Bob_Belville

STEC30 Altitude Hold drifts

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I have a STEC 50 and the ALT function has always operated flawlessly. It modulates the elevator via the pitch servo to hold whatever pressure altitude is when ALT is engaged. (If BARO changes it is necessary to disengage ALT, adjust altitude to desired altitude, engage ALT.) All good.

I have a Mooney friend with a long body model equipped with a STEC 30. NAV & HDG functions work fine allowing GPSS but the owner reports that the ALT HLD does not maintain a constant altitude. The plane climbs, not dramatically, but steadily. If ALT remains engaged the STEC will display the "trim down" light which should mean the pitch servo is fighting the trimmed stabilizer even though the pitch was trimmed to neutral before the ALT was engaged.

Any suggestions as to where to start trouble shooting? 

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Be on the lookout for a static system hose to the Stec...

Various crummy rubber hoses have been used for the connection.

Open the cabin window in flight to see if a wild ride occurs... :)

That would be a strong hint that the static line is no longer connected properly...

Somebody posted a pic around here recently of a disconnected rubber tube...

PP thoughts only, not a mechanic...

Best regards,

-a-

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Lynn says static system might be involved and to check the two static drains as well as the alternate static air.


Sent from my iPad using Tapatalk

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In keeping with standard internet protocol, here's my totally uneducated opinion.  It seems to me that a misbehaving encoder or static system would be sending the servo a "Nose Up" command since it is climbing.  In this case the servo would only ask for nose up trim as needed.    But the servo is asking for "nose down" trim.  This tells me the servo is receiving a "nose down" command and is unable to move in that direction.   Stuck servo? Only operating in one direction? I'd try applying voltage directly to the servo and see if it runs freely in both directions - on the ground.  

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4 hours ago, carusoam said:

Bob, @Bob_Belville and @ValkyrieRider

Check this out...  lifendet didn’t quite have all the details correct, but his pic showed everything... :)

Best regards,

-a-

 

@carusoam, I will be headed to the airport tomorrow evening ... guess what I am going to be looking for :)  

 

I will look under the cowling and see if I can find this transducer and see how it is connected.  

 

I will let you know.  Thank you.

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@ValkyrieRider ...   Expect that it may be hiding under the instrument panel...

The oddity of its behavior indicates it Living inside... and the other MSer’s pic I believe was behind the panel... a short piece of tube that goes to the back of the AP...

Best regards,

-a-

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Altitude hold works by "capturing" the static pressure at the engagement altitude and then adjusting altitude to maintain this pressure. To get a slow, continuous climb caused by a static pressure problem would require a small, constant offset from the "captured" pressure. This is pretty unlikely. I suppose that there might be a failure mode of the pressure transducer that would produce a constant offset voltage, but that wouldn't explain the trim.

The description of the trim indication is exactly what would happen if you were to pull on the yoke while the autopilot was engaged. The pitch servo tries to push the nose down, but can't because you are holding pressure on the yoke and the autopilot tries to relieve the pitch servo pressure by commanding down trim. Since the airplane presumably has no non-autopilot control issues such as binding, the failure must be something within the autopilot that mimics this condition. I would begin troubleshooting by checking all vertical modes to determine if anything works correctly. I would check the pitch servo for bridle tension and clutch slipping. If all that doesn't find a problem then it might be the autopilot itself.

Skip

Edited by PT20J

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