Hacker News Reader

Comment by ams6110 | Hacker News Reader

No, the working theory I've heard is that the hand-crank trim wheels may have been overwhelmed by the air load on the stabilizer given its "nose down" configuration, probable full "nose up" pull on the elevators, and the very high speed they were flying. Speculation is they may have turned the electric trim back on to use the manual trim switches to trim up. By that time possibly too late or even the electric trim was overwhelmed by the load, and of course this would have brought MCAS back into play also.

View on HN

So they'd have to let the plane nose down towards the ground for a bit to then be able to trim back up. This sounds like a cascading failure to me.

Exactly. And Ethiopian was less than 1000ft above ground level at all times. They didn't have the altitude to trade.

After watching Mentour Pilot's demo correcting for runaway trim in a 737 simulator, it seems like a switch that cuts out computer (autopilot, auto-speed, MCAS) control of the elevator trim motors, but keeps the column switches working would be nice. Consider how cumbersome & slow the manual wheels are: https://youtu.be/xixM_cwSLcQ?t=1129

It would be one more layer of complexity, but perhaps when the column trim switches are operated, all computer control could stop until it (autopilot, etc) is re-enabled by a human. The cutout switches in the center are still there if two column switches short out (used to be a single up-down switch, but it was single point of failure to runaway so now there are separate enable and direction switches).

There are switches for auto pilot and auto speed. MCAS is disabled with flaps extended. It's not procedure but had they set the flaps to a single degree it would have saved them. I think the Lion flight experimented with flaps briefly.

This is sensible, but recall that Boeing is still claiming that the MAX flies like any other 737 and requires no special training. Adding a new button would require training in the use of that button.

(In case it's unclear, I don't support Boeing's position here.)

This makes more sense, thanks.

I still don't understand why the trim is "stronger" on these planes than a fully pulled back column. Is this necessary on large planes?

probably mostly due to needing to have a larger operation margin for different center of gravity as load changes depending on the current flight passenger/cargo loadout.

Mentour Pilot (as others have mentioned in various other comments) does a great job explaining this.

On the 737 the trim adjusts the entire rear stabilizer (the entire horizontal structure of the tail), while the elevator is a sub-component of that stabilizer.

So, when you adjust the trim, it's a much larger surface area that is moving. Ultimately, the stabilizer just has more aerodynamic control that the elevator, due to the larger surface area.

I have no idea why the design is that way, though.

I have no idea why the design is that way, though.

Cost, maintenance, weight, etc., probably. The only jet airliner I can think of that uses an "all-flying tail" is the Lockheed L-1011. Nearly every other plane you'd be riding on will have a big stabilizer and a smaller elevator.

Redundancy too.

If both redundant hydraulic systems to the elevator fail, or the elevator gets stuck for other reasons, the emergency procedure is to control pitch with just the stablizer trim alone.

Presumably you could have a redundant drive system for the combined stabilizer/elevator. The L-1011, for instance, had a safety record Boeing should be envious of.

One reason is to recover from "mach tuck":

https://en.wikipedia.org/wiki/Mach_tuck#Design_features

Again, instead of relying on speculation, we should wait to get more information.

Agreed. Hard not to speculate though. I personally find this stuff fascinating from a technical and human factors perspective -- the deaths are tragic of course.

This isn't rumor mongering. This is straight out of Boeing's own documentation on the 737: https://www.satcom.guru/2019/04/stabilizer-trim-loads-and-ra...

When the stabilizer is extended and is being counteracted by elevator inputs, the forces on the control column make manually trimming stabilizers difficult or impossible. Boeing instructs that elevator input on the stick must be eased back in order to manually adjust the stabilizers.

Boeing documents possibilities, not what actually happened.

Yes, and?

If you object to people discussing the technical details of a plane w.r.t. the observed behaviour of that plane in an accident prior to the final report on the accident being released might I suggest....not clicking on the comment section, rather than trolling the comments on news items complaining that people are using the website for its stated purpose: to discuss the news being linked to?

And the news article we're commenting on raises the possibility that this is exactly what happened. If that doesn't make the hypothetical germane to discuss here, what can we talk about?

Why? I tend to enjoy informed speculation, that's one of the reason I visit this website.

Isn't speculation one of the things that characterizes us as humans?

Reply on news.ycombinator.com