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Autopilot not flying courseline on approach

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Kentucky Captain View Drop Down
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    Posted: 13 May 2018 at 10:59pm
I have been having a problem with my IFD540, Aspen, STEC-30 combination during an approach.  Before now, I was mostly hand flying approaches and apparently never completed an approach on autopilot down to the minimums.  Hand flying the approach was fine.  The problem came when I let the autopilot do it.  The autopilot was flying the course set by the IFD just fine until it got to about a one mile final and a couple hundred feet above minimums.  At that point the autopilot would direct a turn to the right and I would have to disengage the autopilot to get back to the runway.  If I hand flew the approach, all was good and I could fly right to minimums on course.

I had s string of emails and a phone call with T.J. Sutton at Avidyne this week and he nailed the issue right on the head. I can't tell how many people I have talked to about this and he is the only one that has had the correct answers.  Thanks T.J.

For anyone else out there that has an STEC-30 - Aspen - IFD combination, here is the low down on it.

While en-route, with a flight plan in IFD540, the autopilot stays on HD mode and the Aspen has GPSS engaged.

When you cross the IAF inbound on the approach, the IFD is in VLOC, LPV, LNAV +V, or whatever approach you're on and your LDI and VDI become active, switch the autopilot to HI TRK mode and then turn off GPSS on the Aspen.

In that configuration, the autopilot flies on the line, all the way down to MDA or DA.

This may be obvious to everyone else, but it was not to me.  I hope this may help anyone else with similar handicaps to mine.

Thanks again to T.J. Sutton.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote HenryM Quote  Post ReplyReply Direct Link To This Post Posted: 13 May 2018 at 11:10pm
Hi KC!

Why does the problem occur if you don’t change to mode? I have an IFD-540 and an STec 30 with GPSS. The few GPS approaches I’ve tried, the AP keeps the plane laterally aligned aall the way down to the MAP and then I guides me through the missed approach. Is this an Aspen issue? What is actually happening behind the scenes to elicit the turn of you don’t change modes?


Edited by HenryM - 14 May 2018 at 5:20am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Kentucky Captain Quote  Post ReplyReply Direct Link To This Post Posted: 14 May 2018 at 4:19am
Originally posted by HenryM HenryM wrote:

Hi KC!

Why does the problem occur if you don’t change to mode? I have an IFD-540 and an STec 30 with GPSS. The few GOS approaches I’ve tried, the AP keeps the plane laterally aligned aall the way down to the MAP and then I guides me through the missed approach. Is this an Aspen issue? What is actually happening behind the scenes to elicit the turn of you don’t change modes?


This was not just with a GPS but also with an ILS approach.  If I leave the STEC-30 on HD mode and the Aspen on GPSS, the autopilot takes a right turn at a one mile final and a couple hundred feet above minimums.

My guess is that it is specific to the Aspen.  Something to do with the ACU and multiple connections between them.  I don't pretend to understand it all.  T.J could probably answer this a lot better than I can.

Here is a portion of the email from T.J. to me about this.  It might help, but if not, maybe T.J. can chime in here:

GPSS should be fine to the IAF, and probably ok to the FAF, but inside the FAF the autopilot should be driven from the main left/right input, rather than the HDG/GPSS input.  A change in the Nav Mode results in a change in the CDI scaling as shown below from the IFD pilot guide.  However, when being driven by the GPSS roll steering label 131, the scaling doesn't change (at least not in the IFD).  It will stay at the "Enroute" value.  The closer you get to the runway, the tighter the scaling should be, but GPSS will not give you that result. 

 

NAVIGATION MODE/CDI SCALE CHANGING

Each navigation mode has an associated CDI scaling associated with it per the table below:

Navigation Mode

CDI Full Scale Value

Enroute

2.0 NM (5.0 NM for non-WAAS)

T erminal

1.0 NM

Approach

0.3 NM or 2°, whichever is less at the FAF


 

Below is from the Aspen Pilot Guide.  There may be some confusion, because they reference a Garmin GNS unit in several places and the GNS transmits ARINC429 Roll Steering Label 131 that actually does change scaling.  The IFD only changes scaling in VLOC/ILS mode, or in GPS mode if the GPS is selected as the Nav Source (not GPSS).

 

From the Aspen manual:

"Whenever GPSS is enabled on the PFD, and the autopilot is in HDG mode, the autopilot will fly the GPS flight plan. The autopilot must be switched to APPR mode in order to fly the VLOC source selected for the CDI and to capture and track the glideslope of an ILS, or even an RNAV approach with vertical guidance.

A couple of miles outside the FAF (TAKLE), we switch to APPR mode on the autopilot, which begins to track the localizer and is armed for glideslope capture. At this point, we could disable the GPSS by pressing the GPSS Hot Key to remove potentially confusing annunciations on the PFD, but we know our equipment well. We understand fully that GPSS is just another form of HDG as far as the autopilot is concerned, and the autopilot will not fly GPSS unless it is in HDG mode. So we leave GPSS enabled on the PFD, planning to use it to fly the Missed Approach if necessary.

With that setup, if we have to fly the Missed Approach, we’ll do so by hitting Go Around mode on the autopilot and climbing straight ahead to 6,700 feet. Then we’ll press the OBS button on the GPS to allow the missed approach flight plan to sequence. Engage the autopilot in the HDG mode and begin the climb out. With GPSS still enabled, the autopilot will navigate the aircraft directly to the FMG VOR. If the GPS supports curved flight plans, such as a WAAS GPS, the autopilot will automatically enter and fly the hold. All we have to do is level off when reaching 11,000 feet - and if we had Altitude Pre- Select on the autopilot, it would even do that for us, too"


 

On the STEC-30 you'll want to have HI TRK mode engaged for either a GPS or ILS approach.  This will give the autopilot maximum authority and try to maintain +/- 1 needle width on the CDI.  and since you'll be using GPS or VLOC as the selected Nav source on the Aspen side, the deviations will be driven from the Main Left/Right outputs of the ACU.

 

S–TEC

3.1.4 High Track (HI TRK) Mode, System Twenty / Thirty

3.1.4.1 LOC Course Tracking

Select the LOC frequency on the Navigation Receiver. Maneuver the aircraft to within ±1 CDI needle width and ±10° heading of the selected course. Engage the high track mode. The HI TRK lamp alone will be illuminated as shown in Fig. 3-4, to acknowledge that this mode is engaged. The autopilot will track the selected course with maximum authority.

3.1.4.2 GPS Course Tracking

Program a predefined course into the GPS Navigation Receiver, comprised of course segments connected by waypoints. Maneuver the aircraft to within ±1 CDI needle width and ±10° heading of each successive course segment. Engage the high track mode. The HI TRK lamp alone will be illuminated as shown in Fig. 3-4, to acknowledge that this mode is engaged. The autopilot will track the selected course segment with maximum authority.


 

 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote HenryM Quote  Post ReplyReply Direct Link To This Post Posted: 14 May 2018 at 5:26am
Thank you for the clarification. As I am still learning the nuances of my system, this is new information for me. I didn't realize I should switch to STEC 30 to HI TRK and disengage GPSS once I cross the FAF, otherwise the guidance is not tight enough as I get close to the runway. I have only done practice approaches in VFR or with relatively high ceilings, so everything seemed to be working if I just left the AP in HDG mode.  
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Catani Quote  Post ReplyReply Direct Link To This Post Posted: 14 May 2018 at 9:07am
I don't know Aspens, and I don't know STec 30s, but generally speaking, this is what I think I know based upon using my STec 55X, Avidyne PFD and Garmin 430, and now IFD440.

A GPS/NAV combo box like an IFD440 or Garmin 430W can navigate using either GPS derived location or VOR/LOC derived location.  As pilot, you get to choose which, just as in the days before combo boxes you could choose whether to have your panel's course indicator accept signals from your KX155 or your King KLN94, depending upon whether you wanted to navigate via VORs or via satellites.  Combo boxes are all the rage now, of course, since you only have to buy one box and they take up less panel space.  Bad news for the King boxes that back in the day ruled GA.

As to which waypoints the early boxes were navigating to, that was also pilot choice, and by turning knobs the pilot could have the boxes navigate from one waypoint on his paper flight plan to the next.  But with the new whiz-bang gizmos that changed GA, like an Apollo GPS or later Garmin 430, it became possible to preload all your flight plan waypoints before takeoff, just like new airliners!  It's already old hat now, but it's worth keeping in mind to help understand what today's boxes are doing in GPSS mode.  Loading an approach is just another automatic (and legal) way of loading the correct waypoints in the correct sequence (as depicted on Jepp or gov't approach charts) in the "flight plan" for an approach.  The navigator part of the box will go from one to another in sequence, using either satellites or ground stations as the pilot directs.  

I think it's key to remember here that whatever the pilot chooses, the GPS part of the box follows right along the flight plan even though the pilot has chosen to navigate by VOR/LOC.  And the VOR/LOC part of the box follows whatever nav radio aid it is tuned to, even though the pilot has chosen to navigate by GPS.  This behavior is just as it was when these capabilities were in separate KX155 and KLN94 boxes.  In either case, the output of the Navigator combo-box, be it right or left of a VOR or GPS course, or high or low on a ILS or LPV glideslope, is fed to a course indicator device on the panel - either a VOR/LOC indicator or a PFD like an Aspen or Entegra or G1000 glass panel. 

As the pilot hand-flying, it's these panel devices you use to stay on course, and what the autopilot uses when in NAV mode.  Unless you are getting vectors, in which case you set the heading bug and put the autopilot in Heading Bug mode so it follows that bug instead of the course indicator.  In GPSS mode (if your autopilot is so equipped, and if your GPS is designed to continuously output heading signals to an autopilot), the autopilot goes into Heading mode and takes heading signals from the GPS and ignores the panel devices.  What's key here is to remember that in GPSS mode, the autopilot is not flying the course indicators on the panel - it's always following the GPS's heading output.

Flying a coupled approach using this "heading bug" GPSS output inside the FAF is illegal, since the autopilot would not be following the final approach course as depicted on the VOR/LOC indicator or PFD. That's why any pilot guide for an autopilot should make clear to the pilot that he must switch to NAV or APR or some other appropriate mode before the FAF, to get the autopilot off that illegal GPSS heading bug mode.  Not to mention the fact that in GPSS mode, the autopilot may not be designed to pick up any glideslope output to follow.  Whatever, it's not legal because the heading bug derived final course is not as accurate as an LPV or ILS derived final approach course.  In GPSS mode you may be right on the heading bug/GPS derived course, but at the same time well right or left of the actual final approach course because of how the "sensitivity" of the output signals from the box change when in final approach course mode.  No doubt somebody more knowledgeable can fill in more accurate technical details here, but all you need to know as a pilot is that GPSS mode inside the FAF is illegal - don't do it.

Also, my understanding is that some autopilots will automatically switch from NAV to APR mode, others the pilot must do that.  If it's not done, the autopilot may not fly the glideslope, and will also fly final with less precision.  And that choice may have some effect on the behavior of the Navigator combo-box - ask somebody more knowledgeable for details.  In any case, all a pilot needs to know is that in GPSS mode, the autopilot is not paying any attention to the panel devices, and will not be switching to APR mode on its own.  And that's bad news.

So there you are, flying an autopilot-coupled GPSS/heading-bug mode on final when the autopilot turns right when 200 feet above the MDA.  What happened?  My guess is, the next waypoint on the approach is off to the right somewhere, or maybe there is no next point and it's going off in some random direction.  Whatever the answer is, it's a mistake - and dangerous if in IMC - to put the autopilot into that circumstance on final in the first place.




Edited by Catani - 14 May 2018 at 9:10am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote nrproces Quote  Post ReplyReply Direct Link To This Post Posted: 14 May 2018 at 10:38am
I have this combination
540/Aspen Pro 1500 system/ With the STC Group A/P.

I don't normally have it fly my approaches, but will give it a shot this week and see what happens.

Sauce
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Kentucky Captain Quote  Post ReplyReply Direct Link To This Post Posted: 14 May 2018 at 1:49pm
Originally posted by Catani Catani wrote:

.......But with the new whiz-bang gizmos that changed GA, like an Apollo GPS or later Garmin 430, it became possible to preload all your flight plan waypoints before takeoff, just like new airliners! 

........If it's not done, the autopilot may not fly the glideslope, and will also fly final with less precision. 


Thanks for the insight.  Two things there struck a chord with me.  Number 2 first.  My STEC-30 has altitude hold but no VNAV.  I might have picked up on the problem earlier if it did, since it wouldn't have captured the glide slop, but maybe not.  I can be thick headed at times.

The first point reminded me of a question from yesterday.  In all the hub-bub of getting ready for the testing, yet again, I inadvertantly not only entered a course in the IFD but I also entered the approach too.  I did not activate it but after taking off, it must have activated itself because that option was no longer available on the FMS page.  In addition, I took off southbound, intending to fly straight out, hit the IAF, do a procedure turn and shoot the LNAV+V approach back to the airport.  The course that the IFD had only went as far as the the IAF outbound.  No PT or approach.  I think I broke it.  I had to delete the approach and re-enter it to get a good setup.  I've never entered the approach while on the ground before and wondered if anyone has seen that behavior?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mfb Quote  Post ReplyReply Direct Link To This Post Posted: 14 May 2018 at 4:37pm
Entering an approach while you're on the ground is not a problem. In fact, it's a good idea because it will reduce your workload in the air. But the IFD suffers from the same curse as every electronic gadget - it will do exactly what you tell it to do (whether it makes sense or not).

What approach transition did you put in the flight plan? If you entered "vectors to final" the IFD would not do a procedure turn because you didn't tell it to do one. It's expecting to intercept the final approach course yourself. It will fly forever until it intercepts the course. And if you intercept going the wrong way the IFD will be very confused.

But, if you tell the IFD to go to an initial approach fix that has a procedure turn, then it will do the procedure turn.

The trick is to load the approach and transition you want, then look at the flight plan and the map representation of the flight plan. Make sure that it shows what you expect it to show. It will clearly indicate if there's a procedure turn or a holding pattern included in the approach. And look at the colors of the course lines on the map. The magenta line is what the IFD thinks it's flying now. The barber pole line is what it wants to do next. Stay aware of these and you won't have surprises.

And there's really no "activating" an approach on the IFD, except for vectors to final. You just load the approach transition you want and fly to a waypoint on the transition. The "activate" thing is a leftover G* concept.

Hope this helps.

Mike
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Kentucky Captain Quote  Post ReplyReply Direct Link To This Post Posted: 14 May 2018 at 6:46pm
Originally posted by mfb mfb wrote:

Entering an approach while you're on the ground is not a problem. In fact, it's a good idea because it will reduce your workload in the air. But the IFD suffers from the same curse as every electronic gadget - it will do exactly what you tell it to do (whether it makes sense or not).

What approach transition did you put in the flight plan? If you entered "vectors to final" the IFD would not do a procedure turn because you didn't tell it to do one. It's expecting to intercept the final approach course yourself. It will fly forever until it intercepts the course. And if you intercept going the wrong way the IFD will be very confused.

But, if you tell the IFD to go to an initial approach fix that has a procedure turn, then it will do the procedure turn.

The trick is to load the approach and transition you want, then look at the flight plan and the map representation of the flight plan. Make sure that it shows what you expect it to show. It will clearly indicate if there's a procedure turn or a holding pattern included in the approach. And look at the colors of the course lines on the map. The magenta line is what the IFD thinks it's flying now. The barber pole line is what it wants to do next. Stay aware of these and you won't have surprises.

And there's really no "activating" an approach on the IFD, except for vectors to final. You just load the approach transition you want and fly to a waypoint on the transition. The "activate" thing is a leftover G* concept.

Hope this helps.

Mike



Well now I'm confused. I entered the KBRY RNAV 3 into the IFD with the initial fix being MAKRZ while I was on the ground at BRY.  I took off of RWY 21 and headed straight out towards MAKRZ (It's Bardstown KY, every fix is either Bourbon or UPS related).  The intent was to go direct MAKRZ, do a teardrop entry and fly the LNAV+V approach back in.

I usually preview my approach in the FMS before switching to the MAP view but was in a hurry and didn't.  After I was airborne and the map started scrolling towards the IAF.  The course line suddenly ended somewhere short of the IAF and didn't show any of the rest of the approach.  I thought that was very strange but dumped the approach and reloaded it and it came up just fine then.  Unfortunately I have enough expeirence with getting very good at doing that quickly.

If I'm not misstaken, any time I enter an approach, I get the ACTIVATE APPROACH bubble on the left side of the screen in the FMS section.  Most of the approaches that I do are direct to the IAF and for GPS approaches so I don't use Vectors To FInal much.

Here is the GPS Rwy 3 approach plate.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote mfb Quote  Post ReplyReply Direct Link To This Post Posted: 14 May 2018 at 7:52pm
I ran the problem in the simulator like this:

1) Entered a flight plan from KBRY to KBRY.
2) Entered the RNAV 3 approach, MAKRZ transition. (It's the RNAV 2 approach in the sim. I must have an old database.)
3) Selected the MAKRZ waypoint and went direct to it.

The course reversal hold came up and the simulated airplane flew the entire procedure, including the hold.

The Activate Approach LSK did come up. I did not use it. I prefer to go direct to a specific waypoint on the approach. The Activate LSK does not always appear and, when it does, I'm not sure what it's going to do. I would rather tell the box exactly what I want to do. In this case, I want to go to MAKRZ.

I don't know why your first attempt didn't work out. But I'm pretty sure that this procedure will always work.

Mike


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Catani Quote  Post ReplyReply Direct Link To This Post Posted: 14 May 2018 at 7:52pm
Originally posted by Kentucky Captain Kentucky Captain wrote:

...I entered the KBRY RNAV 3 into the IFD with the initial fix being MAKRZ while I was on the ground at BRY.  I took off of RWY 21 and headed straight out towards MAKRZ (It's Bardstown KY, every fix is either Bourbon or UPS related).  The intent was to go direct MAKRZ, do a teardrop entry and fly the LNAV+V approach back in....
What was the first waypoint or procedure you entered on the FMS page after the IFD preloaded your departure point KBRY? If you entered anything else, please list them all in order.  What was the last waypoint or procedure you entered into the FMS?  Did you "activate" the flight plan before taking off?  Trying to re-create what your FMS would have looked like before takeoff.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Catani Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2018 at 1:27am
Originally posted by mfb mfb wrote:

I ran the problem in the simulator like this ...
I did as well.  After entering the second KBRY and selecting the approach, the FMS page showed a "connect" LSK.  I did nothing, and the sim never moved.  After waiting, I touched "Connect" LSK and the sim started moving towards MAKRZ from KBRY.  No "Activate Approach" LSK though.

I did it a second time, this time telling the sim to go direct to the second KBRY. Then entered the approach as before.  This time, the sim flew in circles, and then headed off in a random direction.  The FMS page showed "Activate Approach" LSK but no "Connect" LSK, even though there was a gap.

Apparently, the IFD can show an LSK for "Connect" or one for "Activate Approach," but it can't display both at the same time.

The first scenario, the sim had nowhere to go until the gap was connected.  It was at KBRY to start, which was also its destination.  With no other waypoints, it stayed put.  Touching the "Connect" LSK gave it purpose: it headed for MAKRZ.  Not sure why it prioritized the Connect LSK.  I suspect an Activate LSK would have got the sim moving as well.

The second scenario, the only thing different was the direct command.  Not sure why that changed the LSK from Connect to Activate.  

 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Kentucky Captain Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2018 at 2:27am
Originally posted by Catani Catani wrote:

What was the first waypoint or procedure you entered on the FMS page after the IFD preloaded your departure point KBRY? If you entered anything else, please list them all in order.  What was the last waypoint or procedure you entered into the FMS?  Did you "activate" the flight plan before taking off?  Trying to re-create what your FMS would have looked like before takeoff.


I selected -  PROC  -  RWY3 (LNAV+V)  -  MAKRZ. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mfb Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2018 at 9:06am
In cases like this the "connect" and "activate" LSKs are obviously confusing. I don't like them either. They may have uses in other contexts, but not here.

I just know that I want to take off and go direct to the initial approach fix, MAKRZ. So I use the Direct key to do that. It's simple and it works.  Everybody is happy.

That's why I don't like the Activate LSK. It does different things in different contexts. But the Direct key always works. 

I just remember one way of doing what I want to do and I use it every time. That's enough.

Mike




Edited by mfb - 15 May 2018 at 9:06am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote HenryM Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2018 at 11:11am
I went back to read my STEC 30 autopilot and ST 901 GPSS converter manuals, and neither mentions needing to go to Hi Trk mode to shoot an approach. In fact, both show examples of various approaches (LOC and VOR) and use the HDG mode (or GPSS + HDG in the case of the ST 901 manual). They explain the greater authority of Hi Trk vs Low Trk, but when they show examples of using approaches, they don't use those modes.

From what I can tell, the autopilot can use inputs from the HDG indicator (which the GPSS emulates) or the CDI/LDI/VDI. The latter are used when you go to one of the Trk modes. However, the approach examples seem to always rely on the HDG mode of the autopilot.

In my experience, the autopilot keeps the CDI centered very well. The IFD-540 switches to the right mode (say VLOC for an ILS) and shows deviations from the glide path and course. I assume it is indicating information from the NAV radio, not the GPS, when in that mode. Thus as long as the needle is centered, it is on the correct course, regardless of what signal the AP is using to stay there. As long as I monitor the CDI, I'm following the right path and the sensitivity of the needle changes as needed at various points. 

I understand I shouldn't rely blindly on the GPS position, but if the CDI is correct, I can keep going on my merry way. No? 

Is there something I'm missing? Would the GPSS switch that changes the input to the AP from the HDG system to the GPS navigator also affect the IFD-540 and cause it to drive the CDI differently?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Catani Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2018 at 12:27pm
Originally posted by Kentucky Captain Kentucky Captain wrote:

Originally posted by Catani Catani wrote:

What was the first waypoint or procedure you entered on the FMS page after the IFD preloaded your departure point KBRY? If you entered anything else, please list them all in order.  What was the last waypoint or procedure you entered into the FMS?  Did you "activate" the flight plan before taking off?  Trying to re-create what your FMS would have looked like before takeoff.


I selected -  PROC  -  RWY3 (LNAV+V)  -  MAKRZ. 
OK, first question answered.  For the remaining questions, I am going to assume you entered nothing else, the last waypoint or procedure you entered was just the one entry for the approach, and that you did not manually select "Activate Approach" before taking off.

In the sim, you cannot enter an approach if the first fix, KBRY, is entered as an origin, which is what your IFD does when you start it up in the plane. Unless you add KBRY as a waypoint, in the sim you cannot add an approach for KBRY.  So without going out to my plane, I can't duplicate the problem.  It's a mystery.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Catani Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2018 at 12:36pm
Originally posted by HenryM HenryM wrote:

...Thus as long as the needle is centered, it is on the correct course, regardless of what signal the AP is using to stay there. As long as I monitor the CDI, I'm following the right path and the sensitivity of the needle changes as needed at various points. 

I understand I shouldn't rely blindly on the GPS position, but if the CDI is correct, I can keep going on my merry way. No?
If it's keeping the needles centered on an ILS (or even an LPV, but an ILS would make the following true for sure), I doubt the autopilot is remaining in GPSS/HDG mode in order to follow the GPS's heading guidance for GPSS capable autopilots.  Somehow, the autopilot is automatically switching to approach mode (my STec 55X will NOT automatically switch to approach mode from GPSS - it stays in GPSS and does a sloppy job of staying on the final course and can't follow any glidepath).  That must be so since it is following the glideslope, and because of its increased sensitivity to course deviations.  That doesn't happen in GPSS mode. I'd call tech support at the autopilot factory if the manual does not explain that behavior.  In any case, it sounds like you are legal.  Would be good to to understand why and confirm it though.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote HenryM Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2018 at 1:07pm
my STEC 30 AP doesn’t switch anything automatically. It can do three things: keep the airplane stable (a wing leveler that can turn), follow a Heading Input (from a Directional Gyro or the GPSS Converter) or follow the indication of a CDI (with weak or strong authority - Lo Trk or Hi Trk). The modes have to be manually set. 

The GPSS device (an STEC ST-901) connects to the AP through a switch. The switch is on the panel and needs to be manually pressed. It selects between the DG or the GPSS box. I don’t know if the OFD also gets the state of the switch. 

The GPSS device manual describes shooting approaches and in all cases, the AP ends up using HDG mode to follow the path it needs. 

The IFD-540 is connected to the GPSS device and to a Garmin GI-106 CDI that indicates the glide path and the course deviation. 

When shooting an ILS, the IFD will switch to VLOC mode automatically and the GI-106 will provide appropriate indications based on the ILS. 

So far, I’ve been keeping the switch set to GPSS. The AP maneuvers to get on the approach course. Eventually the IFD switches to VLOC mode, and I follow the glide path down manually, with the AP keeping the CDI centered. I do not Switch off GPSS and switch the AP mode to Hi Trk. The latter is what I would use of I didn’t have GPSS and I were following a glide path from a NAV radio. The GPSS seems to do a pretty good job of keeping the CDI centered. 

Kentucky Captain has the Aspen performing the function of my ST-901, along with other functions. In his case, there seems to be a problem inside the FAF, so he has to turn off GPSS and tell the STEC-30 to follow the CDI using Hi Trk mode. 

I want to make sure I’m fully legal and don’t misunderstand what is happening in my plane. I need to understand the IFD-540 operation to make sure it is properly driving the CDI with the required sensitivity. I appreciate the inputs so far. It has been very educational. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Kentucky Captain Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2018 at 1:30pm
Originally posted by HenryM HenryM wrote:

I went back to read my STEC 30 autopilot and ST 901 GPSS converter manuals, and neither mentions needing to go to Hi Trk mode to shoot an approach. In fact, both show examples of various approaches (LOC and VOR) and use the HDG mode (or GPSS + HDG in the case of the ST 901 manual). They explain the greater authority of Hi Trk vs Low Trk, but when they show examples of using approaches, they don't use those modes.

From what I can tell, the autopilot can use inputs from the HDG indicator (which the GPSS emulates) or the CDI/LDI/VDI. The latter are used when you go to one of the Trk modes. However, the approach examples seem to always rely on the HDG mode of the autopilot.

In my experience, the autopilot keeps the CDI centered very well. The IFD-540 switches to the right mode (say VLOC for an ILS) and shows deviations from the glide path and course. I assume it is indicating information from the NAV radio, not the GPS, when in that mode. Thus as long as the needle is centered, it is on the correct course, regardless of what signal the AP is using to stay there. As long as I monitor the CDI, I'm following the right path and the sensitivity of the needle changes as needed at various points. 

I understand I shouldn't rely blindly on the GPS position, but if the CDI is correct, I can keep going on my merry way. No? 

Is there something I'm missing? Would the GPSS switch that changes the input to the AP from the HDG system to the GPS navigator also affect the IFD-540 and cause it to drive the CDI differently?


Thus, my confusion.  Everything I saw or heard from the folks at STEC said to leave it in HD mode.  I was told that Lo TRK and HI TRK were a throw back to an era of VOR approaches and ILS and GPS didn't need anything other than HD mode.  The reality that I experienced was somewhat different witih it toeing the line all the way down to around one mile and then rolling off to the right.

Turning off GPSS steering on the Aspen while the STEC is still in HD mode results in the autopilot following whatever was set in the heading bug of the Aspen.

Setting the STEC to HI TRK and then turning off GPSS steering gives you a straight ride all the way down the final.  All of this after passing the IAF, of course.

All I know is that I'm a happy camper after nearly a year of battling this autopilot issue.  I thought all along that this final issue that I was having, was something left over from when the moron tech fried my panel by plugging a 28-volt GPU into my 14-volt C172, while it was powered up.  It started a cascade of failures in the panel.  For some stupid reason, I allowed the same shop to try to repair their screw-up.  Everything didn't quit at once.  It was a cascade of things over time.  The shop decided that it wasn't their fault and refused to do any more unless I was paying shop rates, so I turned it over to my insurance and went to a reputable shop.

They ended up replacing the Aspen, it's ACU, the IFD540, AXP340, and Garmin GNC255A .  The PSE PMA450 and STEC-30 were sent back to the factory and recertified.  The JPI EDM900 MAP sensor blew up and had to be replaced also.  Not only that but they completely screwed up the installation of the synthetic vision, AOA, and ADS-B unlocks on the Aspen.  It's been almost a year now and I just now feel that the airplane is 100% again.  Thanks go out to the folks at Avidyne and Aspen who fielded so may phone calls and emails from me in the journey to get this bird back to where it was before.

It was easy to assume that this last issue I was having was part of the initial problem but in reality I just hadn't had enough experience with the autopilot thrown into the mix.  I was flying all approaches by hand.

Just to stop the noise, I've had several folks, including Aspen initially tell me that there should be no damage since all of the equipment in question is designed to work at either 14 or 28 volts.  I pointed out that while that may be true, I don't think that applying a 28-volt current to a 14-volt system while it was already powered up is what the manufacturers had in mind.  They all tended to agree.

We had just pushed the airplane into the hanger so that they could install the Skytrax100 and do the IFD 10.1 update.  I remembered that there was a good chance that I would lose all of my configuration settings on the IFD so I told the tech to wait a minute while I powered everything up and get some screen shots.  I was sitting in the airplane with the master switch on and the avionics bus powered up, just snapping pics of the panel, when the engine started turning over on its own.  Scared the crap out of me and I immediately turned the master off.   That's when I saw what the tech had done.  Nearly a year later and all is good....except there is this intermittent issue of the panel mounted power plugs that I had installed with the new panel not working.  Every time I get it to the shop, they work.  Hard to fix intermittent. Someday....


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Post Options Post Options   Thanks (0) Thanks(0)   Quote HenryM Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2018 at 2:29pm
I still don’t understand what is happening with your Aspen commanding a right turn. Where would the next point in the missed approach be? To the right? 

It makes sense that turning off GPSS would make the AP follow the heading bug. The GPSS switch changes from following the GPS signal to following the DG bug. Changing the AP to Hi Trk makes it follow the CDI signal, so that behavior makes sense too. The IFD is driving the CDI in VLOC mode, following the ILS signal. 

When the Aspen is controlling, it should be following the GPSS commands and shouldn’t just get off track like that. It might not be as accurate as the ILS, but it still shouldn’t just turn right like it does. With the GPSS turned on and the AP in HDG mode, it just thinks it has a DG bug telling it how far off the desired track it is. The Aspen is providing that error signal. 

Does the problem happen at one of the approach waypoints where maybe it is changing to a new segment after that point? My system is similar to yours, except that the GPSS is done by an ST-901 instead of the Aspen. The IFD should be sending the same commands to either unit. The Aspen is smarter than the ST-901 and perhaps it is getting ahead of the game too soon. 

Hopefully someone knowledgeable with the GPSS commands can come up with an explanation. I would like to understand the root cause, even if there is a workaround, like changing to follow the CDI instead of the Aspen. 

An experiment you could try is just changing the STEC to Hi Trk mode without messing with GPSS. It should start following the CDI regardless of what the DG says. Then again, in your case the CDI is in the Aspen too, so who knows what its logic is for the CDI output when it thinks it is in GPSS mode. The STEC-30 has no idea what is driving it. It just takes either a bearing ear or from the DG (in HDG mode) or a CDI signal from the NAV (in Lo or Hi Trk). The difference between Lo and Hi Trk is just how quickly it reacts to an error. Lo Trk tries to dampen the scalloping of a VOR signal while Hi Trk tries to follow the signal more quickly. It doesn’t know if the signal is a VOR radial or an ILS. Hi and Lo just determine how fast it reacts to deviations. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Kentucky Captain Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2018 at 6:22pm
Originally posted by HenryM HenryM wrote:



1.  I still don’t understand what is happening with your Aspen commanding a right turn. Where would the next point in the missed approach be? To the right? 

2.  It makes sense that turning off GPSS would make the AP follow the heading bug. The GPSS switch changes from following the GPS signal to following the DG bug. Changing the AP to Hi Trk makes it follow the CDI signal, so that behavior makes sense too. The IFD is driving the CDI in VLOC mode, following the ILS signal. 

When the Aspen is controlling, it should be following the GPSS commands and shouldn’t just get off track like that. It might not be as accurate as the ILS, but it still shouldn’t just turn right like it does. With the GPSS turned on and the AP in HDG mode, it just thinks it has a DG bug telling it how far off the desired track it is. The Aspen is providing that error signal. 

3.  Does the problem happen at one of the approach waypoints where maybe it is changing to a new segment after that point? My system is similar to yours, except that the GPSS is done by an ST-901 instead of the Aspen. The IFD should be sending the same commands to either unit. The Aspen is smarter than the ST-901 and perhaps it is getting ahead of the game too soon. 

4.  Hopefully someone knowledgeable with the GPSS commands can come up with an explanation. I would like to understand the root cause, even if there is a workaround, like changing to follow the CDI instead of the Aspen. 

5.  An experiment you could try is just changing the STEC to Hi Trk mode without messing with GPSS. It should start following the CDI regardless of what the DG says. Then again, in your case the CDI is in the Aspen too, so who knows what its logic is for the CDI output when it thinks it is in GPSS mode. The STEC-30 has no idea what is driving it. It just takes either a bearing ear or from the DG (in HDG mode) or a CDI signal from the NAV (in Lo or Hi Trk). The difference between Lo and Hi Trk is just how quickly it reacts to an error. Lo Trk tries to dampen the scalloping of a VOR signal while Hi Trk tries to follow the signal more quickly. It doesn’t know if the signal is a VOR radial or an ILS. Hi and Lo just determine how fast it reacts to deviations. 


1. I was never able to stay with it that long, only to the threshold.  The view there was not what you want to see at the DA or MAP.  Runway way off to the left.

2. True.  Turn off GPSS mode and the autopilot follows the heading bug on the Aspen.  Real handy in getting vectors to final, around traffic, etc. without losing your flight plan in the IFD.   Once the approach is loaded, you are inside the IAF, and the autopilot is in HI TRK mode, turning GPSS off follows the approach all the way down to minimums like it never has before.

3. No, it only happens around the one mile final mark and a couple hundred feet above DA.  My understanding from Avidyne is that there is more than one path from the IFD to the ACU and depending on what mode you are in, what path it takes.

4.  If T.J. Sutton from Avidyne would chime in here, he could explain it a lot better than I can. He is the one that figured out what was happening and its resolution.

5.  I asked that very question of T.J. and without trying to find the email to quote, he said something like it would probably produce similar results but the presentation that I would be seeing would be GPS derived instead of navaid based,  I think.  I do plan on testing that configuration out on my next flight.

It takes a little more planning when trying to get someone to fly with me to test this out.  Being retired is great but it seems that all the people around me are always working.  I'm not fond of having that much head down time in the cockpit while flying around VFR by myself.  I much prefer having someone with me to keep a lookout for other aircraft and make sure I don't do something stupid while wrapping my head around what's happening on the panel.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Catani Quote  Post ReplyReply Direct Link To This Post Posted: 16 May 2018 at 12:04am
Originally posted by HenryM HenryM wrote:

The GPSS device manual describes shooting approaches and in all cases, the AP ends up using HDG mode to follow the path it needs...
I read the manual, and agree that is what is says, but it is wrong.  Read AIM 1-2-3, paragraph c, Notes 2, 4 and 5.  It's clear you cannot use VOR, NDB, or LOC navaids for final approach guidance unless the approach has "GPS" in the title.

Originally posted by HenryM HenryM wrote:

I do not Switch off GPSS and switch the AP mode to Hi Trk.
I think that's illegal inside the FAF.

Originally posted by HenryM HenryM wrote:

The latter is what I would use of I didn’t have GPSS and I were following a glide path from a NAV radio.
I think you should be using the Hi Trk mode of your autopilot inside the FAF.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Catani Quote  Post ReplyReply Direct Link To This Post Posted: 16 May 2018 at 12:13am
Originally posted by Kentucky Captain Kentucky Captain wrote:

...Everything I saw or heard from the folks at STEC said to leave it in HD mode.  I was told that Lo TRK and HI TRK were a throw back to an era of VOR approaches and ILS and GPS didn't need anything other than HD mode...
Ask an instrument DER or a local experienced CFII what they think about that claim.  Or just read AIM 1-2-3 to them.  If it's illegal for you to handfly inside the FAF following GPS course guidance, I don't see how it could be legal allowing the autopilot to do it.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Kentucky Captain Quote  Post ReplyReply Direct Link To This Post Posted: 16 May 2018 at 12:13am
Originally posted by Catani Catani wrote:

Originally posted by HenryM HenryM wrote:

The GPSS device manual describes shooting approaches and in all cases, the AP ends up using HDG mode to follow the path it needs...
I read the manual, and agree that is what is says, but it is wrong.  Read AIM 1-2-3, paragraph c, Notes 2, 4 and 5.  It's clear you cannot use VOR, NDB, or LOC navaids for final approach guidance unless the approach has "GPS" in the title.

Originally posted by HenryM HenryM wrote:

I do not Switch off GPSS and switch the AP mode to Hi Trk.
I think that's illegal inside the FAF.

Originally posted by HenryM HenryM wrote:

The latter is what I would use of I didn’t have GPSS and I were following a glide path from a NAV radio.
I think you should be using the Hi Trk mode of your autopilot inside the FAF.


Well, that's comforting, non-conflicting opinions about the proper operation.  I've been getting whiplash from different viewpoints or just no information at all.  Aspen has been of little help here and STEC has been no help.  All of the correct procedure information to get my problems solved has come from Avidyne.

I was just a couple of pages over from here when I got notice that you had made a comment.  I was about to get my iPad out with the sim and look at the procedure turn debate going on and the best way to bypass it.  I hate it when you think you know something until everyone starts talking about it.  Back to the books.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Kentucky Captain Quote  Post ReplyReply Direct Link To This Post Posted: 16 May 2018 at 12:16am
Originally posted by Catani Catani wrote:

Originally posted by Kentucky Captain Kentucky Captain wrote:

...Everything I saw or heard from the folks at STEC said to leave it in HD mode.  I was told that Lo TRK and HI TRK were a throw back to an era of VOR approaches and ILS and GPS didn't need anything other than HD mode...
Ask an instrument DER or a local experienced CFII what they think about that claim.  Or just read AIM 1-2-3 to them.  If it's illegal for you to handfly inside the FAF following GPS course guidance, I don't see how it could be legal allowing the autopilot to do it.


After Avidyne used small words to explain how it works, I totally agree.  I was just repeating what I was told when I first took delivery of the new panel.  No wonder I have been so screwed up.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote HenryM Quote  Post ReplyReply Direct Link To This Post Posted: 16 May 2018 at 1:00pm
Originally posted by Catani Catani wrote:

I think you should be using the Hi Trk mode of your autopilot inside the FAF.

I follow the CDI down, handflying the glide path. The CDI is being driven by the IFD-540 in VLOC mode, thus the ILS signal. The AP is connected in HDG mode but I make sure the CDI is centered. Thus, the track being followed is that of the ILS, no? I would take action should there be a deviation. Switching to Hi Trk would make the CDI error be the driving signal for the AP, but it seems leaving the AP in HDG mode should still be ok as long as the CDI remains centered. 

The AP is assisting me, but I am making sure the ILS path is followed. Thus, the lateral guidance is really the CDI I’m monitoring. Vertical guidance is the glide path, since the STEC-30 doesn’t provide altitude navigation. I can’t blindly let the AP fly the approach because of this. 

Is there an FAR that is being broken? The AIM is not regulatory. 


Edited by HenryM - 16 May 2018 at 1:01pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Cruiser Quote  Post ReplyReply Direct Link To This Post Posted: 16 May 2018 at 3:45pm
A couple of things I would like to hear clarified.
1. As far as I know Aspen does not "command" anything. The GPSS signal processor is in the Aspen PFD but it needs an external signal for navigation data.
2. I use GPS inside the FAF all the time. I think what is meant above is that using the GPS as a heading source and flying the heading instead of the approach course is not approved.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote chflyer Quote  Post ReplyReply Direct Link To This Post Posted: 16 May 2018 at 6:04pm
Originally posted by HenryM HenryM wrote:

Originally posted by Catani Catani wrote:

I think you should be using the Hi Trk mode of your autopilot inside the FAF.

I follow the CDI down, handflying the glide path. The CDI is being driven by the IFD-540 in VLOC mode, thus the ILS signal. The AP is connected in HDG mode but I make sure the CDI is centered. Thus, the track being followed is that of the ILS, no? I would take action should there be a deviation. Switching to Hi Trk would make the CDI error be the driving signal for the AP, but it seems leaving the AP in HDG mode should still be ok as long as the CDI remains centered. 

The AP is assisting me, but I am making sure the ILS path is followed. Thus, the lateral guidance is really the CDI I’m monitoring. Vertical guidance is the glide path, since the STEC-30 doesn’t provide altitude navigation. I can’t blindly let the AP fly the approach because of this. 

Is there an FAR that is being broken? The AIM is not regulatory. 

I agree with Catani, but find it an interesting question. I also have an STEC-30 AP, along with the STEC GPSS converter. I assume you also have the GPSS converter since you indicate the AP is in HD mode (there is no HDG mode on the STEC-30)?

If you have the AP in HD mode, then the GPSS switch determines if the AP takes guidance from the heading bug on your HSI or DG (HDG light on the GPSS switch) or the GPS/IFD540 (GPSS light on the GPSS switch). Any other AP mode (e.g. TRK LO or HI) will decouple the AP from the GPSS function.

As long as you have the AP in HD mode and the GPSS converter switch in GPSS mode, then the AP is tracking the GPS roll steering data being received from the IFD540, not the ILS/LOC AC/DC heading error. In the STEC-30 TRK LO & HI modes the AP uses the IFD540 VOR/ILS/LOC AC/DC heading error signals. The TRK LO mode is meant for VOR signals and TRK HI mode is meant for ILS/LOC signals. The difference is how the AP handles deviations as VOR error signals tend to wander back and forth a bit so in the LO mode the AP doesn't react as quickly as in HI mode where immediate reaction is needed to the more precise/stable signal.

Whether you use the STEC-30 in HD mode with GPSS or in TRK HI mode (without GPSS) for an ILS/LOC approach is your call, but in the former case the AP will be following IFD540 GPS roll steering instructions, not the ILS/LOC heading correction signals. If the IFD is showing VLOC for the approach, then the CDI is indicating the actual ILS/LOC deviations regardless of which signal the AP is configured to track.

The obvious question is "What would you do if the ILS/LOC-driven CDI starts to show (lateral) deviations while the AP is engaged, probably because the AP is tracking GPS corrections rather than the ILS/LOC heading error signals?"


Edited by chflyer - 16 May 2018 at 6:05pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Catani Quote  Post ReplyReply Direct Link To This Post Posted: 16 May 2018 at 6:14pm
Originally posted by HenryM HenryM wrote:

I follow the CDI down, handflying the glide path. The CDI is being driven by the IFD-540 in VLOC mode, thus the ILS signal. The AP is connected in HDG mode but I make sure the CDI is centered....

Is there an FAR that is being broken? The AIM is not regulatory. 
You have the option of coupling the autopilot to the localizer, but choose not to do so for reasons I don't get.  Leaving that aside, as I said previously, I don't see how if it's illegal to hand fly a GPS course, I don't see why it would be OK letting an autopilot do it. 

Apparently for this approach, you have flown it and learned that the GPS waypoints and course line are close enough to the localizer to satisfy you.  That will not be the case for many approaches, and you won't find out until you are off course at minimums.  I think it's bad technique, personally, as well as unsafe and illegal.

You hear "the AIM is not regulatory" a lot. But violations of AIM are routinely held to constitute a violation of 91.13, and often others as well.  Generally it's not a good sign for a pilot to be claiming innocence for ignoring the AIM on the grounds it's not regulatory.

I think in your specific case, where you are flying an approach you are familiar with and know from prior experience the GPS course matches the LOC course, which you are monitoring, you would probably be fine if questioned, so long as you indicated it was only for that approach.  I would not say that for other approaches.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote HenryM Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2018 at 12:03am
i appreciate all the responses. Mainly, I want to understand what is going on, not argue for a specific way of doing things. I was flying the approaches using HD mode, because that is what is shown in the STEC manuals, both for the STEC-30 and the ST-901 GPSS. Thanks to this thread, I have a much better understanding of what is going on behind the scenes and will change my habits. 

My understanding of the system is exactly like Vince (chflyer) describes. Before this thread, I would have turned off the AP if I found the CDI deviating. I would not have trusted the AP if I saw it do something I wasn’t expecting. After today, for ILS/LOC approaches, I will put it in HiTrk mode after the FAF as a matter of habit. That is the track I really want to follow when shooting an ILS. In reality, I mostly fly RNAV approaches, which means leaving the AP in HD mode. 

I don’t have a reason for insisting to keep the AP in HD mode for an ILS approach. However, as long as I am following the CDI for the approach, I don’t think I am doing anything illegal either. I am, however, courting a problem that may or may not show up. No good reason to do that going forward. I have no idea how prevalent is the problem of the GPS course not lining up with an ILS/LOC. Most of the airports I use do have RNAV approaches, and they tend to be well aligned with the ILS approaches. I’ll need to search for an approach that doesn’t say (or GPS) to explore what happens. In any case, I can see more clearly now what is going on. 

With my current understanding, I don’t think I want to switch off GPSS, though. If I have to miss the approach, I would want to go back to HD mode on the AP to fly the missed approach and do whatever else I need. I’d switch to Hi Trk for the ILS/LOC approach and then come back to let the IFD fly the rest. I’d rather just be changing the mode of the autopilot than also be changing the heading source. 

It seems prudent to keep the DG heading bug aligned with the lubber line of the DG if you turn off GPSS. Otherwise, you will get an unexpected turn before you switch to Hi Trk mode. I think leaving GPSS on is better, unless you are fully prepared for the switch of heading information driving the AP. If the bug is not set right, you might be in for a surprise. 

The way I see it, switching between GPS and ILS on the STEC-30 is accomplished by changing modes of the AP: HD for GPS (GPSS on), and Lo/HiTrk for using the Nav radio. The GPSS switch determines whether the heading is determined by the DG (really the DG bug) or the GPS. 

For vectors in the middle of my route, i select the given heading with the bug, then turn off GPSS while being vectored. At the end of vectors, I select Direct To whatever waypoint I need to get back to my route, and turn on GPSS again.

I am a newby with the IFD and the autopilot. I am glad Catani, Vince and the rest have been willing to put up with the various scenarios and have been willing to explain the error of my ways! Thanks KC for bringing up the situation to begin with.  


Edited by HenryM - 17 May 2018 at 12:05am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Kentucky Captain Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2018 at 1:58am
Originally posted by chflyer chflyer wrote:

The obvious question is "What would you do if the ILS/LOC-driven CDI starts to show (lateral) deviations while the AP is engaged, probably because the AP is tracking GPS corrections rather than the ILS/LOC heading error signals?"


And that's why I started this whole mess.  The autopilot was driving the airplane to the right of course at one mile even though the Aspen's HSI and LDI were showing it moving from the centerline to right of course.  That was with the STEC in HD mode and the Aspen in GPSS mode.  Flying the approach by switching the STEC to HI TRK and the Aspen off of GPSS mode inside the IAF gives me a really nice approach all the way to the minimums.

You mentioned an STEC GPSS converter and I'm pretty sure that I don't have one because when I was having the panel installed, they mentioned that I didn't need the extra equipment because the Aspen had that already.

One more thing that caught my eye in this thread; all the talk has been about going to HI TRK and off of GPSS when doing an ILS approach.  My understanding from Avidyne was that I was to do the same procedure for a GPS approach too.  My testing bears that out because with GPSS in  and HD mode on, a GPS approach ended with the same dismal results as an ILS.  GPSS off and HI TRK mode on, the autopilot flown GPS is right on the money.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Kentucky Captain Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2018 at 2:16am
Originally posted by HenryM HenryM wrote:

I am a newby with the IFD and the autopilot. I am glad Catani, Vince and the rest have been willing to put up with the various scenarios and have been willing to explain the error of my ways! Thanks KC for bringing up the situation to begin with.  


I've had this panel for almost three years and all was good as long as I wasn't using the autopilot on approaches.  I could never get any satisfaction about the proper way to configure for an approach in that entire time.  Actually, it was worse that that.  I kept getting confilcting information.  I am certainly grateful for all of the insight here to finally be able to peek beneath the covers and find out how things really work.

Trying to do that and test an almost completely replaced panel after what was inflicted on it by a 2nd day on the job avionics tech has been most painful.  Add to that, having the airplane down so long and a medical condition that kept me out of the cockpit for many months creates an environment that made me uncomfortable.

I've been trying to get caught up and am now current and getting close to feeling proficient again.  I also feel that I can now trust my avionics again.  WooHoo!

How many approaches does it take to feel proficeint?  So far, eighteen.  Being away that long also screws with your buttonology.  I knew that I needed the work when on one of my first flights back, I kept trying to get GPS1 to come alive on the Aspen while sitting on the ramp.  Took me a while to realize that I hadn't put in a flight plan on the IFD and without a flight plan, no GPS1 on the Aspen!. Duh! Makes you feel kind of stupid.  I'm feeling much better now.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote HenryM Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2018 at 3:52am
To summarize again, here is my understanding of the system:

1.) The STEC 30 can do two things: turn the plane and hold altitude. The latter is not relevant to our discussion.

2.) The STEC 30 can keep the wings level on its own (ST mode) or turn towards a desired track (all the other modes)

3.) To go to a desired track, the STEC-30 uses one of two inputs: a heading reference error from a heading system (DG or GPSS from a GPS navigator) or a CDI deviation error.

4.) The selection of which error signal is used is made by selecting HD mode for heading or Hi/LoTrk for the CDI.

4.1) The difference between LoTrk and HiTrk is just how aggresively the STEC-30 turns when it detects a deviation in the CDI signal. LoTrk is meant for VOR signals that are not as steady as an ILS signal.

5.) The STEC-30 has no tracking smarts beyond selecting which error signal to watch in order to command a turn.

6.) GPSS is a system to turn digital commands from a GPS navigator into heading errors that are fed to the STEC-30 HD input. The STEC-30 will command a turn as needed to zero this error. It does not know whether the error is indicated by a DG or a GPSS system.

7.) The GPSS on/off switch, whether external in my case or internal in the Aspen, selects where the heading error signal comes from. This source is either a DG or the converted signal from the GPS navigator.

8.) The GPS navigator drives the CDI whether it is using a VOR/ILS signal or a GPS signal. It knows how to make the deviation error indications more sensitive depending on the phase of flight. Thus, the need to be in HiTrk when shooting an approach (ILS or GPS). The DG error signal does not change sensitivity.

9.) The GPS also provides digital commands related to heading indication. A GPSS system is needed to convert this signal to analog signals that emulate a DG with heading bug.

10.) In the approaches I've shot, the GPS track has closely matched the ILS track. The GPS track fed through the GPSS and the STEC-30 in HD mode has closely matched the desired track, such that the CDI has remained centered. This is not always the case, though. To be safe in an approach, you should have the CDI input driving the STEC-30 rather than the HD input. The reason for this is that the CDI input will provide more sensitive error indications about where exactly the plane is. 

11.) The Aspen unit has GPSS built in. It does not need an extra GPSS box. It switches the signal used to drive the autopilot between its DG and the GPS signal as described above. The CDI output is separate. The STEC-30 selects which one to use depending on the mode selected in the STEC-30 (HD or Lo/HiTrk).

I still am unclear about the following:

1.) If I switch GPSS on (in my case a panel switch for the ST-901 GPSS box), does it make the Avidyne IFD-540 drive the CDI differently than when GPSS is off? I believe the Avidyne is still using the NAV radio signal when shooting an ILS or tracking a VOR, but I am not sure. I also don't know if the sensitivity of the CDI indication is changed as I get closer to the runway, just like it would with the GPSS switch off.

In other words, if I leave GPSS turned on, but switch the STEC-30 to HiTrk, will the CDI signal that it uses in that mode be more sensitive as I get closer to the runway?

2.) Why would the Aspen driving the HD input of the STEC-30 all of a sudden get a left of track error indication on its HDG output?

Assuming there is nothing wrong in the programming or anything else in the Aspen, the turn command would only happen if suddenly the GPSS were turned off, and the Aspen suddenly output a heading error because the heading bug was not aligned with the actual heading. I suppose if the Aspen decided a new heading was needed at the MAP and set it up early, thinking the AP was connected to the CDI, then a sudden turn could happen too. No matter what, the Aspen is responsible for putting the right signal at the STEC-30 inputs.

In HiTrk mode, the STEC-30 will be looking at the CDI error output from the Aspen. In HD mode, it is looking at the DG/DG Bug difference error output. In my system, the IFD-540 remains steady as the STEC-30 keeps going straight in HD mode. To me, that shows the IFD is not changing anything a mile from the runway. The Aspen is, though.






Edited by HenryM - 17 May 2018 at 9:00am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oskrypuch Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2018 at 8:55am
"11.) The Avidyne unit has GPSS built in ..."

Presume just a typo, and you mean ASPEN here. There is no GPSS function in the IFD units.

* Orest



Edited by oskrypuch - 17 May 2018 at 9:08am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote HenryM Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2018 at 9:01am
Correct, Orest. It was a mistype. I meant to say Aspen. I corrected my post.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oskrypuch Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2018 at 9:08am
The ILS deviation signals by the IFD are a discrete output. There is an update to the spec for GPS units, where a different output is used to provide BOTH the ILS deviation, and an APV (precision GPS) approach, whichever is active. That is the output that is now normally wired to your autopilot, so you can fly both types of approaches. The autopilot has no idea whether it is flying an ILS or LPV approach, and it reacts identically. The increased sensitivity on an LPV approach is generated digitally by the IFD, to mimic the sensitivity change you get on an ILS, as you near the threshold.

This output does NOT provide the magenta line GPS deviation signals that you use for enroute, and that latter signal is disabled once you pass the FAF. That is why you must switch to an APPR mode on your autopilot, once past the FAF on a precision approach.

* Orest


Edited by oskrypuch - 17 May 2018 at 9:14am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote HenryM Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2018 at 11:11am
The STEC-30 doesn't have an APPR mode. It only knows about a heading source and a CDI. 

There are four selectable settings:
1. ST - Stability mode. Essentially a wing leveler.
2. HD - Heading mode, from the time a DG with heading bug would feed it an error between the heading and the desired track
3. LoTrk - Low authority tracking. Follows the CDI with some damping so it doesn't respond fast to an error. Used to follow a VOR signal.
4. HiTrk - Hi authority tracking. Follows the CDI just like LoTrk, but responds more quickly to any deviation. Used for a LOC or ILS signal.

There is also a separate Altitude Hold function that tries to maintain the pressure altitude recorded when you press an ALT HOLD switch. If the plane is not trimmed properly, it indicates to add UP or DN trim and the pilot must do this manually.

Here's a block diagram:



The ST-901 GPSS box connects only to the HD input of the STEC-30. A panel-mounted switch selects between feeding the deviation from a heading indicator or a deviation derived from the GPS signal. 

Here is a block diagram from the ST-901 manual:


Here are the instructions for flying a GPS approach from the ST-901 GPSS Converter manual:



I do not have an Aspen manual, so I don't know how the STEC-30 is connected to it and the IFD-540. It may be the Aspen provides both the CDI and DG signals or it might be it only provides the DG signal and the CDI is from the IFD-540. 

I was thinking, perhaps naively, that the Aspen had the equivalent of the of the ST-901 inside. In any case, the STEC-30 only knows it is dealing with a HDG Error when it is in HD mode or a CDI deviation when in xTrk mode.

I understand why it is best to switch to HiTrk to correct any CDI deviation. That signal should be more accurate than the HDG Error.

What I don't understand is why a similar electronic setup inside the Aspen would, all of a sudden, command a turn 1 mile from the FAF. The description that Kentucky Captain provided was that the STEC-30 was in HD mode and GPSS was engaged. The Aspen would be providing a heading error as it goes along. Perhaps this error signal is not as sensitive as the CDI, but still, why would it suddenly change to make the AP turn fairly far down the approach path?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote chflyer Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2018 at 12:26pm
Originally posted by HenryM HenryM wrote:


I still am unclear about the following:

1.) If I switch GPSS on (in my case a panel switch for the ST-901 GPSS box), does it make the Avidyne IFD-540 drive the CDI differently than when GPSS is off? I believe the Avidyne is still using the NAV radio signal when shooting an ILS or tracking a VOR, but I am not sure. I also don't know if the sensitivity of the CDI indication is changed as I get closer to the runway, just like it would with the GPSS switch off.

In other words, if I leave GPSS turned on, but switch the STEC-30 to HiTrk, will the CDI signal that it uses in that mode be more sensitive as I get closer to the runway?


I don't have an Aspen, and can't comment on its connectivity and/or behaviour. In the case of a STEC ST-901 GPSS converter, there is a switch that is always active indicating either GPSS or HDG. It is the STEC-30 setting that determines if the GPSS converter comes into play. If the STEC-30 is in any mode other than HD mode (e.g. TRK HI), then the GPSS converter is decoupled and plays no role in AP behaviour, regardless of the setting of the switch (GPSS or HDG).
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Post Options Post Options   Thanks (0) Thanks(0)   Quote chflyer Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2018 at 12:34pm
Originally posted by oskrypuch oskrypuch wrote:

The ILS deviation signals by the IFD are a discrete output. There is an update to the spec for GPS units, where a different output is used to provide BOTH the ILS deviation, and an APV (precision GPS) approach, whichever is active. That is the output that is now normally wired to your autopilot, so you can fly both types of approaches. The autopilot has no idea whether it is flying an ILS or LPV approach, and it reacts identically. The increased sensitivity on an LPV approach is generated digitally by the IFD, to mimic the sensitivity change you get on an ILS, as you near the threshold.

This output does NOT provide the magenta line GPS deviation signals that you use for enroute, and that latter signal is disabled once you pass the FAF. That is why you must switch to an APPR mode on your autopilot, once past the FAF on a precision approach.

* Orest

That would explain and is consistent with Avidyne telling Kentucky Captain that he should switch his STEC-30 from HD (GPSS) mode to TRK HI even for APV approaches. Very useful information !!

BTW, is there any reason NOT to switch to TRK HI as soon as established, rather than waiting for the FAF? I don't see one and would think the earlier the better to keep the workload after the FAF as low as possible.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote HenryM Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2018 at 12:47pm
Vince,

I understand how the STEC-30 switches the sources. My question was about whether the IFD-540 knows that the GPSS is on and somehow drive the CDI signals differently (less sensitive than normal after the FAF) than with GPSS switched off? Like not switching to higher sensitivity when on an approach?

I want to understand in order to determine whether I need to switch off GPSS or if it is enough to switch the STEC-30 to HiTrk inside the FAF.

Theoretically, the STEC-30 could always be in HiTrk mode, but I think GPSS provides smoother fly-by turning instructions as you go from waypoint to waypoint before the approach. 



Edited by HenryM - 17 May 2018 at 12:54pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Kentucky Captain Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2018 at 3:11pm
Originally posted by chflyer chflyer wrote:

That would explain and is consistent with Avidyne telling Kentucky Captain that he should switch his STEC-30 from HD (GPSS) mode to TRK HI even for APV approaches. Very useful information !!

BTW, is there any reason NOT to switch to TRK HI as soon as established, rather than waiting for the FAF? I don't see one and would think the earlier the better to keep the workload after the FAF as low as possible.


Actually, Avidyne told me to switch to HI TRK after the IAF.  In test flying the approach, as soon as I hit the IAF, the IFD went from GPS->LPV to LPV and the LDI and VDI appeared on the Aspen.  At that point, I switched the STEC to HI TRK and turned GPSS off on the Aspen.  The depicted course didn't change and the autopilot stayed on course all the way to the DA.

I haven't had the opportunity to try it out on an ILS.  I like doing this testing in calm air.  With the air so rough lately that you have to put one hand on the overhead and a 20-knot quartering wind, it gets hard to tell what is the airplane and what is the weather.  By the time in the evening when the gale force winds calm down, UPS is cranking up and SDF is the only place close to do an ILS.  They don't appreciate my 90-knot airplane in a 170-knot final.  At least I didn't when I was working it.  I'll wait for the weekend.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote HenryM Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2018 at 3:26pm
I wonder how that would work if you were at DSTL or MAASH. The CDI wouldn’t work so well if you had to make a 90° turn from one of those. The GPSS would make a smoother turn, I think. Once inbound at MARKZ, you’d be OK, I think. 

Edited by HenryM - 17 May 2018 at 3:28pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote paulr Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2018 at 5:25pm
I'm trying to wrap my head around this discussion and apply it to my (ancient) Autocontrol IIIb. Like the STEC30, it doesn't have an approach mode-- it has HDG, LOC, and HI modes though. Normally the AP mode selector stays on HDG and the AP thus flies whatever CDI deviation indications it gets from its nav source. In my particular plane, the AP source is a 2-position switch: NAV1 (which really means "GPSS" or "HDG" modes from the GPSS) or NAV2 (CDI deflection generated by NAV2). 

My flow on an ILS is to use the PSS60-2 in GS mode to capture the glideslope, leave the AP in HDG mode, and leave the GPSS in GPSS mode. I typically disconnect at the FAF and hand fly. I am wondering if I should have been using the HI mode for the reasons described above-- time to give it a try.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oskrypuch Quote  Post ReplyReply Direct Link To This Post Posted: 17 May 2018 at 6:04pm
Originally posted by HenryM HenryM wrote:

The STEC-30 doesn't have an APPR mode. It only knows about a heading source and a CDI. 

There are four selectable settings:
1. ST - Stability mode. Essentially a wing leveler.
2. HD - Heading mode, from the time a DG with heading bug would feed it an error between the heading and the desired track
3. LoTrk - Low authority tracking. Follows the CDI with some damping so it doesn't respond fast to an error. Used to follow a VOR signal.
4. HiTrk - Hi authority tracking. Follows the CDI just like LoTrk, but responds more quickly to any deviation. Used for a LOC or ILS signal.
...

The "HiTrk" setting is typically the equivalent to what other A/P's may call APPR mode. Yes, it provides more sensitivity, but it also enables a pickup of the ILS/APV tracking. Same general comments apply, in that regard.

The ASPEN can select whatever you need to drive the A/P, its heading bug, GPSS (faked out heading bug) based on the magenta line from the IFD, or VLOC signal from IFD or other radio. Any external CDI is driven in parallel, and its presence or absence does not affect the ASPEN switching.

* Orest



Edited by oskrypuch - 17 May 2018 at 10:37pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote HenryM Quote  Post ReplyReply Direct Link To This Post Posted: 18 May 2018 at 2:43am
What could the Aspen be doing a mile from the MAP that would cause the AP to turn? The next waypoint is straight ahead in this case. The only thing I could think of is the heading bug is to the right of the heading, and for some reason the Aspen switches off GPSS. The AP is in HD mode when this happens. Why a change at this point? Is it trying to go to APPROACH mode while the STEC-30 doesn't know to switch to HiTrk automatically? I'd say at this point, Aspen tech support would be the only one that can provide answers.

A separate question, does the IFD-540 know whether GPSS is enabled or not? If it knows, does it change the behavior of the CDI to reflect the GPS course instead of the ILS/LOC. It will take me a week to get back to my plane to try this out.

One last thing. The STEC manuals say to use GPSS to track a GPS approach. HiTrk is used only for Localizer/ILS approaches. Those are the official instructions in the AFM.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote chflyer Quote  Post ReplyReply Direct Link To This Post Posted: 18 May 2018 at 11:44am
When was your STEC/GPSS installed and what is the date on the AFM? Mine was installed back in 2003 and the AFM dates from then too. I'm reasonably certain that was before APV were common. There were no changes to the STEC/GPSS when my IFD was installed, so no updates to the AFM. I plan to do a search to see if STEC/Meggit/Genesys has updated the docs in the meantime. Anyone on the forum know?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Cruiser Quote  Post ReplyReply Direct Link To This Post Posted: 18 May 2018 at 4:26pm
I could be way of base here (easy pickoff) but the Aspen must use a source for the navigation data then apply the GPSS calculations to the output signal to the A/P. 
If the IFD is supplying a route and the Aspen is in HDG mode applying GPSS corrections to that heading data, then there is an interruption in the signal from the IFD (or change of mode) from the flight plan laid in the IFD. Is the approach loaded and activated? is there a break in the sequence to the next waypoint? 
It seems that anytime the NAV source data is interrupted the Aspen is not getting instruction and reverts to the raw heading bug on the HSI. 
Try setting the heading bug to a specific number and see if the autopilot is commanding the turn to that point.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Kentucky Captain Quote  Post ReplyReply Direct Link To This Post Posted: 18 May 2018 at 11:53pm
Originally posted by chflyer chflyer wrote:

When was your STEC/GPSS installed and what is the date on the AFM? Mine was installed back in 2003 and the AFM dates from then too. I'm reasonably certain that was before APV were common. There were no changes to the STEC/GPSS when my IFD was installed, so no updates to the AFM. I plan to do a search to see if STEC/Meggit/Genesys has updated the docs in the meantime. Anyone on the forum know?


My STEC-30 was installed in Feb 2015 along with the rest of my panel.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Kentucky Captain Quote  Post ReplyReply Direct Link To This Post Posted: 18 May 2018 at 11:57pm
Originally posted by Cruiser Cruiser wrote:

I could be way of base here (easy pickoff) but the Aspen must use a source for the navigation data then apply the GPSS calculations to the output signal to the A/P. 
If the IFD is supplying a route and the Aspen is in HDG mode applying GPSS corrections to that heading data, then there is an interruption in the signal from the IFD (or change of mode) from the flight plan laid in the IFD. Is the approach loaded and activated? is there a break in the sequence to the next waypoint? 
It seems that anytime the NAV source data is interrupted the Aspen is not getting instruction and reverts to the raw heading bug on the HSI. 
Try setting the heading bug to a specific number and see if the autopilot is commanding the turn to that point.


In the problem I was having with the deviation at a one mile final, the Aspen was in GPSS mode, the autopilot was in HD mode, and there was no fixes or transitions at that point.  With the Aspen off of GPSS mode, the autopilot in HI TRK mode inside the IAF, the approach was right on the numbers.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote HenryM Quote  Post ReplyReply Direct Link To This Post Posted: 19 May 2018 at 2:45am
Was the heading bug set to the actual course you were flying? As Cruiser suggests, maybe the Aspen is switching the heading source for some reason. 

If you are in HiTrk mode, I’m not sure you need to switch GPSS off.


Edited by HenryM - 19 May 2018 at 2:45am
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