GTN tracks 500 feet right of centerline

[Jim Peace]

I noticed yesterday that my GTN 650 was tracking 500 feet right of centerline shortly after selecting direct to the waypoint. 
 

xwind was from the left but that should not matter. Also I checked my offset in the flight plan page and it was not engaged. 
 

GTN650, dual G5s, Stec30 it’s never done this before. Thoughts?

happened every selection yesterday on different waypoints. 

[Ragsf15e]

2 hours ago, Jim Peace said:

I noticed yesterday that my GTN 650 was tracking 500 feet right of centerline shortly after selecting direct to the waypoint. 
 

xwind was from the left but that should not matter. Also I checked my offset in the flight plan page and it was not engaged. 
 

GTN650, dual G5s, Stec30 it’s never done this before. Thoughts?

happened every selection yesterday on different waypoints. 

I also have an stec30 that tracks slightly left of course.  I heard theres an adjustment but I’ve never messed with it.  I believe it’s your stec doing that.

 

in heading mode does it exactly steer your heading?

[rbridges]

On the stec50 there is a small hole in one of the units on the panel that allows adjustment with a small hex head tool. 

Edited October 28, 2021 by rbridges

[buddy]

If the Stec 30 is similar to the Stec 55X you will need something like a jewelers screw driver to make the adjustment through a small hole on the autopilot. You will need to make the adjustments in the heading mode, probably best while your flying so you can see how it tracks in the nav mode after making the adjustments.

[David Lloyd]

Stec calls the adjustment Roll Centering.  Very small flat blade screwdriver to adjust a hidden pot located under a screw.  Bottom right for the 20 and 30 series, to the left of the display 40, 50 and 60 series.

Picture shows the cover screw out.  Was nav tracking about a needle width off, now perfect.  Also tracked the heading bug a couple degrees off.  The new buttons and a board just cost about $900 to replace.  Read the Stec instructions before jabbing a screwdriver into the internals.

 

[Ragsf15e]

His stec 30 is likely in hdg mode while it’s being guided by the gtn.  The g5s are providing gpss through hdg mode.

For some reason I think the adjustment for the stec 30 is in the back but I don’t have the manual handy.  We’ve definitely talked about this before on MS.

[Ragsf15e]

Oh, sounds like @David Lloyd knew where it is.  We have a picture around here somewhere too.

Dont break anything messing with it… would suck to cause a much bigger problem trying to fix a minor annoyance.

[Jim Peace]

It’s never done this before. It’s always been dead on. So why now?

 

[PT20J]

Parallel track isn’t available in direct to, so it couldn’t be that. Was CDI centered? Did it show a non-zero XTK?

Skip

[Jim Peace]

8 hours ago, PT20J said:

Parallel track isn’t available in direct to, so it couldn’t be that. Was CDI centered? Did it show a non-zero XTK?

Skip

CDI on my G5 and on my iPad shows right of track. 

[Bob - S50]

Don't know if it's the same issue or not, but a pilot on the COPA forum noticed the same issue with his brand autopilot (don't remember what brand).  Turned out to be a yaw issue.  Once he adjusted the airplane so it flew without any yaw, the AP tracked exactly on course.

[Jim Peace]

2 hours ago, Bob - S50 said:

Don't know if it's the same issue or not, but a pilot on the COPA forum noticed the same issue with his brand autopilot (don't remember what brand).  Turned out to be a yaw issue.  Once he adjusted the airplane so it flew without any yaw, the AP tracked exactly on course.

My ball is centered on an old fashion t/c and my G5

[Ragsf15e]

http://n98297.blogspot.com/2020/12/s-tec-40-autopilot-calibration.html?m=1
 

Heresthe calibration for an stec 40.  I’m sure you can find same for a -30 with a little searching.  It doesn’t seem hard.

[PT20J]

Sounds like the GTN is displaying correct Information, so I would investigate the autopilot.

[Jim Peace]

so let me guess,,,,I do not have the ability to adjust this in flight and its going to be a royal pain in the ass going forward from here...

Still have no idea why this all of a sudden started.

 

 

[Jim Peace]

actually I found this in the Stec install manual:

Does anyone think I will be able to get to the POT screw adjustment if I remove the lower screw on the bottom right? Not sure if this will be accessible with the unit mounted behind the panel.  The screw looks so small to have anything behind it....

Has anyone ever done this?  Also wonder if it will work with it in heading mode and me using GPSS steering through the G5

[dzeleski]

If its the same as the one my J had yes you will. It is a VERY small pot adjustment. If you have an iPhone look at the screw at the bottom of your phone near where you charge it. Thats about the size of the pot, totally different fastener but similar in size.

[Jim Peace]

3 minutes ago, dzeleski said:

 If you have an iPhone look at the screw at the bottom of your phone near where you charge it. Thats about the size of the pot, totally different fastener but similar in size.

holy crap....that is smaller than a jewelers driver,,,,,,

[dzeleski]

1 minute ago, Jim Peace said:

holy crap....that is smaller than a jewelers driver,,,,,,

I have a specialty set of small drivers that I used for electronics repair years ago. I dont recall using one that wouldnt be covered by a typical jewelers driver set.

The pot has to be roughly if not a bit smaller then the hole for that screw, which is roughly half the size of that head of the screw. If you have a set of jewelers tools you will be totally fine. Pop that screw out and shine a light in there and you will see it.

[Jim Peace]

18 minutes ago, dzeleski said:

I have a specialty set of small drivers that I used for electronics repair years ago. I dont recall using one that wouldnt be covered by a typical jewelers driver set.

The pot has to be roughly if not a bit smaller then the hole for that screw, which is roughly half the size of that head of the screw. If you have a set of jewelers tools you will be totally fine. Pop that screw out and shine a light in there and you will see it.

Just called their tech support said a normal small jewlers slotted should work...it does have to be an inch long though and said some people including them at times use a paperclip that they flatten the tip out with a hammer and it works great....

[dzeleski]

4 minutes ago, Jim Peace said:

Just called their tech support said a normal small jewlers slotted should work...it does have to be an inch long though and said some people including them at times use a paperclip that they flatten the tip out with a hammer and it works great....

Thats pretty funny actually. Either way youll be fine with either. I will say that pot is seriously sensitive, after making an adjustment fly around with different headings before calling it good. It took a few tries to get it sorted.

[David Lloyd]

Yes, the adjustment is hidden behind the mounting screw on the 20 & 30 series.  Stec warns not to use longer screws for mounting than those supplied (don't crunch the pot with a long screw).  The cheapie small screwdrivers with the fingertip swivel on the end is what Stec shows in a picture.  The smallest size I have worked just fine.  The next larger size would not fit through the screw hole. Shops normally do this on the ground, adjusting until the control wheel reverses direction.  With a Mooney, because of the small control movements may get close but need in-flight adjustment.  Directions say to make small adjustments and wait for the autopilot to compensate.  In my airplane, a bar width out took between a quarter and half turn on the pot.  I don't think it is all that sensitive.  Right of course, adjust CCW.  Mark your screwdriver so you can keep track of how much you turn.  Pick a very smooth day, you can't feel what you're doing.  Like brain surgery in the back of a moving pickup truck.

Jim, have no idea why yours suddenly began tracking off course.  Yes, it can be adjusted in heading mode while on GPSS.

[DXB]

@Jim Peace thanks for posting this - I have the identical issue on STEC30+GTN650 w/GPSS through Aspen PFD.  I'd been too lazy to figure it out, but now I don't have to.

[DXB]

Did this adjustment in flight this eve - definitely a win. I did it in calm air at 8000 ft in GPSS MODE.   My cross-track error went from 1500 feet right of course to pretty much right on (oscillates 200-400 feet to either side).  That’s as good as I could get it though.  @Jim Peace’s 500ft error might not be that far off from what’s possible.  I thought it might do a better job flying an ILS so I gave it a try - nope - it still acted like a drunken sailor down the localizer course.  Oh well, I’m still gonna call it a win 

[Jim Peace]

got mine to go from a solid 500 feet right of track to less than 250 feet bracket left and right of course.

Tried it on a few GPS approaches and it tracked dead on to the runway....

I just need an Acclaim type S now....