Amid statements from airlines that their technicians found loose bolts on the same model of Boeing plane involved in a midair emergency, federal officials indicated an increasing focus on the aircraft's hardware.
In a news conference Monday night, National Transportation Safety Board officials said an examination of a panel that blew out of the plane involved in Friday night's Alaska Airlines flight showed signs of fractured guides and missing bolts, though it's possible the fasteners were lost when the so-called door plug was expelled.
NTSB aerospace engineer Clint Crookshanks said it appeared the door plug moved upward and then out during the explosive decompression event as Alaska Airlines flight 1282 was 10 minutes into its scheduled trip from Portland, Oregon, to Southern California.
The door plug is held in place, in part, by 12 “stop pads” designed to prevent it from blowing outward, but in this case it appeared to slip upward and around the pads.
At least two bolts designed to prevent such vertical movement were missing, as well as two others lower on the panel, NTSB officials said.
"We don’t know if they were there or if they came out during the violent, explosive decompression event," NTSB Chair Jennifer Homendy said of the four missing bolts.
Hinges at the bottom of the panel, designed to open 15 degrees for inspection, were also missing, NTSB officials said. "Guide roller tracks" on the panel, found in a Portland backyard Sunday night by a high school teacher, were fractured, the officials said.
All this may point to crucial weak points that allowed the plug to blow out, but why any hardware may have failed or even gone missing was still a matter of investigation.
Earlier in the day, the two airlines that use the aircraft in question, the Boeing 737 Max 9, reported loose bolts in the area of those plug doors, panels used in place of emergency exits that would be required if the planes were configured to carry more people.
Friday's flight had 171 passengers and six crew members on board, with no major injuries reported after it turned back to Portland International Airport for an emergency landing.
United Airlines said it has been inspecting its fleet of Boeing 737 Max 9 planes to make sure they are safe to fly as federal investigators continued to investigate Friday's near-disaster aboard an Alaska Airlines plane.
Hours later, Alaska said in a statement that reports from its technicians, who had "accessed the area in question" while preparing the fleet for formal inspections, "indicate some loose hardware was visible on some aircraft."
The Federal Aviation Administration on Saturday grounded 171 of the Boeing 737 Max 9 planes worldwide so they could be inspected. On Monday, the agency said the aircraft would remain grounded "until operators complete enhanced inspections" and complete any necessary corrective action.
United Airlines said in its announcement that grounding its fleet of 79 737 Max 9 aircraft led to about 200 flight cancellations Monday, with “significant cancellations” expected Tuesday.
It said switching some flights dependent on the 737 Max 9 to other models will help preserve 30 flights that could have been canceled.
Inspection process
United said it began preparing for inspections Saturday by removing inner panels that lead to the door plugs and then doing visual checks ahead of more formal examination. Five United technicians will be assigned to each inspection, it said.
“To access each door plug, we remove two rows of seats and the sidewall liner,” United said in the statement. “This has already been done on most Max 9s.”
The airline said it will consider installation, hardware, and the structure around the door plugs.
“We’ll then resecure the door, ensuring proper fit and security,” United said. “We’ll document and correct any discrepancies before returning that aircraft to service.”
Alaska Airlines said Monday that it is waiting for final documentation before it can begin the formal inspection process, which will include logging and addressing all findings.
"No aircraft will be returned to service until all of these steps are complete," the airline said in a statement. "The safety of these aircraft is our priority and we will take the time and steps necessary to ensure their airworthiness, in close partnership with the FAA."
Shortly after 5 p.m. on Friday, the Alaska Airlines plane a little higher than 14,000 feet when an explosive sound was heard, the NTSB has said.
Homendy said the flight narrowly avoided tragedy, in part, because the plane was not yet at cruising altitude of 35,000 feet and no one was sitting in the seats next to the door plug. Passengers were still buckled into their seats, two of which experienced such violent force that their frames were torqued and they lost headrests. They were unoccupied.
The NTSB chair said Sunday night that investigators had planned to take a very close look at the door plug that that blew out, its fasteners, and the aircraft structure around it.
Door plug scrutinized
In an interview with NBC News earlier Monday, Homendy said NTSB investigators looked at the door plug on the right or starboard side of the aircraft Monday, seeking to answer major questions.
"How was it put together?" she said. "And how did it perform even when we put some pressure on it? And we really wanted to look at all the components there and see how it related to what we were seeing on the other side."
To the chair's eyes, the door plug that remained intact looked ship-shape, but the one that blew out was being sent to the NTSB's laboratory, where it will be put under a high-resolution microscope and checked out by a metallurgist, she said.
Investigators will check threads on any fasteners, any possible metal-on-metal contact, the state of any sealant used, and any possible deformations or flaws in material, Homendy said during the interview Monday.
"Their whole goal is to really tell a story," the chair said of NTSB investigators. "And that's what the evidence provides — a story of what happened in those moments where the door was expelled."
Pressurization alerts
The 737 Max 9's pressurization system and alerts would also be examined, Homendy said earlier.
On Sunday, the NTSB reported that Alaska Airlines had previously restricted this particular plane from long flights over water, specifically to Hawaii, because an auto pressurization alert light that had illuminated during three prior flights, twice in the days leading up to Friday.
But aviation experts told NBC News on Monday that based on the information provided thus far by federal authorities the light was going off as the result of a computer glitch of some kind and not indicating there was a mechanical problem on the plane.
"It’s not unusual in the aviation world for there to be issues with warning lights and most of the time the issue is with the warning light itself," Jeff Guzzetti, a former NTSB investigator. "It’s not like Alaska Airlines ignored it. The fact that it restricted this plane from making flights over water while they were looking into this warning lights issue points to a robust safety culture."
John Cox, who weighs-in regularly on aviation issues for NBC News, agreed.
"The pressurization system, from what I’ve read, was acting normally," said Cox, who said he flew Boeing 737's for 15 years. "This appears to be more a sensor problem. But Alaska Airlines, being a conservative airline, said this has happened a couple times now and we need to look into, but let’s not do that over the Pacific Ocean."
Homendy said at Monday night's news conference that it does appear the auto pressurization system and its alerts were not involved in Friday night's accident, although she cautioned that the investigation was ongoing.
The system has two backups, and only needed one in each of those cases, she said. The backup system worked successfully, she said, and air pressure was normal on the plane.
"We have no indication whatsoever that this correlated in any way to the expulsion of the door plug and the rapid decompression," Homendy said Monday night.
Manufacturer Boeing said in a statement Monday that it was in touch with domestic airlines that operate 737 Max 9 aircraft and would "help address any and all findings" related to the FAA-mandated inspections.
"We are committed to ensuring every Boeing airplane meets design specifications and the highest safety and quality standards," it said. "We regret the impact this has had on our customers and their passengers."
