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  #1  
Old 05-22-2018, 09:22 AM
tfriendshuh tfriendshuh is offline
 
Join Date: Sep 2016
Location: Duluth
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Default Very strange A/P problem - pls help me troubleshoot!

TruTrak Vizion 385 installed with Garmin GTN650 and HS43 module.

Was on a long cross country over the weekend and discovered the AP will only hold course, not altitude, gotta be the elevator servo or wiring right - here's the kicker - elevator control still works for fully coupled approaches!

Here's a breakdown:

1. Flying along, I select an altitude and vertical speed, engage AP, course is flown, but no elevator changes or fore/aft stick resistance to manual control.

2. I manually fly to the selected altitude and AP displays the normal "approaching altitude" and "altitude hold" messages but again, no exerts no actual control of elevator, no servo hum or resistance to moving the stick fore and aft - doesn't matter if I select existing altitude or dial in an ascent or descent or even if I just use wing level feature - in cruise flight, not elevator control is exerted by AP.

I thought it was possibly a bad elevator servo or a bad connection to the servo but....

3. When on a RNAV approach, I was somewhat startled when between IAF and FAF, (get glide slope indication from GPS) the AP display switched from ALT HLD to CPLD and the AP controled elevator servo all the way down for a successful fully coupled approach with vertical guidance.

This is 100% repeatable. No AP control of elevator in cruise - but once vertical guidance comes from GPS - boom it works.

So the servo is working - it's just not getting any guidance except when coupled.

I sent an email to TruTrak too
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  #2  
Old 05-22-2018, 11:05 AM
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New problem, or something that has been there ??
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  #3  
Old 05-22-2018, 11:58 AM
tfriendshuh tfriendshuh is offline
 
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new problem - was working fine until this weekend...
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  #4  
Old 05-22-2018, 12:00 PM
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Have you checked the shear screw on the servo?
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Rv-10, N210LM.

Flying as of 12/4/2010

Phase 1 done, 2/4/2011

Sold after 240+ wonderful hours of flight.

"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."
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  #5  
Old 05-22-2018, 12:02 PM
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http://www.vansairforce.com/communit...ad.php?t=36476
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Flying as of 12/4/2010

Phase 1 done, 2/4/2011

Sold after 240+ wonderful hours of flight.

"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."
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  #6  
Old 05-22-2018, 12:23 PM
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Quote:
Originally Posted by Mike S View Post
Have you checked the shear screw on the servo?
I'd consider that screw a candidate for investigation if it was software configurable.
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  #7  
Old 05-22-2018, 12:43 PM
Canadian_JOY Canadian_JOY is offline
 
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Good instinctive first call, Mike.

I think the OP has indicated the pitch servo works when on a coupled approach so the shear screw is not likely the culprit.

This isn't intended to impugn the o/p but I know I've made a few fat-finger errors over the years. There is a chance the altimeter setting in the a/p hasn't been entered, or a ridiculous vertical rate has been entered, something that produces a "does not compute" response.

If you have the Straight and Level button installed, I would go for a test flight and press the S&L button and observe what happens.
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  #8  
Old 05-22-2018, 12:55 PM
Ender Ender is offline
 
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I'm a bit of an autopilot guy (for background).

Does ALT HOLD or VS commands come from the same place as the GPS/Navigator? Sometimes the Navigator actually does the pitch and roll commands itself and the AP follows them, but the ALT, VS, etc comes from a different source. Is it possible that the Navigator is properly hooked up but the other part isn't (typically we call this the flight director or outer loops).

Another way to say this is that there may be two different pieces of software and thus interfaces doing the ALT/VS and the GPS part.

Thanks,
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  #9  
Old 05-22-2018, 12:56 PM
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Quote:
Originally Posted by Canadian_JOY View Post
I think the OP has indicated the pitch servo works when on a coupled approach so the shear screw is not likely the culprit.
Aaand that is why I should read a post all the way through before making a comment.

OK, now I have switched to the software or programing camp.
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VAF 909

Rv-10, N210LM.

Flying as of 12/4/2010

Phase 1 done, 2/4/2011

Sold after 240+ wonderful hours of flight.

"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."
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  #10  
Old 05-22-2018, 01:03 PM
tfriendshuh tfriendshuh is offline
 
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Quote:
Originally Posted by Canadian_JOY View Post
Good instinctive first call, Mike.

I think the OP has indicated the pitch servo works when on a coupled approach so the shear screw is not likely the culprit.

This isn't intended to impugn the o/p but I know I've made a few fat-finger errors over the years. There is a chance the altimeter setting in the a/p hasn't been entered, or a ridiculous vertical rate has been entered, something that produces a "does not compute" response.

If you have the Straight and Level button installed, I would go for a test flight and press the S&L button and observe what happens.
Level button engages and holds course but does not maintain altitude or even neutral pitch - i.e. I can still easily deflect stick fore/aft.

I was on a long x-country when I discovered this so I had a lot of time to play with it - believe me, I'm pretty sure I've eliminated all the "fat finger" possibilities
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Last edited by tfriendshuh : 05-22-2018 at 01:18 PM.
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