737-800 Speed trim system
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737-800 Speed trim system
I have some questions regarding the Boeing 738 speed trim system (STS) which I hope someone out there can answer for me. English is also my second language, so please try not to deduct too many assumptions from apparent language errors. 
I am undergoing a type rating for the 738 and cannot find the speed range the STS aims for when it engages. In previous threads I have read:
"Speed trim is applied to the stabilizer automatically at low speed, low weight, aft C of G and high thrust - i.e. on most take-offs. Speed trim is a dual channel system. Sometimes you may notice that the speed trim is trimming in the opposite direction to you, this is because the speed trim is trying to trim the stabilizer in the direction calculated to provide the pilot with positive speed stability characteristics. The speed trim system adjusts stick force so the pilot must provide significant amount of pull force to reduce airspeed or a significant amount of push force to increase airspeed. Whereas pilots are typically trying to trim the stick force to zero. Occasionally these may be in opposition."
When "positive speed stability characteristics" is mentioned I guess it means a speed a set value above Vmd. Would anyone know what value it aims for?
I can neither find it in the FCOM nor the Maintenance Manual.
Does anyone know what the Vmd / Max Endurance - speed range is on the 738?
And in which situations does the STS engage?
"Low speed, low weight, aft C of G and high thrust" is a combination of conditions that makes the airplane unstable around the lateral axis, and I thought it may be a stall protection system, but it also states that the STS can trim the stabilizer towards a lower (!) speed. ("The speed trim system adjusts stick force so the pilot must provide significant amount of pull force to reduce airspeed ... ")
If anyone could help me I would be very grateful.
I am undergoing a type rating for the 738 and cannot find the speed range the STS aims for when it engages. In previous threads I have read:
"Speed trim is applied to the stabilizer automatically at low speed, low weight, aft C of G and high thrust - i.e. on most take-offs. Speed trim is a dual channel system. Sometimes you may notice that the speed trim is trimming in the opposite direction to you, this is because the speed trim is trying to trim the stabilizer in the direction calculated to provide the pilot with positive speed stability characteristics. The speed trim system adjusts stick force so the pilot must provide significant amount of pull force to reduce airspeed or a significant amount of push force to increase airspeed. Whereas pilots are typically trying to trim the stick force to zero. Occasionally these may be in opposition."
When "positive speed stability characteristics" is mentioned I guess it means a speed a set value above Vmd. Would anyone know what value it aims for?
I can neither find it in the FCOM nor the Maintenance Manual.
Does anyone know what the Vmd / Max Endurance - speed range is on the 738?
And in which situations does the STS engage?
"Low speed, low weight, aft C of G and high thrust" is a combination of conditions that makes the airplane unstable around the lateral axis, and I thought it may be a stall protection system, but it also states that the STS can trim the stabilizer towards a lower (!) speed. ("The speed trim system adjusts stick force so the pilot must provide significant amount of pull force to reduce airspeed ... ")
If anyone could help me I would be very grateful.
Last edited by KristianNorway; 2nd Aug 2008 at 21:29.
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Speed trim is there to ensure that the stick force per G is correct; it resolves a minor certification 'issue'. As a pilot, you needn't worry about it too much.
If you have questions like this and are on the course, you really should be asking your instructors...
Good luck. The-800 is the best 737 (aside from the BBJs!).
If you have questions like this and are on the course, you really should be asking your instructors...
Good luck. The-800 is the best 737 (aside from the BBJs!).
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I like the -700 a lot more. And by the way, if speed trim does something stupid (like trimming the "wrong way") a very short manual trim input will inhibit it for around 10 seconds.
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Ok. It seems I thought about it the wrong way.
I thought it aimed for a certain speed, but I see now (if I have understood it correctly this time) that it's a control force augmentation system during conditions where the plane is longitudinally unstable.
I have very good instructors, but I have some days off for individual reading. That's why you guys are consulted.
Thanks for all help!
I thought it aimed for a certain speed, but I see now (if I have understood it correctly this time) that it's a control force augmentation system during conditions where the plane is longitudinally unstable.
I have very good instructors, but I have some days off for individual reading. That's why you guys are consulted.
Thanks for all help!
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FCOM 9.20.9
Speed Trim System
The 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:
• Airspeed between 100 KIAS and Mach 0.5
• 10 seconds after takeoff
• 5 seconds following release of trim switches
• Autopilot not engaged
• Sensing of trim requirement
ha det så fint.
Speed Trim System
The 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:
• Airspeed between 100 KIAS and Mach 0.5
• 10 seconds after takeoff
• 5 seconds following release of trim switches
• Autopilot not engaged
• Sensing of trim requirement
ha det så fint.
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Denti, you wrote:
...and I'm sorry to say that you don't understand your aircraft. The system is there to make the aircraft handle in a very straightforward way, and mucking about with it is neither 'big' nor 'clever'. Let it do its job, and make sure you do yours, and you'll both get along well.
Please find a trainer to explain this for you, and then look to your laurels to find your other areas of weakness before they find you.
Fed up with supposed 'professionals' posting this sort of thing here.
The -700 is OK, but nothing like the money-maker that the -800 can be, if you can fly it full! As a pilot's aircraft they are comparable, though I find the stability of the -800 quite pleasant, as against the relatively short-coupled -700. The downside is that the -800 is slightly unstable in pitch/speed near Vref. The -800 is much easier OEI at low speeds and high weights. The over-powered BBJ doesn't compare with the lovely BBJ2...
By the way, Kristian, your command of English is a pleasure to see. You have a better grasp of the language than most native-speakers, including (I regret to say) many senior managers.
if speed trim does something stupid (like trimming the "wrong way")
Please find a trainer to explain this for you, and then look to your laurels to find your other areas of weakness before they find you.
Fed up with supposed 'professionals' posting this sort of thing here.
The -700 is OK, but nothing like the money-maker that the -800 can be, if you can fly it full! As a pilot's aircraft they are comparable, though I find the stability of the -800 quite pleasant, as against the relatively short-coupled -700. The downside is that the -800 is slightly unstable in pitch/speed near Vref. The -800 is much easier OEI at low speeds and high weights. The over-powered BBJ doesn't compare with the lovely BBJ2...
By the way, Kristian, your command of English is a pleasure to see. You have a better grasp of the language than most native-speakers, including (I regret to say) many senior managers.
Last edited by frontlefthamster; 4th Aug 2008 at 21:33.
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I think the thing you have to remember is the 737-300 onwards were designed to be automated aeroplanes. ie 400 aal auto-pilot in, to an ILS auto land.
It is true you can see Speed Trim System operate in the opposite to pilot inputs. But its ok and works just fine.....
Also when you deploy flap you can see AFS working and (on the ground) can see the controls move forward! strange
It is true you can see Speed Trim System operate in the opposite to pilot inputs. But its ok and works just fine.....
Also when you deploy flap you can see AFS working and (on the ground) can see the controls move forward! strange
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The -700 is OK
but nothing like the money-maker that the -800 can be, if you can fly it full!
In case you have developed a good way to grease the -800 please say so, or send me a PM.
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Inop, you're not the only one who feels like this...
Yes, people have problems greasing the -800. I find that it needs a slower thrust reduction than the -700, and that it is VERY speed-sensitive in the flare. Drop to Vref at the screen, and you'll also drop out of the sky. That said, this aircraft is so runway-hungry that adding a few knots to make the flare easier is not a justifiable option.
You probably know, and use, the 'slight pitch down for touchdown' technique - you know, that little forward movement of the control column to de-rotate the fuselage to a slightly more level pitch attitude just before the MLG touches, which sorts out the oleo alignment, causes the MLG to be travelling up with reference to the C of G, and so on. Well, the -800 seems to benefit from a slightly later and more aggressive pitch down than the shorter Boeings.
Hope this helps. If you're going to refine your landing techhnique, please do so carefully! Smooth landings are possible in the -800...
...but don't forget that a safe landing is on speed, in the right place, and slightly firm.
Yes, people have problems greasing the -800. I find that it needs a slower thrust reduction than the -700, and that it is VERY speed-sensitive in the flare. Drop to Vref at the screen, and you'll also drop out of the sky. That said, this aircraft is so runway-hungry that adding a few knots to make the flare easier is not a justifiable option.
You probably know, and use, the 'slight pitch down for touchdown' technique - you know, that little forward movement of the control column to de-rotate the fuselage to a slightly more level pitch attitude just before the MLG touches, which sorts out the oleo alignment, causes the MLG to be travelling up with reference to the C of G, and so on. Well, the -800 seems to benefit from a slightly later and more aggressive pitch down than the shorter Boeings.
Hope this helps. If you're going to refine your landing techhnique, please do so carefully! Smooth landings are possible in the -800...
...but don't forget that a safe landing is on speed, in the right place, and slightly firm.
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Speed trim and what it does.
First lets assume that the working line of thrust goes trough the CG, so no moments created by the thrust.
1. Suppose you are flying at 250kt and you reduce your speed to 220kts. You will have to trim nose up to do that for aerodynamic reasons only. Remember the thrust line goes trough the CG.
2. If at constant speed you select flaps i.e. increase the camber, then you have to trim nose up again.
If you combine both, flaps and speed reduction the you have to add both trim requirements. If the aircraft is light and the CG aft then the moments created by these forces are smaller than if heavy with forward CG.
Now since the engine thrust does not go trough the CG it may be so that; when reducing speed and setting flaps and increasing thrust to compensate for the increase in drag that the pitch up created by the engines does the trimming work mentioned under 1 & 2 for you.
Theoretically it is even possible that it does it so well that you have to trim FOREWARD iso aft or that the trim requirements become so small that it becomes difficult.
That is why the FAR amongst other reasons sets a min req for stick force / g.
The speed trim takes care of this in a way to trim in the other direction of the required trim. It does so not to trim the aircraft but to fix the Stickforce/g requirement. You can then trim again as usual. Once you get away from this condition the process starts all over again when the system senses that the stick force/g is too low.
Without speed trim the flare would become very difficult bcs the stick force / g would change in a way that makes the required control input a function of too many parameters f(thrust change, rate of decent, runway slope, inertia, wind changes etc)
The reverse happens when you acc.
There is a bit of confusion that arises bcs of the name they gave to this system. On the B777 it works all the time. Fly by wire has the advantage that you can play with this easily. On the B777 the pilot no longer feels the pitch created by the engines at all times. The aircraft behaves as a glider with respect to trimming requirements. In other words you trim for speed changes only.
In fact you see that these things were introduced when engine thrust went up on acft with the engines below the CG.
Bigger issue on the B747 where CG moves horizontal as well as vertical.
First lets assume that the working line of thrust goes trough the CG, so no moments created by the thrust.
1. Suppose you are flying at 250kt and you reduce your speed to 220kts. You will have to trim nose up to do that for aerodynamic reasons only. Remember the thrust line goes trough the CG.
2. If at constant speed you select flaps i.e. increase the camber, then you have to trim nose up again.
If you combine both, flaps and speed reduction the you have to add both trim requirements. If the aircraft is light and the CG aft then the moments created by these forces are smaller than if heavy with forward CG.
Now since the engine thrust does not go trough the CG it may be so that; when reducing speed and setting flaps and increasing thrust to compensate for the increase in drag that the pitch up created by the engines does the trimming work mentioned under 1 & 2 for you.
Theoretically it is even possible that it does it so well that you have to trim FOREWARD iso aft or that the trim requirements become so small that it becomes difficult.
That is why the FAR amongst other reasons sets a min req for stick force / g.
The speed trim takes care of this in a way to trim in the other direction of the required trim. It does so not to trim the aircraft but to fix the Stickforce/g requirement. You can then trim again as usual. Once you get away from this condition the process starts all over again when the system senses that the stick force/g is too low.
Without speed trim the flare would become very difficult bcs the stick force / g would change in a way that makes the required control input a function of too many parameters f(thrust change, rate of decent, runway slope, inertia, wind changes etc)
The reverse happens when you acc.
There is a bit of confusion that arises bcs of the name they gave to this system. On the B777 it works all the time. Fly by wire has the advantage that you can play with this easily. On the B777 the pilot no longer feels the pitch created by the engines at all times. The aircraft behaves as a glider with respect to trimming requirements. In other words you trim for speed changes only.
In fact you see that these things were introduced when engine thrust went up on acft with the engines below the CG.
Bigger issue on the B747 where CG moves horizontal as well as vertical.
Last edited by Pitch Up Authority; 21st Feb 2012 at 11:36.
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Without speed trim the flare would become very difficult bcs the stick force / g would change in a way that makes the required control input a function of too many parameters f(thrust change, rate of decent, runway slope, inertia, wind changes etc)
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Point taken. Probably not as bad as I suggested. But from experience I know that when light on these types of acft it is definitely more difficult to be precise.
When it happened to you, was the acft light and with aft cg?
When it happened to you, was the acft light and with aft cg?
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...and I'm sorry to say that you don't understand your aircraft. The system is there to make the aircraft handle in a very straightforward way, and mucking about with it is neither 'big' nor 'clever'. Let it do its job, and make sure you do yours, and you'll both get along well.
Please find a trainer to explain this for you, and then look to your laurels to find your other areas of weakness before they find you.
Fed up with supposed 'professionals' posting this sort of thing here.
Please find a trainer to explain this for you, and then look to your laurels to find your other areas of weakness before they find you.
Fed up with supposed 'professionals' posting this sort of thing here.
Do you measure the height of that horse in metres or hands?
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Please find a trainer to explain this for you, and then look to your laurels to find your other areas of weakness before they find you.
You probably know, and use, the 'slight pitch down for touchdown' technique - you know, that little forward movement of the control column to bla bla bla...
It should go like this:
1) Fix aim-point in window,
2) Check speed, check aim point, check speed, check aim-point (Make adjustments to flight path and thrust and trim to thrust changes at all time), so that you arrive...
3) ...in trim, on path, on speed... 30' ... thrust idle.... 20'
4) switch view to end of runway.
5) Hold against the thrust reduction induced nose down moment, flare a little bit
5) 10'... Use your vision to judge the vertical speed and adjust input as necessary
6) Let it come down, don't hold it off (or lower the nose)
7) Use your feet to align the aircraft with the direction of travel. Even the lowest vertical speed will feel like a ****ty landing if you come down sideways (NG especially intolerant to drift).
Incidentially the above is exactly what is written in the FCTM only in a slightly different wording
I have written this quite some times before:
Most "firm" arrivals are caused by not looking where you are supposed to… out the window!
Instrument fixation (FD, glide slope, PAPI), then followed by the panic-pull-surprise-over-flare when finally looking out and seeing the rising runway and then the classic late 10 feet thrust reduction followed by endless floating and smack down from 6 feet.
The glideslope, PAPI and Touchdown markings are often not co-located, hence all this information may unstabilize the path at the last moment if switching focus between them in the expectation that it's possible to get them all to magically align.
Actually they are all useless information, compared to the much more precise cues one can obtain from just fixating the aim-point in the window. Being where you want to be in a stabile state is the key to a greaser (point 3 above).
And just to beat who-ever to it... nobody expects you to make a greaser from a hand flow CATII approach (like anybody does those now a days anyway).
