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Pitch trim - poorly understood?

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Pitch trim - poorly understood?

Old 29th Mar 2016, 12:35
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Pitch trim - poorly understood?

Hi all,

The accident to the FlyDubai B738 at Rostov-On-Don recently could have been due to "mistrimming". The vertical profile of this accident is almost identical to that of the Air Tatarstan B735 that crashed at Kazan, Russia in 2013.

In both cases, during a missed approach, the aircraft reached a height of several thousand feet above the aerodrome then rapidly pitched down, diving into the ground beside the runway. Both crashes were fatal. Neither appears to have stalled.

Is the pitch trim system of today's airliners poorly understood to the point that aircraft are crashing?

Please refer to this article on the subject:

Roger-Wilco | Do you really understand how your trim works?

Here is an extract from it:

I have watched, in the simulator, a 737 go-around from a Cat lll fail passive approach (as described above) with its marked pitch up; HP kept his arms locked forward to contain the attitude whilst simultaneously running the trim forward with the thumb switch. I am sure he was expecting the trim to reduce push needed and he either didn’t know, or had forgotten, that it wouldn’t. We duly pitched straight back quickly into the ground as the tailplane incidence ‘bit’.

This account sounds a lot like what could have happened in the two accidents I referred to above.

So, what does actually happen when, in a type like the B737, you hold a particular attitude - against the feel forces - then run the trim. Do the forces reduce to zero, or do they stay the same?

Incredibly, the linked article doesn't actually say! Here is the first paragraph:

Picture yourself in a conventional airliner, say a 737 of any generation. You have to do a low level go-around, perhaps because your fail passive Cat lll has just failed, er, passively. You apply GA thrust, and the aircraft pitches up. If you are low enough, you may already have some extra helpful nose up trim applied thanks to the ‘design feature’ that ensures that in the event of AP failure at low level, the aircraft pitches up not down, and so a few units of nose up trim are applied late in the approach. Your speed is low, about Vapp and the thing is pitching firmly upward. You need ample forward stick/elevator to restrain it. You don’t want to carry this load for long so you retrim. Question: if you run the trim forward while maintaining forward pressure on the wheel, what happens? Hands up all those who think the load reduces to zero. I see a lot of hands. My unscientific polling to date suggests that just about everyone is convinced that this is what happens, but it doesn’t.

Frustratingly, the author doesn't clearly spell out what does actually happen, but reading between the lines, he seems to be saying that the forces DO NOT reduce to zero.

What happens on the B737? And, is it any different to all the other airliners that have trimmable horizontal stabilisers?
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Old 29th Mar 2016, 12:48
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Ok ....pitch main control is the elevator(you use the control column).
Stabiliser is used by automatics or manual to relieve force on the elevator.
Stabiliser is used by the speed trim system to allow speed stability and elevator is used at higher mach number for speed stability and avoid mach tuck.
The stabiliser is the back up for elevator jam,elevator is back up for stabiliser jam.
Easy no?
Dont over trim is the main point,thrust can be used to reduce load,and avoid over trimming.

Some system helps the trimming (STS)and during every t/o ,a nose down trim,to force pilots to trim up at a certain height(about 400 feet or so)just in case an engine fails,so aircraft is a nose up trim.(system is part of mach trim).

Once again,go arounds require very little trimming and so do any other maneuver..flying manually needs also only very few clicks on the stabtrim here and there,normally while banking,1 or 2 clicks NU or when selecting flaps,intially would require nose down,(lift increase),but manul elevator enough,then when stable in speed,1 or 2 click of NU.
Selectionof flaps 40 for example on AP will command a significant trimming about 3 secs or so because the AP is reacting to drag and speed decay,pitching the ND, while on manual flight, not reducing thrust as much as the automatics would before/during selecting flaps 40,would require much less stab trim.
Pilots know in advance what will happen based on their input,,AP are only REACTING to an input.

Last edited by de facto; 29th Mar 2016 at 13:45.
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Old 29th Mar 2016, 12:53
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The stabiliser moves therefore you are always attempting to achieve a control column CENTRAL position. As you trim you should relax pressure on the controls so that they gradually return to the central position without a significant change of attitude.

This is different to most light aircraft as you are trimming the pitch control to a new position NEURTRal.

The two are very different.
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Old 29th Mar 2016, 13:29
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How the trim REALLY works

Salute!

"TNX FG for bringing this over here from "rumor central".

For this old pilot, even tho he flew first operational fully FBW lite a few decades ago, I am fascinated by all the connections with the AP and such.

If I understand other posts, the AP trim "assistance" has two sets of gains and attempts to re-position the stab at different rates depending upon configuration. I am somewhat familiar, as our Viper changed gains, hence physical movement of the stabs, according to the force exerted. However, as with conventional flight control systems, we trimmed to reduce control stick pressure ( not movement).

Would really like to see this thread take a life.
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Old 29th Mar 2016, 14:57
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Gums,
The 737 has an elevator feel computer which provides pilots a feel of their aircraft,via control force column,using the actual position of the stabiliser and aircraft speed.
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Old 29th Mar 2016, 16:44
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I learned about this when I did my original conversion course on the B737-200 in 1980.
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Old 29th Mar 2016, 16:48
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Arent we all very proud you did..
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Old 29th Mar 2016, 16:50
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Arent we all very proud you did..
So is this not taught on conversion courses any more?

Basic a/c control which all pilots should be aware of on this type surely?
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Old 29th Mar 2016, 18:17
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Obviously not everywhere..too busy teaching all failures as quickly as possible and get done with it.
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Old 29th Mar 2016, 18:18
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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?
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Old 29th Mar 2016, 18:18
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So, what does actually happen when, in a type like the B737, you hold a particular attitude - against the feel forces - then run the trim. Do the forces reduce to zero, or do they stay the same?
It is quite simple and the linked article is I guess correct but kind of a storm in a teapot. If you hold the elevator forward in that go around example, say 2kg (just an example) of force is required, to hold 15 deg nose up, if you trim down and continue to hold about 2kg the aircraft will pitch down. If you trim down and reduce forward force commensurate with the trimming (allow the control column to return toward the centre) then the aircraft with remain at 15 deg nose up (all other things being constant of course).

This is because the Elevator Feel System will generate a simulated aerodynamic force for the commanded elevator position and airspeed (and stab position, but how much and in what way isn't documented). As you centralise the control column then the elevator is centralised but the pitch maintained by the stab position.

So if you hold a set pitch attitude and trim then you will need to relax the pressure on the control wheel - the article seems to be making the utterly intuitive reflex an issue.

The article then criticises this:
To recover from a stall, angle of attack must be reduced below the stalling angle. Nose down pitch control must be applied and maintained until the wings are unstalled. Application of forward control column (as much as full forward may be required) and the use of some nose-down stabilizer trim should provide sufficient elevator control to produce a nose-down pitch rate. It may be difficult to know how much stabilizer trim to use, and care must be taken to avoid using too much trim. Pilots should not fly the airplane using stabilizer trim, and should stop trimming nose down when they feel the g force on the airplane lessen or the required elevator force lessen. (my emphasis)
It is absolutely correct. To recover from a nose high condition, the elevator may not be sufficiently effective. It says "the required elevator force lessen". In other words the amount of deflection (i.e force) required to reduce the angle of attack sufficiently to break the stall or recover.

For example, if you try the Turkish AMS event in the sim, recovering from the stall requires immense amounts of forward pressure to prevent the nose from rising with go around thrust. To go from full aft trim to something like say 7 units takes about 14 seconds, which feels like a long time. It takes several seconds of trimming to actually feel the elevator start to have a reasonable amount authority and the amount of required force to reduce.

Last edited by Sciolistes; 30th Mar 2016 at 02:01.
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Old 29th Mar 2016, 18:28
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The feel of the aircraft is 99% of the time as expected.
In some occasions the feel is wrong and maintenance is advised.
The aircraft control column could feel "over sensitive" or "heavy".
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Old 29th Mar 2016, 20:54
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"feel" and control position

Salute!

Lemme see if I have this straight, Betwixt ( Sciol xxxx), Gotta love those cryptic/cute locations, huh?

So the 737 flcs ( flight control system) tries to provide feedback like the old, really old planes ( not fully hydraulic or hybrid systems, but more like a Moth, Chipmunk, Champ, Camel, Warhawk, etc) did with mechanical connections and you could actually "feel" the aero forces or lack thereof. Is this all the time or with only AP engaged? Does it use actual dynamic pressure and static pressure? Are pitch rates integrated? and so forth.

I have a hard time understanding why we are not using a straightforward procedure that requires one to maintain a notional attitude regardless of control pressures or seat of the pants cues. I like trim. I do not like holding "x" pounds of pressure or deflection more than I have to. I also get a good "feel" for the jet when it is trimmed and I note how far off of "normal" the surfaces are positioned.

Looking fwd to more explanations of trim implementations here.
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Old 29th Mar 2016, 21:30
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General
The elevator feel computer receives pneumatic, hydraulic, and mechanical in- puts. With these inputs, the elevator feel computer sends metered pressure output to the elevator feel and centering mechanism.
Physical Description
The elevator feel computer has these components:
- Stabilizer actuated cam (2)
- Q diaphragm (2)
- Droop spring (2)
- Relief valve (2)
- Force balance valve (2)
- Differential pressure switch.
The elevator feel computer has these ports:
- Pneumatic pitot port (2)
- Pneumatic static port (2)
- Hydraulic pressure port (2)
- Hydraulic return port (2)
- Hydraulic computed pressure port (2).
Functional Description
Hydraulic pressure from the system A and B flight control modules goes to the pressure ports.
As the airspeed increases, pitot pressure goes through the pitot port and pushes on the Q diaphragm.
The Q diaphragm moves and pushes the relief valve. This movement is in pro- portion to the airspeed. The stabilizer actuated cam position affects the range that the Q diaphragm can move due to pitot pressure. When the relief valve moves, it pushes the force balance valve. The force balance valve controls the pressure that goes though to the dual feel actuator. The dual feel actuator is part of the feel and centering unit.
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Old 29th Mar 2016, 23:45
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Thanks for the reply, 'facto"

Seems like a lotta stuff to provide basic "feel", but I have never flown a heavy.,

I shall wait until some heavy pilots comment that have a few thousand hours in the 737.
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Old 29th Mar 2016, 23:58
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Originally Posted by Gums
I shall wait until some heavy pilots comment that have a few thousand hours in the 737.
Careful, Defacto's forgotten more about the 737 than you'll ever know...

With respect, Gums, the 737 was designed before your F16 was a twinkle in someone's eye. Companies don't just start completely redesigning FCSs just for stamps when the latest gadget comes along. The 737 works. Why change it?

The message that is reinforced from this discussion, for me, is "don't fly with the trim". Push and pull all you like but only use the trim to remove sustained pressure on the stick after you've been holding the same attitude. I hadn't really appreciated the stick position after trimming the stab, but yes, it's always in the same position when trimmed. I looked at it yesterday!
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Old 30th Mar 2016, 02:37
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... it's always in the same position when trimmed. I looked at it yesterday!
Good info, Bloggsy, very good info.

And the artificial feel? That is a function of the relative positions of the elevator/stab?

What I understand, and would like you to confirm, is that when the elevator and stab are "aligned" (or "neutral") there are no feedback forces. That is, the feedback forces only start when you deflect the elevator away from its "neutral" position with respect to the stab.

This would seem highly logical, and what I'm really after is this confirmation. If confirmed, that would confirm a number of other highly significant things, in relation to trim.
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Old 30th Mar 2016, 04:45
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Salute!
Does it use actual dynamic pressure and static pressure?
Yes it does.
So, if speed data goes wild. As example, an amazing speed due Pitot tube clogged with ice. Are automatically deactivated the "feel" computer and the STS (speed trim system)?
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Old 30th Mar 2016, 09:26
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The FCOM is delightfully vague on this matter. Any sustained differential pressure at the elevator feel unit (as caused by a hydraulic or elevator pitot failure) illuminates the FEEL DIFF PRESSURE light and associated master caution. However, the QRH is a one liner saying "continue normal operation". However, a quick look in the MEL tells us this is a NOGO on the ground.

Hopefully somebody with access to the AMM (de facto?) can enlighten me more. There is no indication that the Speed Trim System is deactivated. However, if you are manually trimming, the speed trim is deactivated and remains so for 6 seconds after you stop trimming.

I do find the complete lack of joined up info in the FCOMs and QRH very frustrating...
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Old 30th Mar 2016, 13:30
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Great stuff, Bloggs, and I am "in the zone" with you and defacto.

I did not intend to diminish the experience and knowledge of defacto, and should have said "waiting for MORE heavy pilots to add to the thread".

Interesting to find use of a "bellows" equivalent to our older fighter flight controls that used them to keep us from ripping the wings off when snatching on the stick.

As far as flying using the trim, I think I understand your position Bloggs. Our problem is that some of the FCS trim implementations move the stab so much that we have almost no elevator authority. The bloggers on the AF447 thread that had sim time and tried to duplicate the deeply stalled, and trimmed stab used very positive manual trim and then 15,000 feet to regain control!!

Back to my question: the trim gains are a function of BOTH the autopilot mode AND the configuration, right?

And BTW, are there indicators that let the pilot know the current trim position of the stab/ailerons/rudder? I am assuming the trim implentation moves the stab to reduce yoke/stick forces required to maintain an attitude or AoA and does not "position" the elevator.
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