STEC 55x pitch and roll issues

[Mooney in Oz]

Hi there everyone.

I have a STEC 55x autopilot fitted in my 80 M20J with a long time pitch issue.  During cruise in ALT mode the nose pitches slightly up and down, but the aircraft does not appear to climb or descend.  It is not noticeable with the VSI or the naked eye, but it is noticeable on the air speed indicator as it fluctuates between 5 and 10 knots upwards before levelling off for a number of seconds then it starts again.  This is a very slow action, but is annoying as it is costing me some speed. I downloaded an STEC information letter which details 11 different possibilities for this issue, but I thought I would ask here to see if there is anything that may be specific to Mooney that can cause this.

Last year I spent money on having the trim motor overhauled and everything was lubed as it should be.  Recently I switched the auto trim switch off to see if it still pitched (as per one of the letter suggestions) but the problem continued.

Another problem I am having is when the autopilot is engaged in level flight, the aircraft very slowly rolls to the left about 2 degrees then straightens again for a few seconds.  This is barely noticeable as looking forward I don't notice it but to the left side and I can see the wing dipping down slightly then returning to level flight.

My avionics tech has serviced the servos but both issues still continue.  He told me not to worry about the roll if it does not bother me, which I'm used to by now.  I was wondering if others also have this other issue with their STEC.  I believe all STEC fitted aircraft have the same servo's, not matter which model autopilot.

 

Thank you.

Victor 

Edited August 7, 2016 by Mooney in Oz

[Marauder]

Victor -- interesting that you mention this issue. On my last flight I noticed a varying airspeed like you described. I will be flying home later and will monitor it again for this.

The STEC autopilot altitude is controlled by a pitch computer, an altitude transducer connected to the static system and obviously the servos. My altitude transducer is located under the pilot's panel. When I do my pre-flight today, I will be paying extra attention to the AP pre-flight steps for altitude to make sure it is functioning correctly. I will also hand fly a good portion of the flight today to see if the issue can be eliminated by hand flying.

If the servos bind a bid, I could envision this playing a part in this. More likely is the altitude or pressure transducer.

I don't have the roll issue. Where did you find the 11 step troubleshooting guide? I only have the STEC service manual.

Let's keep in touch on this. I will try to catch this on video as well.

Sent from my iPad using Tapatalk

[Jerry 5TJ]

Airspeed varies in level flight: Isn't that normal in altitude hold mode as the plane speeds up in rising air? 

[Marauder]

Airspeed varies in level flight: Isn't that normal in altitude hold mode as the plane speeds up in rising air? 

It does to some degree. But in a stable airmass, I normally don't see more than a few knots. This last flight it was a bit more. May have been an anomaly. Will find out later today.

Sent from my iPad using Tapatalk

[mike_elliott]

If you guys haven't already done this, run the servo's in both directions for about a minute. This will require you to make a sub D connector harness and apply 12V to the servo motor pins. I placed an installation manual in the download section at one time here where you can get the pinouts. I do not recall off hand what there are, but there are only 4. A lot of issues are caused because the servo motor only moves slightly and there is a dirt/stiction buildup that by running, can get cleaned up.

Not and A$P

Not a R/E tech

Owned an Stec Mooney for a number of years and was told this back in 2004 when I had the issue by Stec. Every annual I "serviced" the servos this way with an out of date ELT battery and it behaved well for me.

 

[Mooney in Oz]

Thank you all for the helpful replies. I'll be passing these on to my avionics tech.

Marauder;  See attached.

 

 

STEC-55X-Pitch-Problem-Troubleshooting.pdf

[Jeff_S]

I had similar pitch oscillation in my J. For me, it turned out to be old cracked static lines behind the panel that were leaking air. Once we got those replaced the problem went away.

[whiskytango]

I have never experienced this problem, but the 55x in my plane will not couple to a localizer without wallowing all over the sky.  It is being driven by a GNS480, and will absolutely nail a coupled GPS approach, breaking out at minimums right on the extended runway centerline.  Strange that a "precision approach" is less precise than a non-precision approach.  I have heard of other 55x users having this problem, and I am not aware of a fix.

[Mooney in Oz]

2 hours ago, whiskytango said:

I have never experienced this problem, but the 55x in my plane will not couple to a localizer without wallowing all over the sky.  It is being driven by a GNS480, and will absolutely nail a coupled GPS approach, breaking out at minimums right on the extended runway centerline.  Strange that a "precision approach" is less precise than a non-precision approach.  I have heard of other 55x users having this problem, and I am not aware of a fix.

Mine does the same. We don't have many ILS approaches here in Australia outside of the capital cities, so we tend to do more RNAV's and it is rock solid there. I also have a GNS480 and lately installed an Avidyne IFD540. Same behaviour with ILS approaches on both navigators.

 

Victor

[Jeff_S]

The 55x does the same thing with a G1000. When you go into approach mode you take it off of GPSS and I think the nav signals just aren't as precise.

[carusoam]

One thing I notice about the difference between analog signals and GPS based guidance...

The analog signals are not perfectly straight lines.  GPS signals are..

Following a straight line between two VORs, my KAP 150 follows a scalloped pattern compared to my iPad's magenta line.

Following the same VORs using the GPS, the plane overlays the magenta line perfectly.

i'm not sure how you can get the controller tuned better or tune it specifically for the worst case.  But it should do a bit better than what is being described.

Nav and approach modes are examples of two types of tuning modes.  Nav is a soft gentle method of loosely following the magenta line.  Approach mode is tighter control to the magenta line at the cost a harsher more noticeable ride....

Best regards,

-a-

[Mooney in Oz]

On 18 August 2016 at 2:49 PM, carusoam said:

One thing I notice about the difference between analog signals and GPS based guidance...

The analog signals are not perfectly straight lines.  GPS signals are..

Following a straight line between two VORs, my KAP 150 follows a scalloped pattern compared to my iPad's magenta line.

Following the same VORs using the GPS, the plane overlays the magenta line perfectly.

i'm not sure how you can get the controller tuned better or tune it specifically for the worst case.  But it should do a bit better than what is being described.

Nav and approach modes are examples of two types of tuning modes.  Nav is a soft gentle method of loosely following the magenta line.  Approach mode is tighter control to the magenta line at the cost a harsher more noticeable ride....

Best regards,

-a-

You are correct Jeff and Carusoam. I fly a heavy turboprop for a living with full glass mega $$ avionics and LOC intercepts are not that different to our GA navigators, particularly in strong crosswinds.

Victor

Edited August 19, 2016 by Mooney in Oz