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Pitch trim - poorly understood?

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Pitch trim - poorly understood?

Old 30th Mar 2016, 14:10
  #21 (permalink)  
 
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Back to my question: the trim gains are a function of BOTH the autopilot mode AND the configuration, right?
Well, yes in a way. It is actually a function mainly of configuration. Both the autopilot stabilizer trim circuit and the main electric trim stabilizer circuit have two speeds, slow and fast. Fast is used with the flaps extended, slow with flaps retracted. The main electric trim (manual trim) is faster than the autopilot trim iirc. And of course, the fallback mode, manual trim, is as fast as you can turn that trim wheel.
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Old 30th Mar 2016, 14:46
  #22 (permalink)  
 
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hec7or - your post is really interesting.

I posted this extract from the B737-800 FCOM in R & N but it was largely ignored,
During high AOA operations, the Stall Management/Yaw Damper (SMYD) reduces yaw damper commanded rudder movement. The EFS module increases hydraulic system A pressure to the elevator feel and centering unit during a stall. This increases forward control column force to approximately four times normal feel pressure. The EFS module is armed whenever an inhibit condition is not present. Inhibit conditions are: on the ground, radio altitude less than 100 feet and autopilot engaged. However, if EFS is active when descending through 100 feet RA, it remains active until AOA is reduced below approximately stickshaker threshold. There are no flight deck indications that the system is properly armed or activated.
the crucial part is the increase in control column force provided by the EFS Module by increasing system A pressure to the feel and centering unit which IMO could result in the PF over trimming. What is not clear to me, is if the feel force is increased, will the aircraft feel more out of trim than it actually is?
I would love to know the answer to that one, potentially in the low speed high AOA option the pilots recovering from it and maybe the effects of those quick blips on the trim having a magnified effect..... especially if it is combined with any possible thought process of use the trim if not getting full authority using elevators only !
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Old 30th Mar 2016, 16:11
  #23 (permalink)  
 
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Originally Posted by xollob View Post
hec7or - your post is really interesting.




I would love to know the answer to that one, potentially in the low speed high AOA option the pilots recovering from it and maybe the effects of those quick blips on the trim having a magnified effect..... especially if it is combined with any possible thought process of use the trim if not getting full authority using elevators only !
The opposite xollob. The amount of required force to push the control column forward during a AoA event is significantly reduced making it far easier to reduce the angle of attack and so less trimming will be required.

Just don't stall upside down!
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Old 30th Mar 2016, 17:10
  #24 (permalink)  
 
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Originally Posted by Sciolistes View Post
The opposite xollob. The amount of required force to push the control column forward during a AoA event is significantly reduced making it far easier to reduce the angle of attack and so less trimming will be required.

Just don't stall upside down!
You see, every day is a learning day, rather learn that one away from a live environment

I can see myself asking to experience that in my next sim to see if there is any noticeable difference.
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Old 30th Mar 2016, 18:30
  #25 (permalink)  
 
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The elevator feel shift module will (as far as I understand) INCREASE the force required to move the control column further aft in order to prevent a worsening situation. Never heard of it reducing forward force.
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Old 30th Mar 2016, 18:45
  #26 (permalink)  
 
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sciol

your explanation would make sense only if there was a reduction in hydraulic pressure to the feel and centring actuator during forward control column displacement.

the FCOM says that the EFS module commands higher system pressure to the elevator feel and centring unit during a stall resulting in higher forward control column force.

In normal flight, feel is provided at higher airspeeds by the feel control computer which increases hydraulic pressure to the feel pressure hydraulic actuator which increases the centring force on the control column.

It appears from the FCOM, that the EFS module commands higher "forward control column force" at low airspeeds by increasing hydraulic system pressure to the feel and centring unit.

Could it be poorly worded or do they really mean that centring force is increased at high AoA or are we actually talking about an increase in forward control column force?

I agree with nick14, i think what they mean by the term "forward control column force" is in fact "aft column displacement feel force" but as with everything in the manuals, it is too condensed to make any sense except to the person who wrote it.

Last edited by hec7or; 30th Mar 2016 at 19:31.
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Old 31st Mar 2016, 03:42
  #27 (permalink)  
 
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My apologies fellas,

I think I am correct in what I wrote, but I think I got two separate elements of the system confused. Yes, I think you are correct, the EFS will make it more difficult to move the stick aft. The crucial bit is that the aircraft is however automatically trimmed nose down by the speed trim system.

I interpret therefore that the net result would seem to be that, fortunately as I wrote, in terms of the required force, it is easier to lower the nose and that less trim would be required. Do you guys concur with that explanation?
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Old 31st Mar 2016, 04:17
  #28 (permalink)  
 
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Flying the fly-by-wire Airbus in direct law indeed makes this much clearer.
The situation is the same, artificial feel on the control, and a completely separate trimming of the horizontal stabilizer.
But the sidestick is just centered by a spring so it is obvious that it's load will not change until you release it (and it centers). Just running the trim can't and won't change the load while holding it deflected.
With a conventional yoke somehow the expectation remains that the trim would relief the load.
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Old 31st Mar 2016, 06:42
  #29 (permalink)  
 
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I think the main issue being missed/not appreciated, is that the artificial feel unit is present exactly because without it there would be no feedback whatsoever! It is there to provide a dynamic load against which the controls are moved, but a critical point is it generally does not reflect trim condition (ignoring aircraft response, the controls feel the same whether trimmed or out of trim, for a given airspeed).

The hint that the aircraft is not trimmed is in the fact you must hold a pressure against the artificial feel unit in order to maintain a desired pitch.

Last edited by ECAM_Actions; 31st Mar 2016 at 06:53.
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Old 31st Mar 2016, 14:18
  #30 (permalink)  
 
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I have done many go-arounds in a 737-300 full flight simulator and that includes from well below Vref. Have also simulated the Turkish Airlines 737 coupled approach at Amsterdam where the radio altimeter defect caused the thrust levers to close allowing the Vref to reduce to Vref minus 30. On that occasion the stab trim winds back a long way as the AP attempts to hold the ILS glide slope.
Providing the stab trim is toggled forward for a few seconds at the same time the GA commences and the aircraft pitches up because of the effect of GA thrust, the GA is relatively straight forward.

But if you are not up to scratch with a manual GA on instruments in IMC, especially if you are slow to operate the stab trim in conjunction with forward elevator to ease the stick forces as the pitch up occurs, then the pitch angle can quickly increase to a dangerous angle.

If the GA starts at an already slow airspeed well below VREF, then the trick is not to call for Flap 15 until passing VREF in the climb. Calling for Flap 15 at Vref minus 30 from a Flap 30/40 landing approach is most unwise as the aircraft will almost certainly stall. Treat the GA like a Windshear escape manoeuvre and don't touch the gear or flaps until the climb is safely under control and speed is beyond VREF and increasing slowly.
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Old 31st Mar 2016, 14:18
  #31 (permalink)  
 
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Originally Posted by Gums
are there indicators that let the pilot know the current trim position of the stab/ailerons/rudder?
Here're my indicators:


This is on one of the centre displays. Whether one would have the wherewithal to look at the stab position if fighting the beast on a dark and stormy night is debateable....

In the cruise the stab is around 0-1; on approach 5-9.
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Old 31st Mar 2016, 17:55
  #32 (permalink)  
 
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It's a fundamental procedure that you do not retract flaps until above Vref surely?

Approach to stall recovery first then when above Vref "return to desired flight path" IE start the missed approach procedure.
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Old 31st Mar 2016, 18:17
  #33 (permalink)  
 
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Thnx, Bloggs, what I had hoped.

My thot was having a way to tell if the A/P had run in a gob of trim due to fuel balance, icing, whatever. Then, upon disconnect you are confronted with an unusually large amount of required stick/yoke force.

Agreed about prolly not noticing when very BZ. But you might take a peek if holding in or close to icing conditions.
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Old 31st Mar 2016, 19:12
  #34 (permalink)  
 
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Haven't seen that kind of display on any 737 though (yes, i'm cautious, there are more options than anybody knows). Usually it is just the stab trim scale next to the big clunky trim wheels, and a small white pointer moving mechanically above it. Not at all in the primary field of view, but usually not needed either.
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Old 1st Apr 2016, 00:31
  #35 (permalink)  
 
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I am unfamiliar with the aircraft and the control systems of which you speak, but the system you describe OK465 sounds like it may be in place to assist the pilot to avoid the 'super stall' regime common to T tail aircraft?
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Old 1st Apr 2016, 02:47
  #36 (permalink)  
 
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Possibly, but more specifically the special system on the MD-90 was directed at providing handling qualities that would more closely match the previous iterations of the DC-9 (-80s included) and allow for a common type rating from the DC-9-10 all the way through the MD-90.
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Old 1st Apr 2016, 03:06
  #37 (permalink)  
 
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OK465,

Hmm, for cross crew qualification requirements the appropriate advisory material is the
AC 120-53.

Common handling, i.e. recovery technique required, in the stall regime is one thing but if the system of which you speak is only active in that regime, I am not sure it would be required to make the advisory requirements.

I will have to think about that.
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Old 1st Apr 2016, 09:36
  #38 (permalink)  
 
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I own up to being the author of the original article quoted by FGD135 at the start of the thread. I apologise to him if I didn't make myself totally clear about what happens when the pitch trim is moved while still holding the column forward or aft at a constant force; the forces do not change (though as has been pointed out by others in extreme situations when as hec7or says the Stall management/Yaw damper system may intervene, so everything may well feel different - I've never been there). I had hoped the message was clear enough without inelegant repetition. I am pleased that this little article has caused some head scratching, which is exactly what it was intended to do. The Big Lesson was meant to be 'Know your control system'. Sadly far too many people, including especially the trainers who should know better, don't think it through thoroughly. I hope people read the whole article and reflect on the other situations I quote and think what they would do. I note Sciolistes' view that this is all a storm in a teacup... I wish it were, but I continue to see examples of crew actions that show understanding could be improved.
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Old 1st Apr 2016, 12:53
  #39 (permalink)  
 
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It certainly got me thinking gonebutnotforgotten.
I took off this morning in a light 737-800 with plenty of thrust and paid close attention to what I was doing with the trim and yoke as I cleaned up.
A fair amount of fwd trim was required, but I noticed that I was automatically ( without thought) reducing the pressure on the yoke so that it came back towards neutral as the trim was going in.
It seemed it is a natural thing to do even though I confess my understanding of what was happening was low prior to reading your article.
Do you think, or is it your experience, that in an extreme ANU situation I would have kept the yoke forward and then as the nose came down towards the horizon had to apply large back pressure to prevent an extreme AND?
Is that what you see in the sim?
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Old 1st Apr 2016, 13:06
  #40 (permalink)  
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gonebutnotforgotten,

Thank you so much for joining us in this thread and thank you so much for that original article.

Just a few minutes ago, I read an update on the FlyDubai crash. That update came from a short report, in Russian, issued today by the MAK. It said that "on reaching 900 m, there was recorded stab travel to the nosedive position, resulting in aircraft beginning descent and hitting runway".

That must be a sadly familiar scenario to you, as it sounds exactly like the simulator account you gave in your article.
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