B-737 Speed Trim System

[ManaAdaSystem]

Speed Trim SystemThe Speed Trim System (STS) is a speed stability augmentation system designed to improve flight characteristics during operations with a low gross weight, aft center of gravity and high thrust when the autopilot is not engaged. The purpose of the STS is to return the airplane to a trimmed speed by commanding the stabilizer in a direction opposite the speed change. The STS monitors inputs of stabilizer position, thrust lever position, airspeed and vertical speed and then trims the stabilizer using the autopilot stabilizer trim. As the airplane speed increases or decreases from the trimmed speed, the stabilizer is commanded in the direction to return the airplane to the trimmed speed. This increases control column forces to force the airplane to return to the trimmed speed. As the airplane returns to the trimmed speed, the STS commanded stabilizer movement is removed.

STS operates most frequently during takeoffs, climb and go-arounds. Conditions for speed trim operation are listed below:

STS Mach gain is fully enabled between 100 KIAS and Mach 0.60 with a fadeout to zero by Mach 0.68


10 seconds after takeoff




5 seconds following release of trim switches


Autopilot not engaged


Sensing of trim requirement






I though I had this annoying system nailed, but when I read what the FCOM says, I’m just getting confused. How can a system trim to return the aircraft to the trimmed speed by trimming opposite the speed change? If I increase the speed this means the STS will trim aft. At the same time increased speed will increase the lift. Aft trim will bring the aircraft into a more out of trim state. I always have to trim opposite the STS which is a nuicance, but it looks like the STS is made to do this.
On top of this, the STS is trimming on all my take offs, not only in the high thrust, aft CG, light aircraft configuration, and I’m pretty sure it trims outside the 10 seconds window as well.
My opinoin, we would be safer without this system working agains us when we hand fly. Those of you who engage the auto pilot passing 400 ft will not understand what I’m talking about, but the few of you who hand fly on a regular basis will.
Comments?

[ManaAdaSystem]

I don’t understand the meaning of «return the airplane to the trimmed speed». And if you need force on the controls, in my head, this makes the aircraft anything but speed stable. Or in trimmed speed, whatever that means.
If I let the aicraft STS do it’s job without interference and let go of the controls, the aircraft will pitch up very fast.
It does not make sense to me.

[Jwscud]

I can’t find my old 737 FCOM but my understanding of the system was that trim was inhibited the moment you applied the control column in the opposite direction, and that included speed trim.

[Vessbot]

I though I had this annoying system nailed, but when I read what the FCOM says, I’m just getting confused. How can a system trim to return the aircraft to the trimmed speed by trimming opposite the speed change?

That's self-explanatory, anything that changes is returned to the original state by making a change opposite to the first change (i.e., stable behavior). If you added a change in the same direction as the first change, the state will only move further away from the original (unstable behavior).

I think I understand where you're coming from, and if I'm right, it's due to a confusion between two different things that are meant by "speed stability." They're superficially similar (it's what, um, keeps the speed stable, right? duh!) but actually very different. And the distinction is a little tricky and more than a little subtle, but nonetheless important; and if you're having a hard time applying what I pointed out as self-explanatory to speed and trim, it's probably because you're thinking based on the first definition, while the system was designed (and explained in the FCOM) based on the second.

1. The "speed stability" in some pilot training materials:

This assumes that you're controlling for a constant altitude, be it with autopilot altitude hold, or manual elevator control. Whatever is controlling the elevator, and, in turn, the AOA (be it your arm or the autopilot) is free to do whatever it takes with the AOA to maintain that level flight path. Think of it as altitude-fixed, AOA free. So if a gust speeds you up a little bit, the extra drag will then slow you back down to the original speed. If a gust (or momentary dip in thrust, or anything) slows you down, the reduction in drag will speed you back up to the original speed. This works on the front side of the thrust curve. If you're slow enough already to be on the backside, however, then a momentary slowdown will see more drag, which will slow you down even more, which will cause more drag, etc. in a runaway cycle. So this type of speed stability only exists above Vmd. It's unstable when you're slower than that.

2. The "speed stability" in aeronautical engineering:

For this definition you have to completely forget about holding altitude. The airplane is free to climb/descent at the whims of the other factors at play. However, without elevator changes, AOA is maintained. So, opposite the fist definition, think AOA-fixed, altitude free. (Also for the moment, set aside the effects of thrust-pitch couple like the 737 is famous for, or propwash like in most bugsmashers.) This AOA-fixed behavior is the natural behavior of normal planes. After any disturbance (assuming the elevator and/or trim returns to the original position) the AOA will also return to its original position. It may be after a period of alternating overshoots (phugoid oscillation), but eventually it will.

Let's say you're cruising level in the middle of the speed envelope, and you reduce some power (permanently). The power reduction will cause a speed reduction, which will cause a lift reduction, which will cause the beginning of a descent, which will cause a component of weight to align forward parallel with thrust, which will cause the plane to speed up, and where it finishes speeding up will be its original speed.. (If it finished at any other speed, that speed difference would cause a lift other than the weight, and the plane would go through more repetitions of this same cycle until lift=weight. Since weight is fixed, lift is fixed, and the only 2 remaining variables of the lift equation are speed and Cl, and Cl is a stand-in for AOA. So the speed will settle at that speed which is corresponds only to the particular AOA you're flying at.) This is why there's a fundamental relationship between AOA and speed in vertically unaccelerated flight, and why AOA stability implies speed stability under the engineer's definition. To summarize, we pulled the power, but the speed remained the same (albeit under a descent, but we don't care about that). Vice versa, if we increase power the airplane will begin a climb, at the same speed.

This speed stability (definition 2), by the way, is required to exist throughout the entire certified envelope all the way down to stall speed. Remember, so far no electronic or mechanical trickery is involved. This is the natural, aerodynamic behavior of every stable plane from a hand-thrown model to the 747. To change the speed that is held constant we must change the AOA, which is done either by moving (and holding in a new position) the elevator or moving the stabilizer, i.e., actuating the trim. And in some engineering contexts, they don't care whether you end up having to hold a force with your arm or not. (This can be kind of confusing to us, who are used to thinking of trim as the-thing-that-removes-the-arm-force.) It's all just considered changing the trim point, or trim speed, based on the new position of elevator or stab. Now, this new speed will be maintained (again, maybe climbing or descending now, but that's irrelevant).

If I increase the speed this means the STS will trim aft. At the same time increased speed will increase the lift. Aft trim will bring the aircraft into a more out of trim state.

It appears that your scenario is based on the premise of the first definition. You're increasing speed while maintaining altitude, (intentionally) so the trim required is forward. (Faster speed = forward elevator and/or forward trim) But the STS, sensing that you're faster than original, trims aft in an attempt to slow you down to the original speed. This causes the plane to tend to climb which annoys you. (Of course, we're used to the premise of maintaining altitude) But this is where the engineering definition kicks in and you have to realize that the airplane doesn't care about the climbing tendency. It only wants to return to the original speed, which, being slower than the new speed, mandates a climb. This is the behavior of a normal airplane, only a little stronger with the augmented speed stability of STS. Understanding that (non-Airbus) airplanes really behave in accordance with the engineering (#2) definition, will hopefully let you come to terms with the "opposite" behavior.

[ManaAdaSystem]

I can’t find my old 737 FCOM but my understanding of the system was that trim was inhibited the moment you applied the control column in the opposite direction, and that included speed trim.

No, it will happily continue to trim even if you push (you need to) forward on the controls after take off when you accelerate. It will stop when you apply opposite trim. Which I do on every take off since STS trims the aircraft out of trim. It normally takes 2-3 turns of trim, depending on when you start to trim.
Weird system!

[Switchbait]

The last part of Vessbot’s description is on the money for the STS.


Another cause for a stab trim change requirement in the early initial climb, is normally due to large Assumed Temp Method derates. The calculated Stab Trim setting is for Rated Thrust. Less thrust = less pitch up moment

[Derfred]

Yes, that was a great explanation, Vessbot.

I suppose it would be fair to say that if the speed trim appears to be working against you during an acceleration or deceleration phase, it’s simply doing what it is designed to do - speed stability augmentation. It doesn’t exist to second-guess what trim inputs you may desire, nor does it exist to auto-trim out your elevator force - that’s Airbus ****.

I have certainly noticed the speed trim trimming the opposite direction to the trim input I was about to apply manually, but it has never concerned nor annoyed me. The speed trim inputs are minute compared to the real trim changes required.

Just man-up and fly the aircraft. As soon as you put in a trim input, the speed trim goes away for a while.

At the risk of thread drift, I can’t believe the number of posters on the Lionair crash thread trying to suggest speed trim as a contributing factor.

[de facto]

Manada...super easy...think bout it like that...high speed low thrust...stab trimmed nose up.....low speed high thrust..stab trimmed nose down.....just how a pilot would react when flying manually....ohh manually.....

[Capn Bloggs]

Vessbot's description = My Head Hurts! :)

I shouldn't complain though, I don't even have a trim wheel... :ok:

[ManaAdaSystem]

Vessbot's description = My Head Hurts! :)

I shouldn't complain though, I don't even have a trim wheel... :ok:

Take off today. 76 tons. So not low gross weight. I let the STS do it’s job (it was not supposed to do anything due to high gross weight?). It was trimming way longer than the claimed 10 seconds band it should trim after take off. I had to really push forward on the controls after it had finished trimming aft. So much that if I had let go, the nose would have pitched up really fast.
I had to trim 5-6 turns forward before the aircraft was stable. That’s a lot!
So, we have a system that brings the aircraft out of trim, and trims when it is not supposed to trim.
Not super easy. Dangerous. I don’t think about this when I fly manually since the first thing I do is to trim forward in order to cancel out the STS.
This system is plain stupid. Try it, and see for yourself.

[ManaAdaSystem]

It appears that your scenario is based on the premise of the first definition. You're increasing speed while maintaining altitude, (intentionally) so the trim required is forward. (Faster speed = forward elevator and/or forward trim) But the STS, sensing that you're faster than original, trims aft in an attempt to slow you down to the original speed. This causes the plane to tend to climb which annoys you. (Of course, we're used to the premise of maintaining altitude) But this is where the engineering definition kicks in and you have to realize that the airplane doesn't care about the climbing tendency. It only wants to return to the original speed, which, being slower than the new speed, mandates a climb. This is the behavior of a normal airplane, only a little stronger with the augmented speed stability of STS. Understanding that (non-Airbus) airplanes really behave in accordance with the engineering (#2) definition, will hopefully let you come to terms with the "opposite" behavior

Not maintaining altitude, just a normal take off and acceleration. The STS is not supposed to do anything past 10 seconds after take off, but it does. If I follow your logic, it would trim towards the speed I had at lift off. It doesn’t make sense. Nothing with this system does.
If STS was completely disabled during take off, nobody would notice. When it works it tries to bring the aircraft into a stall.

[MickG0105]

The STS is not supposed to do anything past 10 seconds after take off, ...




You might want to re-read your FCOM. The STS only becomes active 10 seconds after take off and remains active while the autopilot is disengaged up to Mach 0.68.

[Derfred]

Take off today. 76 tons. So not low gross weight. I let the STS do it’s job (it was not supposed to do anything due to high gross weight?). It was trimming way longer than the claimed 10 seconds band it should trim after take off. I had to really push forward on the controls after it had finished trimming aft. So much that if I had let go, the nose would have pitched up really fast.
I had to trim 5-6 turns forward before the aircraft was stable. That’s a lot!
So, we have a system that brings the aircraft out of trim, and trims when it is not supposed to trim.
Not super easy. Dangerous. I don’t think about this when I fly manually since the first thing I do is to trim forward in order to cancel out the STS.
This system is plain stupid. Try it, and see for yourself.

Who told you to fly like that?

When you accelerate, you apply forward pressure to the control column, and trim out that pressure with forward trim as it accelerates. This forward manual trim stops the speed trim for a bit. Try it, and see for yourself.

[wiedehopf]

It's not trying to stall the plane. The system does not know about you retracting the flaps and it does not need to.

Retracting flaps and accelerating always means you need to trim, with the STS you just need to trim a little bit more.
You are blaming the STS for the need to trim, but that requirement is always there.

So if you assume no flap changes (for which you will always need to trim) the system does indeed try to keep the speed stable.

Accelerating the STS will support the natural pitch up tendency of the plane and the pitch up will reduce the speed increase.
That is what speed stable is, the plane pitches up when you increase the speed.

The system is not meant to assist you trimming.
It is meant to emulate a plane with a bigger elevator in which you need to trim more for speed changes.

[hans brinker]

What are the inputs to the STS? If that input was wrong is it possible that (without the pilot using trim to stop it) the STS would keep trimming till it hit the stops? Not on a Boeing myself.

[Vessbot]

Not maintaining altitude, just a normal take off and acceleration. The STS is not supposed to do anything past 10 seconds after take off, but it does. If I follow your logic, it would trim towards the speed I had at lift off. It doesn’t make sense. Nothing with this system does.
If STS was completely disabled during take off, nobody would notice. When it works it tries to bring the aircraft into a stall.


As MickG pointed out, takeoff+10sec is where it starts, not ends, being active, at last according to the quoted snippet from the manual. If the trim is moving prior to 10 seconds, I have no answer to that. 737 guys, (I'm not one) is there another system that's supposed to be automatically moving the trim prior to 10 seconds?

As far as the high weight and forward CG... the manual isn't saying that STS is only active at low weight. It's saying that the reason it exists (in general) is to fix some conditions that are more prevalent at low weight, etc. But it's always active. Kinda like the yaw damp, it's there to fix dutch roll, but that doesn't mean it's only active at high altitude; it's always active.

But most importantly, have I been able to answer your confusion about trim and speed stability in general? In your first 2 posts, you noted some really basic misunderstandings, such as not knowing what "returning the airplane to trimmed speed" means. Do you understand that now, and do you understand why a speed stability augmentation system would be expected to increase the stick force you're holding, if you're holding it away from trimmed speed?

[Vessbot]

What are the inputs to the STS? If that input was wrong is it possible that (without the pilot using trim to stop it) the STS would keep trimming till it hit the stops? Not on a Boeing myself.

The input is "sensing of trim requirement," can't you read? ;)

Overly dumbed down **** in manuals like this pisses me off. That provides zero value in trying to understand how the system works and why it's doing what it's doing... they might as well have saved the ink and replaced that whole section with "The trim is gonna move by itself a lot, don't worry about it it's normal."

[hans brinker]

The input is "sensing of trim requirement," can't you read? ;)

Overly dumbed down **** in manuals like this pisses me off. That provides zero value in trying to understand how the system works and why it's doing what it's doing... they might as well have saved the ink and replaced that whole section with "The trim is gonna move by itself a lot, don't worry about it it's normal."

Yup, I’m on the 320, lots of “there two redundant computers for this, they get inputs from the coffee maker, WOW switch, and ADIRU. Please don’t touch it, because no one will know what will happen”.

[Rozy1]

Ok.. think of it this way. It IS NOT AN AUTO TRIM.
It is not attempting to trim the aircraft for you. It is attempting to go for the last trimmed for an set airspeed and this is why it is so obvious during acceleration. It was last set 'for' 160kts but you are speeding up. It can only return to this speed by commanding some sort of nose up, i.e trim! You start manually trimming and it has a new target. You are NOT meant to be letting it trim for you. That is not what it is designed to do.

p.s the system becomes active 10 seconds after take off until mach 0.68

I don’t understand. What does attempting to go for mean if not attempting to trim the aircraft?

He who said taking the feature away would lose nothing is correct. I’ve been flying the fluf for 26+ years and I don’t appreciate the function at all. In fact, on takeoff my technique is to periodically move the left half of the trim switch, which inhibits the annoying trim from even happening.

[Vessbot]

To be more succint, "auto trim" means it trims to reduce stick force.

STS does the opposite, it trims to increase stick force.

"Auto anti trim," how bout dat?

[Ex-Brazilian]

What I understand from the STS, is that the system prevents you of being in a severe out-of-trim situation, helping to keep a good level of elevator authority in all situations. At least at my previous airline, where I flew the NG 700/800, a old instructor told me that on every takeoff we were taking off with plus 1 unit of forward trim, and that was to prevent tail strike, he said. He even showed me a page on the DFDAU showing the actual position of every flight control surface, and he proved me that the actual trim position was always exactly 1.0 unit forward compared to the wheel trim that we usually use to set the T/O trim.

Since we were taking off with a more forward trim setting, that caused a tendency of the airplane to accelerate after takeoff, and so all pilots were over accelerating, most of the time unable to keep the liftoff speed, simply because the aircraft was slightly out-of-trim. Then STS kick in, moving the trim to the up position. Then the acceleration altitude comes, and you don’t understand how the **** the STS is trimming the plane up, but the F/D is calling you to pitch down and you as well are commanding elevator nose down.

Bull**** or not, that was what I heard about it. Anyway, everytime I saw the STS working, it was very smooth, making just small inputs, easy to identify. I can’t see how it could be dangerous in anyway.

[Vessbot]

At least at my previous airline, where I flew the NG 700/800, a old instructor told me that on every takeoff we were taking off with plus 1 unit of forward trim, and that was to prevent tail strike, he said. He even showed me a page on the DFDAU showing the actual position of every flight control surface, and he proved me that the actual trim position was always exactly 1.0 unit forward compared to the wheel trim that we usually use to set the T/O trim.



This sounds bizarre to me, why wouldn't they just make the performance data tell you to set the correct trim, instead of lying to you by 1 unit, and then having the indicator lie to you by one unit the other way?

Paging Nigel Tufnel...

https://www.youtube.com/watch?v=KOO5S4vxi0o

"Ours goes 1 unit forward..."

[SOPS]

I have 9000 hours on the 737. 200/300/400/700/800. I thought I understood the STS. Having read and re read this thread..I am now convinced I know nothing. I must say that in day to day operations..I just let it do its thing..and it never seem to worry me. Perhaps ignorance is bliss!!

[Derfred]

Perhaps another way of thinking about it is that the STS doesn’t exist to help you fly the aircraft. It exists to help the aircraft fly itself, if you let go of the control column.

That’s what stability is. It could have been achieved by other means (and it largely is, because the aircraft is naturally stable by design). The STS simply makes it slightly more stable. The more stable an aircraft is, the harder it is to change flight path or speed. That is why STS always works against you when you attempt to change speed.

Boeing could have done away with STS by making the aircraft more stable aerodynamically, and we wouldn’t be having this discussion, because it would have been completely transparent to the pilot.

I’ve been flying the thing for decades, and I’ve never heard a colleague complain about STS, nor has it ever bothered me.

But Vessbot’s explanation is really good, and provides a lot more knowledge than the manuals do.

[Vessbot]

For full disclosure I've never flown the 737, and I only learned about the existence of the STS probably the day before I made the post. I only slotted the info from the quoted manual section, as well as Mad (Flt) Scientist's blurb from the Lion Air thread, into the basic framework of how trim and speed stability works in flight dynamics. So if there are any subtleties to the system, beyond what's written here, that might change how it works in a non-obvious way, I wouldn't know about them.

[Switchbait]

In summary, it works how it’s supposed to, don’t worry about it, fly the airplane. :)

[downwind]

Hi all,

Although the STS demo is captured from flight sim it might be a good description of the workings of the system;

https://www.youtube.com/watch?v=AXr0PExNqBA

See what you think.....

[Capn Bloggs]

Cool video! Looks like a bit of a Heath-Robinson setup though...

Boeing could have done away with STS by making the aircraft more stable aerodynamically, and we wouldn’t be having this discussion, because it would have been completely transparent to the pilot.
I suspect Boeing won't/can't because more stability more fuel burn means means less efficiency.

[Derfred]

I suspect you're right.

[infrequentflyer789]

Boeing could have done away with STS by making the aircraft more stable aerodynamically, and we wouldn’t be having this discussion, because it would have been completely transparent to the pilot.

I suspect Boeing won't/can't because more stability more fuel burn means means less efficiency.

I think it's more complicated than that - this is about stability in response to control movement (which is also why it's off when AP is on). On the R&N Lion Air thread I think there was a post about certification reqs for minimum stick force per knot (of speed change), although I've also seen "stick force per g" elsewhere. As far as I can see the Vessbot's description makes sense, and STS helps meet that cert. req. by increasing stick force (in effect, trimming against you), in certain phases of flight.

Since you're not pulling directly with cables, I suspect Boeing could have met the requirement by just making the control force larger, but that might have made them too hard to move in other parts of the envelope, where STS is inactive. But I'm not sure, I might have that wrong - my aerodynamics is probably past it's use-by date.

[m39462]

Although the STS demo is captured from flight sim it might be a good description of the workings of the system;
At the end of that video STS is disabled when flaps are retracted, that's not something Mr. Boeing put in his FCOM. Any experts to confirm or deny?

[Vessbot]

I suspect Boeing could have met the requirement by just making the control force larger, but that might have made them too hard to move in other parts of the envelope, where STS is inactive. But I'm not sure, I might have that wrong - my aerodynamics is probably past it's use-by date.

I asked myself the same thing earlier, why can't they fix the stick force per knot problem by tweaking the artificial feel unit? That's literally why it exists. Then as I wrote that post I figured out a possible answer... (but, again, this is just supposition.)

There are two aspects to the problem: First, the stick force per knot gradient, i.e., how much pull does it take to hold away from the trim speed. This aspect by itself is easily fixable with artificial feel setting.

But the second one is changing of the trim speed itself. When you increase the thrust like the 737 if famous for, the low thrustline offset tends to pitch the plane up, which is effectively the same as applying aft elevator and/or trimming aft. Say you're approaching at 140 knots, after you apply TOGA the trim speed becomes 100 knots (these are only example numbers). This is only fixable by retrimming.

[Capt Quentin McHale]

There has been a lot of theorising, hand wringing, head scratching, chin rubbing etc on this subject. I quite like Switchbaits post #29... The system works fine, just fly the aircraft. Too much supposition can lead to misinformation and mistakes. What would be ideal is the system description and operation from the Aircraft Maintenance Manual to prove once and for all as to how the system operates and lay this topic to rest. Any engineers out there able to assist this think tank?

[FCeng84]

Please allow another try at describing pitch trim on the 737. Notice that I chose the word "pitch" trim rather than "speed" trim to be more general. I will address the speed trim system more directly below.

One of the sources of confusion (at least for me and maybe others) is that the word "trim" is often used to describe any motion of the horizontal stabilizer. This gets confusing in that "trim" or "pitch trim" also carries the notion of "moving the horizontal stabilizer so as to offload column forces" as in getting to a state where steady column input is not required to continue flying at the desired steady state condition. It is unfortunate that the word "trim" has been used for both of these.

Now we need to consider the FAR speed stability requirement. In general terms, the FAR states that an airplane should exhibit positive speed stability with a stick force of at least 3 lbs per 10 knots. That means that once trimmed (i.e., no column input required for steady flight) reducing speed by 10 knots (without a throttle or configuration change) should require 3 pounds of pull force to maintain steady flight. Similarly increasing speed (again without a throttle or configuration change) should require 3 pounds of push force to maintain steady flight. In this manner, the airplane (absent a column input) will tend to nose up/down appropriately to resist changing speed. If an airplane does not exhibit sufficient speed stability to meet this requirement, one solution is to have a control system function that moves airplane surfaces as needed to provide sufficient pitching moment as a function of speed changes to require the specified levels of column input to balance moments.

Meeting the stick force per knot FAR is the motivation for the speed trim system on the 737. For a limited portion of the flight envelope the bare airplane without any stability augmentation did not meet this FAR. To compensate, the 737 speed trim system moves the stabilizer in response to speed changes to augment the stability. Note that while this system is only needed to help meet the FAR at a limited set of conditions, it is for simplicity sake designed to respond to speed changes regardless of CG or weight. As a result, the speed stability is increased over a much wider range of conditions than just those where the airplane would not meet the FAR without it. For instance, at aft CG where the function is needed it will require just enough column when speed is changed. At forward CG where the airplane already has sufficient inherent speed stability, the STS will add more and could be seen as being an unnecessary nuisance.

As many have noted in previous entries to this thread, the STS "trims" (i.e., moves) the stabilizer opposite the direction that the pilot has to move to the stabilizer to relieve column forces. In a sense, the STS "un-trims" the airplane thus requiring the pilot to re-trim it. Whether one sees this as an unnecessary bother or a tool providing the flight crew with positive awareness of speed deviations is a matter of opinion. This gets to the heart of the A vs. B differentiation between their respective C* and C*U pitch augmentation systems - another topic that has been discussed in other PPRUNE threads over the years.

It is important to recognize that there are a number of other factors that impact pitch trim for which the speed trim system does not take any consideration. Changes to thrust, flap position, speedbrake setting, and gear position will generate pitching moment changes that must be balanced via the elevator (i.e., column when flying with autopilot disengaged) and followed up via pilot pitch trim inputs (i.e., movements of the stabilizer) to allow releasing the column. The 737 speed trim system essentially adds an increment of stabilizer motion in the positive speed stability direction as a function of a change in airspeed. It does not maintain a target airspeed that it seeks to return to regardless of other pitching moment disturbances that the airplane may experience. Other airplanes with higher levels of pitch augmentation (777 and 787 for example) do provide control surface inputs to counter such pitching moment disturbances and thus return to a specific airspeed, but that is another story for another thread.

[Vessbot]

It does not maintain a target airspeed that it seeks to return to regardless of other pitching moment disturbances that the airplane may experience.

Interesting, do you know why not? And if it only increases the stick force without maintaining the trim speed, why it uses the trim system instead of just adding more resistance to the artificial feel?

[Capn Bloggs]

FCEng84: Good job. Since you are in town, maybe you could zip down to the 737 office and re-write their Systems description? :ok:

[downwind]

Excerpt of 737-300/400/500 MAINTENANCE MANUAL STS logic.....

The Speed Trim system provides automatic stabilizer trim for positive speed stability during low speed, high thrust conditions. The speed trim
system is only operational while the autopilot is off, and on 737-300 airplanes only, the speed trim system is automatically terminated when
the flaps are up. Either FCC can provide the speed trim commands and only one channel will be engaged at any one time. The system which will be in command is alternately chosen as a function of the squat switch input, i.e., if FCC A was in command on the last flight, the FCC B will be in command this flight.
The engine No. 1 and 2 N1 inputs are used to control the gain of the trim command signal. The two engine inputs are summed, divided by 2 to get the average and then summed with stabilizer position gain control. The gain, as a function of the N1 signal, will go from zero at 60% N1 to a gain of 100% at 80% N1 and above.
The stabilizer position input is used as the reference at the time that the speed trim system was engaged and also to provide position inputs as it changes as a result of the computed airspeed input. The stabilizer reference input also provides a gain control function which is combined with the N1 input to provide overall circuit gain.
737-300 AIRPLANES;
The Digital Air Data computer provides computed airspeed and altitude rate inputs. The computed airspeed is used to generate a stabilizer
command input as a function of airspeed. The altitude rate input provides an in-phase signal referenced to the CAS command to increase the
nose down command as the altitude rate increases.
ALL EXCEPT 737-300 AIRPLANES;
the digital air data computer provides computed airspeed. The computed airspeed is used to generate a stabilizer command input as a function of airspeed. Inertial vertical speed from the Inertial Reference Unit provides an in-phase signal referenced to the CAS command to increase the nose down command as the altitude rate increases.
The flaps input provides a reference angle of airflow program for any particular flap position which is then compared to the actual alpha angle
of the airplane. If the airplane approaches the stall condition, switch S1 will open, which will remove any more inputs from the speed trim
system until the condition has been corrected.
To enable speed trim system operation, the following conditions must all be met:
Autopilot not engaged
Flaps not up (For 737-300 airplanes only)
Flight Control computer, speed trim circuit valid
More than five seconds elapsed after cessation of manual trim
More than 10 seconds after unsquat.

Prior to speed trim operation, STAB trim, CAS and altitude rate (or inertial vertical speed on ALL EXCEPT 737-300 airplanes) are synchronized. The synchronized signal is used as a reference and any differance from that reference is the output signal. When the speed
system is operational and the airspeed increases, there would be a command generated as a result of the difference between the airspeed
reference and present airspeed. This is compared to the stabilizer position along with any assist from the altitude rate input. This signal
passes through relaxed S1 to the gain amplifier. The amplifier has a variable gain which is dependant on stabilizer position at the time the
speed trim system engaged (hold logic generated). The amplifier gain ranges from 100% at zero units of trim to zero gain at 5 units of
stabilizer trim.
The polarity detector determines whether the command is for nose up or nose down. The trim detector will not put out a command until the error signal is at least equal to 0.072 degrees of stabilizer command. This also provides the other required input to either gate 2 or 3 which
provides the nose up or down command. The output of the trim error detector remains until the error gets down to less than 0.070 degrees of
command error. The output then goes to zero and the stabilizer trim stops. Gates 2 and 3 also require that the speed trim system is in operation and that the local FCC has been selected for command of the system. The signal then passes to the nose up or nose down relay in the stabilizer trim servo.
Selection of FCC A or B is altered at each squat or selection of the operational system, if one system fails.

[FCeng84]

First let me thank downwind for the STS details that show that I had a mistake in my earlier description. I had stated that STS operation was not a function of thrust setting. This is not correct as noted in the details. I will correct my earlier entry to align. In response to Vessbot's comment about why not adjust column feel instead of move the stabilizer to provide missing stick force per knot I see two reasons.

First, if the unaugmented airplane were neutrally speed stable it would not require any steady elevator for a speed change and thus stiffening the column feel would not help as none would be needed to fly faster or slower. Further, if the unaugmented airplane were actually unstable with regard to speed it would require a push force to keep the nose from rising after slowing down and a pull force to keep the nose from falling after speeding up. These are clearly not the desired situation. Stiffening the column feel in that event would actually make the speed stability handling characteristics worse.

Second, if the column feel were stiffened, it would not only change the resulting stick force per knot (the measure of speed stability) but also the stick force per g (a measure of maneuver stability). The problem that STS was introduced to address is limited to speed stability and thus stick force per knot. There was neither need nor desire to change the stick force per g maneuver characteristics. The implementation of STS that moves the stabilizer in the direction to increase speed stability does not impact stick force per knot and thus leaves the maneuver stability characteristics essentially unchanged.

[Derfred]

Boeing could have done away with STS by making the aircraft more stable aerodynamically, and we wouldn’t be having this discussion, because it would have been completely transparent to the pilot.

A few people have jumped on this comment - I didn’t really mean it to be taken literally.

I was just trying to point out that the STS is merely a tool Boeing have used to meet the required speed stability. Perhaps in theory it could also be achieved aerodynamically, perhaps not.

But a visible trim wheel spinning contrary to a pilot’s desired trim input seems to be upsetting the opening poster and others. My point was, accept it.

[RetiredBA/BY]

Just an interested bystander as it’s a long time since I flew the 737, 200 and the very first 300s. I can’t remember the 73s or the 75 and 76 having a speed trim system as described here. Found them all delightfull aircraft to fly manually throughout the speed range.

So, may I ask if it so vital, to meet the arbitrary speed stability requirements of the FAA that another system , with potential for malfunction with potentially serious consequences is really needed. Do we really NEED such a system and since it IS installed is there an instant cutout switch.

When were the values of 3 pounds per 10 knots decided on and why are these exact values so important. Are current models of the 73 significantly more speed unstable than the earlier versions ?

I still remember so clearly the problems caused by runaway tailplanes on my first jets, the Canberra and Valiant. Of course I know the flight control electronics, electrics, were nowhere near so advanced or capable as on present jets.

Is the FAA using, or requiring Boeing to use a sledgehammer to crack a nut?

Just curious!

[Hi_Tech]

Has any one in this forum have access to B737 MAX AMM (Pages from AMM Chap 34-20-00) and if you can post the same system info for B737 MAX redundancy management of AOA signals.

I just cannot believe that one faulty AOA sensor can make the aircraft trim nose down. There has to be more protection in the system design for this not to happen. I think the FAA AD more or less confirms a poor design in B737 MAX.
In the B777 which I am familiar with, each of the two ADIRUs (Air Data Inertial reference unit) receive both AOA inputs (There are two AOA sensors on most aircraft, same config on B737 also). This is compared with 'Calculated AOA' and a mid value is used. This is the redundancy built in the system on B777. Also each of the AOA sensor has two outputs, feed into two different computational channels. See the redundancy. There are actually 4 signals from two AOA sensors.
The full text from the B777 AMM is as below.
AOA Redundancy Management
The AOA redundancy management logic uses a modified midvalue selection.
The modified mid-value selection chooses the mid-value of these three AOA values:
* Left corrected AOA
* Right corrected AOA
* Calculated AOA.
The AOA redundancy management logic receives inputs from the inertial and air data systems to calculate the calculated AOA.

So I am a bit curious about the B737 MAX AOA signal Logic.

[Hawala]

Take off today. 76 tons. So not low gross weight. I let the STS do it’s job (it was not supposed to do anything due to high gross weight?). It was trimming way longer than the claimed 10 seconds band it should trim after take off. I had to really push forward on the controls after it had finished trimming aft. So much that if I had let go, the nose would have pitched up really fast.
I had to trim 5-6 turns forward before the aircraft was stable. That’s a lot!
So, we have a system that brings the aircraft out of trim, and trims when it is not supposed to trim.
Not super easy. Dangerous. I don’t think about this when I fly manually since the first thing I do is to trim forward in order to cancel out the STS.
This system is plain stupid. Try it, and see for yourself.

Never flown 73s, but always thought it would have less behind the scene stuff:eek:...wasn't it suppose to be hands-on & no computer interference machine?:rolleyes:

[ManaAdaSystem]

I thank you all for your contributions to this discussion. I think I get what you are trying to explain, but I am a simple soul and by my logic, any system that works agains my control inputs when I fly manually is making my work harder.
And the STS works when the aircraft is heavy or light, GC fwd or aft, derated or full thrust. That is not how the STS is explained. I have flown the NG since they made it. I have never bothered with the STS before, I simply cancel it with manual trim. I was just when I saw the system discussed that I started to check what it did.
So to all of you 737 NG drivers out there. Next time you take off, fly manually and let the STS work without interference. Clean up the aircraft. Tell me how the pitch controls feel. Let go of them (if you dare) and see what happens.
For those of you saying «just let the STS do it’s job». No. You can’t because it will bring the aircraft well out of trim.

It can’t be the regulators job to make the aircraft trim itself into a situation where if you let go of the controls, the aircraft will fly itself into a stall!

[Capn Bloggs]

It can’t be the regulators job to make the aircraft trim itself into a situation where if you let go of the controls, the aircraft will fly itself into a stall!
No it won't, as the speed reduces, the nose will drop, helped by the STS. The STS ensures the aircraft is speed-stable.

[Hi_Tech]

Just an interested bystander as it’s a long time since I flew the 737, 200 and the very first 300s. I can’t remember the 73s or the 75 and 76 having a speed trim system as described here. Found them all delightfull aircraft to fly manually throughout the speed range.

So, may I ask if it so vital, to meet the arbitrary speed stability requirements of the FAA that another system , with potential for malfunction with potentially serious consequences is really needed. Do we really NEED such a system and since it IS installed is there an instant cutout switch.

When were the values of 3 pounds per 10 knots decided on and why are these exact values so important. Are current models of the 73 significantly more speed unstable than the earlier versions ?

I still remember so clearly the problems caused by runaway tailplanes on my first jets, the Canberra and Valiant. Of course I know the flight control electronics, electrics, were nowhere near so advanced or capable as on present jets.

Is the FAA using, or requiring Boeing to use a sledgehammer to crack a nut?

Just curious!
Just read this good article by Bjorn Ferhrm which explains why the MAX requires this MCAS. The aircraft is more unstable than the NG with the larger engines.
https://leehamnews.com/2018/11/14/boeings-automatic-trim-for-the-737-max-was-not-disclosed-to-the-pilots/

[ManaAdaSystem]

No it won't, as the speed reduces, the nose will drop, helped by the STS. The STS ensures the aircraft is speed-stable.

Forgive me, I have only been flying for 30 years and nearly 20 on the NG, but all my departures in this period have been about raising the landing gear, accelerate and retract the flaps before I head off for far away places.
It has never been about flying at 150 kts in a «speed stable» condition.
I admit I don’t know how far towards a stall the STS will bring the aircraft, but with the amount of back pressure it creates, I think it will get pretty close before anything happens. If it tries to trim the aircraft back to the «stable speed» of 150 kts in clean config, things will get interesting.
By the way, which aircraft type do you fly, Bloggs?

[Derfred]

Forgive me, I have only been flying for 30 years and nearly 20 on the NG

So, after 20 years of 737NG experience, you suddenly have a problem with STS because you read something on PPRuNe.

So to all of you 737 NG drivers out there. Next time you take off, fly manually and let the STS work without interference. Clean up the aircraft. Tell me how the pitch controls feel. Let go of them (if you dare) and see what happens.

At the risk of repeating myself, who told you to fly the aircraft like that? That is not how it is intended to be flown. It is designed with speed stability, which means that an intentional speed change requires trim, like any non-FBW aircraft. So TRIM FFS. Like you’ve been doing for 20 years.

For those of you saying «just let the STS do it’s job». No. You can’t because it will bring the aircraft well out of trim.

No-one is saying that. They’re just saying don’t expect it to trim to what you want. That’s not what it’s there for. Trimming is your job. Boeing designs aircraft for pilots.

So either be a pilot or go fly an Airbus. Believe me, you’ll have a lot more “why is it doing this” questions on an Airbus.

[ManaAdaSystem]

So, after 20 years of 737NG experience, you suddenly have a problem with STS because you read something on PPRuNe.



At the risk of repeating myself, who told you to fly the aircraft like that? That is not how it is intended to be flown. It is designed with speed stability, which means that an intentional speed change requires trim, like any non-FBW aircraft. So TRIM FFS. Like you’ve been doing for 20 years.



No-one is saying that. They’re just saying don’t expect it to trim to what you want. That’s not what it’s there for. Trimming is your job. Boeing designs aircraft for pilots.

So either be a pilot or go fly an Airbus. Believe me, you’ll have a lot more “why is it doing this” questions on an Airbus.


Why so agressive? I have used the trim to cancel out STS for nearly 20 years, without it I would need to trim much less.
I have just not bothered to see what it really does before now. What is wrong with that?
Keep defending a system that works continously against the pilot. Never had any other aircraft do that before.
I have never flown an Airbus. Why bring it into this discussion? And why are you implying that pilots who fly Airbus aircraft are not pilots?
Do you work for Boeing?

[Vessbot]

Keep defending a system that works continously against the pilot. Never had any other aircraft do that before.

By the sounds of everything, the Cessna 172 behaves the same way: When you get off the trim speed, a stick force develops. The STS only increases this stick force because otherwise it's too weak to meet certification.

[ManaAdaSystem]

By the sounds of everything, the Cessna 172 behaves the same way: When you get off the trim speed, a stick force develops. The STS only increases this stick force because otherwise it's too weak to meet certification.

And in three sentences, you have explained the STS perfectly. Thanks!

But I still don’t like it.

[Vessbot]

And in three sentences, you have explained the STS perfectly. Thanks!

But I still don’t like it.

Thanks, but my aim wasn't to explain the system. I think that part has been thrashed out thoroughly enough in this thread by now. It was a reaction to your characterization of it as "work[ing] continuously against the pilot," unlike other aircraft. I mean, I don't see the difference between this, and any other transport airplane that meets the stick force per knot requirement naturally without add-on systems. Do you see those as working against the pilot? Maybe your argument is that the 1 pound per 6 knots requirement is too heavy, and it should be, for example, 1 pound per 8 knots (and I might even agree with you!) but that's a different argument entirely... and it would, again, apply the same to any transport airplane.

[EIFFS]

Thanks, but my aim wasn't to explain the system. I think that part has been thrashed out thoroughly enough in this thread by now. It was a reaction to your characterization of it as "work[ing] continuously against the pilot," unlike other aircraft. I mean, I don't see the difference between this, and any other transport airplane that meets the stick force per knot requirement naturally without add-on systems. Do you see those as working against the pilot? Maybe your argument is that the 1 pound per 6 knots requirement is too heavy, and it should be, for example, 1 pound per 8 knots (and I might even agree with you!) but that's a different argument entirely... and it would, again, apply the same to any transport airplane.


I’ve been fly the MAX for 16 months now and it’s a delightful bit of kit and in general more stable than the NG to fly manually, of course news of this stall system was news to me and there is nothing in the Boeing FCTM or FCOM that mentions it, soon to change no doubt....

The interface between the auto throttle logic and speed seems more balanced with less of the RoC change that one sees on the NG where one minute the RoC will melt away to zero and the next ( normally with a 1000’ to go) is back up at 1500fpm

Due to the approach speeds the aircraft is cat D and you notice that it eats up runway unless you are spot on speed and aggressive with auto brakes, anything under 2500m LDA my default is AB3 wet or dry, I predict that there will be overuns this coming winter season.

[ManaAdaSystem]

Thanks, but my aim wasn't to explain the system. I think that part has been thrashed out thoroughly enough in this thread by now. It was a reaction to your characterization of it as "work[ing] continuously against the pilot," unlike other aircraft. I mean, I don't see the difference between this, and any other transport airplane that meets the stick force per knot requirement naturally without add-on systems. Do you see those as working against the pilot? Maybe your argument is that the 1 pound per 6 knots requirement is too heavy, and it should be, for example, 1 pound per 8 knots (and I might even agree with you!) but that's a different argument entirely... and it would, again, apply the same to any transport airplane.

I have no clue as to pounds pr knots, but I am familiar with trim down when speed increases and trim up when speed decreases. I am not familiar with the trim up when speed increases STS action. You have explained that weird behaviour with just a few words.
So, I will just continue to trim against it, just as as before.
Have a nice weekend!

[ManaAdaSystem]

I’ve been fly the MAX for 16 months now and it’s a delightful bit of kit and in general more stable than the NG to fly manually, of course news of this stall system was news to me and there is nothing in the Boeing FCTM or FCOM that mentions it, soon to change no doubt....

The interface between the auto throttle logic and speed seems more balanced with less of the RoC change that one sees on the NG where one minute the RoC will melt away to zero and the next ( normally with a 1000’ to go) is back up at 1500fpm

Due to the approach speeds the aircraft is cat D and you notice that it eats up runway unless you are spot on speed and aggressive with auto brakes, anything under 2500m LDA my default is AB3 wet or dry, I predict that there will be overuns this coming winter season.



NG -800 has a bad rep when it comes to landing mishaps. I really hope your predicitons about the Max does not come through.

[MD83FO]

According to boeing how is the Lior air accident prevented?

[EIFFS]

NG -800 has a bad rep when it comes to landing mishaps. I really hope your predicitons about the Max does not come through.i


There is more risidual thrust and Vref is higher with a MLM of 69308, then you have another “sub system” new to the max called the LAM to ensure adequate nose gear clearance ( the NLG is 20cm longer than the NG) on landing.

The Landing​ Attitude Modifier (LAM) system performs two​ functions. The first​ ​
LAM function applies​ when the flaps are in​ the 30 or 40 position. To maintain ​
acceptable nose landing gear contact margin, LAM symmetrically deploys flight ​
spoilers on approach to reduce lift and force the airplane to use a higher angle of ​
attack. The amount of spoiler deflection depends on the approach speed. ​
Deflection begins at approximately 10​ knots above VREF.​
The second LAM function applies​ when flaps are positions 15 through​ 30​ and​ the ​
thrust levers are near idle. This function also symmetrically​ deploys flight ​
spoilers, in order to generate additional drag

The max Vref increment on the MAX is now 15 knots down from 20 on the NG ( because we fly both we have 15 max across both fleets)

Provided you follow the FCTM ie reduce thrust to idle by touch down and touch down in the correct zone ( many don’t ) and you’ve correctly completed a landing distance calculation ( again many don’t) the NG SFP & 737-800 (MAX) model have excellent stopping capability.

As you say there have been a few incidents with the NG, the risk is higher with the MAX IMHO

[ManaAdaSystem]

i


There is more risidual thrust and Vref is higher with a MLM of 69308, then you have another “sub system” new to the max called the LAM to ensure adequate nose gear clearance ( the NLG is 20cm longer than the NG) on landing.

The Landing​ Attitude Modifier (LAM) system performs two​ functions. The first​ ​
LAM function applies​ when the flaps are in​ the 30 or 40 position. To maintain ​
acceptable nose landing gear contact margin, LAM symmetrically deploys flight ​
spoilers on approach to reduce lift and force the airplane to use a higher angle of ​
attack. The amount of spoiler deflection depends on the approach speed. ​
Deflection begins at approximately 10​ knots above VREF.​
The second LAM function applies​ when flaps are positions 15 through​ 30​ and​ the ​
thrust levers are near idle. This function also symmetrically​ deploys flight ​
spoilers, in order to generate additional drag

The max Vref increment on the MAX is now 15 knots down from 20 on the NG ( because we fly both we have 15 max across both fleets)

Provided you follow the FCTM ie reduce thrust to idle by touch down and touch down in the correct zone ( many don’t ) and you’ve correctly completed a landing distance calculation ( again many don’t) the NG SFP & 737-800 (MAX) model have excellent stopping capability.

As you say there have been a few incidents with the NG, the risk is higher with the MAX IMHO

Thanks for your explanation on the Max. We have max 15 kts addon on our NGs, this was just changed. We operate SFP NGs and regular ones. There is a marked difference between the two models when it comes to landing distance.
-800 and FL 40 = pitch nearly 0 degrees. I see this as one of the reasons why pilots have a tendency to go above the glide when they get close to the runway.

[LEOCh]

Thanks Mana for starting for a great STS thread, but also Vessbot and FCeng84 for some very high quality explanations of speed stability and augmentation.

I don't have much to add especially to Vessbot's original Post#5, but I do have an interest in a general question: can a feedback system based on speed input and stab trim output like the STS actually create a longitudinally stable aircraft, or is it just simulating one for certification reasons? I would argue the answer is somewhere between the two.

Creating a longitudinally stable aircraft (in oversimplified terms) is achieved by placing center of gravity in front of a calculated neutral point. Highly stable aircraft effectively waste lift and lower efficiency by generally having their horizontal stabilizer in negative AoA/lift, which is fine on a C172 at forward COG limit but wasteful on a modern air transport. Maximum efficiency should be achieved at cruise if aircraft COG is assigned so tail AoA is approximately 0'. At cruise a 737 will be operating at a few degrees of wing AoA so the aircraft will still be stable, but probably marginally so, as compared to certification requirements originally developed for an older generation of aircraft. The closer the aircraft gets to neutral stability, the more problematic it's control characteristics become. At true neutrality the AoA of the tail and wing are equal, and the aircraft is effectively trimmed simultaneously for any AoA/speed (i.e. it is just as suited to VNE at a very low AoA, or stalling AoA at low speed). Not to mention that the tail might stall before the wing.

FAR states that a transport airplane should exhibit positive speed stability with a stick force of at least 3 lbs per 10 knots, one characteristic of a longitudinal stability that the STS assists with achieving. As others have noted, as thrust in the 737 is increased the plane will tend to not accelerate but instead pitch up and settle to the same speed in a climb (thrust line effects are a factor here also) just like a C172. As other posters noted it just feels odd that if you want to accelerate in level flight you will have to remove some trim that the STS just obviously applied against your intention.

However, is the aircraft now fully longitudinally stable in the same way the C172 is? The COG has not changed position, and the AoA differential between tail and wing is still low (i.e the same just-stable configuration as before). The aircraft is not any more resistant to pitch changes from updrafts and downdrafts. The C172 has more pitch stability in this sense as the greater difference between trimmed wing and tail AoA means the tail creates a stronger opposition to an airflow disturbance that increments AoA equally at the wing and tail.

The fact the STS has met the speed stability requirement for certification, but only created effectively half of a stable aircraft, is more of an interesting theoretical issue than a real one. The aircraft will be likely to have sufficient pitch stability to make manual flying perfectly acceptable and pleasant. However, I think this would change if pilots had to fly whole sectors in STS assisted manual mode, hour after hour and through turbulence. Like a 1940s transport pilot, there would be a strong push to demand aircraft loaded further forward with stronger pure aerodynamic longitudinal stability.

[George Glass]

Vessbot's analysis is excellent. The only other thing he should added is that it works fine. I've only ever had one significant failure in several thousand hours on the type. Dont over think it. Runaway Stab. Checklist works fine if you have a problem and all else fails put the little feet of the PFD aircraft symbol on the horizon and set 80% N1 then work the problem. Fly the f@#king aircraft. Not that hard.

[ManaAdaSystem]

Thanks Mana for starting for a great STS thread, but also Vessbot and FCeng84 for some very high quality explanations of speed stability and augmentation.

I don't have much to add especially to Vessbot's original Post#5, but I do have an interest in a general question: can a feedback system based on speed input and stab trim output like the STS actually create a longitudinally stable aircraft, or is it just simulating one for certification reasons? I would argue the answer is somewhere between the two.

Creating a longitudinally stable aircraft (in oversimplified terms) is achieved by placing center of gravity in front of a calculated neutral point. Highly stable aircraft effectively waste lift and lower efficiency by generally having their horizontal stabilizer in negative AoA/lift, which is fine on a C172 at forward COG limit but wasteful on a modern air transport. Maximum efficiency should be achieved at cruise if aircraft COG is assigned so tail AoA is approximately 0'. At cruise a 737 will be operating at a few degrees of wing AoA so the aircraft will still be stable, but probably marginally so, as compared to certification requirements originally developed for an older generation of aircraft. The closer the aircraft gets to neutral stability, the more problematic it's control characteristics become. At true neutrality the AoA of the tail and wing are equal, and the aircraft is effectively trimmed simultaneously for any AoA/speed (i.e. it is just as suited to VNE at a very low AoA, or stalling AoA at low speed). Not to mention that the tail might stall before the wing.

FAR states that a transport airplane should exhibit positive speed stability with a stick force of at least 3 lbs per 10 knots, one characteristic of a longitudinal stability that the STS assists with achieving. As others have noted, as thrust in the 737 is increased the plane will tend to not accelerate but instead pitch up and settle to the same speed in a climb (thrust line effects are a factor here also) just like a C172. As other posters noted it just feels odd that if you want to accelerate in level flight you will have to remove some trim that the STS just obviously applied against your intention.

However, is the aircraft now fully longitudinally stable in the same way the C172 is? The COG has not changed position, and the AoA differential between tail and wing is still low (i.e the same just-stable configuration as before). The aircraft is not any more resistant to pitch changes from updrafts and downdrafts. The C172 has more pitch stability in this sense as the greater difference between trimmed wing and tail AoA means the tail creates a stronger opposition to an airflow disturbance that increments AoA equally at the wing and tail.

The fact the STS has met the speed stability requirement for certification, but only created effectively half of a stable aircraft, is more of an interesting theoretical issue than a real one. The aircraft will be likely to have sufficient pitch stability to make manual flying perfectly acceptable and pleasant. However, I think this would change if pilots had to fly whole sectors in STS assisted manual mode, hour after hour and through turbulence. Like a 1940s transport pilot, there would be a strong push to demand aircraft loaded further forward with stronger pure aerodynamic longitudinal stability.

But STS works primarily in low speed situations, it’s not active during normal cruise:

STS Mach gain is fully enabled between 100 KIAS and Mach 0.60 with a fadeout to zero by Mach 0.68

How does this pound pr knot certification requirement work on FBW aircraft where the aircraft is kept in a trimmed state at all times? Or do 777 drivers trim against a STS as well when they fly manually?

[FCeng84]

Mana - When Boeing developed the C*U control law that is the augmented pitch axis of the 777 a very deliberate design decision was made to provide positive speed stability that would require pilot action to "trim" column forces that build up when airspeed changes. C*U speed stability is implemented by managing within the control law a reference speed and requiring steady column force to fly at speeds away from that reference. Pitch trim on the 777 works to slew the reference speed up and down. The 777 does not drive the stabilizer directly to create speed stability, but it does augment the response characteristics in such a way that column force builds up that must be trimmed off. Without trim, the 777 seeks to return to the reference speed. 777 pilots must trim against the speed stability provided by C*U whenever they change speed regardless of the unaugmented speed stability characteristics of the bare airplane itself. In this way, the pilot task with regard to speed changes is not that different between 777 and 737.

[ManaAdaSystem]

Mana - When Boeing developed the C*U control law that is the augmented pitch axis of the 777 a very deliberate design decision was made to provide positive speed stability that would require pilot action to "trim" column forces that build up when airspeed changes. C*U speed stability is implemented by managing within the control law a reference speed and requiring steady column force to fly at speeds away from that reference. Pitch trim on the 777 works to slew the reference speed up and down. The 777 does not drive the stabilizer directly to create speed stability, but it does augment the response characteristics in such a way that column force builds up that must be trimmed off. Without trim, the 777 seeks to return to the reference speed. 777 pilots must trim against the speed stability provided by C*U whenever they change speed regardless of the unaugmented speed stability characteristics of the bare airplane itself. In this way, the pilot task with regard to speed changes is not that different between 777 and 737.

Thanks! So there is a difference between US and European certification in this area? I don’t fly Airbus but I believe Airbus aircraft are kept in trim (pitch).
If I understand the 777 system correctly, you do not need at lot of trim if a short trim activation resets the speed reference?

I know what I would prefer my aircraft to do.

[Denti]

I don’t think that it is a major difference between European and US certification rules as such, just a difference in control design. After all there are no changing stick forces in an airbus, the only force is a spring setup in the stick itself to return it to neutral, completely independent of control law and speed. Trim is always automatic except in direct law, and most people forget at first to trim manually if they happen to find themselves in direct law, which is the last fallback level available.

That said, the airbus FBW design is older than the Boeing one, and I think that Boeing learned from what happened with the Bus.

By the way, it is absolutely astonishing how fast one de-learns trimming on switching from Boeing to Airbus. Just my personal observation. Simply because it is never needed in normal line operation.

[Jwscud]

The 777 flight control system is excellent. It flies like a large, more stable 737. Trimming is natural and intuitive and exactly like flying a non-few aircraft. It also keeps you in practice for the two reversionary modes, where manual trim is required. It also means the handling characteristics in reversionary modes are very similar to normal mode and require less capacity in a non-normal situation. C* U is excellent - Boeing certainly as Denti says looked hard and learned lessons from the Airbus approach.

[gums]

Salute Denti!

A great point about how fast one de-learns trimming on switching

The Boeing control laws are different than the 'bus, but do not seem to have as many reversion sequences. I would have to fly one to see how it 'feels". A lotta difference between trimming for speed/AoA versus a gee command that is biased by pitch attitude and bank angle.

As denti says, the move from a "conventional" plane to the new, improved one is not always a big deal. We old curmudgeons that resisted the Viper limiters abandoned the prejudice real fast. We also had no problem NOT TRIMMING for speed/AoA. It was more trimming for attitude, and the 'bus does that without a "coolie" hat button or whatever. If you are holding stick fore or aft, the the FLCS tries to reduce that pressure/displacement/force. AF447 showed one problem with that philosophy, in that we had a fully deflected stab after "x" seconds since the pilot held back stick forever and the FLCS tried to reduce the back stick displacement.

Gums...

[Lonewolf_50]

Intermim Report's Out.
http://knkt.dephub.go.id/knkt/ntsc_aviation/baru/pre/2018/2018%20-%20035%20-%20PK-LQP%20Preliminary%20Report.pdf

As mentioned elsewhere, the MCAS system didn't get a mention.
As also mentioned elsewhere, multiple previous sectors were flown using manual trim wheel, apparently, to handle the defect that never got quite fixed.
The CVR still not recovered.
I encourage your attention to pages 14 and 16 of the report: the difference in the parameters on the accident flight and the previous flight.

[gearlever]

MCAS is mentioned in the BOEING letter.

Chapter 5.12, p62

[wiedehopf]

As also mentioned elsewhere, multiple previous sectors were flown using manual trim wheel, apparently, to handle the defect that never got quite fixed.


Could you point to the page where "multiple" previous sectors were flown using the trim wheel?
Looking at FR24 data the typical altitude excursions associated with the MCAS trim and stickshaker are only present on the accident and previous flight DPS to Jakarta.

Reading pages 7, 8 and 9 it appears to me the plane intermittently didn't show any speed or altitude on the captains side.
If i understand correctly this could happen if the left AoA signal was missing as static pressure which is needed for speed and altitude gets corrected by AoA.
So intermittently failing AoA sensor might have been the problem.

Now in Denpasar the AoA sensor was replaced. This is the most likely point for the introduction of the 20 degree offset.
A stuck sensor that just transmits one value might happen as a failure. But tracking a 20 degree offset is very unlikely to occur without some change being made.

Possible reasons for the 20 degree offset i can imagine are:
Wrong part number
Faulty manufacturing
Very creative attachment to the plane
AoA vane bent after installation and test


To quote the report:

For troubleshooting due to repetitive problem perform replaced angle of attack sensor in accordance with Aircraft Maintenance Manual (AMM) Task 34-21-05-000-001 and task 34-21-05-400-801 carried out. Installation test and heater system test result good.


Some people said the AoA sensor needs to be tested with a special jig to orient it while someone in the cockpit is checking the output value.
Now either that test was not performed or they made a mistake here.

[MickG0105]

Some people said the AoA sensor needs to be tested with a special jig to orient it while someone in the cockpit is checking the output value.
Now either that test was not performed or they made a mistake here.

Or it's not a raw data (sensor) problem rather it's a processed data (ADIRU) problem. The Left ADIRU seems to pop up quite a bit in the AFML Resolution Descriptions. I've got a fiver on a fault in the Left ADIRU featuring as a contributing factor.

[FCeng84]

The difference between the C* pitch control approach taken by Airbus and the C*U approach taken by Boeing is not so much a difference in cert authority requirements as it is a difference in how these two groups chose to address speed stability requirements. Boeing with C*U chose to continue to meet the existing requirements for stick force per knot as speed changes. Airbus chose to seek a special condition to allow their design not to require any force for speed changes within the normal envelope. Airbus was able to convince cert authorities that because their system provides protections for both overspeed and underspeed conditions there was no need to provide pitch controller force cues for speed changes that do not exceed upper or lower limits. As a result, the Airbus C* system does not require the pilot input to manage pitch trim where the Boeing C*U system does. The Airbus system presents the pilots with less workload. The Boeing system is more conventional when compared with non FBW airplanes. As noted earlier, the Boeing system promotes the pilot maintaining pitch trim skills that come in very handy if failures result in dropping to a degraded mode. It may be easy for pilots to unlearn their pitch trim skills when transitioning to a system that does not require pitch trim. Hopefully those same crews are not caught out too far when a failure drops them to a mode that requires manual control of the stabilizer to keep the airplane in pitch trim.

[ManaAdaSystem]

With that logic all FBW aircraft should operate in degraded mode at all times. That would keep the pilots ready always.

[infrequentflyer789]

Some people said the AoA sensor needs to be tested with a special jig to orient it while someone in the cockpit is checking the output value.
Now either that test was not performed or they made a mistake here.

On the classic the last step of the install procedure 27-32-11/401 is to perform system test 27-32-00/501 which includes testing the stall warning system end to end by using the calibrator,
turning the vane and checking at what point stick shaker triggers.

I don't have the relevant bits of NG or MAX AMM to see if it is the same, but it seems likely it will be similar. We know from the log that they recorded doing an "installation test", but we don't know exactly what. However, if as with the classic, the test is specified as the last step of the install procedure, do you actually need to note the test number given that you have recorded completing the install procedure? Not sure.

There is further oddity in the traces too - the 20deg offset is actually not consistent. At the beginning of the previous flight, while on the ground, it looks to be about 10, falling and briefly going -ve, then rising to a constant 20 once the airspeed goes up. At the end of that flight, when airspeed drops, the offset appears to increase a little, and that higher offset is the same at start of next flight, until airspeed comes up and then it goes back to 20.

So we may be looking at a sensor fault that is dependent on airspeed and therefore wouldn't have been found on a ground test anyway. On the other hand the vanes are not alive without airspeed and may settle at different points anyway. But that drop at the start of the previous flight may be significant, probably at the end of taxi, the right AOA bounces a bit too, but the left much more. Something came loose? Image to show what I am rambling on about:

https://cimg9.ibsrv.net/gimg/pprune.org-vbulletin/297x218/80-untitled_d476c1fec00b36ff24db9f89012b49aa36e8b203.png

Note that if you are thinking sticky vane (I was), the test procedure on the classic contains the following: slowly rotate sensor vane between stops using light finger pressure. Check that vane rotates without binding or variations in torque

[infrequentflyer789]

Or it's not a raw data (sensor) problem rather it's a processed data (ADIRU) problem. The Left ADIRU seems to pop up quite a bit in the AFML Resolution Descriptions. I've got a fiver on a fault in the Left ADIRU featuring as a contributing factor.

Yeah, but, stick shaker goes off too, and assuming SMYD (which provides stick shaker) hasn't changed from the NG, it gets raw sin/cos signal for AOA which is what the sensor outputs, not the ADIRU.

Also, on the NG at least, the data acquisition point for AOA is downstream of SMYD, so we are seeing, in the traces, what SMYD saw/calculated, not ADIRU output.

Unless the MAX air data architecture is significantly different, but I don't think it will be.

[gearlever]

Concerning MCAS on the accident flight.

In the tech log no report of SS on previous flight...!?

Anyhow "the DFDR showed the stick shaker activated during the rotation and remained active throughout the flight."

So the first activation of SS was after replacement of LH AoA sensor.
Is it possible to install a RH AoA sensor in the LH position?

Part number confusion...?

Just asking.

[Jwscud]

With that logic all FBW aircraft should operate in degraded mode at all times. That would keep the pilots ready always.



Not at all, it is simply a design feature that degradation is graceful and intuitive.

[infrequentflyer789]

There is further oddity in the traces too - the 20deg offset is actually not consistent. [...]
https://cimg9.ibsrv.net/gimg/pprune.org-vbulletin/297x218/80-untitled_d476c1fec00b36ff24db9f89012b49aa36e8b203.png


Well, since I wrote that Peter Lemme over at satcom.guru has done an entire piece about failure modes in AOA sensors, and he picked up on that bit of the trace too, as well as much more. Worth a read: https://www.satcom.guru/2018/12/angle-of-attack-failure-modes.html

In summary: he doesn't have a failure mode that fits all the trace data either.

[Machinbird]

Infrequentflyer, I think we can easily explain the WTF point on the graph you presented.
Siince the aircraft is not yet airborne, that is probably brief jet blast impingement on the nose of the aircraft.

Additionally, Mr. Lemme has called attention to the difference in signal between left and right AOA while taxiing. The left AOA was noisy and the right AOA was rather smooth in that segment of the FDR data.
IMO that would be typical of a sneak circuit that is being excited by vibration transmitted by the landing gear into the fuselage.
Sneak circuits commonly originate at bent connector pins, conductors that are chafed by adjacent structure, or wires that become pinched when other components are installed, but there are also many other possibilities.

[infrequentflyer789]

Infrequentflyer, I think we can easily explain the WTF point on the graph you presented.
Siince the aircraft is not yet airborne, that is probably brief jet blast impingement on the nose of the aircraft.


Thanks, hadn't thought of that. Could the asymmetry between sides then be due to the aircraft turning at the time?


Additionally, Mr. Lemme has called attention to the difference in signal between left and right AOA while taxiing. The left AOA was noisy and the right AOA was rather smooth in that segment of the FDR data.
IMO that would be typical of a sneak circuit that is being excited by vibration transmitted by the landing gear into the fuselage.

Yep I've been thinking that way too, Mr Lemme doesn't seem to have picked up on this possibility - but that's maybe because his article was about sensor failure modes (depends at what point he picked his title I guess).

An intermittent wiring or connection fault, at or near the sensor, could account for the issues that triggered the sensor replacement and the worsening of the problem after it was replaced. If it is vibration triggered then ground testing might fail to find it (and the AOA sensor might test out fine). Be interesting to know if that AOA sensor had been replaced before or was factory fitted.

[safetypee]

From the descriptions in this thread and external inks, #97, and particularly https://www.satcom.guru/2018/11/stabilizer-trim.html, the STS is a ‘crutch’ on late series 737 to meet low speed stability requirements.
MCAS appears to be a similar ‘crutch’ but addressing more specific nose up issues when approaching or at stall (25.203), and when turning where the trim would be more nose up, and where there may be a greater pitching moment associated with the new engines -737 MAX.
However, it is difficult to understand the effects of input failures (e.g. AoA) amongst the complex computations and interactions in these trim systems.

(Forgive the ‘Non PC’ quip, but a failure in heavily ‘crutched’ systems, literally leaves you with no leg to stand on.) - ‘close coupled’ systems.

The association of the Elevator Feel and Centering Unit with trim has been described, and the reasons for separating the (independent?) pneumatic ‘muscle’ from the electronic logic in the trim system - mechanical, elevator feedback.
Is it possible that a false AoA will trigger a change in the trimmed - elevator neutral shift unit due to a false change in the feel / centering unit ?
If so, then a consequence might be an erroneous trim datum, which may also be misplaced by MCAS and / or electric pitch trim, so that no stable trim condition can be achieved. A pilot will continuously ‘chase’ a stable pitch condition - there is no trimmed position for speed (trying to fly in-trim with the elevator offset).
Furthermore with similar mechanism, would trim / AoA offset cause the feel unit to position at it’s maximum extent, and so doing limit the available amount of stabilizer / elevator for control ? (and / or stick force limit).


Aerodynamic slat operation also appears as a connection within the various ‘trim’ computations, relating to AoA input.
Is slat extension possible based on a false AoA, but at much higher air speeds those normally expected ? Possibly with significant adverse pitching moments competing with pitch control and trim.
Or even with the ‘handed’, separate-side AoA inputs and computations, could there be asymmetric slat deployment.
(The slat thoughts above come from the apparent roll control issues in the Lion accident.)

[infrequentflyer789]

From the descriptions in this thread and external inks, #97, and particularly https://www.satcom.guru/2018/11/stabilizer-trim.html, the STS is a ‘crutch’ on late series 737 to meet low speed stability requirements.
MCAS appears to be a similar ‘crutch’ but addressing more specific nose up issues when approaching or at stall (25.203), and when turning where the trim would be more nose up, and where there may be a greater pitching moment associated with the new engines -737 MAX.

There was/is a similar mechanism added to the NG as part of speed trim function, my guess is MCAS evolved from it and replaced it. The NG AMM says:

Near stall, the speed trim function trims the stabilizer to a nose down condition to allow for trim above the stickshaker AOA and idle thrust. The trim continues until the stabilizer gets to its limits or the aft column cutout position is exceeded

However it looks like the activation conditions for MCAS may be wider (more of the envelope) and the trimming is more aggressive and ignores the aft column cutout, there are probably other differences too. It isn't clear (to me) that the NG function can kick in from just one AOA.

However, it is difficult to understand the effects of input failures (e.g. AoA) amongst the complex computations and interactions in these trim systems.

(Forgive the ‘Non PC’ quip, but a failure in heavily ‘crutched’ systems, literally leaves you with no leg to stand on.) - ‘close coupled’ systems.


I agree but I do think there is a wider more general issue (shown nicely in AF447) with close coupling / interdependence of airdata from "independent" probes that are in reality all of the same type and sat in the same outside environment. The redundancy is illusory, and the interdependence means that when we do get failures they cascade - bad AOA fails altitude and speed, bad speed fails altitude and AOA, etc. Tying everything together and correcting for everything to put "perfect" data in front of pilots is great when it works - when it doesn't, the pilots are left trying to disentangle multiple failures, with less data they can trust, and may end up focusing on the wrong thing.

Essentially the automation paradox again.


Is it possible that a false AoA will trigger a change in the trimmed - elevator neutral shift unit due to a false change in the feel / centering unit ?
[...]
Is slat extension possible based on a false AoA, but at much higher air speeds those normally expected ?

Been wondering about those two as well - can't be sure if it's possible or not on the NG from the AMM, MAX could do anything since this is an area we know they've changed.

[safetypee]

More thoughts about system’s architecture and input sensors; primarily relating to MCAS, but perhaps some may relate to STS.

Although the aircraft airspeed system has ‘triplex’ integrity, this depends on the pilots selection of the best 2 out of 3 displays. However, I assume that the inputs into the trim computations are only dual (digital), and thus with a ‘speed disagreement’, in might be possible for an erroneous low value to activate trim.
Similarly an erroneous high thrust input, affecting MCAS.

Comments?

[de facto]

My opinoin, we would be safer without this system working agains us when we hand fly. Those of you who engage the auto pilot passing 400 ft will not understand what I’m talking about, but the few of you who hand fly on a regular basis will.
Comments?

i fly manual on a daily basis and the 737 flies like a Cessna. A pure joy. No issue with the speed trim sysyem at all.

[ManaAdaSystem]

i fly manual on a daily basis and the 737 flies like a Cessna. A pure joy. No issue with the speed trim sysyem at all.

Yes, because you cancel it by trimming against it. As I have done for nearly 20 years. My point is, if STS was not there, you would not notice. As the aircraft accelerates you would trim down. Same as with a Cessna.

[TOGA Tap]

MCAS is not a system at all - it is just a few lines of code in the flight control computer - other than that it does not exist in any form, part or shape. This code was supposed to give one single and small movement of the horizontal stab leading edge upwards when the aircraft enters one corner of the flight envelope and move the neutral point slightly back in order to satisfy the static stability regulation requirement.
Then during certification testing code was changed and instead of one activation new code allowed stabilizer to move more than once.
That effectively created a never ending loop - most common cause od any computer program gets stuck and only way to clear it is to reset the computer. It was a begginer's mistake. Only this time it was the aircrafts crashed not just code.
I do not know how deep the FAA can go into the code but they should have not ever ever approve this change.

[megan]

Question if I may from a SLF, if the STS were to fail does it impose any operating limitations?

[Gin Jockey]

if the STS were to fail does it impose any operating limitations?

It’s not listed in our DDG (manual that tells us what defects we can fly with), which means if it’s inoperative, we can’t fly. On the practical side, if it wasn’t working and there was no warning light to tell you it wasn’t working, no-one would ever notice. In flight we would just continue as normal if it failed.

[Denti]

At least it used to be in the MEL. According to that one of the two speed trim systems can be inoperative. Don't have access to a current MEL anymore, so if that has changed, please correct me.


https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/613x99/20201123_speedtrim_mel_19d4913041d4d21f3734baff2d84f66011e90 85d.jpg
Speedtrim MEL entry

[FlyingStone]

MMEL still shows exactly the same as your post Denti, dispatch with one STS inoperative (out of two installed) is most definitely permitted.

[Gin Jockey]

dispatch with one STS inoperative (out of two installed) is most definitely permitted.

Not at our outfit, DDG sections 22-09 and 22-10 (speed trim light and system respectively) have been removed from our DDG. It may be different at other companies obviously.

[megan]

Thanks folks, but I'm interested in if the complete STS system fails in flight, any guidance given as to how to proceed, and what might it be?

[vilas]

Aircraft with dangerous stall charactreistic is difficult to get certified. STS is a work around to help that. Same as it happened with B727 series. Boeing was able to get it through FAA(just like the max) but D P Davies refused to certify for CAA UK till a stick pusher was installed seven months later. MCAS is extended part of the STS. It was concealed to avoid expenditure and delays due recertification. If aircraft is flown well within the envelope no one may notice it's presence. Boeing had it in mind to upgrade the MCAS software in their own time but postponed it. Perhaps if a certain number of hours are flown without a problem then it may not even be required as it happened In case of B727. It kept flying in the US without the stick pusher. But whether 707 or 737 trim wheel all the time going crazy one way or the other is very noisy for sure.

[FlyingStone]

Thanks folks, but I'm interested in if the complete STS system fails in flight, any guidance given as to how to proceed, and what might it be?

https://cimg7.ibsrv.net/gimg/pprune.org-vbulletin/724x199/image_3425e8be9ca34785d554b6754ccf2e9902d046c8.png

[safetypee]

vilas, you should reconsider your use of the phrase 'dangerous stall characteristics', and implication that stall avoidance is the design objective of STS.

There are significant differences between the requirements for aircraft aerodynamic and flight handling characteristics when approaching and at stall, and for crew awareness when approaching a stall; e.g. provide the pilot with cues of approaching a stall with stick force.
Or alternatively and unrelated, reducing workload with auto trim in some situations.

The effect of STS failure will depend on the objective of STS; poor awareness might be tolerated for low exposure, low risk situations, but if used to alleviate weak aerodynamic characteristics or control effects, not so acceptable.

[vilas]

vilas, you should reconsider your use of the phrase 'dangerous stall characteristics
Shall I say undesirable pitch up near stall?

[safetypee]

vilas, 'Shall I say undesirable pitch up near stall?'
Only if correct; not proven to be so.

MCAS restored a less than desirable change in stick force with decreasing speed (stability - but appears to be more AoA and manoeuvre related).
There is no evidence that stick force reversed or became negative - hands off pitch up.

None of the above appears related to STS except that MCAS is now labelled as being part of STS; perhaps more as it is engineered / certificated opposed to the function.

[RobTek]

The STS system is a FCC function that when engaged calculates the climb air speed (trimmed speed), external conditions like wind speed and direction affect air speed, the FCC moves the stabilizer to hold its calculated climb speed (provided the pilot is not moving the stab or auto pilot is not engaged), if air speed increases the only way the FCC can slow down the aircraft is to increase drag by moving the stab areoplane nose up or if air speed decreases the only way the FCC can speed up the aircraft is to decrease drag by moving the stab areoplane nose down. FCC reaction time are very low so this should make it easier for the pilot to maintain that constant climb speed. Boeing believe that once the climb speed is set by the pilot is should remain constant.

The system in not about the pilot changing the speed (it will oppose pilot speed change if the stab is not used). Boeing believes the if the pilot wants to change speed they will operate the stab trim switches (disabling STS) 5 seconds after release of the trim switches FCC will recalculate climb speed and trim accordingly.

All take off’s, climbs and go arounds are high thrust according to the FCC.

The stab should not move automatically until 10 seconds after takeoff.

The STS is there to cover all calibers of pilot.

[RobTek]

That means the aircraft is accelerating diring the climb, and STS is trying to slow you down.
Dont forget that stabiliser movement cause the column to move if you are not holding it and STS will stop trimming when the aft column cutout switched are activated.
If you are holding the column you will need more muscles to hold it and would be opposing what the STS is trying to do.

[RobTek]

STS trim stops if the column cutout switches are activated, in the direction that will oppose what the stabiliser is doing.

[RobTek]

Just to carify
The STS system is a FCC function, there 2 FCC (A and B), Either FCC will provide STS, if there a single failure (speed trim fail light will illuminate on recall) and as noted continue flight normally.
However to dispatch with this failure, that STS must be deactivated (by pulling the FCC Dc circuit breaker, this will disable the whole FCC) you will now be operating with only 1 FCC.
If the speed trim fail light illuminates automatically, this means both STS functions have failed and the STS system will not work. The pilot will hanble this, after landing dispatch is not allowed, you cannot deactivate both systems if you did there would be no FCC's, therefore you must fix at least 1 of the FCC systems

[RobTek]

The only commonality between STS and MCAS is that they are both functions of the FCC and Both systems move the Stab.
They are different and seperate operating logic