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B-737 Speed Trim System

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B-737 Speed Trim System

Old 7th Dec 2018, 12:23
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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.

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Old 7th Dec 2018, 13:25
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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.
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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.
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Old 7th Dec 2018, 15:02
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Originally Posted by de facto


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.
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Old 24th May 2019, 20:37
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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.
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Old 23rd Nov 2020, 04:12
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Question if I may from a SLF, if the STS were to fail does it impose any operating limitations?
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Old 23rd Nov 2020, 10:40
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Originally Posted by megan
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.
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Old 23rd Nov 2020, 11:05
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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.



Speedtrim MEL entry
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Old 23rd Nov 2020, 11:16
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MMEL still shows exactly the same as your post Denti, dispatch with one STS inoperative (out of two installed) is most definitely permitted.
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Old 23rd Nov 2020, 21:04
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Originally Posted by FlyingStone
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.
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Old 24th Nov 2020, 01:12
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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?
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Old 24th Nov 2020, 07:57
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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.
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Old 24th Nov 2020, 08:46
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Originally Posted by 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?


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Old 24th Nov 2020, 08:48
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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.
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Old 24th Nov 2020, 09:21
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vilas, you should reconsider your use of the phrase 'dangerous stall characteristics
Shall I say undesirable pitch up near stall?
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Old 24th Nov 2020, 10:44
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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.

Last edited by safetypee; 24th Nov 2020 at 11:08.
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Old 1st Dec 2020, 23:55
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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.
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Old 2nd Dec 2020, 00:06
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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.
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Old 2nd Dec 2020, 00:09
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STS trim stops if the column cutout switches are activated, in the direction that will oppose what the stabiliser is doing.
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Old 2nd Dec 2020, 23:37
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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
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Old 2nd Dec 2020, 23:46
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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
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