Alaska Airlines said on Wednesday (Jan 10) it would cancel all flights on 737 Max 9 jets through Saturday as it continues to wait for regulatory approvals to resume flying after a cabin panel blowout Boeing suggested was caused by a "quality" issue.
The US Federal Aviation Administration (FAA) on Saturday grounded 171 Boeing jets installed with the same panel after the emergency landing, including Alaska Airlines' 65 Max 9s.
The move has forced the cancellation of about 20 per cent of the Seattle-based airline's daily schedule for five consecutive days and 150 flights for Thursday.
United Airlines, the other US 737 Max 9 operator with 79 of the planes in its fleet, said it had cancelled 167 flights on Wednesday and expected "significant" cancellations on Thursday as well.
US Transportation Secretary Pete Buttigieg declined to say on Wednesday when the FAA may allow the planes to resume flights but said it would only be when safe.
"The only consideration on the timeline is safety," Buttigieg told reporters. "Until it is ready, it is not ready. Nobody can or should be rushed in that process."
Boeing CEO Dave Calhoun told CNBC on Wednesday that a "quality escape" was at issue in the Max 9 cabin blowout that left a gaping hole in the plane that had been in service for just eight weeks, but added key questions remained.
"What broke down in our gauntlet of inspections? What broke down in the original work that allowed for that escape to happen?" Calhoun said.
A quality escape was "a description of what people are finding in their inspections... anything that could potentially contribute to an accident," he added.
Boeing on Tuesday told staff the findings were being treated as a "quality control issue" and checks were under way at Boeing and supplier Spirit AeroSystems, Reuters reported previously.
Calhoun said he had spoken to Buttigieg and FAA Administrator Mike Whitaker to ensure there was no repeat of the cabin panel blowout on any Max 9 after Friday's incident.
The grounding has rekindled frustration among airlines over the planemaker's struggle to contain a series of safety and supply crises, industry officials and experts said.
Alaska Airlines and United said on Monday they had found loose parts on multiple grounded aircraft, raising new concerns among industry experts about how Boeing's best-selling jet family is manufactured.
The carriers still need revised inspection and maintenance instructions from Boeing that must be approved by the FAA before they can begin flying the planes again.
"We will only return these aircraft to service when all findings have been fully resolved and meet all FAA and Alaska's stringent standards," Alaska Airlines said.
The International Association of Machinists (IAM), a union representing workers at fuselage maker Spirit and at Boeing's factory in Renton, Washington that produces the 737, has been appointed a party to the US National Transportation Safety Board's (NTSB) investigation, an IAM spokesperson said on Wednesday evening.
The spokesperson did not provide further details. The NTSB confirmed the IAM's involvement.
Spirit's interim CEO Patrick Shanahan was in Seattle over the weekend at Boeing's invitation in a crisis room "listening to all of what we are trying to collect in the production process, taken it back to his team, interrogating their processes, alongside of our people," Calhoun said.
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Spirit said on Wednesday a company team was now supporting the NTSB's investigation directly, adding it remained "focused on the quality of each aircraft structure that leaves our facilities".
The NTSB is focusing in part on whether the recovered cabin panel that blew off had been properly attached.
Buttigieg said he told Calhoun how important it was for Boeing to address the issue in the Max 9 and to ensure every plane it delivered was "100 per cent safe".
Buttigieg said the FAA "will continue with a very, very strict level of oversight" to ensure that.
The transportation secretary said he had also spoken to the United and Alaska Airlines CEOs and they agreed to take care of passengers whose flights have been cancelled because of the grounding.
Buttigieg said the dramatic photos of the gaping hole led him to think what he would have done in that circumstance, which NTSB Chair Jennifer Homendy described as "terrifying".
"I was on a plane a day or two later, with my two-year-old son sitting in the window seat next to me," Buttigieg said. "No passengers should go through what those passengers went through."
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