Aviation safety experts increasingly see only a few plausible reasons why a China Eastern Airlines Boeing 737-800 plunged into the ground on 21 March, killing all 132 people aboard.
Details about the crash remain sparse, but the information available seems to suggest either that the jet suffered some type of incredibly unusual flight-control problem, or that one of its pilots put the 737 into a dive.
The Civil Aviation Administration of China (CAAC), which is leading the investigation, has not yet said what it thinks caused the crash.
Although aviation safety experts caution against drawing conclusions based on incomplete information they see a limited number of possible causes.
“That kind of vertical dive, without a radio call of any kind from the flightcrew, could clearly indicate a human activity to make that happen,” says aviation safety consultant and former National Transportation Safety Board (NTSB) member John Goglia.
By “human activity”, he means a pilot deliberately commanding the jet to dive. “Nobody can come up with a mechanical failure mode that would make the airplane behave the way it did,” Goglia adds.
That theory had been circulating virtually since the narrowbody came down, but China’s release of an initial accident report on 20 April included no details to dispel it.
Aviation safety expert John Cox, who works at consultancy Safety Operating Systems, says “it is entirely possible that it is a deliberate act… I would not take it off the table”.
However, Cox, a former airline pilot who flew 737s, speculates that some unusual type of mechanical issue, or perhaps inaccurate flight data, could also be the cause.
“Could it have been an autopilot-induced trim runaway? Theoretically, I can make that case,” Cox says. “But this airplane doesn’t have a history [of] doing anything like that.”
The 737-800, part of the 737NG family, is among the most common airliners in operation, with Boeing having delivered more than 5,100 of the variant since 1998, according to the airframer’s data. Of those, nearly 4,300 remain in service, according to Cirium.
In addition, 737NGs are not equipped with the Maneuvering Characteristics Augmentation System – the troubled flight-control system implicated in two crashes of the newer 737 Max.
China Eastern flight MU5735 took off on 21 March from Kunming, heading east and climbing to a cruise altitude of 29,100ft.
Suddenly, about 1h after taking off, it dived steeply, at one point descending at more than 30,000ft per minute, according to publicly available ADS-B data from aircraft tracking site Flightradar24.com. However, investigators have not verified the ADS-B data.
“It simply nosed over and came down damn near vertically,” says Goglia.
Airliners do not “come straight down” unless someone in the cockpit commands the aircraft to do so, he says, and even then, that person must keep pressure on the controls or the jet will start levelling off. “In other words, you have to make the airplane go straight down.”
Goglia says an explosion or rapid decompression would likely have caused the jet to break apart. Closed-circuit video purporting showing the final seconds of the flight depicts an aircraft, seemingly largely intact, in a vertical dive.
But Cox warns against placing too much confidence in the descent rates as derived from the ADS-B numbers; in particular he highlights a sequence in the data which appears to show the aircraft stopping its dive at 7,425ft, and then climbing, to 8,600ft, before pitching down again and continuing its descent.
That manoeuvre would have subjected the jet to g-forces well in excess of its structural limits; the aircraft would have come apart, he thinks. “The numbers aren’t reasonable”, says Cox.
Still, data does suggest an extreme rate of descent and safety experts say only a handful of other crashes involved comparable descent rates, two of which resulted from deliberate pilot action.
They include the October 1999 crash into the Atlantic Ocean of EgyptAir flight 990, a 767-300ER that plummeted at a descent rate reaching 39,000ft per minute. Before crashing, that aircraft also briefly recovered some altitude.
Though disputed by Egypt, the NTSB concluded the first officer caused the crash by pushing the jet into a dive and cutting its engines. It said flight data indicated a struggle in the cockpit.
The NTSB also attributed the rapid descent and crash of 737-300 operated by Singaporean carrier SilkAir in December 1997 to “intentional pilot action” – specifically, “sustained manual nose-down flight-control inputs”, according to a letter from the investigation agency.
That aircraft came down in Sumatra at a rate that neared 39,000ft per minute, according to Indonesia’s accident report. Indonesian investigators said they found insufficient evidence to determine the cause.
Goglia says the China Eastern accident appears “amazingly similar” to the EgyptAir and SilkAir crashes.
But events involving such extreme descent rates do not always result from deliberate pilot action.
Cox cites the rapid descent and crash in Indonesia in January 2007 of an Adam Air 737-400, which dived from 35,000ft at a descent rate that reached 53,760ft per minute, according to Indonesian investigators. They concluded the jet’s pilots, preoccupied with troubleshooting an inertial reference system problem, responded improperly after the autopilot disengaged. The jet banked steeply and the pilots were unable to recover.
Cox also cites two similar 737 crashes caused by a rudder servo valve problem: USAir flight 427 (a 737-300) in 1994, and United Airlines flight 585 (a 737-200) in 1991. Boeing, however, has long since addressed that problem, without any further incidents.
Goglia notes that the China Eastern pilots made no contact with air traffic control. “There should have been some sort of a call,” he says.
But Cox cautions that emergencies do not always afford pilots time to issue “mayday” calls or to otherwise notify controllers of trouble.
“Talking to ATC is about third on the list,” he says. “If I have a major flight-control problem, I am going to deal with that [first].”
China Eastern responded immediately to the crash by grounding its 737-800s. But other Chinese airlines continued operating the type. In mid April, China Eastern returned some 737-800s to service, data from flight tracking sites and Cirium shows.
“Notice the Chinese returned the airplane to service,” says Cox. “That says the Chinese are convinced that the 737-800, as a fleet type, does not have a problem.”
The CAAC’s initial accident report says air traffic controllers tried to call the 737’s pilots during the descent but received no response.
At the crash site, investigators found the jet’s horizontal stabiliser, vertical tail, engines, wings, landing gear and parts of the cockpit, the report says. The jet was airworthy and the crew were qualified, and investigators have no evidence the 737 was carrying dangerous cargo.
“It does sound as though the airplane was sound. It was doing everything it was supposed to do,” says Cox.
The US Federal Aviation Administration (FAA), NTSB and Boeing are assisting with China’s investigation.
The FAA declined to comment about its role, deferring to the NTSB, which is the US representative to the investigation. Both the NTSB and Boeing also declined to comment, deferring to the CAAC. They cited ICAO guidelines that assign the country in which the crash occurred with responsibility for releasing information.
Reticence to comment is typical during crash investigations, but observers say US officials are handling all China affairs with particular delicacy due to recent global events.
The US is cautious against acting in any way that might push China closer to Russia or encourage Chinese support for Moscow’s invasion of Ukraine, say observers.
More broadly, China and the USA have for several years been fighting a trade war and battling over industrial intellectual property, fallout from which already landed on Boeing’s 737 Max.
China waited one year after the FAA cleared the Max before issuing its own order lifting the grounding, in December 2021.
Still, no Chinese airlines have re-started Max flights.