Alaska Airlines flight on Friday night leaving a gaping hole on the side of the aircraft. Investigators have said the consequences could have been much worse if it had happened at cruising altitude of 35,000 feet rather than around 16,000 feet.

The blown out door plug, supplied by Boeing’s former subsidiary, Spirit AeroSystems, is now the focus of the probe. Spirit builds the fuselages and plugs for the Max and then ships them to Boeing. The US plane maker, citing the ongoing investigation, has declined to comment on whether the panels are removed at its facility outside Seattle in order to complete the cabin before being reinstalled during final assembly.

Investigators from the National Transportation Safety Board (NTSB) on Monday said they had not yet recovered the four locking bolts that should have been on the door plug nor yet “determined if they existed there”. The plug on the right hand side of the aircraft remained intact and no discrepancies were found by the NTSB.

Investigators are also examining why pressurisation warning lights on the same aircraft went off three times in the month before the incident, leading the airline to restrict the plane from long flights over water. The incidents were described to the NTSB as “benign” it has said. It is unclear if these were related to the accident.

Why are some airlines still flying the Max 9?

The FAA has only grounded Max 9 models that have plugged their emergency exit doors. Door plugs have been used with the 737 for decades, said John Cox, retired pilot and head of Safety Operating Systems, an aviation consultancy. They also feature on older models of the 737, the 737-900ERs. “There is nothing new about it. It makes sense as [it enables] different configurations for airlines.”

But the incident, Cox said, “brings into focus [Boeing’s] quality assurance process and how robust that is”.

Boeing, which has called a company-wide meeting to take place on Tuesday, has stressed that safety remains its top priority.

What happens next?

While investigators have a clear understanding of what happened, they do not yet know why it happened. The NTSB probe is focused on the Alaska accident but it could still widen.

The length of the grounding remains unclear. Analysts said the incident could yet further delay the certification of newest Max models, the Max 7 and the Max 10.

It could also have wider implications notably in the all-important Chinese market. No Chinese airline has taken delivery on a new Max since the grounding in 2019. “This event is unlikely to help Boeing’s case in China, where the government has yet to approve the delivery of more new Max aircraft for Chinese airlines,” noted Rob Stallard of Vertical Research Partners.