B-737 Speed Trim System
Thread Starter
Join Date: Aug 2000
Posts: 1,499
Likes: 0
Received 0 Likes
on
0 Posts
B-737 Speed Trim System
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 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?
Thread Starter
Join Date: Aug 2000
Posts: 1,499
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
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.
Join Date: Sep 2016
Location: USA
Posts: 803
Likes: 0
Received 0 Likes
on
0 Posts
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.
Last edited by Vessbot; 5th Nov 2018 at 18:53.
Thread Starter
Join Date: Aug 2000
Posts: 1,499
Likes: 0
Received 0 Likes
on
0 Posts
Weird system!
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
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
Join Date: Jun 2006
Location: Brisbane
Posts: 265
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Apr 2010
Location: Home soon
Posts: 0
Likes: 0
Received 0 Likes
on
0 Posts
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.....
Thread Starter
Join Date: Aug 2000
Posts: 1,499
Likes: 0
Received 0 Likes
on
0 Posts
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.
Thread Starter
Join Date: Aug 2000
Posts: 1,499
Likes: 0
Received 0 Likes
on
0 Posts
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
If STS was completely disabled during take off, nobody would notice. When it works it tries to bring the aircraft into a stall.
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.
Last edited by MickG0105; 4th Nov 2018 at 01:29. Reason: Added speed condition
Join Date: Jun 2006
Location: Brisbane
Posts: 265
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Jul 2014
Location: Germany
Posts: 344
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
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.
Join Date: Sep 2016
Location: USA
Posts: 803
Likes: 0
Received 0 Likes
on
0 Posts
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 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?
Join Date: Sep 2016
Location: USA
Posts: 803
Likes: 0
Received 0 Likes
on
0 Posts
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."
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."
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."
Join Date: May 2011
Location: Metropolis
Posts: 1,270
Likes: 0
Received 0 Likes
on
0 Posts
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
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
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.
