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View Full Version : Very strange A/P problem - pls help me troubleshoot!


tfriendshuh
05-22-2018, 10:22 AM
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. :confused:

I sent an email to TruTrak too :)

Mike S
05-22-2018, 12:05 PM
New problem, or something that has been there ??

tfriendshuh
05-22-2018, 12:58 PM
new problem - was working fine until this weekend...

Mike S
05-22-2018, 01:00 PM
Have you checked the shear screw on the servo?

Mike S
05-22-2018, 01:02 PM
http://www.vansairforce.com/community/showthread.php?t=36476

Bob Kuykendall
05-22-2018, 01:23 PM
Have you checked the shear screw on the servo?

I'd consider that screw a candidate for investigation if it was software configurable.

Canadian_JOY
05-22-2018, 01:43 PM
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.

Ender
05-22-2018, 01:55 PM
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,

Mike S
05-22-2018, 01:56 PM
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.

tfriendshuh
05-22-2018, 02:03 PM
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 ;)

tfriendshuh
05-22-2018, 02:14 PM
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

Ender
05-22-2018, 05:14 PM
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!

Ender
05-22-2018, 05:17 PM
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.

bharral
05-22-2018, 06:26 PM
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

Canadian_JOY
05-22-2018, 07:17 PM
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.

Jesse
05-22-2018, 09:50 PM
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.

tfriendshuh
05-23-2018, 03:52 AM
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.

tfriendshuh
05-23-2018, 03:54 AM
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

Jesse
05-23-2018, 06:27 AM
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.

flytoday
05-23-2018, 07:01 AM
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
..

B Cunningham
05-23-2018, 09:03 AM
Following this thread with interest. I have the same hardware configuration and I am currently trying to troubleshoot the opposite issue -- Vizion TruTrak captures and holds altitude just fine but will not couple to a glideslope on any approach vertical path... :confused: I have ruled out all the obvious reasons as well. One thing I was told by TruTrak was that the commands the autopilot uses for alt hold are the same ones it uses to control pitch on a glide slope or VNAV path.

rleffler
05-23-2018, 09:13 AM
Hard to provide feedback without understanding more about your panel.

1. Who's EFIS do you have?

2. Is the ARINC line from the 650 wired directly to the AP or the EFIS?

3. Are you attempting to do an altitude hold from the AP head or your EFIS?

4. I suspect you're doing from the AP head. The GTN doesn't have any altitude capability, so I assume that you're setting an altitude in the AP head. Like a previous poster mention, I suspect that you didn't sync the altimeter in the AP head if it isn't stopping at the correct altitude.

tfriendshuh
05-23-2018, 09:23 AM
Hard to provide feedback without understanding more about your panel.

1. Who's EFIS do you have?

2. Is the ARINC line from the 650 wired directly to the AP or the EFIS?

3. Are you attempting to do an altitude hold from the AP head or your EFIS?

4. I suspect you're doing from the AP head. The GTN doesn't have any altitude capability, so I assume that you're setting an altitude in the AP head. Like a previous poster mention, I suspect that you didn't sync the altimeter in the AP head if it isn't stopping at the correct altitude.

1. AP isn't connected to the EFIS (Dynon D100)

2. ARINC direct to AP

3. From the AP head

4. Yes and no. It's not that it "isn't stopping at the correct altitude", it's not engaging at all (in pitch) - i.e. no "hold" when AP LVL button is pushed, no u/down motion when + or - vertical speed is selected.

tfriendshuh
05-23-2018, 09:26 AM
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.

I'll be at the hangar tonight so of course, I'll check it out :):D after that, I'm also going to fly out to a nearby airport with an LPV approach and do a coupled approach or two to ensure the full spectrum of observed behavior is 100% repeatable.

tfriendshuh
05-23-2018, 09:34 AM
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
..

This was my first thought too (ITSM manager natch) :D

Two changes since it was last working:

I swapped out a bad ignition coil - very unlikely to have an effect

I updated the Garmin database - more likely IMHO to break the LPV/approach glide slope feed to AP than the AP head control in cruise flight - but I am going out tonight to review config of all components to make sure nothing got changed somehow

Nobody else worked on it or flies it :cool:

The only other thing out of the ordinary was that I spent enough time trip planning and playing with the GTN prior to the flight that I had to put a charger on the battery for about half hour to get it started.

tfriendshuh
05-23-2018, 10:21 AM
TruTrak says check the "VRT Activity" setting and (like Jesse :cool:) wants ground confirmation / listening for servo activity so I'll be checking all that out tonight.

Jesse
05-23-2018, 05:20 PM
Following this thread with interest. I have the same hardware configuration and I am currently trying to troubleshoot the opposite issue -- Vizion TruTrak captures and holds altitude just fine but will not couple to a glideslope on any approach vertical path... :confused: I have ruled out all the obvious reasons as well. One thing I was told by TruTrak was that the commands the autopilot uses for alt hold are the same ones it uses to control pitch on a glide slope or VNAV path.

Ben, what gps do you have. If it?s a 430/530W, then make sure you have ?VNAV: Enable Labels? ser in the ARINC settings. Hold Ent when you power it up until you see the Garmin logo to get into settings mode. If you have a non-W, then you won?t get glideslope controls on the auto pilot.

tfriendshuh
05-23-2018, 08:20 PM
Ok so I'm a moron....

According to TruTrak, shear screw can in fact shear and it is not unpossible for it to "stick" at some point and have AP function.

Tonight I went out to reproduce the "works on coupled approach" I had experienced on my long XC and was unable to reproduce - sure the head would indicate coupled but contrary to my earlier experience there was no movement by the AP on the stick in pitch - AP wouldn't fly glide slope at all.

Got back on the ground and dialed in +/- 500 FPM and there was a slight catch to fore / aft stick / elevator movement with an accompanying strange zzzt zzt zzt sound from somewhere.

Sigh...

Ok - it's guess its the shear screw and I just got lucky/unlucky and on that one approach it was "stuck" enough to maintain glide slope until I cleared the clouds.

Haven't pulled the floor off to inspect yet, but likely everybody who said shear screw was correct after all :o

thanks all for the input and assistance

Jesse
05-23-2018, 08:23 PM
There?s nothing moron about it. You?re a genius. You did the exact right thing and asked the brain trust, VAF!

flytoday
05-23-2018, 08:34 PM
For my education, why does/did this screw shear? Is this common? Is the screw life-limited?

Carl
..

Jesse
05-23-2018, 08:39 PM
It is a fail safe item that protects the function of Tye control system if the Servo somehow locks up. The shear screws have had periods of weakness in my experience. It?s not uncommon.

BobTurner
05-23-2018, 08:53 PM
For my education, why does/did this screw shear? Is this common? Is the screw life-limited?

Carl
..

I see in your sub-line that you have a Trio. It uses a clutch that can slip under enough force - a better solution, imho, than using a shear screw for the same function.

Mike S
05-23-2018, 09:26 PM
For my education, why does/did this screw shear? Is this common? Is the screw life-limited?

Carl
..

As I recall, it can fail due to trying to let the A/P fly in heavy turbulence. There are a number of older threads on this----------if you do a search, be sure to set the time frame to "any date"