STEC-55X Problem

[irishpilot]

Anyone have issues with an STEC autopilot? I have an STEC-55X in my plane and it's causing some issues. The plane is new to me and wasn't flown much before I bought it, so I'm ironing out the squawks. If I engage HDG or NAV + ALT and the plane is already on steering, it will hold altitude and heading for a while. If it has to make a steering correction, it will get into an oscillation and then turn on with "FAIL" listed on the Alerter page.  I can reengage it and it will do the same, but then the system will finally display fail and not work unless I cycle power. Does anyone know the cause of this and is there anything I can troubleshoot before taking it to an avionics shop (which is a ways away)? Thanks!

[Marauder]

The fail indication is usually the result of either a failure during power up or when the NAV engagement can't hold the desired course within 50% needle deflection. When the latter happens, the NAV indicator is also blinking. Do you see that?

What bothers me is that you are also seeing this in HDG mode. That suggests something is going on with the roll computer. Wish I could offer troubleshooting help.

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[irishpilot]

I think it may be the roll because I only engage the system with the needle centered. Small changes in heading cause it to hunt and then fail. Is there anything I can do besides take it in?

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[Marauder]

I think it may be the roll because I only engage the system with the needle centered. Small changes in heading cause it to hunt and then fail. Is there anything I can do besides take it in?

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It really sounds like an internal issue with the roll computer. You could pull the unit and make sure the contacts are clean. I believe your 55X may be self contained unlike my 60 which has separate boxes in the avionics bay.

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[Marauder]

This site has the installation manual for these autopilots. It includes some self tests that you may be able to conduct without the calibration tools.

http://www.csobeech.com/files/S-TecServiceManual.pdf

On page 1-6, it does look like the roll computer is separate.

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[irishpilot]

Thanks so much for the link! I had no idea the site had those. I will adjust the roll on the ground and see if it stablizes.

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[irishpilot]

So I went out to the plane today and adjusted the roll sensitivity per the instructions and that seemed to work, but I still have a fail indication after the initial BIT. I think I'll have to get it to the avionics shop. Any other ideas? Does the computer store fault codes?

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[Marauder]

So I went out to the plane today and adjusted the roll sensitivity per the instructions and that seemed to work, but I still have a fail indication after the initial BIT. I think I'll have to get it to the avionics shop. Any other ideas? Does the computer store fault codes?

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Really sounds like you have something flakey going on with the computer. I doubt the computer stores codes, but the 55X is the latest gen of the series, so who knows...

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[irishpilot]

I'll the company tomorrow and see what they say. Thanks again and I'll post what I find.

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[mitch0537]

A couple of things come to mind for you to check. The rear connectors on a 55X are made of plastic, there are screws going into them from the tray side. Sometimes these plastic backshell screw-holes get stripped and when the unit is slid into the tray and locked, the connectors push out because they're not secured to the tray properly. 2) I've seen this scenario before: There are four screws that secure the entire backplate to the tray. The backplate is what the connectors are secured to. These screws are I believe #2 and are screwed inward from the outside way in the back of the tray. If they are too long they will prevent the 55X from sliding all the way into the tray and make proper connections.

 

Also check that the tray is mounted close enough to the radio rack that the connectors secure before the faceplate bottoms out against the radio rack. If you just go ahead and send the 55X to S-Tec, which could indeed be defective, it'll be north of $1500 flat fee. That's why you want to make sure it's not a connectivity issue.