Mentour Pilot has acknowledged that the forces on the stabilizer can be so great that it might be possible that manual trim cannot be performed
I'm assuming, perhaps incorrectly, that the trim wheels are mechanically connected to the stabilizer jack screws so that one turn on the trim wheel corresponds to one turn on the jackscrew.QuoteMentour Pilot has acknowledged that the forces on the stabilizer can be so great that it might be possible that manual trim cannot be performedUnder abnormal, worst case, stabilizer loading conditions how much torque do you need to put onto the stabilizer jack screws ? Quite a lot I would have thought, but to make matters worse frictional losses along the entire length of the mechanical link would increase under high load conditions, so more torque would be required on the trim wheels that at the jackscrews.
Just trying to get my head around the losses in a mechanical system at maximum load. I don't think trim wheels were designed to work at these sort of loads, not only does friction increase but stiction comes into play as well.
@langwadt, forgot about the grab and hold thing, thanks. So it isn't a simple 1:1 mechanical linkage. That makes sense because I couldn't see it working mechanically on an aircraft that size. So it's all servo driven and the pilots have no direct control of air surfaces, it's just haptic feedback. More control systems and more rabbit holes.
EDIT: Probably wrong about the haptic feedback, I was second guessing.
@langwadt, forgot about the grab and hold thing, thanks. So it isn't a simple 1:1 mechanical linkage. That makes sense because I couldn't see it working mechanically on an aircraft that size. So it's all servo driven and the pilots have no direct control of air surfaces, it's just haptic feedback. More control systems and more rabbit holes.
EDIT: Probably wrong about the haptic feedback, I was second guessing.
Pretty sure it *is* a mechanical linkage, and of course it's not 1:1. It's many to 1, which you can see in the videos when the motor is turning the jackscrew to move the stabilizer a few degrees, and the wheel is going round and round.
Still, even with the mechanical advantage, it might be difficult or maybe impossible. To me, this new information makes a lot more sense to me than MCAS-pushed-the-nose-down-and-we-crashed scenario by itself ever did. Not being able to turn the trim at all or fast enough makes more sense than pilots forgot to disable the electric trim.
Several hours after Ethopian investigators found that the crash of Ethiopian Airlines Flight 302 was not the result of pilot error (hence, it was the result of Boeing error), and demanded a full review of the Boeing 737 Max flight control system, just after 3pm, Boeing CEO Dennis Muilenburg took to social media where in what passed as an attempt at a a "heartfelt" apology, the CEO of the most important, for the Dow Jones, company said that Boeing was "sorry for the lives lost" and essentially admitted that it was the company's software that was responsible for the crashes, saying that "with the release of the preliminary report of Ethiopian Airlines Flight 302accident investigation, it's apparent that in both flight the Maneuvering Characteristics Augmentation System, known as MCAS, activated in response to erroneous angle of attack information."
Here are Muilenburg's prepared remarks:
We at Boeing are sorry for the lives lost in the Boeing 737 MAX accidents. These tragedies continue to weigh heavily on our hearts and minds and we extend our sympathies to the loved ones of the passengers and crew on board Liion Air Flight 610 and Ethiopian Airlines Flight 302. All of us feel the immense gravity of these events across our company and recognize the devastation to the families and friends of the loved one who perished. The full details of what happened in these two accidents will be issued by the government authorities in the final reports.
But with the release of the preliminary report of Ethiopian Airlines Flight 302accident investigation, it's apparent that in both flight the Maneuvering Characteristics Augmentation System, known as MCAS, activated in response to erroneous angle of attack information.
The history of our industry shows most accident are caused by a chain of events. This again is the case here, and we know we can break one of those chain links in these two accidents. As pilots have told us, erroneous activation of the MCAS function can add to what is already a high workload environment. It's our responsibility to eliminate this risk. We own it, and we know how to do it.
At 05:39:55, Autopilot disengaged,
At 05:39:57, the Captain advised again the First-Officer to request to maintain runway heading and that they are having flight control problems.
At 05:40:00 shortly after the autopilot disengaged, the FDR recorded an automatic aircraft nose down (AND) activated for 9.0 seconds and pitch trim moved from 4.60 to 2.1 units. The climb was arrested and the aircraft descended slightly.
At 05:40:03 Ground Proximity Warning System (GPWS) “DON’T SINK” alerts occurred.
At 05:40:05, the First-Officer reported to ATC that they were unable to maintain SHALA 1A and requested runway heading which was approved by ATC.
At 05:40:06, left and right flap position reached a recorded value of 0.019 degrees which remained until the end of the recording.
The column moved aft and a positive climb was re-established during the automatic AND motion.
At 05:40:12, approximately three seconds after AND stabilizer motion ends, electric trim (from pilot activated switches on the yoke) in the Aircraft nose up (ANU) direction is recorded on the DFDR and the stabilizer moved in the ANU direction to 2.4 units. The Aircraft pitch attitude remained about the same as the back pressure on the column increased.
At 05:40:20, approximately five seconds after the end of the ANU stabilizer motion, a second instance of automatic AND stabilizer trim occurred and the stabilizer moved down and reached 0.4 units.
Sorry guys, but this pilot... should have flipped the cutout switches just after restoring (*) trim between 5:40:12..5:40:20 and they would all be alive now. It's sad.
I don't know the details of the published procedures then in effect. Under exactly what circumstances do they require putting those switches into cutout position, and what other things are supposed to be tried first? How much were the official procedures modified after the Lion Air crash?
100% sure.
My point is that those switches are probably inputs to a logic system, who will later decide what to do based on 20 other inputs, all controlled by software
GeorgeOfTheJungle is 1000% correct (yes one thousand). These switchs cut the electrical power to the Horizontal Stabilizer trim mechanism.
Still 10000000% correct?
It's not entirely clear what they are saying. If they are saying that attempts to trim the aircraft with the trim wheel did not work after the trim motors were put in cutout, then Boeing has a much bigger problem on its hands than MCAS.
If they are saying that manual trim switches on the yoke didn't work after the trim motors were put into cutout, then, yeah, duh. The question then becomes whether the pilots tried to trim the aircraft with the wheel, and if they did, then was there just not enough time to make the adjustment, etc.
As a pilot myself, I would not be particularly scared of an automated system that moves the trim, as long as I could disable it. On the other hand, there's no way in hell I'd launch in an aircraft where I felt that manual trim might not work.
It's not entirely clear what they are saying. If they are saying that attempts to trim the aircraft with the trim wheel did not work after the trim motors were put in cutout, then Boeing has a much bigger problem on its hands than MCAS.
If they are saying that manual trim switches on the yoke didn't work after the trim motors were put into cutout, then, yeah, duh. The question then becomes whether the pilots tried to trim the aircraft with the wheel, and if they did, then was there just not enough time to make the adjustment, etc.
As a pilot myself, I would not be particularly scared of an automated system that moves the trim, as long as I could disable it. On the other hand, there's no way in hell I'd launch in an aircraft where I felt that manual trim might not work.
The mechanical trim system of the Boeing 737 have been installed in more than 10 000 Boeing 737 since the '60s.
The mechanical trim system of the Boeing 737 have been installed in more than 10 000 Boeing 737 since the '60s.
[quote
afaiu the trim wheels are connected to the jack screw with cables, I image the clutch being between the electric motor
and the jackscrew, so that by grabbing the wheels the motor cannot move the jack screw
Yes, there's definitely a clutch between the motor and jackscrew. Demonstrated by being able to halt auto-trim by grabbing and holding the heel.