Question if I may from a SLF, 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.

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.


https://cimg4.ibsrv.net/gimg/pprune....6011e9085d.jpg
Speedtrim MEL entry

MMEL still shows exactly the same as your post Denti, 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.

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?

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.

https://cimg7.ibsrv.net/gimg/pprune....9902d046c8.png

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.

Shall I say undesirable pitch up near stall?

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.

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.

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.

STS trim stops if the column cutout switches are activated, in the direction that will oppose what the stabiliser is doing.

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

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