What Lion Air pilots may have had to do to avoid an accident

Investigators and experts do not know why Lion Air Flight 610 collapsed in the Java Sea last month, killing 189 people aboard. But they are focusing on an automatic system designed to prevent the plane, a Boeing 737 Max 8, from going into "stall".

A stall may occur when the nose of the aircraft is too inclined upward, depriving the craft of the aerodynamic lift that allows it to stay in the air. But if the 737 receives incorrect data on the angle – like the same plane during the flight just before the accident -, the system designed to save the plane can instead force the dive towards down, potentially sending him into a fatal dive.

The situation in this case is further complicated by the installation of the system by Boeing, which the company did without explaining it in the user manual of the new model. It may be that the pilots have misunderstood.

In a statement, Boeing said he was confident in the safety of the Boeing 737 Max and added, "Although we can not discuss the details of an ongoing investigation, we have provided two updates to the operators who are putting again focus on existing operating procedures. – the series of steps required – for these situations. "

If the Lion Air 610 pilots had actually faced an emergency with this type of anti-stall system, they should have taken a series of quick and complex steps to understand what was happening and ensure the smooth running of the aircraft. line. These steps were not included in the manual and the pilots were not trained.

Approximate speed and altitude data from the aircraft over the 11 minutes in the air suggest that the first sign of trouble would have been just over 2,000 feet, as its trajectory began to stabilize.

At that time, said John Cox, former executive chairman of the Air Line Pilots Association and now a security consultant, an unexpected event occurred: instead of stabilizing momentarily, the altitude of the Apparatus dropped about 600 feet. "It may be the beginning, the first time something has happened," said Mr. Cox.

At this point in the flight, the pilots would generally have moved the flaps of the main wings from the low position required for take-off to an adjusted position to be able to fly at higher speeds. The Boeing anti-stall system can only be activated when the flaps are raised.

After the 600-foot drop, the pilots climbed to 5,000 feet, probably to give themselves more leeway if another unexpected dive occurred. They requested and obtained permission to return to the airport, but for unknown reasons, they did not appear to have attempted to do so. When the aircraft stabilized just above 5,000 feet above sea level, there was another indication that something was wrong: instead of the straight and smooth flight that would produce the usual autopilot setting, the aircraft pitched from top to bottom, indicating manual operation.

This could indicate that the pilot just was not very good driver in manual mode. More likely, said Les Westbrooks, an associate professor at Embry Riddle Aeronautical University, the pilot was already struggling to use a system that would divert the aircraft from its straight course.

In this case, said Mr. Westbrooks, it would be like trying to drive a pulling car in one way or another – the driver can neutralize it, but the path is erratic. Aircraft movements continued, including a larger dive and a recovery of about 1,000 feet in the last minutes of the flight, which could have given the impression of flying. a turbulent turbulence for passengers, said R. John Hansman Jr., a professor of aeronautics and astronautics and director of the International Air Transport Center at the Massachusetts Institute of Technology.

Then, suddenly, the plane fell.

No official verification has been made that the anti-stall system – known as the Maneuverability Enhancement System, or M.C.A.S. – has been activated. But if the 737's sensors incorrectly indicated that the nose was pitched up dangerously, the pilot's first warning could have been a "stick trembling": the yoke – the steering wheel-shaped handles in front of the pilot and the co-pilot – would vibrate.

If the false alert in turn activated the automatic anti-stall system, the pilots should have taken a series of quick and not necessarily intuitive steps to keep control – a particular challenge, as these steps were not included in the manual use of the aircraft and the pilots had not been trained on how to react.

If it had detected a stall, the system would have automatically pushed up the front edge of the outriggers, the larger horizontal surfaces on the tail of the aircraft, in order to exert pressure towards the down on the nose.

To counter the dive movement, the pilot 's natural reaction would probably have been to use his yoke, which displaces the other smaller surfaces of the aircraft' s tail, the elevators. But trying this maneuver might have lost precious time without solving the problem, because the force exerted on the nose by the stabilizer is greater than the opposing force that the pilot would try to exert through the elevator. said Pat Anderson, professor of aerospace. engineering at Embry Riddle.

"After a while, the elevator will lose and the stabilizer will win," he said.

With only piecemeal data, Hansman said he suspected a leak in front of Mr.C.A.S. system played a central role in the crash. "The system basically canceled the pilot in this situation," Hansman said.

If the anti-stall system actually escaped with the stabilizer control, only a quick sequence of pilot and first officer steps could have saved the aircraft, instructions later given by the Boeing show.

On the outside of the carcass in front of the pilot and the first officer, a switch allows to electrically control the attitude – the angle of the stabilizers. If the pilot understood what was going on, he could have used this switch for a few seconds at a time to counteract what the pilot M.C.A.S. did to the stabilizers. But that would have been only a temporary solution: the pilot had to release the switch or the nose could go too high. But if he releases the button, the anti-stall system will reactivate a few seconds later, according to a bulletin published by Boeing.

The crucial step, according to the Boeing newsletter, would be to reach out and switch off a pair of switches (sometimes protected with covers to open). These switches disable the motor's electric drive which moves the outriggers up and down, preventing the anti-stall system from controlling their position.

The final step would complete the pilot physical check process. The cables for the manual operation of the outriggers pass over a wheel – actually two wheels, one on each side of the console, next to the driver's and the first officer's ankles. One of the pilots must spin the wheel to bring the stabilizer back to the correct position.