An advanced control system designed to stop Boeing's top-selling 737 MAX 8 from stalling in mid-flight is coming under scrutiny following Sunday's Ethiopian Airlines crash, which killed 157 people.
The aircraft's state-of-the-art Maneuvering Characteristics Augmentation System (MCAS) was also implicated in the downing last year of Lion Air Boeing 737 MAX 8 in Indonesia, when all passengers and crew members also died.
In both cases, the aircraft were just a few months old and crashed minutes after take-off. Both planes are reported to have flown erratically, climbing and descending steeply before finally crashing.
"We are hearing that the problem was similar to the Lion Air accident; 'unreliable airspeed' and/or difficulty in controlling the aircraft, and both had asked to return to the airport," Gerry Soejatman, an aviation analyst based in Indonesia's capital, Jakarta, told Al Jazeera.
Boeing in the spotlight after Ethiopian Airlines crash
An initial investigation in the Lion Air accident suggested a faulty sensor had triggered the plane's automated MCAS system possibly resulting in the nose of the aircraft being pushed down.
"New planes do have issues," said Soejatman. "But they are usually solved before delivery."
"These two accidents, potentially with similar causes, are definitely raising eyebrows within the industry."
The similarity of the two accidents has also put the aviation industry into a tailspin, with a number of countries grounding their Boeing 737 MAX 8 fleets, even before the results of an investigation are known.
China, a major buyer of the aircraft, was one of the first to ground its 737 MAX 8 aircraft until further notice.
"Given that two accidents both involved newly delivered Boeing 737-8 planes and happened during take-off phase, they have some degree of similarity," China's Civil Aviation Administration said in a statement.
Henry Harteveldt, a travel industry analyst at the Atmosphere Research Group, said Beijing's move "will carry enormous impact across the global airline community".
"China's decision may be premature, [but] it's one that cannot be ignored," he added."China is an extremely important market for Boeing."
Boeing 737 MAX 8 under scrutiny
Cayman Airways and Ethiopian Airlines have also grounded their 737 MAX 8 fleet.
Boeing has delivered about 350 of the approximately $120m each aircraft to date, with more than 4,500 in total ordered since 2017, making it one of Boeing's most important and profitable aircraft.
What is MCAS?
The MCAS, introduced with the launch the 737 MAX 8 in May 2017, was the result of an effort to equip the new model with larger and more fuel-efficient engines.
These were placed further forward and higher on the wing, altering the aircraft's balance. This resulted in the plane's nose pitching upwards in certain circumstances.
To solve the problem, Boeing added the MCAS flight-control system, which automatically pushed the aircraft's nose down when a sensor indicated the plane was at risk of stalling.
The MCAS is designed to only activate when the wing flaps are retracted after take-off and the aircraft has gained altitude.
However, it is speculated this system may have played a part in both accidents, with the climbing and diving behaviour of the flight the result of the pilot's struggle to overcome the automated system.
Pilots in the dark?
Following the Lion Air crash in October 2018, which killed 189 people, it was reported that the addition of the MCAS flight-control system had not been adequately explained to pilots, possibly making them ill-prepared if it were to activate during flight.
At the time, Boeing denied the company had "intentionally withheld" information about the new system. According to digital aviation publication Air Current, citing an internal Boeing memo, company CEO Dennis Muilenburg told staff the "relevant function [of MCAS] is described in the Flight Crew Operations Manual."
"We routinely engage with customers about how to operate our airplanes safely," Muilenburg said in the memo, according to an Air Current report.
But pilots flying the aircraft for Southwest Airlines were reported to have been told that it was likely they would never see the system at work as "it operates in situations where the aircraft is under relatively high g load and near stall … designed to assist the pilot during recover, and likely going unnoticed by the pilot."
Air Current, citing a Q&A document provided to pilots at SouthWest Airlines, said staff were told this was why "Boeing did not include an MCAS description in its Flight Crew Operations Manual."
In the wake of the Lion Air crash, Boeing stepped up its efforts to make pilots familiar with the new system, but the relations between to two companies soured, with Lion Air threatening to cancel billions of dollars of orders.
"Normally, manufacturers would be supportive of their customer in times of accidents," said Soejatman.
"This crash has raised questions on the real reason behind the breakdown in relations between Lion Air and Boeing."
The US aerospace giant is also understood to be working on updated software for the MCAS system but it is unclear if the Ethiopian Airlines accident will delay this.
As investigators from Boeing, Ethiopia Accident Investigation Bureau and US National Transportation Safety Board sift through the remains of the Ethiopian Airlines Boeing 737 MAX 8, the focus now turns to understanding the cause of the accident.
"At the very least, this accident will dampen the Boeing 737 MAX 8 sales campaign from this point on, until the completion of the accident investigations," said Soejatman.
For Boeing, a great deal hangs on the findings of those investigations.
On Tuesday, the aerospace giant's shares plunged in value by nearly 13 percent on the New York Stock Exchange.
The dive left the company poised for their worst day of trading since September 17, 2001, when several aerospace firms saw their share prices plummet in the wake of the 9/11 attacks in the US.