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 Post subject: Porpoising A36 with S Tec 55
PostPosted: 25 Oct 2010, 06:51 
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Aircraft: bonanza G36
I am looking for some help with an autopilot problem. With the A36 in altitude hold (S TEC 55), the aircraft is porpoising up to 200 feet up and down and it seems to get worse if left in altitude hold. My avionics shop has overhauled the elevator servo and tried a new STEC altitude sensor capsule. Neither has made any differece to the problem. I have also checked the static lines for blockage and also lubricated all the elevator and trim linkages. Has anyone any more ideas? :scratch:


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 Post subject: Re: Porpoising A36 with S Tec 55
PostPosted: 25 Oct 2010, 07:06 
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Joined: 12/13/07
Posts: 1983
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Location: FL KHEG, KSGJ, TN KGCY
Aircraft: F33A, J3 Cub
I am going through this same problem now but the plane is only porpoising only 20 feet each way. But it is trying to correct itself at 200 feet per minute. Are you sure yours is not doing the same thing? To cross out other problems try:

1. While in level flight and trimmed out with auto pilot turned off turn off the auto trim. Then turn on the Stec 55x. If it still porpoises then its not the auto trim which is known to cause this problem.

2. While in level flight and trimmed out with auto pilot turned off open the alternate static line valve and then turn on the Stec 55x. If it still porposes then again the problem is somewhere else.

3. Water in the static system. I bought a dehumidifyer which got a ton of water out of the plane but it did nothing. I then had the lines blown out backwords with nitrogen which did nothing.

4. Rick's point on the transducer. If it is blue it needs to be replaced with a metal one.

:scratch: Any other ideas let me know.


Username Protected wrote:
I am looking for some help with an autopilot problem. With the A36 in altitude hold (S TEC 55), the aircraft is porpoising up to 200 feet up and down and it seems to get worse if left in altitude hold. My avionics shop has overhauled the elevator servo and tried a new STEC altitude sensor capsule. Neither has made any differece to the problem. I have also checked the static lines for blockage and also lubricated all the elevator and trim linkages. Has anyone any more ideas? :scratch:

_________________
Doug

"Please help spay & neuter homeless cats & dogs"


Last edited on 25 Oct 2010, 09:10, edited 2 times in total.

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 Post subject: Re: Porpoising A36 with S Tec 55
PostPosted: 25 Oct 2010, 07:14 
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Joined: 06/17/09
Posts: 627
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Company: WCVS
Location: Asheville, NC
Aircraft: 1976 Bonanza A36
I had a similar issue with my s tec 65. It turned out to be my flight director feeding bad info to the a/p. I rebuilt the KI-256 and now it works great! Bob Bramley at aerolab is a superstar!


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 Post subject: Re: Porpoising A36 with S Tec 55
PostPosted: 25 Oct 2010, 07:56 
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Location: FL KHEG, KSGJ, TN KGCY
Aircraft: F33A, J3 Cub
Username Protected wrote:
I had a similar issue with my s tec 65. It turned out to be my flight director feeding bad info to the a/p. I rebuilt the KI-256 and now it works great! Bob Bramley at aerolab is a superstar!


David, Is Aerolab in the Asheville area?

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Doug

"Please help spay & neuter homeless cats & dogs"


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 Post subject: Re: Porpoising A36 with S Tec 55
PostPosted: 25 Oct 2010, 08:02 
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Company: M.B. Kahn Construction Co.
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Aircraft: 2003 Bonanza A36
I don't have a STEC 55 but let me give you a tip that I learned from my Avionics guys. There were at least two types of transducers that STEC used. They made a blue transducer and a metal one. If you have the Blue transducer it needs to be replaced. Look under the panel (likely on the copilots side) and see if you have a blue one.

Apparently there was a SB issued by STEC but I've found very few people are aware of the issue including STEC technical Reps.

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 Post subject: Re: Porpoising A36 with S Tec 55
PostPosted: 25 Oct 2010, 08:22 
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Aircraft: F33A, J3 Cub
Thanks Rick, I'm going panel diving later today.

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"Please help spay & neuter homeless cats & dogs"


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 Post subject: Re: Porpoising A36 with S Tec 55
PostPosted: 25 Oct 2010, 09:39 
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Aircraft: BE95 Travel Air
I had this problem once with my 50, and it turned out to be my wifes cell phone left on in the nose baggage compartment, right next to the transducer. Could there be some RMI causing problems? Maybe that is what the SB is for, to add a metal shield to the transducer to prevent RMI issues. Good luck.


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 Post subject: Re: Porpoising A36 with S Tec 55
PostPosted: 25 Oct 2010, 10:09 
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Location: Houston (LVJ)
Aircraft: 1965 Debonair C-33
Quote:
I had this problem once with my 50, and it turned out to be my wifes cell phone left on in the nose baggage compartment, right next to the transducer.


Ah yes, proof to those "cell phone deniers" that a cell phone actually can interfere with an aircraft's instrumentation.


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 Post subject: Re: Porpoising A36 with S Tec 55
PostPosted: 25 Oct 2010, 10:12 
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Username Protected wrote:
.... Could there be some RMI causing problems? Maybe that is what the SB is for, to add a metal shield to the transducer to prevent RMI issues. Good luck.

I still have a blue one. The SB dealt with specific s/n's of them, not mine. sTec told me to wrap it in MuMetal for a pitching from transmitting on com1. Haven't yet.

New metal ones NOT cheap either!


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 Post subject: Re: Porpoising A36 with S Tec 55
PostPosted: 25 Oct 2010, 10:23 
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Location: Charlotte, NC (KUZA)
Aircraft: 1968 Bonanza V35A
Don't forget the pitch cable tension, the bridle cable tension, and even though the servo has been overhauled, have the startup voltage checked.

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John D. Collins CFI, CFII, MEI
68 V35A N7083N Home Base KUZA
Charlotte, NC
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 Post subject: Re: Porpoising A36 with S Tec 55
PostPosted: 25 Oct 2010, 10:36 
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Aircraft: BE95 Travel Air
Username Protected wrote:
I still have a blue one. The SB dealt with specific s/n's of them, not mine. sTec told me to wrap it in MuMetal for a pitching from transmitting on com1. Haven't yet.

New metal ones NOT cheap either!


That's great info. I've always had pitching when transmitting from my old com 2 KX175, especially the higher frequencies (higher energy as well), this should resolve both issues.


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 Post subject: Re: Porpoising A36 with S Tec 55
PostPosted: 25 Oct 2010, 10:39 
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Joined: 12/10/07
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Location: Minneapolis, MN (KFCM)
Aircraft: 1970 Baron B55
Username Protected wrote:
Don't forget the pitch cable tension, the bridle cable tension, and even though the servo has been overhauled, have the startup voltage checked.

John, do you know if the gain and/or deadband is adjustable on a Stec-55? Or is the gain set by the factory for a given model aircraft? If the latter I wonder if it's possible that the wrong unit got installed by mistake. Also a problem with the wiring or plumbing to the altitude sensor could cause pitch problems. Finally AFaIK, the S-Tec's all use a vertical accelerometer to provide high bandwidth feedback in pitch and I believe that's located in the control head, not the remote altitude sensor. A problem there might manifest itself as porpoising.

That said, the common causes for pitch oscillation (loose control cables, sticking control movement, loose bridle, static blockage, high breakout on the pitch servo, overly sensitive autotrim) have already been covered.

WRT the AI, AFaIK that can't be a factor on a 55 series because it has nothing to do with the autopilot.

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Advice in this post may contain errors. Using said advice is totally at the risk of the user.


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 Post subject: Re: Porpoising A36 with S Tec 55
PostPosted: 25 Oct 2010, 15:10 
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Aircraft: F33A TNIO-550
isn't porpoising a feature of stec's in general?

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 Post subject: Re: Porpoising A36 with S Tec 55
PostPosted: 25 Oct 2010, 15:14 
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Joined: 09/13/08
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Location: Bakersfield, CA
Aircraft: S35, 260B Comanche
Username Protected wrote:
I am looking for some help with an autopilot problem. With the A36 in altitude hold (S TEC 55), the aircraft is porpoising up to 200 feet up and down and it seems to get worse if left in altitude hold. My avionics shop has overhauled the elevator servo and tried a new STEC altitude sensor capsule. Neither has made any differece to the problem. I have also checked the static lines for blockage and also lubricated all the elevator and trim linkages. Has anyone any more ideas? :scratch:


Is there any chance that your airplanes alternate static source is venting to the cabin? This is a common cause of porpoising when there is really no problem. Sometimes the knob will be pulled by somebody else and often follows maintenance, or a pitot static check. If this turns out to be your problem throttle the avionics shop that's been working on your autopilot. Another thing you can do to help reduce porpoise is to trim the airplane just enough out of trim that you have to hold constant but slight pressure on the yoke. When engaged the alt. hold will be much more stable with this slight out of trim condition. Of course this isn't a fix, but it can possibly make it usable until you find a shop that can find the problem.


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 Post subject: Re: Porpoising A36 with S Tec 55
PostPosted: 25 Oct 2010, 15:30 
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Joined: 11/27/07
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Company: BeechTalk
Location: Pontiac, MI (KPTK)
Aircraft: 1991 Bonanza A36
Username Protected wrote:
isn't porpoising a feature of stec's in general?

Mine doesn't.


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