S-Tec Servo Issue

Toobuilder · #1 11-12-2013, 06:59 AM

I know it's not the most popular autopilot for homebuilts due to the breathtaking price that a TSO'd part commands, but for some reason the builder of my Rocket shelled out the cash and installed a System 30. Now that I got that pesky "over center and lockup issue" fixed, I tried turning it on for the first time for a functional test. The pitch servo grabs hold and is extremely strong compared to the Tru Trak I'm used to. The roll servo on the other hand follows the turn knob, but is not connected to the output arm apparently. In other words, the motor runs, but no movement of the arm/stick. Does anyone know if the S-Tec servos use a shear pin as the TT units do?

And yes, in case you were wondering, as soon as I can verify the system is functional it's going up for sale to a certified driver so I can buy a proper experimental autopilot.

Radomir · #2 11-12-2013, 08:23 AM

Does the clutch engage? There's a separate solenoid that engages the motor to the controls.. apply power to pins 1 and 2 (power on 1 and ground on 2) and you should hear a click and the stick should get stiff (clutch engaged).. If this doesn't happen, the motor can spin disengaged all day long and it won't move the control surfaces. Motor movement is on pins 3 and 4 (reverse polarity to move the motor in other direction).

To my knowledge they do not use shear pin but it's a slip clutch instead.. but not 100% sure.

rocketbob · #3 11-12-2013, 09:01 AM

The problem with selling used S-TECs is they won't reissue or transfer the STC for certified installations. What a wonderful company!

Toobuilder · #4 11-12-2013, 09:10 AM

Quote:

Originally Posted by Radomir

Does the clutch engage? There's a separate solenoid that engages the motor to the controls.. apply power to pins 1 and 2 (power on 1 and ground on 2) and you should hear a click and the stick should get stiff (clutch engaged)...

Sounds like this is not happening. Motor runs (follows the turn knob), but no mechanical force on stick at all. Will troubleshoot IAW your guidance and see what happens. Thanks.

Toobuilder · #5 11-12-2013, 09:16 AM

Quote:

Originally Posted by rocketbob

The problem with selling used S-TECs is they won't reissue or transfer the STC for certified installations. What a wonderful company!

Well, maybe yes, maybe no. I ran into this issue selling my PA-22 tailwheel conversion kit which had been issued to a specific airplane SN. I called Univair to see if it could be transferred and they (Univair) made me aware that the Feds had recently cracked down on the practice of "serializing" the STC paperwork. Because the STC paperwork belongs to the Feds, it amounted to "tampering with a Federal document" by placing a SN restriction on it. So today, STC's are generic in nature and can't be tied to a particular aircraft or person. If you have the parts required and the STC exists for your airplane, you should be good to go.

I will certainly check deeper however.

rocketbob · #6 11-12-2013, 10:49 AM

I'd be very surprised if that were true. STC's are considered proprietary and that's why they are not available to the public for one, and I wouldn't expect the FAA to do away with per S/N STC's.

Toobuilder · #7 11-12-2013, 11:22 AM

I think the rub is the fact that the Feds own the document (the paper), while the company has the intellectual rights. Univair told me straight out that they got busted for overstamping a federal document with the aircraft serial number. The only way that would be OK is if the STC was granted for just one airplane rather than a series.

I'm sure the companies like it the "old way", but I think that ship has sailed.

rocketbob · #8 11-12-2013, 11:45 AM

Hopefully so.

I do know that is difficult DAR's/DER's to look in the FAA's systems for STCs to be used as the basis for "approved data" for the sake of intellectual property. They have to do this by finding thru word of mouth an airplane that has an STC approved, get the $10 CD and use that as a basis. The reasoning being so that someone that went thru the trouble of getting an STC can't be easily copied verbatim as a basis for 337 approvals, thus protecting their intellectual property at least somewhat.

Toobuilder · #9 11-25-2013, 06:45 PM

Update:

Clutch was not engaging due to a bad connection. Fixed up the connector and the A/P works just like its supposed to. Thanks for the insight, Radomir.

Also, talked to my avionics guy today and he has some experience transferring the S-tec system from airplane to airplane. It can be done, but it can get expensive. There's the paperwork to buy, then all the airframe specific bits and pieces from the "installation kit"... And you guessed it, S-tec is the exclusive seller of those too.

Time to throw it on that "online auction place" (that rhymes with "fleabay") and see what happens.

Radomir · #10 11-25-2013, 07:25 PM

What is your roll servo's part number? I may be interested in that piece only if it matches what I have..