S-tec 30 altitude hold oscillating

[Mooneyjet]

Hey guys flying the mighty m20 C at 9.5k smooth, with the s-tec selected in nav and altitude hold selected. After about an hour into the three hour xcountry trip,the altitude hold started to oscillate. I retrim the mooney manually. Once again when the altitude hold was selected it started to oscillate to the point that it got to almost plus or minus 100 feet. I did notice the Altimeter and vsi had a bit of wiggle. Anyone ever experience this snag before.

[KSMooniac]

Try opening your alternate air valve next time and see if it holds... if so you might have moisture in your static system.

[1964-M20E]

Mine will oscillate some but not 100 feet.  I can see the nose go up an down on about 3 to 4 second cycles and no noticeable altitude change.  I've found for that I take alt hold off see where the aircraft is trimmed and then trim slightly nose down engage alt hold and it seems to help.

[BigTex]

Mine does that as well if I don't have it trimmed out. In my case I've found my S-Tec 50 isn't very tolerant of not being trimmed well.

[UFOPILOT]

I had this problem with my M20J, it got worse over time, to the point the altitude delta was 200 feet.   After much fruitless effort, it was finally found that the King HSI was sending bad pitch data to the KAP 150 autopilot, and the autopilot was dutifully chasing that bad data.   I believe the 55A HSI has electromechanical brushes doing the sensing, and they can wear out, or otherwise go astray on you.    This was solved by an overhaul of the HSI.   Suggest you rule out any cheaper possible fixes first, as the King HSI overhaul is not cheap.  I believe a solid state HSI such as the Aspen would eliminate this problem, by sending more reliable data to the autopilot than any electromechanical unit might be capable of.   Compare the projected HSI overhaul cost against a solid state replacement unit. 

[Marauder]

The STEC series uses a pitch transducer in the static system. Moisture as described above by Scott could be one culprit. Another could be a failing pitch transducer or a problem with the pitch computer. I would pursue the moisture path first before having a qualified shop look at it,

[harrispa]

Stec 30 - If I don't have it trimmed down almost to the point of the "trim up" alarm, it will oscillate. Since I learned that trick, it has been flawless.

Paul

[DaV8or]

  On 1/24/2014 at 4:00 AM, Marauder said:

The STEC series uses a pitch transducer in the static system. Moisture as described above by Scott could be one culprit. Another could be a failing pitch transducer or a problem with the pitch computer. I would pursue the moisture path first before having a qualified shop look at it,

 

This is not true of the S-Tec 30. I don't know about other models. The Altitude hold on the 30 is completely independent of your pitot static system, or any other system on the plane except a power supply. It does not function on any numerical value, or data supplied from any other device.

 

The way it works is, it has it's own barometric pressure transducer. When you push the button on the yoke, it locks down the barometric pressure at that moment, not the altitude that you want to stay at. It will do it's best to maintain this pressure by pitching the plane up and down. It has no idea how high you are over sea level.

 

The only components to the system are, the pitch servo attached to the elevator, the pressure transducer, the computer that controls the servo and the switch on the yoke. The OP's problem likely lies in the transducer. I think it can be adjusted. There may also be a gain adjustment at the computer, so possibly the gain is set too high.

 

I know all this because my 30 was screwing up, but in a very different way than the OP's. After a process of elimination, my problem was the computer. Only solution there is an expensive trip back to S-Tec.

 

Take the plane to a shop that knows S-Tec and they will likely fix you up. I bet it's just an adjustment. If the transducer does need work, at least it's cheaper than the computer. I know.   In No. California, I recommend Executive Autopilots at KSAC.

[Marauder]

STEC Pressure Transducer.pdf

  On 1/25/2014 at 5:35 PM, DaV8or said:

This is not true of the S-Tec 30. I don't know about other models. The Altitude hold on the 30 is completely independent of your pitot static system, or any other system on the plane except a power supply. It does not function on any numerical value, or data supplied from any other device.

 

The way it works is, it has it's own barometric pressure transducer. When you push the button on the yoke, it locks down the barometric pressure at that moment, not the altitude that you want to stay at. It will do it's best to maintain this pressure by pitching the plane up and down. It has no idea how high you are over sea level.

 

The only components to the system are, the pitch servo attached to the elevator, the pressure transducer, the computer that controls the servo and the switch on the yoke. The OP's problem likely lies in the transducer. I think it can be adjusted. There may also be a gain adjustment at the computer, so possibly the gain is set too high.

 

I know all this because my 30 was screwing up, but in a very different way than the OP's. After a process of elimination, my problem was the computer. Only solution there is an expensive trip back to S-Tec.

 

Take the plane to a shop that knows S-Tec and they will likely fix you up. I bet it's just an adjustment. If the transducer does need work, at least it's cheaper than the computer. I know.   In No. California, I recommend Executive Autopilots at KSAC.

Dave -- not sure either of the altitude hold differences between the 30 and the 60 I have. But I do know for certain that my pressure transducer is in the static system flow path as per the attached STEC drawing. I believe it is there specifically to provide an active drainage spot for the transducer. Not sure why there would be a difference over the series. Perhaps someone else can chime in.

 

 

[uscgdoc]

I had the exact problem on my 20c.  I had had the unit more than 10 years without any issues.  Required servo service with stec.  It was about $400.  works like a champ now with my GPSS and coupled garmin in IMC.

[chrixxer]

3RM's servo was pulled yesterday at Marana (S-Tec authorized service center and a shop I trust, even if it is ~350 nm from home). They tested the whole S-Tec 30 system and found high start voltage on the pitch servo, which was causing it to pitch hard up and climb at like 1000 fpm to like 300' above the specified altitude, then dive-bomb back down, rinse, repeat. Happens due to worn brushes on the motor, apparently. Frustrating, in that it was intermittent; most of the time it worked perfectly. Flew like 1300nm in one day, and it was rock solid until the very last (100 nm) leg...  I'm told now there's two tiers of S-Tec service, $500(ish) and $1300(ish).

Edit: And it was doing the +/- 600' (?!) oscillation even when trimmed perfectly (I took video to make sure I wasn't imagining things).

Edited June 14, 2018 by chrixxer

[KSMooniac]

You can make a harness to connect to a servo (mechanically disconnected from plane!) and feed it with a variable power supply to exercise the servo in both directions. That helps with this issue.

Sent from my LG-US996 using Tapatalk

[M20S Driver]

I had a similar problem which turned out to be low cable tension on the pitch servo. 

[Piloto]

The pitch servo clutch may be worn out. With the A/P engaged try to see if you can override it when pushing and pulling the yoke. If normal force is required to override it the clutch may need adjustment, replacement or cleaning. If the servo was lubricated the clutch will slip. Try cleaning it by spraying contact cleaner on the clutch area.

José 

[jetdriven]

  On 1/24/2014 at 4:00 AM, Marauder said:

The STEC series uses a pitch transducer in the static system. Moisture as described above by Scott could be one culprit. Another could be a failing pitch transducer or a problem with the pitch computer. I would pursue the moisture path first before having a qualified shop look at it,

Expand  

If the static line Is disconnected in the tailcone it can cause some weird behavior. Changing airspeed raises or lowers the pressure in there which causes the alt hold to pitch up or down.