B-737 Speed Trim System
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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.
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.
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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.
Paging Nigel Tufnel...
"Ours goes 1 unit forward..."
short flights long nights
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!!
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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.
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.
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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.
Cool video! Looks like a bit of a Heath-Robinson setup though...
I suspect Boeing won't/can't because more stability more fuel burn means means less efficiency.
Originally Posted by 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.
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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.
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.
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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.
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.
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?
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B737 pitch trim description
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.
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.
Last edited by FCeng84; 7th Nov 2018 at 02:35. Reason: correct error: STS is a function of thrust but not weight
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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?
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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.
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.
Last edited by T28B; 7th Nov 2018 at 18:29. Reason: formatting clean up
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Why move stab and not adjust column feel to spd stability?
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.
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.
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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.
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!
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!
Last edited by RetiredBA/BY; 7th Nov 2018 at 11:55.
