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Comment by wereHamster | Hacker News Reader

> Aerodynamic loading prevented manual trimming using the trim wheels.

Are the trim wheels mechanically connected to the trim mechanism in the back? Is there no way to trim the horizontal stabilizer electrically without also enabling MCAS? What would happen if the pilots continually pressed the trim up switch on the yoke? That should've prevented MCAS from activating, right?

I thought Boeing airplanes were built such that "the pilot is always in control". That doesn't seem to be the case anymore, if by cutting the power to certain subsystems you can't control the airplane anymore.

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Yes they are manually connected but not mechanically assisted in any large (maybe any) part and the nose down trim and full back on the yoke made the manual wheel impossible to use. Thumbing the switch will work to disengage the mcas temporarily yes.

I think the only way to survive this is to use the electrical trim assist to get to level flight and then shut off the system. After it’s level you can go ahead and trim using the manual wheel. Well the other way to survive this is to not have Boeing design the mcas to read from one sensor only and also have mcas operate in a narrow authority.

Yes, pilots selecting "up" on the trim switch should stop and override MCAS. But it also "resets" MCAS so that it will pitch down again on its next cycle. So you'd have to be constantly correcting it, or switch the electric trim off entirely (i.e. treat it as runaway trim).

Or toggle the trim control switch when MCAS starts up again.

this video[0] from "mentour pilot" on youtube explains the stabilizer system quite well.

it seems the trim wheels are connected manually to the trim mechanism in the back, but when the trim stabilizers are in an extreme position the wind loading on them makes it very hard/impossible to rotate the trim wheels manually.

from what i understand, one way to relieve/lessen the wind loading so that the manual wheels can be used again is to (in the case of trim down) dive the plane. this would lessen the wind loading on the stabilizer and allow the manual wheels to move again. but you would need "room" to dive, something they did not have in this case.

[0]https://www.youtube.com/watch?v=xixM_cwSLcQ

Its possible and I think it was demonstrated in one of Mentour Pilot's videos that both pilots can work together to manually trim. Boeing put notes in the manual that this may be needed and it would not break the manual trim system to apply that much force to the trim wheels.

the fact that the combined strength of two humans would not be enough to _break_ the manual trim system does not tell us whether that combined strength would be enough to _turn_ the trim wheels at all.

https://news.ycombinator.com/item?id=19575318 "option 2)" (on this same page) refers to a situation where a "dive" could be executed in a situation where manual strength is not enough to trim the aircraft.

Recall that at the same time it's taking the strength of both pilots on the yoke to keep the plane approximately level. You're saying they should both let go at low altitude and see if they can crank a trim wheel together?

> Is there no way to trim the horizontal stabilizer electrically without also enabling MCAS?

Correct. One wonders if this is a consequence of trying to maintain the type rating and therefore not being able to introduce any pilot training about the MCAS feature, and therefore not being able to introduce any control features for it.

Good question. This is what makes these accidents so bad in my eyes: they weren't caused by a defect or flaw in the traditional sense, but rather by a system that seems to have been conceived solely to allow Boeing to skirt retraining/recertification requirements, to sell more planes.

Such a system should have been subject to extreme scrutiny, and honestly it beggars belief that the potential for these accidents wasn't seen. "This system can trim the nose down beyond the authority of the manual controls, based on readings from a single non-redundant sensor" should have been more than enough to get the gears turning, and it seems that the deeper one looks it only gets worse.

I thought all jets had to be capable of performing non powered glides (as in no electrical power, not just engine power).

I guess that's no longer a requirement.

The planes when setup correctly have a pretty good glide slope actually. When there is no power the pilots can deploy the apu to restore basic power for flight controls.

The 737 has a better glide ratio than a Cessna 172 when both set up correctly.

I assume you mean the Ram Air Turbine (https://en.wikipedia.org/wiki/Ram_air_turbine)?

ah yeah. got my initialisms mixed up. the apu is not an emergency source of power.

Being capable of performing a glide without electricity does not necessarily imply that a no-power glide can safely be entered from extreme conditions like diving too fast with trim set to insane.

But being able to actually utilize all your control surfaces without electrical power must be a basic requirement.

They could make the trim control wheel mechanically assisted, for example (since there are cases where electronic trim control is required because because of the amount of force needed to turn the wheel)

> They could make the trim control wheel mechanically assisted

I'd imagine it already IS mechanically assisted. There is a limit to how much you can amplify a pilot's input mechanically, and I'd imagine the load on the aircraft exceeded the amount of amplification needed.

I.e. your car has power steering, so the car manually adds power to the turn based on the user's input. That doesn't mean that something in the road can't override the input of both the driver and power steering and jerk the wheel.

Why is it possible to set the trim to insane? Is there a legitimate scenario where that would be used?

If the AoA was truly way too high then insane trim would be required to correct that. MCAS and similar systems came about in response to AF 447 where the pilots entered a high AoA situation and stalled the plane into the Atlantic.

Is it necessarily true that you have to use the stabilizer for MCAS? Would implementing a stick pusher instead fail to avoid some stalls?

It's true in the case of the 737 MAX where the pitch-up moment caused by the engines exceeds the elevator authority in a low-speed, high-thrust situation if the pitch trim is also set too high.

MCAS will evidently work with less maximum nose-down trim than the current version has, as the proposed update places a limit on it.

Well, the plane might be at an insane angle, so then you need an insane trim setting to correct it :)

Sure, but that's assuming the plane was trimmed somewhat correctly to start with. The only reason the pilots had problems manually moving the trim was because the electric motor moved it far out of the correct trim range.

Also, it's not zero power. A ram air turbine deploys and provides hydraulic pressure to move the controls. And there are batteries to keep a small number of essential flight instruments powered.

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