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  #11  
Old 05-22-2018, 01:14 PM
tfriendshuh tfriendshuh is offline
 
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Location: Duluth
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Quote:
Originally Posted by Ender View Post
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,

this is my thinking as well - input for course and coupled glide slope comes from the GTN650 via the arinc module

I'm not an expert but it's my understanding that the AP control head has it's own internal altimeter and VSI and that's what it uses when not in coupled (approach) mode.

The odd thing is that it does detect and display the correct altitude (when properly calibrated on the ground of course) and it appropriately notes when I am (manually) approaching target altitude and then displays ALT-HOLD which is also expected when I reach the pre-set target altitude.

It's like the internal software isn't generating output for the elevator servo except when being fed by the GTN.

I'm expecting TruTrak to want me to pull the unit and sent it to them for testing, but hoped maybe someone here would have some experience with something similar
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Last edited by tfriendshuh : 05-22-2018 at 01:19 PM.
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  #12  
Old 05-22-2018, 04:14 PM
Ender Ender is offline
 
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I'm not familiar with your setup, so bear with me (I'm only in early stages of my build, I do Autopilots for work). But the Autoflight System is usually Flight Director -> Autopilot -> Servos. So I'm concerned that your FD is working right (you are getting mode transitions), but that the FD command isn't getting to your AP. It could be as simple as missing data (i.e. the wires are disconnected).

The odd thing here is that it seems (from what I've read) that the elevator servo isn't engaged, as you aren't backdriving it. Depending on how the Servo works (or if the AP is inside the servo) this could be the default operation if it is missing Pitch Command data from the Flight Director. So when you go for Approach, valid data shows up and it engages the servo and starts working.

Again, I'm not familiar with your equipment, but these are just some of the things jumping out at me. Hope it helps!
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  #13  
Old 05-22-2018, 04:17 PM
Ender Ender is offline
 
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Also, if the AP does live outside the servo, you've effectively ruled out the AP to Servo connection as a culprit as the approach scenario works. So it's still pointing at the FD to AP/Servo connection.

Looking at your docs, see "who" actually calculates the commands for the ALT and VS etc and see if that box is connected to the servos. check that connection.
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  #14  
Old 05-22-2018, 05:26 PM
bharral bharral is offline
 
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If the RNAV approach you flew has GPS based-vertical guidance, then perhaps there is a problem with the Baro Altitude input to the A/P?\

If Baro Altitude is routed through the GTN (which does not directly measure it), then make sure the input/output formats on the GTN support the Baro Altitude messages, and the AP is configured to understand the format transmitted by the GTN. It is likely that you can use the GTN maintenance/setup page to look at at any Baro Altitudes inputs, so I would start there.

Regards,
Blake
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  #15  
Old 05-22-2018, 06:17 PM
Canadian_JOY Canadian_JOY is offline
 
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The TruTrak autopilot has its own internal air data sensors for pitot and static, as well as rate gyros. With the symptoms described, the altitude hold / rate select is effectively not functioning. This sounds almost certainly like an internal sensor failure.

By all means check your pitot/static connections but this one sounds like a failure of the control head itself in establishing the link between the air data sensor and the circuit that uses this air data to actually drive the servo. If you talk to TruTrak I'd love to hear their recommendations and, ultimately, find out what is the cure. Yes, I have a Vision 385 so want to be aware of its failure modes.
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  #16  
Old 05-22-2018, 08:50 PM
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Jesse Jesse is offline
 
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You can test it on the ground. Sitting in the hangar or on the ramp, boot up the system and just hit Continue once on the GTN so you see ?Instrument Panel Self Test? at the top of the screen. Turn on the auto pilot by pushing the knob once. Push the knob again to underline SVS and dial a vertical speed of 500+ rpm and The stick should move aft. Then, push mode and it will go into GPSS and GS Flg then GS Arm mode and will hold the stick steady in pitch and roll will move. Report back your findings. My thought is a shear screw that is broken but still causing a little bit of drag. The GPSS commands work better than without when there isn?t enough torque in the servo, which sounds like what is happening.

The next thing to do is test your pitch servo and see, when the AP is engaged in level mode, if you can move the stick fore and aft and the servo arm moves independently of the ?barrell? that the arm is attached to.

Please report back what you find or give me a call and we can discuss.
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  #17  
Old 05-23-2018, 02:52 AM
tfriendshuh tfriendshuh is offline
 
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Quote:
Originally Posted by Jesse View Post
You can test it on the ground. Sitting in the hangar or on the ramp, boot up the system and just hit Continue once on the GTN so you see ?Instrument Panel Self Test? at the top of the screen. Turn on the auto pilot by pushing the knob once. Push the knob again to underline SVS and dial a vertical speed of 500+ rpm and The stick should move aft. Then, push mode and it will go into GPSS and GS Flg then GS Arm mode and will hold the stick steady in pitch and roll will move. Report back your findings. My thought is a shear screw that is broken but still causing a little bit of drag. The GPSS commands work better than without when there isn?t enough torque in the servo, which sounds like what is happening.

The next thing to do is test your pitch servo and see, when the AP is engaged in level mode, if you can move the stick fore and aft and the servo arm moves independently of the ?barrell? that the arm is attached to.

Please report back what you find or give me a call and we can discuss.
Thanks Jesse, the servo is not engaging, the stick is not steady fore/aft when the leveler button is pushed and no pitch deflection occurs when I dial in a + or - vertical speed. On the ground or in the air.

The shear screw cannot be broken because when I capture glide slope on an RNAV approach with LPV, pitch control abruptly starts working and it will hold glide slope all the way to minimums. Similar logic dictates that the servo and wiring are also functional.
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  #18  
Old 05-23-2018, 02:54 AM
tfriendshuh tfriendshuh is offline
 
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Quote:
Originally Posted by Canadian_JOY View Post
The TruTrak autopilot has its own internal air data sensors for pitot and static, as well as rate gyros. With the symptoms described, the altitude hold / rate select is effectively not functioning. This sounds almost certainly like an internal sensor failure.

By all means check your pitot/static connections but this one sounds like a failure of the control head itself in establishing the link between the air data sensor and the circuit that uses this air data to actually drive the servo. If you talk to TruTrak I'd love to hear their recommendations and, ultimately, find out what is the cure. Yes, I have a Vision 385 so want to be aware of its failure modes.
Yes - I think this is the only possibility. I haven't heard back from TruTrak yet, but will let you know when I do.

Thanks
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1943 Boeing Stearman N19BB
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  #19  
Old 05-23-2018, 05:27 AM
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Jesse Jesse is offline
 
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Quote:
Originally Posted by tfriendshuh View Post
Thanks Jesse, the servo is not engaging, the stick is not steady fore/aft when the leveler button is pushed and no pitch deflection occurs when I dial in a + or - vertical speed. On the ground or in the air.

The shear screw cannot be broken because when I capture glide slope on an RNAV approach with LPV, pitch control abruptly starts working and it will hold glide slope all the way to minimums. Similar logic dictates that the servo and wiring are also functional.
Have you tried it on the ground? Even with what you are seeing in Flight, I would still test on the ground as I described.
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  #20  
Old 05-23-2018, 06:01 AM
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flytoday flytoday is offline
 
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Default What recent changes?

How about a different line of inquiry?

What was changed in the plane just prior to the ?failure??

Was any work done on the plane since the last ?successful? flight?

Were there any software upgrades? New equipment installed? Wiring installed or changed?

Anyone else work on or fly the plane?

Carl
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