S-TEC autopilot servo -- quality product?
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Interesting - and chilling - read indeed.
Slight thread drift: how many of us actually LOOK BACK at he trim tab during the checks or confirm during walk-around that the 'take-off' position on the wheel actually corresponds to the correct trim tab setting?
Perhaps not an issue on you own a/c which you know well, but a good idea on a rental!
Slight thread drift: how many of us actually LOOK BACK at he trim tab during the checks or confirm during walk-around that the 'take-off' position on the wheel actually corresponds to the correct trim tab setting?
Perhaps not an issue on you own a/c which you know well, but a good idea on a rental!
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I would like to try out a takeoff run with the trim tab fully deflected to either position and see how that feels. I wonder if that is a dangerous experiment.
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I would try a simulated t/o at altitude. However, I believe what gets you in this kind of situation is not the actual flying, but the WTF 
factor. Pushing full down with all your force is totally counter-intuitive at t/o and I guess for most pilots the time to figure out what has gone wrong would be too long.
After once discovering the trim tab position way out of agreement with the wheel, I've made it a habit of looking back at the tab immediately before lining up. Not sure that would have saved the day in the accident described, as it is conceivable that the trim ran away during the t/o run. In this case, all bets are off.
factor. Pushing full down with all your force is totally counter-intuitive at t/o and I guess for most pilots the time to figure out what has gone wrong would be too long.After once discovering the trim tab position way out of agreement with the wheel, I've made it a habit of looking back at the tab immediately before lining up. Not sure that would have saved the day in the accident described, as it is conceivable that the trim ran away during the t/o run. In this case, all bets are off.
Last edited by 172driver; 4th Aug 2012 at 12:18.
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That 2x fatal is well known.
It has been speculated that since their AP was mounted at the bottom of the centre stack, they may have engaged it (pressed the AP button) with a knuckle, when advancing the throttle for takeoff.
However, with a correctly functioning autopilot, the system doesn't ever drive the pitch trim servo except as a response to a signal from the torque sensor in the pitch servo. The KS270C pitch servo has a -3V to +3V output, corresponding to max torque one way / the other way, and the AP computer uses this to drive the pitch trim servo to reduce that signal back to within certain limits.
So, to get the pitch trim servo to run at all, during the takeoff phase, you would need to engage the autopilot (by pressing AP) and then ignore the fact that it is driving the yoke back towards you (or away from you; the direction depends on the actual pitch angle at the time AP was pressed, and on the subsequent pitch angle, because merely pressing AP captures and tries to maintain the pitch angle, and it does it solely by driving the pitch servo) and you would either have to resist that motion (which would be bl00dy obvious, since during the takeoff phase the elevator hardly moves, apart from its initial "self-alignment" when you go to full power) for several seconds, or ignore the motion until it hits one of the stops, and only then will the pitch trim servo get actuated.
The above assumes the system was working as it should be.
I have no reason to say this relative to that accident but loads of people fly with defective or partly defective autopilots because (a) the costs of fixing it and (b) the extreme scarcity of avionics shops in Europe who know anything about them. Most planes I flew in during training, that had autopilots fitted, had them placarded INOP or something similar, but that doesn't mean the CBs were pulled
It is a pre-takeoff checklist item on the TB20. It's obviously critical to have the elevator trim in the right place (more or less) prior to takeoff, on any aircraft type.
The TRIM IN MOTION is IIRC what you get after 9 seconds of continuous trim servo activity, and after 15 seconds (IIRC) the AP will disconnect and hand the plane back to you, sir
The system I fly behind that had annunciation years before that accident, but there may be autopilots which didn't have it.
as it is conceivable that the trim ran away during the t/o run
However, with a correctly functioning autopilot, the system doesn't ever drive the pitch trim servo except as a response to a signal from the torque sensor in the pitch servo. The KS270C pitch servo has a -3V to +3V output, corresponding to max torque one way / the other way, and the AP computer uses this to drive the pitch trim servo to reduce that signal back to within certain limits.
So, to get the pitch trim servo to run at all, during the takeoff phase, you would need to engage the autopilot (by pressing AP) and then ignore the fact that it is driving the yoke back towards you (or away from you; the direction depends on the actual pitch angle at the time AP was pressed, and on the subsequent pitch angle, because merely pressing AP captures and tries to maintain the pitch angle, and it does it solely by driving the pitch servo) and you would either have to resist that motion (which would be bl00dy obvious, since during the takeoff phase the elevator hardly moves, apart from its initial "self-alignment" when you go to full power) for several seconds, or ignore the motion until it hits one of the stops, and only then will the pitch trim servo get actuated.
The above assumes the system was working as it should be.
I have no reason to say this relative to that accident but loads of people fly with defective or partly defective autopilots because (a) the costs of fixing it and (b) the extreme scarcity of avionics shops in Europe who know anything about them. Most planes I flew in during training, that had autopilots fitted, had them placarded INOP or something similar, but that doesn't mean the CBs were pulled
how many of us actually LOOK BACK at he trim tab during the checks or confirm during walk-around that the 'take-off' position on the wheel actually corresponds to the correct trim tab setting?
The TRIM IN MOTION is IIRC what you get after 9 seconds of continuous trim servo activity, and after 15 seconds (IIRC) the AP will disconnect and hand the plane back to you, sir
The system I fly behind that had annunciation years before that accident, but there may be autopilots which didn't have it.
Last edited by peterh337; 4th Aug 2012 at 13:08.
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However, with a correctly functioning autopilot, the system doesn't ever drive the pitch trim servo except as a response to a signal from the torque sensor in the pitch servo.
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I can see that, yes, but mine (a similar system except that the altitude preselect is integrated) doesn't do that, and I cannot see why this type of autopilot (with a pitch servo) should drive the pitch trim when on the ground.
On mine, the only way to make the pitch trim servo run is to either subject the pitch servo to a "torque" (in flight, or by pushing/pulling on the yoke either on the ground or in flight for several seconds), or of course by operating the electric trim switch.
I will make some enquiries in the USA regarding this, to see if Honeywell changed the firmware since 2001, or whether the KAP140 really does this.
On mine, the only way to make the pitch trim servo run is to either subject the pitch servo to a "torque" (in flight, or by pushing/pulling on the yoke either on the ground or in flight for several seconds), or of course by operating the electric trim switch.
I will make some enquiries in the USA regarding this, to see if Honeywell changed the firmware since 2001, or whether the KAP140 really does this.
Interesting read.
I've been toying with the idea of upgrading my Mooney with either a S-TEC 30 as a minimum solution or a 60-2 combined with an Aspen, the latter as I don't see the point of having a FD compatible PFD without an AP which delivers the FD signal.
Any experiencs with the -30 and 60-2 will be appreciated.
Friend of mine used to fly a Twin Com with a -30 over most parts of the world for thousands of hours and told me he never had as much as a twitch, much less a real problem. Of course, the -30 is a rather simple system with only an alt hold function.
I did my initial IR on a KFC150 equipped Seneca II and later also flew on a Seneca I with the old Piper Alticontrol system. Never had any problems with either. Never flew with an S-TEC system at all.
I've been toying with the idea of upgrading my Mooney with either a S-TEC 30 as a minimum solution or a 60-2 combined with an Aspen, the latter as I don't see the point of having a FD compatible PFD without an AP which delivers the FD signal.
Any experiencs with the -30 and 60-2 will be appreciated.
Friend of mine used to fly a Twin Com with a -30 over most parts of the world for thousands of hours and told me he never had as much as a twitch, much less a real problem. Of course, the -30 is a rather simple system with only an alt hold function.
I did my initial IR on a KFC150 equipped Seneca II and later also flew on a Seneca I with the old Piper Alticontrol system. Never had any problems with either. Never flew with an S-TEC system at all.
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I wonder if wigglyamp might comment on that AAIB report?
He knows about autopilots.
I have been asking around US forums and they seem to be 50/50 on the AAIB report being confused.
Personally, and not knowing the KAP140 or the particular C182 installation, I cannot be sure that the accident installation was not faulty.
There is a curious way in which a fully functioning autopilot might drive the trim during the takeoff run: when full power is applied for takeoff, the airflow "straightens" the elevator into a neutral-ish position. If you engaged the autopilot, by pressing say the AP button just prior to that moment, it would "grab" the yoke instantly, and the centralising-airflow effect on the elevator would apply a torque on the pitch servo which would cause the pitch trim to be driven. However, this sequence of events would commence at the moment full throttle is applied, and I wonder if there would be anywhere near enough time for the Cessna electric trim to wind the trim all the way UP in the duration of the takeoff run.
He knows about autopilots.
I have been asking around US forums and they seem to be 50/50 on the AAIB report being confused.
Personally, and not knowing the KAP140 or the particular C182 installation, I cannot be sure that the accident installation was not faulty.
There is a curious way in which a fully functioning autopilot might drive the trim during the takeoff run: when full power is applied for takeoff, the airflow "straightens" the elevator into a neutral-ish position. If you engaged the autopilot, by pressing say the AP button just prior to that moment, it would "grab" the yoke instantly, and the centralising-airflow effect on the elevator would apply a torque on the pitch servo which would cause the pitch trim to be driven. However, this sequence of events would commence at the moment full throttle is applied, and I wonder if there would be anywhere near enough time for the Cessna electric trim to wind the trim all the way UP in the duration of the takeoff run.
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Peter, according to the AAIB it's the other way round, i.e. the LACK of airflow initiates this sequence of events. Quoted from the AAIB report, my bold:
If any one of the autopilot AP, HDG or ALT buttons had been pressed before the take-off roll commenced, the autopilot would have engaged and attempted to maintain the vertical speed existing at the time of engagement. In effect, since the vertical speed would have been zero, the autopilot signals would have been consistent with attempts to maintain level flight. In the absence of aerodynamic loads, the elevator would have drooped under its own weight, causing a difference in tension in the elevator servo capstan cables. This difference would have caused the pitch trim servo to be signalled in the normal manner, thus causing movement of the trim tab. With insufficient airflow to produce any aerodynamic force on the tab that would lift the elevator and eliminate the tension differential, the auto-trim function would have continued to apply nose-up trim until the trim actuator cables reached their stops.
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Dear freinds,
A freind of mine has the S-Tec auto pilot in his E-AB Lancair 360 A/C.
My freind asked me to help him identify the problem in the auto pilot:
There is an external toggle switch 3 positions: ON - OFF - TEST.
In test position it operate the auto pilot in test mode but in ON position nothing works. no lights no eny sign of operation.
Has any one of you met this problem? of have any idea what can be wrong?
Thanks,
Ariel
A freind of mine has the S-Tec auto pilot in his E-AB Lancair 360 A/C.
My freind asked me to help him identify the problem in the auto pilot:
There is an external toggle switch 3 positions: ON - OFF - TEST.
In test position it operate the auto pilot in test mode but in ON position nothing works. no lights no eny sign of operation.
Has any one of you met this problem? of have any idea what can be wrong?
Thanks,
Ariel
