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Comment by cjbprime | Hacker News Reader

> I don't find 1 particularly compelling, it would have been noted by now if it was impossible to trim the aircraft using the manual trim wheels.

It has been noted:

737 Flight Crew Training Manual, chapter Non-Normal Operations/Flight Controls, sub heading Manual Stabilizer trim:

"Excessive air loads on the stabilizer may require effort by both pilots to correct mis-trim. In extreme cases it may be necessary to aerodynamically relieve the air loads to allow manual trimming. Accelerate or decelerate towards the in-trim speed while attempting to trim manually."

In this case the excessive air load would be caused by the yoke control pulling the elevator up while the trim controlled stabilizer is doing the opposite.

Like you say, it's hard to know whether this actually happened.

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I've seen that before. It certainly doesn't say it's impossible. I need to have look in the certification requirement but I'm pretty sure it would be uncertifiable if the trim couldn't be neutralised.

> It certainly doesn't say it's impossible.

What would it have to say? It says you might have to do X before Y, where Y is a thing that you need to do quickly while at low altitude to avoid dying and X is a thing that gives you even less time to do it in and makes the problem worse.

Ok let's look at it another way. Do you think it's even remotely likely that Boeing haven't had one of their 737 Max aircraft try this out at a safe altitude? They staked the future of the aircraft by saying they believe they are safe to fly.

I've never suggested having a trim runaway close to the ground is trivial. It's certainly going to result in brown trousers when it happens, but it should be within the abilities of a well trained crew to handle.

Following the outcry over Lion air I just don't believe a pilot on type wouldn't have known the procedures for getting the aircraft to a safe altitude and disabling the stab trim.

As a pilot your hands are on the controls. You notice pretty instantly that your having to pull the nose up. You instinctively reach for the trim control on the yoke. This disables the MCAS system for 5 seconds. You get the pitch forces under control. A few seconds later MCAS decides you've still got the nose too high because of a faulty sensor and tries to trim it back down once more.

How many times before you remember that fatal bug from that crash a few months ago. Maybe twice? Three times tops.

I mean, the answer could be that you do that three times, then you remember that the emergency AD said that you should trip STAB TRIM CUTOUT when this happens, so you do, and then you realize that you're mistrimmed at less than 1000ft AGL, you just disabled the motor that was allowing you to fix the trim, and now you can't fix it without losing 600ft in the process while you release the yoke.

I am not saying this is likely to be what happened! But given that Boeing didn't even mention any of this in the manual before Lion Air, it seems plausible that they either didn't test it carefully, or tested it at high altitude and imagined that it would never happen at low altitude?

Let's work through an example. You are in level, trimmed, manual flight and the trim system suddenly starts pitching the nose down. As a consequence, the aircraft will begin descending and accelerating. Your first response, even before you have recognized that you have a trim runaway situation rather than just some turbulence, is to pull back, to restore level flight level and bring the speed down to where it is supposed to be. In other words, you are decelerating towards the in-trim speed, which is precisely what the quoted section of the manual is saying you should do.

Unfortunately I think you may be misinterpreting that section in the manual.

From your senario when the trim moves nose down. You apply nose up on the elevator which makes the trailing edge of the elevator go up which applies a downwards force on the trailing edge of the stabilizer. In order to move the trim back towards neural you need to lift the trailing edge against this force.

If the trim is extreme you may have to relax back pressure to reduce that force.

By in trim speed they mean the speed at which the aircraft would naturally settle at given the current trim setting.

You have a point, but I am not sure that this is a complete analysis of the situation, because it does not consider the changing moments as the centers of pressure of the wing and stabilizer move in response to the changing airspeed. Furthermore, when one moves the elevator of a subsonic airplane, it does not just change the force generated by the elevator, but also the force generated by the stabilizer it is attached to (by affecting the airflow over it), so that is another potential source of changing moments.

Perhaps the phrase "aerodynamically relieve the air loads" means to minimize the aerodynamic force generated by the stabilizer? The stabilizer of a conventional airplane will invariably generate a downwards force when the aircraft is in pitch equilibrium at high speed, but it may be upwards at low speed, even with the C of G within limits. In such cases, the speed at which it has no load will be an intermediate one, and therefore could be either above or below the speed to which the airplane is currently (mis-)trimmed.

The quoted passage says it is referring to correcting what it calls a mis-trim, so if your interpretation is correct, then where it says "in-trim speed", it actually means "the speed to which the airplane is mis-trimmed", while the general usage of the term "in trim" means trimmed to fly at the intended speed, and I would take the phrase "in-trim speed" to refer to that speed - the speed that the pilots intend to fly at, or, in other words, the speed that this out-of-trim airplane would tend to fly at when put back in trim.

Furthermore, if things are as you describe, then the rule could have been more simply and clearly phrased as "reduce your pressure on the control yoke". Maybe your interpretation is correct, but if so, it seems to me to be an unnecessarily confusing way to describe it.

For what it's worth, my reading matches VBprogrammer's, though I agree with you that much of the language seems unnecessary.

Frustrating to have a question that could be answered quickly by a 737 pilot! (Whether the trim wheel becomes much harder to turn after you set STAB TRIM CUTOUT to cutout, mistrim down, and pull up.)

You make a couple of very good points. And your right, if it were that simple they hopefully would have described it that way.

In general pitch trim runaway incidents are easiest to control at low speeds. It gives you more aircraft configuration options available to try and reduces the aero loads general.

But perhaps they were just not ruling out either.

I have asked around, and the consensus is that you are correct.

This appears to be the case for 737 NG models, as well as the MAX. If it is a dangerous flaw, it is so for all of them.

The bit about the 'effort by both pilots' raises the issue of the 200-hour first officer.

https://www.pilot18.com/wp-content/uploads/2017/01/B737NG_FC...

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