I think the jackscrew itself (not counting the MCAS) is a single point of trouble. I doubt there is an another system in modern airplane which is not backuped with something automatic.
Imagine the screw breaks (as it happened due to missing lubrication in past), or the servo get jammed in an fatal position, etc. There should be a hydraulic, or pneumatic system (pyro generator?) which sets the stabilizer into a neutral position upon single button push..
I don't see how a hydraulic or pneumatic system would be able to move the stabilizer with the jackscrew still linked to the stabilizer. There would need to be an explosive bolt linkage or something. The problem is MCAS uses the stabilizer in a way where a malfunction can put it in an extraordinarily unusual position before the pilots might be able to react, whether that's due to insidious onset or due to protocol that is too slow for the situation or whether it's just unreasonable to expect a human pilot to be ready to decisively react to runaway trim within a few seconds, at all times. The reason the stabilizer is moved with a jackscrew rather than hydraulics, to begin with, is that under ordinary conditions, you want the stabilizer to stay where it was in case of any failure. If it were moved by hydraulics, then a leak of the hydraulic fluid would leave the stabilizer flopping in the breeze, and the elevators, say they're on an isolated hydraulic system, would have no effect. Move the elevator up, and the entire stabilizer would just flop down and vice versa.
It might require careful gearing to maintain sync, but I suppose one could have two redundant jackscrews, each of which is sufficient to support the horizontal stabilizer on its own.
If you had two jackscrews working in parallel, you'd have two points of failure. If one screw stuffed, then it could jam and prevent the other from working. And the forces would be even higher trying to manually trim the thing. But... explosive bolts?
Indeed it’s a single point of failure, but then again, screws are fundamentally reliable things. Any automatic backup is likely to be less reliable than the screw and nut.
+1. There have been several warplanes that have returned home using the stabilizer to control the plane after the hydraulics/elevators failed.
I kinda doubt any plane would have a backup to the jackscrew, anymore than they have a backup if the wings fall off. The fact there's a backup (manual trim wheel) to the motors/electrical turning the jackscrew is great that it's there, at all. But apparently the manual trim might not even work under a high stabilizer loading, which an unusually high stabilizer load is what you get with the the new engine placement plus the abnormal MCAS response plus pilots not wanting the plane to crash.
Others have inquired, and I'm also curious about it, whether under such a high load if it's possible the motors won't even move the stabilizer. The report suggests that
the pilots pressed the trim up button for 3 seconds at one point... but it only moved the stabilizer by 0.2 degrees. Math says 3 seconds should move it roughly 0.7 degrees! Maybe something was slipping, whether a belt/pulley or an actual clutch. Going back a few posts, this is another potential reason the pilots made only a small correction... because they realized the trim wasn't moving and something was slipping. And they were just smoking a belt/pulley or whatnot. In the original Jakarta crash a similar event was recorded. The last correction attempt before the crash was only a very small correction at the jackscrew. This might not have been due to switching pilots prior to this. Perhaps by this point, the belt that had already corrected this unusual condition 20 times had already started to fail, heating and stretching and smoking, having been pushed beyond its maximum expected load and duty cycle. So by the end, the motor is turning but the trim isn't moving, because the belt has become too loose and/or is melting at the surface. And now the jackscrew does what it's supposed to do. What it was designed to do. To stay put.
It's feasible that the pilots realized exactly what was happening, by the end, and that they knew exactly what had to be done. That their only hope was to temporarily reduce stabilizer load in order to trim the plane back up to stop losing altitude. But at the altitude they were at, they needed the speed of the motors to have any hope to trim the plane in time.... and hopefully not burn out the belt in the meantime, hence why they are not spamming the trim up button the entire time for naught. So when they finally attempted this, letting go of the yoke, MCAS decided to fire again at that moment just as they are pressing the up trim. Or maybe they were already too low/lost by the point they tried this. This would explain the severe downward angle at the end of each crash, in the last attempt to move the damn stabilizer. They might have realized they were stuck between a slow but sure death and a fast and nearly certain one with at least a sparkle of hope.
The trim wheel, itself, would most likely have a separate belt and pulley directly to the jackscrew. Then the motor has its own shorter belt and pulley. The manual method might be too slow, but if the motor belt is smoked, the manual trim wheel procedure might be the better chance. Ideally you could use both, but I think the speed of the motor would be too high to do both, simultaneously.
So ideally, perhaps. 1. Cut stab trim 2. Let go pressure on the yoke 3. Both pilots manually crank the wheel. 4. When static friction is broke and the wheel starts to turn, press trim up button while putting stab trim back on, and letting go the handles when the motor kicks on. 5. Resume pulling back on yoke. 6. cut stab trim when the the stabilizer is corrected. 7. Clip a few trees 8. Don't stall the plane during or after the hard nose up maneuver. (I think this last part is going to be extremely difficult after/during step 7, esp on a plane that is aerodynamically unstable in the specific way that the MAX is reported to be).
Yeah, that sounds pretty complicated. It would help if the pilots had telepathy and a third arm. And there might be a helpful other step, to cut back throttle to 50% in situation like this, where the plane is losing altitude and gaining speed. The higher speeds made this problem worse. Then putting throttle back up somewhere between 1 and 7, depending on the delay.
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