B737NG - Speed Trim System

[B737NG_Pilot]

Hi can any one explain the STS on the B737NG. I have very low time on the B737, and have never really understood the function of the STS.

As per FCOM

The speed trim system (STS) is a speed stability augmentation system designed to improve flight characteristics during operations with a low gross weight, aft centre 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

- What is Speed Stability ?

- What is Trimmed Speed?

- What does direction opposite the speed change mean?

Thanks

[de facto]

If you have low speed high thrust:stabiliser will be trimmed nose down.
High speed low thrust:stabiliser will move upwards.
To feel the speed stability of your airplane,when in descent,disconnect AP and note the IAS(ie 250 kts),then add SLOWLY some thrust,you will see that the speed is stable at 250 and the thrust increase results in an increase of pitch.
If you increase the thrust faster,the speed will initially tend to increase and the stab trim will trim the stab down and vice versa.

[B737NG_Pilot]

Thank you de facto... that makes my understanding of the system much better.

[RVF750]

B7, who did your ground school course? Just wondering because you post a lot of questions that should have been answered by your instructor....


No offence intended at all, but any knowledge gaps are due to training generally not the trainee.

[Kirks gusset]

Forgive me for paraphrasing text from 2008, but this is a complete answer

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

[JammedStab]

Originally Posted by Turkish Delight

B7, who did your ground school course? Just wondering because you post a lot of questions that should have been answered by your instructor....


No offence intended at all, but any knowledge gaps are due to training generally not the trainee.

Most ground schools are computer based training. After having been on 9 turbine types I find that every one of the ground schools whether with an instructor or CBT leaves one with many questions if they are the questioning type.

Many pilots probably just accept not knowing smaller details and the more modern types frequently exclude a lot of system detail and can get along just fine like that.

Sometimes a question gets asked and the answer was there in the manual but other interesting information gets posted on the thread which may or may not be relevant. Subsequently, one can quickly do a google seach with pprune and subject matter and find several threads with good information.

Unfortunately, some responses(without specifying any threads) come from posters that basically question a persons abilities because of the initial question was asked. I find those people to themselves have a questionable personality in terms of whether they themselves should be flying while the people starting the posts to likely be the kind of pilot you want to fly with because they are furthering their knowledge.

This is a forum for technical questions. If you don't like it because sometimes the subject should be well known then go somewhere else. You won't be missed.

[flyboyike]

Originally Posted by JammedStab

Most ground schools are computer based training. After having been on 9 turbine types I find that every one of the ground schools whether with an instructor or CBT leaves one with many questions if they are the questioning type.

Many pilots probably just accept not knowing smaller details and the more modern types frequently exclude a lot of system detail and can get along just fine like that.

Sometimes a question gets asked and the answer was there in the manual but other interesting information gets posted on the thread which may or may not be relevant. Subsequently, one can quickly do a google seach with PPRuNe and subject matter and find several threads with good information.

Unfortunately, some responses(without specifying any threads) come from posters that basically question a persons abilities because of the initial question was asked. I find those people to themselves have a questionable personality in terms of whether they themselves should be flying while the people starting the posts to likely be the kind of pilot you want to fly with because they are furthering their knowledge.

This is a forum for technical questions. If you don't like it because sometimes the subject should be well known then go somewhere else. You won't be missed.

It has nothing to do with one's abilities, and I, for one, have no problem with someone who wants to get into the finer, more advanced details of systems or procedures. However, when someone who claims to have been flying his aircraft for over a year apparently (among other things):

1. Doesn't know how to interpret F/D commands;

2. Doesn't know how to set the pressurization panel;

3. Doesn't know what speed stability is (which is a concept not dependent on a particular aircraft type);

4. Doesn't know how the anti-ice system operates;

Can I be blamed for maybe not entirely buying such a person's story?

Or when another someone gets people who don't buy their story banned (twice now), can I be blamed for thinking said someone is a fraud, especially when they:

1. One week are flying a 737NG into Anchorage and are all up in arms about how the FMS is 0.0025 degree different from the ILS G/S;

2. The very next week are flying a 747-400 out of China and are all sorts of hung up about what a "high runup" is while not being sure what TOGA switches do.

3. The week after that they're operating some PW100-powered something or other?


Feel free to get me banned a third time, you'll just prove me right (again).

[punkalouver]

Originally Posted by flyboyike

It has nothing to do with one's abilities, and I, for one, have no problem with someone who wants to get into the finer, more advanced details of systems or procedures. However, when someone who claims to have been flying his aircraft for over a year apparently (among other things):

1. Doesn't know how to interpret F/D commands;

2. Doesn't know how to set the pressurization panel;

3. Doesn't know what speed stability is (which is a concept not dependent on a particular aircraft type);

4. Doesn't know how the anti-ice system operates;

Can I be blamed for maybe not entirely buying such a person's story?

Or when another someone gets people who don't buy their story banned (twice now), can I be blamed for thinking said someone is a fraud, especially when they:

1. One week are flying a 737NG into Anchorage and are all up in arms about how the FMS is 0.0025 degree different from the ILS G/S;

2. The very next week are flying a 747-400 out of China and are all sorts of hung up about what a "high runup" is while not being sure what TOGA switches do.

3. The week after that they're operating some PW100-powered something or other?


Feel free to get me banned a third time, you'll just prove me right (again).

Well, after that semi-coherent rant, I can see why this statement was made earlier.

"Unfortunately, some responses(without specifying any threads) come from posters that basically question a persons abilities because of the initial question was asked. I find those people to themselves have a questionable personality in terms of whether they themselves should be flying while the people starting the posts to likely be the kind of pilot you want to fly with because they are furthering their knowledge.

This is a forum for technical questions. If you don't like it because sometimes the subject should be well known then go somewhere else. You won't be missed."

[flyboyike]

Just calling a spade a spade.