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https://www.omaha.com/money/wall-st...cle_900d74a1-c7b3-5ba1-b76b-d58101640c2b.html
In a November television interview on the Fox Business Network, Boeing Chief Executive Dennis Muilenburg, when asked if information had been withheld from pilots, cited the procedure as “part of the training manual” and said Boeing’s bulletin to airlines “pointed to that existing flight procedure.”
Vice president Mike Sinnett repeatedly described the procedure as a “memory item,” meaning a routine pilots may need to do quickly without consulting a manual and so must commit to memory.
But Lemme said the Ethiopian pilots most likely were unable to carry out that last instruction in the Boeing emergency procedure — because they simply couldn’t physically move that wheel against the heavy forces acting on the tail.
“The forces on the tail could have been too great,” Lemme said. “They couldn’t turn the manual trim wheel.”
The stabilizer in the Ethiopian jet could have been in an extreme position with two separate forces acting on it:
MCAS had swiveled the stabilizer upward by turning a large mechanical screw inside the tail called the jackscrew. This is pushing the jet’s nose down.
But the pilot had pulled his control column far back in an attempt to counter, which would flip up a separate movable surface called the elevator on the trailing edge of the tail.
The elevator and stabilizer normally work together to minimize the loads on the jackscrew. But in certain conditions, the elevator and stabilizer loads combine to present high forces on the jackscrew and make it very difficult to turn manually.
As the jet’s airspeed increases, and with nose down it will accelerate, these forces grow even stronger.
In this scenario, the air flow pushing downward against the elevator would have created an equal and opposite load on the jackscrew, a force tending to hold the stabilizer in its upward displacement. This heavy force would resist the pilot’s manual effort to swivel the stabilizer back down.
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In the test, the two European pilots in the 737 simulator set up a situation reflecting what happens when the pre-software fix MCAS is activated: They moved the stabilizer to push the nose down. They set the indicators to show disagreement over the air speed and followed normal procedures to address that, which increased airspeed.
They then followed the instructions Boeing recommended and, as airspeed increases, the forces on the control column and on the stabilizer wheel become increasingly strong.
After just a few minutes, with the plane still nose down, the Swedish 737 training pilot flying as Captain is exerting all his might to hold the control column, locking his upper arms around it. Meanwhile, on his right, the first officer tries vainly to turn the stabilizer wheel, barely able to budge it by the end.