Data from the fatal Oct. 29 flight that killed 189 people, and from the prior day's flight of the same jet, raises questions about three factors that seem to have contributed to the crash.
A key instrument reading on Lion Air flight JT610 was faulty even as the pilots taxied out for takeoff. As soon as the Boeing 737 MAX was airborne, the captain’s control column began to shake as a stall warning.
And from the moment they retracted the wing flaps at about 3,000 feet, the two pilots struggled — in a 10-minute tug of war — against a new anti-stall flight-control system that relentlessly pushed the jet’s nose down 26 times before they lost control.
Though the pilots responded to each nose-down movement by pulling the nose up again, mysteriously they didn’t do what the pilots on the previous day’s flight had done: simply switched off that flight-control system.
The detail is revealed in the data from the so-called “black box” flight recorder (it’s actually orange in color) from the fatal Oct. 29 flight that killed 189 people and the prior day’s flight of the same jet, presented last Thursday to the Indonesian Parliament by the country’s National Transportation Safety Committee (NTSC).
This data is the major basis for the preliminary crash-investigation report that was made public Wednesday in Indonesia, Tuesday evening in Seattle.
The flight-recorder data is presented as a series of line graphs that give a clear picture of what was going on with the aircraft systems as the plane taxied on the ground, took off and flew for just 11 minutes.
The data points to three factors that seem to have contributed to the disaster:
A potential design flaw in Boeing’s new anti-stall addition to the MAX’s flight-control system and a lack of communication to airlines about the system.
The baffling failure of the Lion Air pilots to recognize what was happening and execute a standard procedure to shut off the faulty system.
And a Lion Air maintenance shortfall that allowed the plane to fly repeatedly without fixing the key sensor that was feeding false information to the flight computer on previous flights.
Anti-stall system triggered
Peter Lemme, a former Boeing flight-controls engineer who is now an avionics and satellite-communications consultant, analyzed the graphs minute by minute.
He said the data shows Boeing’s new system — called MCAS (Maneuvering Characteristics Augmentation System) — “was triggered persistently” as soon as the wing flaps retracted.
The data confirms that a sensor that measures the plane’s angle of attack, the angle between the wings and the air flow, was feeding a faulty reading to the flight computer. The two angle-of-attack sensors on either side of the jet’s nose differed by about 20 degrees in their measurements even during the ground taxi phase when the plane’s pitch was level. One of those readings was clearly completely wrong.
On any given flight, the flight computer takes data from only one of the angle-of-attack (AOA) sensors, apparently for simplicity of design. In this case, the computer interpreted the AOA reading as much too high an angle, suggesting an imminent stall that required MCAS to kick in and save the airplane.
When the MCAS system pushed the nose down, the captain repeatedly pulled it back up, probably by using thumb switches on the control column. But each time, the MCAS system, as designed, kicked in to swivel the horizontal tail and push the nose back down again.
The data shows that after this cycle repeated 21 times, the captain ceded control to the first officer and MCAS then pushed the nose down twice more, this time without a pilot response.
After a few more cycles of this struggle, with the horizontal tail now close to the limit of its movement, the captain resumed control and pulled back on the control column with high force.
It was too late. The plane dived into the sea at more than 500 miles per hour.
Previous crew handled similar situation
Remarkably, the corresponding black-box-data charts from the same plane’s flight the previous day show that the pilots on that earlier flight encountered more or less exactly the same situation.
Again the AOA sensors were out of sync from the start. Again, the captain’s control column began shaking, a stall warning, at the moment of takeoff. Again, MCAS kicked in to push the nose down as soon as the flaps retracted.
Initially that crew reacted like the pilots of JT610, but after a dozen cycles of the nose going down and pushing it back up, they turned off MCAS using two standard cutoff switches on the control pedestal “within minutes of experiencing the automatic nose down” movements, according to the NTSC preliminary investigation report.
There were no further uncommanded nose-down movements. For the rest of the flight, they controlled the jet’s pitch manually and everything was normal. The jet continued to its destination and landed safely.
Because the cockpit voice recorder has not yet been recovered from the sea bed, it’s a myste