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Trutrak Deviation

R

RV9Otter

Active Member
I have a Trutrack Digiflight IIVS getting its GPS input from a Garmin GNS430w. Recently, I have noticed approximately a 20 degree variation between the selected track dialed into the autopilot and the actual track that the aircraft flies. For example, today, I was directed by the controller to fly a heading of 70 degrees. When I dialed 70 degrees into the autopilot, the track displayed on my GRT EFIS, on the GNS430w and the autopilot itself showed 90 degrees. I had to dial in 50 degrees on the autopilot to get the aircraft to fly the 70 degree heading.

The variance is always 20 degrees more than I've dialed in. In other words, if I dial in a track of 60 degrees in the autopilot, the plane flies a track of 80 degrees.

This is a rather recent but consistent event.

When the autopilot is in the GPSS mode, if flies the correct track. In other words, if the waypoint that I have dialed in requires a 40 degree track, that is what the autopilot flies and is also what is displayed in the EFIS, the Garmin GNS430W and also the autopilot.

So, the autopilot always displays an actual track that agrees with the Garmin GNS430W and the GRT EFIS (which has its own built-in GPS), but whenever I dial in a manual track for the autopilot to fly, the actual track flown is approximately 20 degrees more than what I have dialed in.

I am thinking of recalibrating the magnetometer per the manual's instructions, but, my understanding is that the magnetometer is only involved when the autopilot is not receiving a GPS signal. When I manually dial in the track to be flown, my understanding is that the unit will use the GPS signal from the GARMIN GNS430w as its reference. Is this correct?

Or when I dial in a track manually, is the autopilot using its internal magnetic data as the reference? If this is the case, I can understand why I could get such a large difference between track dialed in and track flown, that is, if the unit is using its magnetic data.

Again, all of this assumes no wind.

Thanks,

Curt
 
Just to note, if a controller gives you a heading, that's what he means. Not a ground track.
Sounds like you are controlling the autopilot directly? What happens if you control it thru the GRT EFIS? (That makes it easy to fly a heading)
 
Your GPS senses movement in order to determine direction, therefore that direction is always track (direction the airplane is going), not heading (direction the nose is pointing).

When you dial in a specific heading on your AP, I believe the magnetometer is what would be used to determine the heading.

When you use your GPS to specify where you want to go, and with the AP coupled, then you will be flying the track to that destination.

Sounds like a calibration of your magnetometer may be in order.
 
Thanks

Bob: I realize that the controller's heading is magnetic, not track, that is why I mentioned the "no wind" condition, where, track and heading should be pretty close. I'm not sure how to use the GRT EFIS to drive heading through the autopilot but I'll research it. Thanks.

SMO: Thanks for the confirmation...I'll recalibrate and post the results. Thanks.
 
That's what I was thinking too. Recalibrate the EFIS magnetometer.

But I'm pretty sure the EFIS has nothing to do with driving the TruTrak; that's getting its instructions directly from the GPS, I believe. You'd only be determining whether the problem is that your EFIS is giving you wrong numbers.

I am curious about the "no wind" note since -- at least where I fly -- there's always wind, and -- as you've noted -- the TruTrak is going to fly either a ground track (when on nav mode) or a heading (when manually entering a number).

Does your EFIS confirm that there's no wind? And if it shows a wind, if you change altitude (and presumably a change in wind), is the difference between the two (manual v. auto) still the same number of degrees?

If you fly a route to test that, and then do a 180, is the difference the same? If so, is the same difference... in other words, if you're seeing, say, a +20 degree difference, when you fly the same route on a 180, are you still +20 or -20?
 
That's my point, though. I'm pretty sure the EFIS isn't driving the autopilot at all, heading or otherwise. I could be wrong on this point since I use a Dynon D-100 and not a GRT unit.

The part of your note that is the most interesting is that you have a consistent difference between the displays in heading and track mode, presumably regardless of wind. That's why I think the first step is just to find a compass rose and confirm that EFIS calibrated correctly.
 
With the GRT-TruTrak-430W combo, he has several options.
TT 'stand alone'
TT controlled by the 430W (gpss mode)
TT controlled by the GRT HX or HXr. Most builders install a 3 pole, double throw switch to go between the first and third options.
I highly recommend #3. The EFIS will pass thru the 430W signal (arinc) so that option is preserved, but you can also track VOR or ILS, and have easy heading control.
 
Autopilot track error

It would appear that your autopilot requires re-calibration. It too can be "swung"...

RV9Otter said:
I have a Trutrack Digiflight IIVS getting its GPS input from a Garmin GNS430w. Recently, I have noticed approximately a 20 degree variation between the selected track dialed into the autopilot and the actual track that the aircraft flies. For example, today, I was directed by the controller to fly a heading of 70 degrees. When I dialed 70 degrees into the autopilot, the track displayed on my GRT EFIS, on the GNS430w and the autopilot itself showed 90 degrees. I had to dial in 50 degrees on the autopilot to get the aircraft to fly the 70 degree heading.

The variance is always 20 degrees more than I've dialed in. In other words, if I dial in a track of 60 degrees in the autopilot, the plane flies a track of 80 degrees.

This is a rather recent but consistent event.

When the autopilot is in the GPSS mode, if flies the correct track. In other words, if the waypoint that I have dialed in requires a 40 degree track, that is what the autopilot flies and is also what is displayed in the EFIS, the Garmin GNS430W and also the autopilot.

So, the autopilot always displays an actual track that agrees with the Garmin GNS430W and the GRT EFIS (which has its own built-in GPS), but whenever I dial in a manual track for the autopilot to fly, the actual track flown is approximately 20 degrees more than what I have dialed in.

I am thinking of recalibrating the magnetometer per the manual's instructions, but, my understanding is that the magnetometer is only involved when the autopilot is not receiving a GPS signal. When I manually dial in the track to be flown, my understanding is that the unit will use the GPS signal from the GARMIN GNS430w as its reference. Is this correct?

Or when I dial in a track manually, is the autopilot using its internal magnetic data as the reference? If this is the case, I can understand why I could get such a large difference between track dialed in and track flown, that is, if the unit is using its magnetic data.

Again, all of this assumes no wind.

Thanks,

Curt
Click to expand...
 
TruTrak

There is a toggle switch on the panel near the autopilot that has two positions, one so that the autopilot receives its guidance from the EFIS and the other so that the autopilot receives its guidance from the Garmin 430W. I've always used the Garmin GPS to drive the autopilot...just my preference as the Garmin is what I use to navigate and fly approaches, etc. On the EFIS, I usually engage the synthetic approach to most approaches as a reference, but, use the Garmin "localizer" and "vertical guidance" as the primary.

On the question regarding "no wind". When this happened the other day the EFIS was indicating practically no wind. Certainly not enough to give a 20 degree variation between heading and track.

I did not try the 180 degree test to see if the deviation stayed the same. Will do that.

Again, input from all of you is very much appreciated. I'm going the re-set the internal "magnetic" references per the manual and report what happens.

Curt
 
Let me see if I can clear up a few things and figure out what's happening.

Quick question first, do you see this discrepancy regardless of the GPS source? Does your source switch change the RS232 input from the EFIS to the 430W?

What do you see in the top left of the autopilot display? Is it TRK or HDG? If you're seeing HDG then you're using the magnetometer in the autopilot, which is not meant as a means of primary navigation. If you're seeing that, then we need to figure out why the RS232 signal from either your GPS or EFIS is not being received. The magnetometer could be causing the heading error.

If you're seeing TRK and still seeing this discrepancy, first try a gyro set. With the AP powered up and the aircraft stationary, press and hold the knob for about 45 seconds. Then release the knob and you can move the airplane at that time.
 
Problem Seems Resolved

Lucas:

Did the 45 second reset as you suggested and there is no longer a discrepancy between the track dialed in and the track flown.

I don't think the autopilot was ever in the heading mode...I think it was receiving the GPS signal all along as indicated by the TRK in the upper left hand corner.

This is not the first time that this longer gyro reset has been used to correct a deviation. Some time ago, the autopilot was consistently tracking to the right of course, and this same reset technique seemed to resolve the issue.

Is there something going on that might be causing this reset to be needed periodically?

Thank you for your input.

Curt
 
Hi Curt,

If at any time a gyro set is inadvertently done, it could cause it to overwrite with the new set value. Also, make sure you power up the autopilot while you're on the ground and stationary. If you power up while moving, it can cause the offset you see.
 
Hi Lucas - in an earlier post in this thread you asked, as a troubleshooting step, if the RS232 input to the A/P is switched between GNS430 and EFIS.

The TruTrak wiring diagram pretty clearly shows only the ARINC429 signals going through the switch.

The TruTrak A/P manual is really, really skinny on describing what the RS232 is used for and why it needs to be connected, let alone where it should be connected. It would be tremendously helpful if this type of info could be shared here, and added to the product documentation.

I'm currently helping a friend in Europe troubleshoot his installation and it's clear that at least some of his troubles are being driven by the paucity of information on this topic in the product documentation.

Thanks in advance for any info you might be able to share.
 
Canadian_JOY said:
Hi Lucas - in an earlier post in this thread you asked, as a troubleshooting step, if the RS232 input to the A/P is switched between GNS430 and EFIS.

The TruTrak wiring diagram pretty clearly shows only the ARINC429 signals going through the switch.

The TruTrak A/P manual is really, really skinny on describing what the RS232 is used for and why it needs to be connected, let alone where it should be connected. It would be tremendously helpful if this type of info could be shared here, and added to the product documentation.

I'm currently helping a friend in Europe troubleshoot his installation and it's clear that at least some of his troubles are being driven by the paucity of information on this topic in the product documentation.

Thanks in advance for any info you might be able to share.
Click to expand...

I actually don't have any diagrams in the DII manual that specify an input switch of any kind, so that's left up to the end user completely.

As to the function of the RS232, what problems are you having? Please feel free to email me if you'd like. lucas (at) trutrakap (dot) com.

Also, if you read the pinout chart, it specifies that the RS232 provides navigational input to the autopilot.

We can get you and your friend fixed up. Thanks!
 
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