STOLman, thanks a lot for sharing this. My Stec3100 is going im this April and I agree the EP features are not described properly at all in the POH. One report I have is that it’s linked to the FD. A pilot with the same plane as me found out he had to disengage FD on approach (which is kind of ok) and take off (which is not) otherwise the EP would lower the nose at 95kias (short final speed is 90kias, threshold and take off speed is 80kias so this is NOT GOOD).
I’m also worried (and surprised) by your report that data entered on the stec doesn’t feed into the G500. I will be very interested to know if you find out more about the expected behaviour of this data exchange (i.e. is it supposed to be like that, in which case all entry should happen on the G500, or is it something that can be solved with installation).
In essence an EP should be active as long as the AP isn’t completely and fully disengaged and disconnected.
In a King AP you have 4 buttons dealing with AP activation in different ways:
1) the big red button that disconnects the AP completely from the servos, which you press to handfly the approach etc,
2) the CWS button that allows you to override the AP but keeps the input so the AP picks up pitch and roll once you release it,
3) the AP button which essentially switches the AP off, but keeps the FD which has a different switch.
4) the FD button which activates the Flight Director.
My understanding is that the EP acts a bit like a flight director “plus” : in addition to displaying the FD triangle in the AI it would keep the servo grip and protect the pilot from maneauvering the plane outside the envelope.
In this scenario the EP is always active except for if you disengage the Flight Director.
Does that make sense ?
New S Tec 3100 autopilot now fitted. Did a two hour flight test with it yesterday and everything functioned correctly and precisely as you’d expect from a fully digital autopilot.
However, what is clear is that S Tec need to improve their documentation. The installer was driven mad trying to work out things because of poorly written installation manuals. Several features, such as the envelope protection, have also been poorly described in the POH.
With regards to the envelope protection @lionel was dead right. The disconnect button does disconnect the servos but the envelope protection is still active. The claxon and aural warnings are not disconnected and on my first landing slower than the envelope setting I got these horrible loud warnings just when you don’t want them. Turns out you can turn these off as well by holding down the disconnect button for two seconds. This disconnects the AP and the FD and all warnings. Bingo. I’m happy now.
Another strange effect was the G500 or the AP can accept an assigned altitude. Putting the altitude in the G500 (or the vertical speed) with a press of the select button on the AP the numbers go across from the G500 to the AP. However, if you put the same numbers manually into the AP that data cannot go the other way to the G500 so they end up out of sync. It’s no big thing really but why does the data only go one way?
So far I’m very happy with the 3100. It’s a lot more precise than the 55X and will be a powerful safety addition once a get used to it. The LVL button is awesome. Cannot get my head around IAS yet.
@lional; as I said in my last post above I believe you are correct. The latest information is saying exactly that disconnect button does disengage the servos so there can be no intervention if outside the envelope. What you do get is a message on the G500 and an audible warning.
I’m picking my plane up next week where the first flight is a comprehensive flight test before its signed off. I’ll be more knowledgeable then but I am much more relaxed about it now.
It is my impression that some APs (such as Garmin GFC700/600/500) have envelope protection on all the time, even if the autopilot is not engaged, but others (such as as the S-Tec 3100) have it on only when the autopilot is engaged, meaning it protects only about the autopilot bringing the plane out of the flight envelope (e.g. if VS mode is on with an unreasonable setting or not enough power, or IAS hold mode is on with not enough power), not when hand-flying.
@STOLman, this might actually mitigate your fears, since you are getting the S-Tec 3100 installed? But check that I’m right about this! I might have misunderstood.
Peter wrote:
Obviously, for EP the servo power must be maintained even though the red button is pressed
On the SR22G6 I fly, the EP is disabled while you are pressing the red button
Yes; a very good point.
This should also be reflected in the POH under Emergency Procedures.
Peter wrote:
for EP the servo power must be maintained even though the red button is pressed
Not necessarily while the button remains pressed.
Note to self – before next flight, look at the wiring diagram…
the disconnect switch on the yoke should not only send a “disengage” command to the autopilot, but also remove all power from the servos while held down
Exactly correct for the King boxes. The red button switch interrupts the power to the servo solenoid-operated clutches.
Obviously, for EP the servo power must be maintained even though the red button is pressed. This introduces a big risk… if the software crashes, you cannot overpower the autopilot unless the slipping clutches work correctly. On many installations they are stuck (corrosion, debris, misadjustment) and the owners don’t know because they never do the preflight “overpower” test.
There must be a Master switch, or maybe just a circuit breaker, and you better know where this is in case you have to pull it in a hurry 
Someone told me the EP disable is in the G1000 menus and one hopes this is not the only place…
You cannot overpower the servos, because the gearing is something like 1000:1. There is just the slipping clutch.
With autopilots, it is important to understand exactly how much “off” it is for a given “off” switch, or if it merely “disengages” it.
As I dimly recall, the disconnect switch on the yoke should not only send a “disengage” command to the autopilot, but also remove all power from the servos while held down, but this may also be installation specific. In the Columbia pre G1000, the big autopilot switch did that, too. After G1000, there was only the circuit breaker, IIRC.
Envelope protection is supposed to work even if the autopilot itself is “disengaged”, i.e., when you hand fly.
