F.A.O.:Gabiee- Re: What's the trim doing now?

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3WE
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F.A.O.:Gabiee- Re: What's the trim doing now?

Postby 3WE » Wed Mar 13, 2019 6:37 pm

Late in my 172 training...

Lift off...Instructor reaches down...cranks trim wheel full nose up...

3BS pushes (the damn trim tab can generate some fairly amazing forces)- nose stays on the horizon, speed at 80 MPH, climb at ~800 FPM.

Then instructor reverses and proceeds to crank it full nose down.

3BS pulls (the damn trim tab can generate some fairly amazing forces)- nose stays on the horizon, speed at 80 MPH, climb at ~800 FPM.

After another minute, 3BS says, "May I now trim the aircraft?" Instructor: "Yes".

Lessons:

1- Damn glad something in the control linkage didn't break.
2- Maintain attitude.
3- Fly the GD airplane.
4- Ignore trim forces.

Realism disclaimers:

1. Yes, I saw the instructor reach down and my SA told me, "he's going to dick with the trim to teach me a lesson"...no startle factor
2. Crummy performance, simple airplane with a ton less stuff to keep track of and less of a tendency for crazy speed excursions...not nearly as critical as a 737-MiniLav.
3. "Ignore trim forces" will not give you the elevator authority you need to overcome a lot of things (including busted jack screws and apparently whats-MCAS-doing-now- Even though the trim supposedly still works).

Questions to Gabieeee:.

1. Did you ever do something similar.
2. Did your wimpy ass 2-seat 115 HP trainer generate any significant control forces (in contrast to my big manly 172?) I would testify that 150 control forces were a LOT less (never had the trim demo, but to make those two stall....let's say that the 172 required fairly relentless PRESSURES while the 150 was more like relentless DISTANCES...

/Parlour-what-I-would-have-done-if-I-was-the-Lion-air-pilot.

(Your reply is genuinely appreciated, thanks, in advance)
Commercial Pilot, Vandelay Industries, Inc., Plant Nutrient Division.

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Gabriel
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Re: F.A.O.:Gabiee- Re: What's the trim doing now?

Postby Gabriel » Wed Mar 13, 2019 7:19 pm

I never practiced a simulated out-of-trim situation.

But as part of the before engine-start preparation you have to put the trim in neutral. Because the trim position indicator is not very reliable, the procedure at my flight school was to trim all the way up to the stop and the trim down 4 and 1/2 strokes. I can say that pushing the yoke forward when trimmed all the way back required interesting but not outstanding forces.

The trim system of the Tomahawk, the Cessna and the 737 are quite different.

The tomahawk is simply a spring that pulls in one direction or another, adding a force to the aerodynamic forces.
The Cessna is the tab that you mentioned.
The 737 (and all planes with hydraulic elevator and then some others like the J3 and the DC-9) have a movable horizontal stabilizer.

There are important differences between these systems not only in their design but also in how they affect the pilot's ability to control the pitch.

First of all, forget about the force, or say that the pilot will have as much strength as it will take and the system will withstand whatever force.

The spring and trim tab doesn't restrict the control authority*. As long as you can move the elevator, you will have always all* of the authority and the elevator in any given position will mean* one given AoA. Conversely, if you keep the yoke/elevator fixed in one position, applying trim will have no effect*.

With the movable stabilizer, it is different. If you keep the elevator "fixed", the change in the AoA of the fixed stabilizer will change the lift in the tail as a whole.

That means, in the Cessna/Tomahawk, if your instructor moves the trim you just need to fight the force to keep the stabilizer in place.
In the 737, I am not sure how the artificial feel forces act when you just move the trim (because hydraulic systems don't have real feedback from the elevators to the column), but keeping the yoke and elevator in place against the forces will not be enough. If you want to keep the same AoA and apply nose-up trim, you need to actually displace the elevator forward.

There is like a "shift" in the neutral point of the elevator. That makes that, with the trim full nose-up. your nose-down authority is reduced even if you have enough force to push full nose-down and the elevator responds accordingly by displacing itself to the full nose-down position. There will not be as much nose-down action as there would be if you did the same thing with the elevator in neutral.

And then you have the forces. The Lion Air pilot was pulling with up to 100 lb (per side???) and that was not enough to put bring the plane back to 1G fight (it kept pitching down and increasing its sink rate).

Finally, for the reasons explained above, your "relentless distances" assessment regarding the 150 doesn't make much sense. If you keep the elevator fixed against the trim forces the plane will never note* that the trim was changed.

* Almost, since the tab in a fixed-elevator situation does act a little bit like a mini elevator, but in the opposite direction as the trim. I.e, if you trim up the tab will go down with the "intention" of moving the elevator up, but if the elevator remains "welded" in place the trim tab displaced down acts as a nose-down mini-elevator, so if anything you will need to compensate the application of nose-up elevator with pushing down quite a bit in terms of force but letting the elevator displace a little back/up.

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Re: F.A.O.:Gabiee- Re: What's the trim doing now?

Postby Chris Foss » Wed Mar 13, 2019 8:48 pm

That's a very good response Gabriel. Agree with everything.

A little addition to the 737 system:

There is also a balance tab as well as stab angle adjustment.
When I give a pilot a CG calculation, the loadsheet will also give the corresponding stab angle for departure.
The balance tabs balance the forces for elevator and sensor simulate the forces to give pilots feedback.

My question for Boeing is: why does MCAS control the stab angle instead of the balance tab?

Their answer would be: because we reserve the right to have authority over a pilot trying to stall, and we made the 737 MAX too easy to stall with funny feedback.

As I pilot I would respond: "Who's in charge here?"

And it seems it's not the pilots.

also, some aircraft change CG and trim automatically already.
During take off, 747 400s have up to 8,000 kg of fuel in the stab tanks, once fuel feed is switched from wing to centre tank and the centre tank has burned enough fuel, the pumps transfer the stab fuel to the centre tank to be burned from there.
This causes a significant change in CG as fuel is burned from tanks on different moment arms, then when up to 8 tonnes is transferred from stab tank to centre tank which have opposing CG effect, it is the aircraft that automatically changes the stab angle in flight through these changes, to ensure the aerodynamics are trimmed to the desired speed and pitch authority.
As the stab moves in flight, the elevator deflection is controlled to maintain constant attitude (unless commanded otherwise) and the balance tab will adjust to balance the forces.

So when that trim wheel rolls away, I believe it's the directly linked the the trim tab which is being moved because the stab is being adjusted because the CG is changing.

Same tiny effect when 5 people go to the loo or crew is wheeling heavy carts up the aisle: you hear the trim clicking away as this whole system is adjusting to fuel burn and speed changes.

Am I correct in assuming full manual only gives you trim authority to trim tab and not the stab angle?

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Re: F.A.O.:Gabiee- Re: What's the trim doing now?

Postby Chris Foss » Wed Mar 13, 2019 8:55 pm

I believe the MCAS links to the system that automatically adjusts stab angle, which if subject to runaway is beyond the authority of the elevator to overcome,
If it were linked to the balance tab, it would provide strong forces to the stick enough to counter the effects of the big engines and placement and give a good indication that it doesn't want to stall, but not strong enough to resist the authority of a pilot.

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Re: F.A.O.:Gabiee- Re: What's the trim doing now?

Postby Chris Foss » Wed Mar 13, 2019 9:19 pm

There is another incident that may have a small bearing on MCAS.
How does it handle at the limits of CG position?

Let's say for some bizarre reason the aircraft is loaded at the fed CG limit (16% ish depending on act wgt)
But because of notional weights the fat dudes and heavier bags were up front of each cabin and all the skinny women at the rear of each cabin, Etc. So the actual CG is even further forward.
But the aircraft has safety margins to cope with this defined during testing and certification and the conditions of notional weights.

But now the loadsheet numbers are fed into the FMC and take off trim commanded which moves the stab to full up trim for that Nose heavy CG position.

That's where the authority of the whole rear stabiliser to produce enough down force at low speed for its size limits the forward CG. Any further forward and you'd have to get a bigger stab/elevator as the existing size will start to stall.

Nose heavy CG also makes the aircraft fly with a slightly higher alpha than CG on the aft limit so slightly more drag.

But in terms of MCAS, with the stab needed at that incidence for the elevator to have authority, even a momentary reduction of stab incidence could place the elevator in a position where it is stalling (losing attached flow on the low pressure side).

The opposite happened to the Cargo 747 in Iraq when the cargo shifted so far back, despite full pitch forward command possible, the rear stab could not produce the sufficient forces to overcome the CG so far aft on the MAC.

I think there may be connection to CG, but that's an ignorant hunch.

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Re: F.A.O.:Gabiee- Re: What's the trim doing now?

Postby 3WE » Wed Mar 13, 2019 9:20 pm

Finally, for the reasons explained above, your "relentless distances" assessment regarding the 150 doesn't make much sense. If you keep the elevator fixed against the trim forces the plane will never note* that the trim was changed.
Eye roll emoji...

I didn't ask for the lecture on trim systems (I am familiar with those that you describe)

I don't think Lion Air is simply a matter that the dudes were too stupid to pull up.

What I do think is that pulling up should buy them SOME time...more than enough time to say, "oh crap, let me get some nose-up trim going

I don't know what the exact middle ground is here...I think we (yes italicized) don't like it that ONE SINGLE stall indicator should turn on a slightly relentless nose-over mechanism...

I also think we think that an ordinary (or even a slightly dumb) pilot should be able to keep the 737-Minilav in the air- even if they forget the exact checklist...that maybe using trim and elevator inputs is pretty adequate.

The 172 vs 150 trim situation was more of a joke-based-on-truth...A 172 sort of needed trim to fly comfortably...A 150 sort of needs trim "only" so you can take your hand off the wheel and scratch your nair regions without it climbing or diving...but it's not that much effort to fly untrimmed or even to stall it...just gently pull back...until you have a yoke in your stomach...To stall a 172 is a good bicep exercise...

Comprende?

Footnote: It is interesting that El Presidente de Dom Trumpo has grounded the 737-MiniLav...maybe not curtains....at least not yet.
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Re: F.A.O.:Gabiee- Re: What's the trim doing now?

Postby 3WE » Thu Mar 14, 2019 2:07 pm

***more than enough time***
Lazy question: Anyone know how fast this MCAS thingie puts in nose-down trim?...I always thought that trim progressed kind of slow and gentle...But maybe if you need to fight stall that has nose up tendencies :shock: , maybe it needs to kick in faster?
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Re: F.A.O.:Gabiee- Re: What's the trim doing now?

Postby Gabriel » Thu Mar 14, 2019 4:15 pm

***more than enough time***
Lazy question: Anyone know how fast this MCAS thingie puts in nose-down trim?...I always thought that trim progressed kind of slow and gentle...But maybe if you need to fight stall that has nose up tendencies :shock: , maybe it needs to kick in faster?
I might be wrong but I believe that the trim motor in the 737 has only 2 speeds (that I would call slow and slower) and what speed is used depends on whether the flaps are extended or retracted. Extended? Slow. Retreated? Slower. And the MCAS only works with the flaps retracted.

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Re: F.A.O.:Gabiee- Re: What's the trim doing now?

Postby ocelot » Sun Mar 17, 2019 8:09 am

I don't think Lion Air is simply a matter that the dudes were too stupid to pull up.

What I do think is that pulling up should buy them SOME time...more than enough time to say, "oh crap, let me get some nose-up trim going
There is so much wrong stuff circulating...

Examine the FDR traces from the interim report (they're also attached to the avherald article) -- for most of the accident flight there are automatic trim down inputs followed by manual trim up inputs. It's only at the very end that it gets away from them. So there are only really two important questions: (1) why didn't they follow the trim runaway procedure? and (2) what happened at the end that was different?

it will be very VERY depressing if what happened at the end is that they thought they'd stalled and therefore they pulled_up.

Also,
Am I correct in assuming full manual only gives you trim authority to trim tab and not the stab angle?
No. As is customary for Boeing A/C the automation drives the manual controls. The trim wheel on the center console controls the stab angle.

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Re: F.A.O.:Gabiee- Re: What's the trim doing now?

Postby 3WE » Mon Mar 18, 2019 12:40 am

There is so much wrong stuff circulating...
I think we are saying the same thing (except for me being confusing).

Hard to understand what happened at the end.

I hate what we are hearing, but conversely, I’m not sure why you wouldn’t just sit there all day trimming up as needed (stall warning acknowledged as complicating that thought.)
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Re: F.A.O.:Gabiee- Re: What's the trim doing now?

Postby Gabriel » Mon Mar 18, 2019 1:48 am

Hard to understand what happened at the end.

I hate what we are hearing, but conversely, I’m not sure why you wouldn’t just sit there all day trimming up as needed
Exactly, especially when you have been doing that quite successfully for some 5 minutes. Why did they stop doing what was working? It's not like it was a panic reaction to the stall warning which had been shaking (on one side) since rotation.

Have a close look at the FDR charts in page 14
http://knkt.dephub.go.id/knkt/ntsc_avia ... Report.pdf

And why do we have a second crew falling in this trap again? (IF that's what happened, which becomes more certain but I still hope that it was something else) Every airline and pilot flying a MAX should have had a great interest in the Lion Air crash and learn from it to avoid that the same thing happens to them.

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Re: F.A.O.:Gabiee- Re: What's the trim doing now?

Postby ocelot » Wed Mar 20, 2019 3:45 am

Hard to understand what happened at the end.

I hate what we are hearing, but conversely, I’m not sure why you wouldn’t just sit there all day trimming up as needed
Exactly, especially when you have been doing that quite successfully for some 5 minutes. Why did they stop doing what was working? It's not like it was a panic reaction to the stall warning which had been shaking (on one side) since rotation.
I figure something else happened, maybe as simple as they got distracted, maybe some other technical failure, maybe medical (must have been quite a rollercoaster after all) and that was that.

One also wonders why they sat there trimming up and didn't use the trim cutout; maybe they were afraid it would keep trimming down and they'd need to use the wheels to trim back. That was before anyone had heard of MCAS or had got hold of wrong ideas about it; but on the other hand it also meant that they wouldn't know where it was coming from.
And why do we have a second crew falling in this trap again? (IF that's what happened, which becomes more certain but I still hope that it was something else) Every airline and pilot flying a MAX should have had a great interest in the Lion Air crash and learn from it to avoid that the same thing happens to them.
If they did. avherald's now saying that the supposedly independently confirmed reports of unreliable airspeed were bogus, in which case I think all we know is that the jackscrew was found full-forward and that a politician with access but maybe not that much technical knowledge characterized the events as similar. No, I guess the pitch excursions shown by the ADS-B data still stand...

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Re: F.A.O.:Gabiee- Re: What's the trim doing now?

Postby 3WE » Thu Mar 21, 2019 12:28 am

Exactly, especially when you have been doing that quite successfully for some 5 minutes. Why did they stop doing what was working?
Very simple- they switched pilots.

(Yes, that’s not a complete explanation, but a partial one).
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Re: F.A.O.:Gabiee- Re: What's the trim doing now?

Postby elaw » Thu Mar 21, 2019 11:13 am

Am I the only one who's doing some serious head-scratching about the postflight actions of the prior crew of Lion Air?

You'd kind of think maybe they'd mention to someone that the airplane tried to dive into the ground of its own accord and nearly succeeded, and maybe someone should have a look at it before some less-capable flight crew takes it up with a load of pax...

Or maybe said crew did provide proper notification to proper people, but said notified people dropped the ball?
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Re: F.A.O.:Gabiee- Re: What's the trim doing now?

Postby 3WE » Thu Mar 21, 2019 11:40 am

Am I the only one who's doing some serious head-scratching about the postflight actions of the prior crew of Lion Air?...
I think there is a collective “loss of words”.

Would it help if I did an Evan rant about procedures, regulatory oversight and screening?

Conversely- it’s consistent if the crew one botches a plane handoff and crew two botches control hand offs.
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Re: F.A.O.:Gabiee- Re: What's the trim doing now?

Postby Gabriel » Thu Mar 21, 2019 5:13 pm

Am I the only one who's doing some serious head-scratching about the postflight actions of the prior crew of Lion Air?
You are not alone. I asked that a few times in this forum and there. Other people too.


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