View Single Post
Old 04-11-2019, 08:34 AM
  #1  
sgrd0q
Gets Weekends Off
 
Joined APC: Jan 2006
Posts: 222
Default Extreme Out Of Trim Flight

Looking at the Ethiopian preliminary accident report and the one from the Lion Air accident, I am starting to think there is generally lack of training and understanding of how to trim in an extremely out of trim condition. I should start by saying that I am not bashing the crew in either accident; I blame Boeing 100%. But I think everyone may be somewhat deficient in this area.

Generally, trimming in the usual scenario is an easy balancing act. There are three key points, the action is quick, it is done automatically/subconsciously and the feedback is instant. This is similar to driving a car and keeping it in your lane – you observe the car getting closer to one side, you make a quick small adjustment automatically/subconsciously and you get an instant feedback when you see the car move to the other side. This is repeated many times per minute. It is a balancing act. Another analogy is riding a bicycle, particularly at slow speed – you make small inputs, subconsciously (it is much harder to explain to your kid how to do it, vs just doing it) and the feedback is instant. It is the same with walking - also a balancing act. So the action of trimming is very straight forward and everyone does it reasonably well, just like driving, or riding a bike, or walking, whichever analogy you prefer.

The situation is different in an extremely out of trim state. Trimming is now not a small, instant, automatic adjustment, and even worse - the feedback is grossly distorted. What I mean by this is this – imagine you are pulling hard at the control column, say 100 pounds. Nobody, no matter how strong can sustain this for any significant period of time. You get tired. Your muscles want to give out. So you trim in the usual way, for a short burst, the back pressure is so much that you get very small relief comparatively, but you may not even perceive it. The feedback is distorted. In fact as you get tired, you may even feel like you are pulling harder, even though you got a small relief. So the feedback loop is broken. Your brain is wired to not keep on if there is no positive feedback, and you stop and move on to the next thing.

The failure to trim properly is astounding in both accidents.

With the Ethiopian accident, you have the captain (pilot flying) doing very little to trim after the MCAS kicks in and while they still have the electric trim. His feedback is distorted and he likely assumes his trim is not working so he asks the first officer to help trim. The first officer does a great job pulling up with him, but is also not nearly aggressive enough with the trim. All the while they both recognize the out of trim condition. Then, of course, per the procedure they cut off the electric trim which is great because that kills MCAS, but not so great because they are now grossly out of trim, not controlling the speed (unfortunately), and ultimately finding that the manual trim wheel is not working in that configuration. At some point after that one of them (presumably the CA) turns on the electric trim. I don’t think this was a bad idea as others have speculated. At this point they are exhausted holding the back pressure, the manual trim wheel is not working presumably due to the speed and they have no other options. Throwing the procedure out of the window is the right call as there is no more procedure to follow. You have to do what you have to do to recover. So I would assume you turn the electric trim on and you absolutely lay on the thumb switch. It has been shown that not only you can interrupt MCAS but you can actually override it – as the Lion Air CA did repeatedly on that flight. The most disappointing thing with the Ethiopian accident is that they turned on the electric trim and did very little trimming. There is one blip on the graph where they tried, it changed little and they let go, then another blip, then nothing. They assumed it was not working. They gave up. Then MCAS kicks in and finishes the job. This is the most disappointing aspect.

With the Lion Air accident, the captain actually does a great job trimming nose up repeatedly, for many cycles of MCAS activation and for many minutes. Then he presumably gives control to the first officer, who is not nearly as aggressive and crashes.

This is astounding, 3 out of 4 pilots, 75%, failed to use the trim aggressively enough in a severely out of trim state. As for the Lion Air captain (again pilot flying) – to his credit he catches the first MCAS activation early enough, so he never gets in truly severe out of trim state – so the feedback loop is not as distorted and he is successful in trimming up. This then repeats many times and he gets into a groove with this, into rhythm, going back and forth and staying on top of it. We don’t know how he would have fared if he started with an extreme out of trim condition to begin with.

One may think this is all training, and a student pilot should be able to trim, but it goes beyond that. Everybody trims and does a good job under normal circumstances when the feedback loop is there. Once you go into an extreme out of trim situation and the feed back loop is distorted then this is an entirely different skill – you will need to trim, your only feedback will be the trim wheel spinning, and you will have to be persistent and not give up.

Not many were put in that situation in real flying before MCAS, save for a few test pilots who do that sort of thing on purpose.

I believe there is a training opportunity here – not how to trim under normal conditions, and not on any specific procedure either, but rather training to be persistent and aggressive with the trim even when it feels like it is not working. Hold the damn button for five, ten, fifteen seconds - whatever it takes.
sgrd0q is offline