Boeing has asserted the pilots on the next-to-last flight of the same Lion Air aircraft that crashed encountered a similar, if less severe, nose-down problem. They addressed it by flipping off the stabilizer cutout switches, in keeping with the emergency checklist. Still, Indonesian investigators found, the pilots broke from the checklist by flipping the switches back on again before turning them off for the rest of the flight. That flight, with different pilots from the flight that crashed, landed safely.
Older 737s had another way of addressing certain problems with the stabilizers: Pulling back on the yoke, or control column, one of which sits immediately in front of both the captain and the first officer, would cut off electronic control of the stabilizers, allowing the pilots to control them manually.
That feature was disabled on the Max when M.C.A.S. was activated — another change that pilots were unlikely to have been aware of. After the crash, Boeing told airlines that when M.C.A.S. is activated, as it appeared to have been on the Lion Air flight, pulling back on the control column will not stop so-called stabilizer runaway.
The preliminary results of the investigation, based on information from the flight data recorder, suggested that the pilots of the doomed flight tried a number of ways to pull the nose back up as it lurched down more than two dozen times. That included activating switches on the control yoke that control the angle of the stabilizers on the plane’s tail — and when that failed to stop the problem, pulling back on the yoke.
There is no indication that they tried to flip the stabilizer cutout switches, as the emergency checklist suggests they should have. Findings from the cockpit voice recorder could establish in more detail what culpability, if any, rests with the Lion Air pilots.
Boeing’s position that following the established emergency checklist should have been sufficient understates the complexity of responding to a crisis in real time, pilots said.
Referring to Boeing’s focus on the need for pilots to flip the stabilizer cutout switches, Dennis Tajer, the spokesman for the American Airlines pilots union and a 737 pilot, said, “They are absolutely correct: Turning those two switches off will stop that aggressive action against you.”