News Ethiopian 737 crashed on way to Kenya, 157 people on-board

Urwumpe

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And in certain situations, you can apply enough aerodynamic load on the stabilizer with the elevators to lock up the jackscrew that serves as the actuator, at least with the amount of torque the crew can provide with the trim wheel. The electric motor on the jackscrew can provide significantly more torque and is basically impossible to lock up, provided the jackscrew is properly maintained.


The issue is, the crew is supposed to overrule the electric motor by holding the hand wheel. This should be implemented by electronic torque limiter in the electric trim motor assembly.



But I have no idea about the specifics there - does it only work on the manual control wheel?
 

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...Boeing's management is prioritizing personal enrichment over passenger safety...


 

Linguofreak

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As I understand it, a jackscrew is a mechanically irreversible mechanism? So once the motor has driven the stabiliser to wherever it has been commanded, it stays there till something breaks or changes?
Its odd that this none-reversible mechanism exists only in pitch control, not the others?

N.

Well, it's only pitch trim that moves the entire airfoil in question (as opposed to just a control surface), which is necessary because of large changes in trim needed for the range of Mach numbers airliners operate at in the course of a single flight.

The jackscrew is used because the forces across the whole stabilizer can be very large and you need something where the control end (electric motor or manual trim wheel) can easily move the thing being controlled (the stabilizer), but the forces on the controlled end can't easily overpower the control end and move everything back to where it started, or beyond. A screw converts rotational motion into linear motion, so it works very well for this: you can easily move a screw into a hole by rotating it, but if you try prying it back out directly, the threads will hold it in place until something in the assembly undergoes structural failure. But if you want to remove it, it's easy to get it back out by turning it the other way.

The same goes for a jackscrew: the motor and trim wheel operate on the jackscrew by rotating it, so they can move it easily, but since the lever that moves the stabilizer is screwed into the jackscrew, forces on the stabilizer act perpendicularly to the threads and this prevents stabilizer forces from forcing the mechanism.

The problem that was encountered on the Ethiopian flight was that the elevators were acting one way on the stabilizer (the crew pulling up), the general forces in the stabilizer were acting in the opposite direction (from down), and the high airspeed meant that the forces involved were, in general, larger than had been designed for, and this caused the jackscrew to bind, which made it difficult to move the jackscrew assembly even by rotation. The motor could put out the force needed, but the crew couldn't, especially while simultaneously trying to haul back on the yoke, so they gave up trying and turned the trim motor back on so that they could use the extra power it had available, but since MCAS was still going off, it was regularly signaling the motor to pitch down.

The solution was for the crew to release the yoke, which would lessen the forces on the stabilizer and ibid the jackscrew, and dedicate their full strength to rotating the trim wheel, then to haul back on the yoke again until the nose was above the horizon, then let go of the yoke and work on trim again, and so on with that roller coaster ride until trim was in an acceptable position.
 

steph

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Yup...apparently, despite the crew knowing the procedure for the MCAS (this was rolled out after the first crash), it's very easy for them to get stuck 'between a rock and a hard place' , as one expert put it. They kept the engines revved because they were at low altitude AND because of the pitch up effect. Meanwhile, MCAS kept trying to pitch the nose down. When they decouple the electric trim, they find out they're now nose down AND moving way too fast to manually move the trim, due to the engines being revved up trying toto compensate for the pitch down.

Iidling the engines seems counter-intuitive, especially if the plane is at low altitude and/or already falling. In short, due to MCAS always trying to lower the nose, it's very easy to get it in a state where it's impossible to control it manually. What Boeing failed to mention in the procedures is actually how little time there is to act between MCAS activating and the moment when the plane is descending too fast for manual trim to work. Then again, telling them to reduce throttle would have exposed the seat-of-your-pants flying needed to bring it under control and it would have highlighted the fact that it's an inherently unstable plane.

https://www.bbc.co.uk/news/extra/sd9LGK2S9m/battle_over_blame
 

MaverickSawyer

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Well, this certainly doesn't bode well for Boeing...

http://www.npr.org/2019/06/19/73424...aying-737-max-should-never-have-been-approved

One of the nation's best known airline pilots is speaking out on the problems with Boeing's 737 MAX jetliner. Retired Capt. Chesley "Sully" Sullenberger told a congressional subcommittee Wednesday that an automated flight control system on the 737 MAX "was fatally flawed and should never have been approved."

...

"These crashes are demonstrable evidence that our current system of aircraft design and certification failed us," Sullenberger told lawmakers. "The accidents should never have happened."

...

Sullenberger says he recently experienced scenarios similar to those facing the pilots of the doomed Ethiopian and Lion Air jetliners in a simulator, and says he understands the difficulties they had trying to maintain control of the planes. "Even knowing what was going to happen, I could see how crews could have run out of time and altitude before they could have solved the problems," he said.
 

MaverickSawyer

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Local NBC affiliate in Seattle sent their camera helicopter out over Boeing's facilities for this video...

 

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Europe will not accept US verdict on 737 Max safety - BBC

Europe's aviation safety watchdog will not accept a US verdict on whether Boeing's troubled 737 Max is safe.

Instead, the European Aviation Safety Agency (Easa) will run its own tests on the plane before approving a return to commercial flights.

The 737 Max has been grounded since March after two fatal crashes.

But Easa told the US Federal Aviation Administration (FAA) there would be "no delegation" on safety approval in a letter sent on 1 April.
 

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In fact this is not new, EASA has been doing this "no direct acceptation" for a long time, mainly since FAA-EASA TIP R6 was issued with some changes and different interpretation of some items, this "new" interpretation is becoming a nightmare for cargo modified (STC) aircraft operators mainly, but for some pax operators as well.

Some of us believe that this is more of a commercial issue than a technical one.
 

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737 Max may be as much of a blunder for Boeing as A380 was for Airbus.

Difference is that Airbus missed the way the aviation model will change in the future and Boeing screwed up the tech.
 

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Notebook

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