STEC-30 Altitude Hold

[wings_level]

So I took the plane up for a little cross country for the holiday and out of no where the altitude hold on my STEC 30 autopilot would not hold altitude.  The blue light came on indicating it was on but it would not engage and hold altitude.  Anyone ever heard of this?  Got to get  some good experience hand flying again.

[Rwsavory]

This happened with our STEC a little while ago.  The pitch servo probably needs to be repaired or overhauled.  Your avionics tech will most likely need to send the servo to Genesys 

[1964-M20E]

Double check the mode select button on the turn coordinator.  This is the button to select heading mode or track mode.  Make sure it springs out after you push it in.  Simple low cost item before digging deeper. 

[DXB]

On 11/21/2018 at 9:43 PM, wings_level said:

So I took the plane up for a little cross country for the holiday and out of no where the altitude hold on my STEC 30 autopilot would not hold altitude.  The blue light came on indicating it was on but it would not engage and hold altitude.  Anyone ever heard of this?  Got to get  some good experience hand flying again.

The ops check in the manual for the altitude hold while on the ground is a good place to start.  Mine checks out fine on the ground but goes into the same "blue light on but no ones home"  mode intermittently that you describe.  Turning it off and then on seems to correct, but the benefit is often short lived. Engaging it when out of trim seems to help.   It happens to me pretty consistently when flying right at nightfall, making me think it's some kind of condensation issue somewhere in the system as the air cools, or maybe when the air becomes smoother?   Lately it's also started oscillating altitude in smooth air only by about 50 feet - very annoying and possibly a separate problem - resetting or adjusting trim has no benefit for this issue.

[DMM]

I agree with all the above comments.  The same thing happened to mine several years ago. Altitude hold got intermittent and would often oscillate.  As a last ditch effort I sprayed the pitch servo with electrical contact cleaner (sprayed the end with the brushes).  Its been working just fine since then.  I read somewhere that the brushes wear and all the dust causes problems. 

[buddy]

I have 55X and about a year ago my airplane started oscillating in level flight with the alt. hold on and found that the control wheel shaft was binding right in the area that the control wheel shaft would be in level cruise flight. The shaft should not bind even the slightest bit when moving the yoke in and out. My mechanic sprayed the shaft with ??? and the binding stopped along with the oscillating. If this sounds like your problem I will give my mechanic a call and let you know what he sprayed on the shaft.

[Andy95W]

Probably Silicone Spray, then wipe with a clean rag or paper towel.

[John Mininger]

I had a chronic problem with my STec 55 (non X) porpoising. It went up and down about 30' from its assigned altitude. It was barely noticeable in any turbulence. But in smooth air it was monotonous. It turned out the pitch servo needed be overhauled. And my A&P did also spray Tri-Flow on the yoke shaft, which was binding a little. 

My avionics tech had a neat trick. He jumped the power terminals on the trim servo, and then the pitch servo with a D-cell battery. The trim servo started fine with the D-cell, the pitch servo did not. He then removed the pitch servo and connected it to a power supply on the bench. It took about 3.5 volts to start, which is out of spec. What was happening was, the baro pressure would go up or down as the plane drifted up or down from the assigned altitude. The transducer would than call for up or down pitch from the servo to hold the assigned altitude. But the servo required too much voltage to start. So, when it did start, it would go past the point, up or down where it needed to be. And the altitude would oscillate up and down by about 30’.

It seemed to me a little counterintuitive at first, that the servo that has the least amount of work to do, the pitch servo, is the one that first causes problems most of the time. But it seems the lack of work for the brushes are what cause the issues.

I asked Genesys if there was anything I could do preventably to extend the life of the pitch servo. The tech suggested that while on the ground, turn on the autopilot and run the vertical speed up 1500’ and then down 1500’. VS is controlled by the pitch servo and doing that would give the brushes some movement. (Obviously, be sure the autopilot is off and the airplane is trimmed up before takeoff.)

[Jerry 5TJ]

1 hour ago, John Mininger said:

...It seemed to me a little counterintuitive at first, that the servo that has the least amount of work to do, the pitch servo, is the one that first causes problems most of the time.....

The pitch servo is the most complicated servo of the autopilot system.  In addition to applying elevator up/down forces continuously it must also sense average force to signal the pilot to roll in manual elevator trim (as in the STEC 30) or to control electric pitch trim in other autopilot systems.  

[wings_level]

Well I have an update.  It was found to be the pitch servo.  Sent it off for overhaul.  Then should be good as new.

[Nippernaper]

Well I have an update.  It was found to be the pitch servo.  Sent it off for overhaul.  Then should be good as new.

What is the estimate for an overhaul on an stec pitch servo? My stec 30 is starting to porpoise, and it seems this will be in my future


Sent from my iPhone using Tapatalk

[wings_level]

Manufacture charged $1375 for overhaul.  And then about 7 hours labor for removal, install, and bench check.

[MikeOH]

OUCH!

Glad my STEC-30 is NOT porposing...yet, anyway.  7 hours, YIKES!

[Supercop0184]

On 1/16/2019 at 2:49 PM, nosky2high said:

Yet another reason I’m installing a TruTrak when able.

I’m leaning this direction as well.