Boeing's troubled 737 MAX planes — which have twice crashed, killing 346 people — have experienced at least six mid-air emergencies and dozens of groundings in the year after an extensive probe cleared them to fly.
The incidents, pulled from US government air safety databases, are among more than 60 mid-flight problems reported by pilots in the 12 months after the US Federal Aviation Administration (FAA) recertified the plane's airworthiness in late 2020.
Former employees of both Boeing and the FAA characterised the reports — which included engine shutdowns and pilots losing partial control of the plane — as serious and with the potential to end in tragedy.
In one incident in December 2021, a United Airlines pilot declared a mayday after the system controlling the pitch and altitude of the plane started malfunctioning.
The US government has also announced a new audit examining oversight of Boeing's 737 production, including the controversial MAX model.
It will focus on how the the FAA identifies and resolves production problems and how it is assessing allegations of undue pressure on employees at the factory.
In an email obtained by ABC Investigations, the US air safety investigator the National Transportation Safety Board (NTSB) said the Inspector-General's office of the US Department of Transportation (DOT) would carry out what it described as "vitally important" work.
"The DOT Inspector-General's office [has] confirmed … that Congress requested an audit of Boeing's production oversight and that the review of the production of the 737 MAX will be a part of this audit," the NTSB email said.
Virgin Australia — which declined to comment on the new data — has ordered four of the same MAX-8 model that has crashed twice, and 25 of a newer MAX-10 model, which has yet to take to the skies.
Both planes in the disasters were less than four months old and all MAX planes are manufactured at Boeing's factory in Seattle.
The first crash was a Lion Air flight that plunged into waters off Indonesia in October 2018.
In March 2019, a MAX jetliner operated by Ethiopian Airlines went down 6 minutes after take-off from the capital, Addis Ababa.
Air crash investigator reports pointed to a malfunction caused by the MAX's flight control software system known as MCAS, in both crashes.
Boeing was charged by the US Department of Justice and paid $US2.5 billion ($3.5 billion) in fines and compensation after it was found to have deceived authorities over the system's complexities and removed references to the MCAS from its pilot training manual.
All MAX planes worldwide were grounded after the second crash as a 20-month safety review was carried out.
But in April last year, five months after they were cleared to fly again, 100 MAX jets were again withdrawn from service after the discovery of an electrical fault in the cockpit that could result in the loss of critical flight functions.
Boeing told the ABC it traced the problem back to a change in production processes at its Utah factory.
Now, an ABC Investigation has unearthed dozens of other mid-flight incidents on MAX planes during the aircraft's first year back in service.
The safety report data was extracted from the FAA Service Difficulty Reporting System as well as anonymous reports submitted to NASA's Aviation Safety Reporting System.
Pilots declared mid-air emergencies at least six times last year — including one United Airlines flight en route to Houston, Texas in October, which was not in the database.
The MAX's flight control system also failed on 22 separate flights, a problem which became the primary focus of the FAA's 20-month recertification effort after the two fatal crashes.
More than 42 incidents involved equipment malfunctions, and on more than 40 occasions, flight crews chose to ground the affected aircraft while problems were fixed.
In one incident on an American Airlines flight in April last year, multiple systems including both autopilot functions stopped working soon after take-off.
On landing, the crew found the backup power unit, considered vital for safe flight, had failed and was emitting a strong electrical smell.
Another report from the NASA system detailed how in December a plane had "multiple system failures" and suddenly lost altitude as the nose of the aircraft pitched downwards and its speed changed rapidly. The crew was unable to provide an explanation.
Some planes also had a multitude of problems. One Alaskan Airlines MAX-9 was grounded seven times over five months due to malfunctions with its navigation or communication equipment.
A Boeing spokeswoman told the ABC, "none of the reports indicate a trend".
"In fact, the in-service reliability of the 737 MAX is consistent with other commercial airplane models," the spokeswoman said.
"Since November 2020, the 737 MAX has flown more than 1.5 million flight hours in more than 580,000 revenue flights. The overwhelming majority of these flights have been conducted without any incident."
Virgin is expected to take delivery of its jets next year.
Australia's Civil Aviation Safety Authority (CASA) told the ABC in a statement it could not respond to incidents reported to the FAA because they fell within the FAA's jurisdiction, unless the incidents involved an Australian aircraft or pilot.
"The FAA addresses unsafe conditions on American-designed aircraft by issuing Airworthiness Directives which are mandatory for those aircraft operating in Australia," CASA said in its statement.
"Any new type and model of aircraft must be issued a type acceptance certificate by CASA before it can operate in Australia.
"Type certificate holders provide airworthiness and safety documentation as part of this process."
Issues 'could lead to a tragedy', says former Boeing employee
Former senior Boeing manager Ed Pierson – who worked at the 737 MAX factory in Seattle between 2015 and 2018 — said many of the incidents across the 28 planes identified in the dataset were "serious in the right circumstances" and "could lead to a tragedy".
"There are a lot of similarities between what we're seeing in some of the reports with what happened during these two crashes," he said.
He testified in 2019 to US Congress about "incredible pressures" faced by workers to meet demands at the Boeing plant and has come forward again because he fears history repeating itself.
The ABC sought independent analysis of the safety reports from former pilots and crash investigators.
They cautioned the data related only to US incidents and may not indicate fleet-wide problems.
Kwasi Adjekum, an assistant professor of aviation at the University of North Dakota and a former air crash investigator, identified seven of the mid-air emergencies as being very serious and said Boeing had cut corners with the design of the MAX and suffered persistent manufacturing problems.
But he said the high number of electrical failures reported on MAX planes could be explained by wiring and components degrading while the aircraft sat in storage for up to 20 months.
"It may be a far stretch to conclude that there are serious system flaws in an aircraft type that traces back to an original equipment manufacturer using localised or limited data such as these. It could also be an issue with the operator's maintenance practices/procedures," he said.
Guido Carim Jr, a senior lecturer in aviation at Griffith University, said some of the incidents in the dataset were not uncommon, such as errors involving communication equipment and anti-ice valves, and pilots were trained to deal with them as they arose.
"Those incidents can become a problem, but those equipment malfunctions are not such a concern for me because they all have redundancies," he said.
However, he said incidents involving flight control and the stabiliser trims, which help control the rotation of the plane, were "very, very serious".
He said some other reports raised by the ABC were bizarre, such as an incident involving a United Airlines flight last August where fuel was found leaking into an overhead passenger bin. Fuel reserves are located in the wings.
"I've never seen that before," Dr Carim Jr said.
Mr Pierson has been working with the families of the crash victims in calling for a more comprehensive investigation into Boeing's troubled production history and its deceptive practices about its safety compliance.
Nadia Milleron and Michael Stumo's 24-year-old daughter Samya was on her way to Uganda to work in public health when she died in the Ethiopian Airlines crash.
The couple have campaigned to ground the planes, believing authorities did not comprehensively examine all the causes of the two crashes.
"There have been numerous emergencies in the air that are documented in the United States … and we don't even know the ones that are in the international realm," Ms Milleron said.
"It's entirely possible based on the pilots' reports that another crash could happen and that would be even more devastating to us."
Mr Pierson, a navy captain of 30 years and retired flight officer, said many of the issues were likely linked to production quality problems he witnessed at the factory.
He first raised fears with senior management that planes were being built with embedded defects before the first crash. His pleas to shut down the factory were ignored.
"These are brand new planes, you can't explain it other than production quality issues," Mr Pierson said.
"Every one of our factory health metrics was hitting the worst record … and it was trending in the wrong direction."
The ABC has spoken with two other former Boeing employees involved in the production of the MAX aircraft. They raised significant concerns around quality assurance of the aircraft but wished to remain anonymous, citing fears of retribution from Boeing.
One mechanic said they observed sub-standard manufacturing and testing of the planes, which resulted in wires being left exposed and debris such as rubbish, metal slivers and washers lodging itself inside various parts of the plane, which could lead to electrical short circuits or fires.
An engineer working with the test flight team told the ABC the crew did not have enough equipment for all the aircraft it was handling and faced schedule pressures to certify the airworthiness of the planes faster.
Boeing's spokeswoman said it encouraged employees to raise safety issues and that it had "well-established" internal controls to remove debris from its aircraft before delivery.
In response to an engineer's claims about schedule pressure, the Boeing spokeswoman said: "We gather and analyse all required data to meet requirements before certification of a new plane."
'A D-grade airplane'
Joe Jacobsen, who has 37 years of experience as an aerospace engineer with the Federal Aviation Administration and Boeing and who has also worked on air crash investigations, found the incidents unnerving.
He was most concerned about the reported incidents involving stabiliser trims, which help with the directional rotation of a plane, including during take-off and landing.
"It's a D-grade airplane", Mr Jacobsen said.
"A poor design combined with manufacturing defects is a recipe for disaster.
"Typical failures on this airplane are more of a concern than typical failures on some higher-grade airplanes [because] it's less resilient to these typical type failures.
"Airspeed anomalies can cause crashes, horizontal stabiliser malfunctions can cause crashes, flight crew alerts that don't notify the pilots in time cause crashes."
Griffith University's Dr Carim Jr reviewed several incidents from last year involving malfunctions with the MAX's stabiliser trim and said failures involving the component were "quite concerning".
"This system is pivotal to keep the aircraft flying," he said, adding the consequences could be "disastrous" if the component failed.
While overseeing the safety certification of the MAX plane for the FAA until April 2021, Mr Jacobsen questioned Boeing's assurances of the plane's airworthiness.
He was alarmed at the number of serious failures still being experienced by the planes, particularly given the scrutiny they faced before they were allowed to take off again.
Over the past two decades, Mr Jacobsen watched the regulator gradually delegate large portions of its independent oversight of safety certification to airline manufacturers like Boeing.
In 2018, according to FAA data, 94 per cent of certification activities to determine compliance for four US aircraft manufacturers — Boeing being the largest — were undertaken by internal divisions within the companies rather than by the regulator.
"It's scary to have airplanes with defects flying, it's scary to have a design that's not what it could be," he said.
The FAA said in a statement that when it put the planes back in service in November 2020, it advised the jetliner would experience "routine in-flight issues".
US air safety investigator ignored production problem warnings
Mr Pierson said America's independent crash investigator, the National Transportation Safety Board (NTSB), should have examined production practices at Boeing's 737 factory as part of its role in assisting the international investigations of the Lion Air and Ethiopian Airlines crashes.
Last month, the agency confirmed to him it never did.
"The NTSB completely violated the most fundamental principle of investigation and that is to follow the evidence," Mr Pierson said.
The Lion Air crash investigations cited poor pilot training and design flaws but found the key factor was a problem with the MAX's MCAS, which pushed the plane's nose down, leaving pilots unable to regain control.
The Lion Air probe found a crucial sensor was incorrectly installed the day before the crash and was responsible for triggering the MCAS malfunction.
The plane's original sensor, which was installed at the Seattle factory, was replaced because it was delivering faulty readings to flight crews.
Testing of the part eight months after the crash showed it was defective.
Mr Pierson said the NTSB never investigated why the original sensor of the Lion Air plane failed nor examined whether the faulty part was a fleet-wide problem.
Boeing said investigators for the two fatal crashes did not find production conditions at the Seattle factory contributed to the tragedies.
"The Lion Air and Ethiopian Airlines accidents have been reviewed by numerous governmental and regulatory entities, and none of those reviews have found that production conditions in the factory contributed to the accidents," a Boeing spokeswoman said.
The spokeswoman said following the crashes, the company had implemented "comprehensive quality and productivity initiatives" and was strengthening the review of its supply chain.
"Safety and quality are Boeing's highest priorities," the spokeswoman said.
"We conduct regular audits internally with suppliers … we proactively and transparently keep the FAA fully aware of our efforts."
An interim report into the Ethiopian crash in March 2019 found the aircraft had no known technical problems before take-off but that the same sensor that malfunctioned in the first crash also failed, leaving pilots unable to control the plane.
Just weeks before the Lion Air tragedy, US carrier Southwest Airlines replaced two of the same sensors because they were malfunctioning.
"I do not believe the Ethiopian Airlines flight 302 crash would have occurred if the NTSB pursued the evidence that was in front of them," Mr Pierson said.
An NTSB spokeswoman said while the agency gave several safety recommendations about the MAX planes to the FAA, it concluded the original faulty sensor was not material to the facts of the Lion Air crash.
"NTSB thoroughly reviewed the information from whistleblowers regarding the production facility and determined that the Department of Transportation's [DOT] Inspector-General was the best office to review," she said.
Design changes have been made to the MAX, training for flight crews has been bolstered and Boeing has been forced to pay out billions in damages.
Ms Milleron believes her daughter Samya would still be alive if Boeing had responded after the first crash five months earlier.
She describes the NTSB's investigation as a "travesty" because the agency did not include production problems in its probe.
"What we're looking at is bureaucrats checking off boxes, covering their butts and not actually doing their job, which is to protect the public in the future," she said.
"This is another moment where we should be waving our hands and saying you shouldn't be allowed to ramp up production," Ms Milleron said.
"We will keep doing it, if it prevents another crash."
Editor's note on 8/7/22: The article was updated to represent new information from the US government about a planned audit related to the Boeing 737 MAX.
