Track deviation

[flyingforfun2004]

My AP holds TRACK great most of the time (unless the 296 looses signal but that is another thread) however just now and again even though its still locked to track, say after 15 mins of been engaged, it just wanders to port about 30 degrees (heading not bank) and then corrects its self. This coincides with the AP having to make a severe correction to height. I get the impression if it needs to make a severe correction to height it just temporary wanders its accuracy of heading. Is it my imagination or may there be something else. Its a bit annoying as it is fine for ages but for no reason just goes off.....

regards
Kev

 

[dynonsupport]

Just to clarify what you're seeing:

While holding a track, you ask the AP to change altitude, and while this is happeneing, the AP wanders off heading?

Are you observing any servo slips on either axis when this happens (are the TRK and/or ALT annunciators turning yellow?)

I see in another thread that you've been having some DSAB problems as well; It's probably best to sort those out first.  =)

 

[flyingforfun2004]

Hi, there is no slipping and no yellow out of trim indications. I am finding...Lets say the AP is set for 090 deg at 2000ft, it gets a bit of lift and raises 30 or 40 feet or so the AP sticks the nose down to loose this height. It is at that point that as it starts to descend the aircraft may wander to 060 deg (say 30 deg left but still showing TRK been received) before then correcting its self back to 090 deg. It always drifts left. As I say the majority of the time it is fine it just tends to do this randomly but not often.

In regard to the DSAB problem I mentioned this was a strange one. It happended at start up. I cant help but feel this was software related as the D100 could not see the D120 or AP servos despite me trying a configure test lots of times, UNTIL I removed the power to the AP servos as soon as i did that then doing a configure found the D120 straight away. Once it found the D120 and I applied power back to the AP servos a configure found them straight away too. Its ok again following that sequence.

It was having a bit of a wobble. :-[

 

[dynonsupport]

Kev,

How does the AP do in HDG mode?

When you observe the error in heading, are you seeing the heading on the compass change, or are you seeing the purple ground track indicator move?

How much does the airplane bank during these occurrences?

 

[flyingforfun2004]

Hi, I do not use tit in heading mode as its all over the place. I suspect this is calibration of the compass. So i just use track only mode from the gps. This is magenta and has a yellow block around it sometimes if i recall correctly.

regards
Kev

 

[dynonsupport]

Kev,

A poor compass calibration can cause the compass to show a false heading change with changes in bank or pitch angle.  

Since it sounds like what you are seeing is the heading change with little to no bank change during an altitude bump, it's probably the compass showing an error rather than the AP turning the airplane.

Before calibrating your compass, verify that:

• The EFIS pitch angle and compass pitch angle are the same relative to the aircraft centerline.
• The PITCHADJ value is set such that when the airplane centerline is at a known value on level ground, the EFIS reads that value.  (For example, a taildragger with an 8 deg nose up on the ground should have  PITCHADJ set so that the EFIS reads 8 deg nose up.)

After calibrating the compass verify in flight that the compass shows heading changes only when the airplane changes heading.  An easy way to test this is to choose a point on the horizon and bank back and forth while holding the nose on the point.  The heading value should not change.  Likewise, pitch changes should not cause the heading to change.

If TRK is turning yellow while the AP is engaged, it indicates that the roll servo is slipping.  If this happens often we recommend turning up the roll torque.

 

[flyingforfun2004]

Hi,

Not quiet,

When it deviates it does a medium turn to the left and does bank and does change headign by 30 or 40 degress before correcting its self. This is confirmed by the change in heading/track when checked visually and on the mag compass and gps. Ill keep an eye to see how often the track yellow box comes on identifying its slipping.

Its just odd. It can be fine for 15 mins or more and then for no reason while in track mode and still in track mode just does a 30 or 40 deg turn to the left.

Ill check its not slipping as well but i knwo when it does as i can feel if behind me as the motor is right by the back seat and i can fell it if it slips.

regards
Kev

 

[dynonsupport]

Have you noticed whether the GPS ground track icon (triangle on either the heading tape or on the HSI page) is correct (and therefore also 30-40 degrees off where the autopilot is taking you) when this is happening?

If you have the ability to shoot a video with a digital camera, or even take a few pictures of the PFD + HSI screen, we might be able to see what's going on in more detail.

 

[DBRV10]

As you may have seen so far from my comments we have had a very good result with our Dynon system.

We did also experience this roll to the left of about 30 degrees for no apparent reason. It did it twice in one half hour. We were in Heading mode and with an ALT captured.

Also did it once while VOR tracking.

I thought at the time it may have been a compass error problem, as it made sense to me that could be the likely culprit and having seen a warning in the manual somewhere, even if not apparent on the display, it could have happened with data quicker than we saw it.

Anyway we did another and we think more successful compass calibration and its been playing nice ever since.

I would be interested to hear any comments back. I am going to take a closer look at itagain some timeby just flying in heading mode.

 

[dynonsupport]

I can't say we have an obvious answer. But, if you can capture some data of the AP doing this, this would help us see what you're seeing. You'll want to turn datalogging on at 1hz, and set it to turn on at boot so it's effectively always on. After you capture an event, download the datalog to your computer (turn datalogging off after that flight to keep that data fresh if you can't pull it off right away).

 

[DBRV10]

Ok I will try to remember to turn on the data logging in the D100 and see if it does it again. Mind you we have the compass calibrated better now.

I am doing around 15 hours of flying in the next week so there will be plenty of heading mode options to play with.