S-Tec 50 Porpoise

[Chief]

Has anyone had an issue (AND FIXED) with S-Tec 50 Altitude Hold producing a porpoise of the plane while in flight? It holds altitude, but the porpoise of a hundred feet, up and down is aggravating.

[David Pyle]

Brett,

Something "fishy" going on. Call STEC support.

[Chief]

I hear you, that's pretty good!

I'll try and S-Tec and post their response.

[N3322G]

Brett,

I've seen posts like this from others for several types of autopilots. Usually, not always, it turns out to be a problem from wherever the autopilot gets its altitude sensing information. - for Century, that's the attitude gyro - Fix that and it fixes the problem. Other possible sources are loose trim tab assembly and the autopilot altitude pitch servo.

Hope this helps.

[Zach Grant L1011jock]

Absolute pressure transducer! It is a life limited item, and usually fails every 7-10 yrs.
-Zach

[Chief]

Thanks Zach,

any idea where best to find one??

[Chief]

Update,

Just spoke with Autopilot Central. Not sure on why our S-Tec is porpoising, but they believe it to be most likely a pitch servo power issue. As the altitude hold flies the plane, it asks the pitch servo motor to start, normally the motor starts with less than 2 on the power flow, if the pitch servo is bad, it could take more to start the motor, all the while the plane is deviating off altitude in a slow porpoise. Once the motor receives necessary power, it engages and begins to stabilize the plane at the proper altitude. That lag time of power applied and power needed causes a lazy porpoise. A power meter check by our A&P of the Pitch Servo will give us the answer, we simply apply power until the servo motor starts. Again, should be below 2. If not consistently below 2, I'll need to have the servo rebuilt.

I'll keep everyone updated

[tomburke1]

I had a similar situation with the altitude hold on a PSS system. Banyon radio shop came up with the servo question. I had them send the barometer to S-TEC and it came back bad. New one fixed it. I believe the $$$ makes them suggest the pitch servo before the barometer.

[Chief]

Thanks Tom, I'll look into it. How much was barometer?

[tomburke1]

It was a while ago, but as I remnember it was about $375.00 back then. The rebuild of the servo was over a $1000.00.

[Chief]

Thanks, good insight.

[VANBLADEREN]

If you autopilot has been in for a number of years, the brushes wear and the start voltage for the motor goes up. The result of this is porpoising you talk about.

[tomburke1]

Hi Jon.
The porpoising he described was exactly like my situation. I had taken it to Banyon at FXE, and the servo was their first guess. I discussed the altitude senser and they kind of talked around it..
One of the questions I asked of the tech was how did the computer get it's altitude information. When he explained it to me I thought that logical place to start was at the sense part.
They disagreed but I insisted thast they send it for a check, and it came back bad. Installed a new one and off I went,,,That was 7 yrs ago and it still works fine. I didn't buy the unit new so I don't know off hand how old the PSS is but I woul;d guess that the original part lasted at least 15 years.
Another thought is that you check the continuity of the line to the barometer as well.

[skipsouthernsky]

Dear John,

I am assuming that the brushes you are talking about are in the servo motor. If so, and assuming that the servo is reasonably accessible, are the brushes hard to get at? Is it hard to replace the brushes or clean the commutator? Is it the same situation on the Century autopilots?

Sincerely,
Skip Dykema

[VANBLADEREN]

Hello Skip, the brushes I am talking about are the ones in the motor. If the start voltage is more than 2 volts, then they need to be replaced or serviced. Century autopilots have the same issues with their servos. It is like any DC motor.

I do want to stress, that the brushes are one issue. Other issues could be the transducer in the STEC and the Horizon on a Century. The brushes are the easiest to check.

Hope this helps.

Regards

jvb