WASHINGTON (NYTIMES) - Investigators and experts are uncertain why Lion Air Flight 610 plummeted into the Java Sea last month, killing all 189 people on board. But they are focusing on an automatic system designed to keep the plane, a Boeing 737 Max 8, from going into a stall condition.
A stall can occur when the plane's nose points upward at too great an angle, robbing the craft of the aerodynamic lift that allows it to stay aloft.
But if the 737 receives incorrect data on the angle - as the same plane did on the flight just before the crash - the system designed to save the plane can instead force the nose down, potentially sending it into a fatal dive.
The situation in this case is further complicated by Boeing's installation of the system, which the company did without explaining it in the new model's operating manual. So the pilots might well have been unfamiliar with it.
In a statement, Boeing said it was confident in the safety of the 737 Max, and added: "While we can't discuss specifics of an ongoing investigation, we have provided two updates to operators that re-emphasise existing operating procedures - the series of steps required - for these situations."
If the pilots of Lion Air Flight 610 did, in fact, confront an emergency with this type of anti-stall system, they would have had to take a rapid series of complex steps to understand what was happening and keep the jetliner flying properly. These steps were not in the manual, and the pilots had not been trained in them.
Approximate data on the plane's speed and altitude on the 11 minutes it spent in the air suggest that the first indication of trouble may have come just above 2,000 feet, when its trajectory was beginning to level off.
At that point, said Mr John Cox, the former executive chairman of the Air Line Pilots Association and now a safety consultant, something unexpected occurred: Instead of levelling off momentarily, the plane dropped around 600 feet.
"This may have been the onset, the first time something happened," Mr Cox said.
By this point in the flight, the pilots typically would have moved the flaps on the main wings from the down position needed for takeoff into a trimmed up position for flying at higher speeds. The Boeing anti-stall system cannot activate until the flaps are up.
After the 600-foot drop, the pilots climbed to 5,000 feet, possibly to give themselves more manoeuvring room if another unexpected dive occurred. They sought and received permission to return to the airport, but for reasons not yet known, they did not appear to have tried to do so.
When the plane levelled off just above 5,000 feet, there was another indication that something was amiss: Instead of the smooth, straight flight that the usual autopilot setting would produce, the plane pitched up and down, indicating manual operation.
That could indicate that the pilot simply was not very good at flying in manual mode. More likely, said Associate Professor Les Westbrooks, from Embry Riddle Aeronautical University, the pilot already was struggling with some system causing the plane to veer from its straight path.
In that case, Prof Westbrooks said, it would be like trying to drive a car that is tugging one way or another - the driver can counteract it, but the path is jagged.
The plane's up-and-down motion continued, including a larger dip and recovery of about 1,000 feet in the last few minutes of the flight that might have felt like a bit of rough turbulence to passengers, said aeronautics and astronautics professor R. John Hansman Jr, who is also director of the international centre for air transportation at the Massachusetts Institute of Technology.
Then, suddenly, the plane went down.
There has been no official finding that the anti-stall system - known as the manoeuvring characteristics augmentation system, or MCAS - was activated. But if the 737's sensors were indicating erroneously that the nose had pitched dangerously up, the pilot's first warning might have been a "stick shaker". In that case, the yoke - the steering wheel-like handles in front of the pilot and co-pilot - would vibrate.
If the false warning in turn activated the automatic anti-stall system, the pilots would have had to take a series of rapid and not necessarily intuitive steps to maintain control, a particular challenge since those steps were not in the plane's operating manual and the pilots had not been trained on how to respond.
If it sensed a stall, the system would have automatically pushed up the forward edge of the stabilisers, the larger of the horizontal surfaces on the plane's tail section, in order to put downward pressure on the nose.
With only fragmentary data available, Prof Hansman said he suspected that a runaway of the MCAS system played a central role in the crash. "The system basically overrode the pilot in that situation," he said.
If the anti-stall system indeed ran away with the stabiliser control, only a fast sequence of steps by the pilot and first officer could have saved the aircraft, instructions later issued by Boeing show.
On the outside of the yoke in front of both the pilot and the first officer, there is a switch for electrically controlling the trim - the angle of the stabilisers. If the pilot understood what was happening, he could have used that switch for a few seconds at a time to counteract what the MCAS was doing to the stabilisers. But that would have been only a temporary solution: The pilot has to release the switch or the nose could go too high. But if he releases the switch, the anti-stall system would reactivate a few seconds later, according to a bulletin issued by Boeing.
The crucial step, according to the Boeing bulletin, would be to reach across to the central console to a pair of switches (sometimes protected with covers that must be opened), and flip the switches off. Those switches disable electric control of the motor that moves the stabilisers up and down, preventing the anti-stall system from exerting control over their position.
The final step would complete the process for giving the pilots physical control. Cables for manually operating the stabilisers run over a wheel - actually two wheels, one on either side of the console next to the ankles of the pilot and first officer. One of the pilots must rotate the wheel to pull the stabiliser back into the correct position.