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STEC 55x ILS capture


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Folks,

 

we had a discussion here about the correct way to intercept and capture an ILS on the STEC 55x, as the manual is not very clear on this.

As far as I understand the manual, the procedure is like this:

- On intercept course to the LOC press the NAV button once to arm NAV/APR mode (NOT the APR button).

- The AP should display NAV/APR and ALT at this stage. AP determines by itself it is tracking a LOC? (How?)

- Once established on LOC, the GS will only arm if you are in ALT hold, loc is established and you intercept from full deflection below.

 

I am particularly interested in the function of the NAV and APR buttons on the AP controller. For me, it has worked like that, but some folks tell me that I would have to press the APR button. But the S-Tec manual sais differently. So what is the story here?

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Since you are usually in NAV/GPSS mode in the preliminary stages of an approach, the method I learned is to press APR when established on final. This will activate the ability for the system to capture and track the glide slope. What has always been annoying to me is that when you go from GPSS to ILS, the tracking ability seems to decline and the plane will wander a lot more, especially if there are cross winds.  Proper "help" with the rudder is essential. It never goes full-scale deflection, but it does seem to wander a bit.

On the other hand, the GS capture has always been very smooth for me.

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Just now, Jeff_S said:

Since you are usually in NAV/GPSS mode in the preliminary stages of an approach, the method I learned is to press APR when established on final. This will activate the ability for the system to capture and track the glide slope.

Ok, this I understand perfectly. However, let's say you are on vectors to final. "Fly Heading 120 to intercept the localizer".

For me, in this situation, you'd have to press NAV and HDG simultaneously (plus whatever vertical mode you need) and not APR/HDG. Correct?

 

For some folks who are used to other AP's like the Kings or even Airbus/MDC/Boeing types this is very confusing as APR is usually the mode for both loc and gs.

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Folks,
 
we had a discussion here about the correct way to intercept and capture an ILS on the STEC 55x, as the manual is not very clear on this.
As far as I understand the manual, the procedure is like this:
- On intercept course to the LOC press the NAV button once to arm NAV/APR mode (NOT the APR button).
- The AP should display NAV/APR and ALT at this stage. AP determines by itself it is tracking a LOC? (How?)
- Once established on LOC, the GS will only arm if you are in ALT hold, loc is established and you intercept from full deflection below.
 
I am particularly interested in the function of the NAV and APR buttons on the AP controller. For me, it has worked like that, but some folks tell me that I would have to press the APR button. But the S-Tec manual sais differently. So what is the story here?


I read the 55X manual and it was an interesting read. I have the 60-2 and I don’t have the APR button. To fly a precision approach, I am in HDG and ALT mode and all that I need to do is hit the NAV button for the glideslope intercept.

What it appears to me is that the APR button on the 55X is only used on a precision approach to able or disable the glideslope if you need or want to. On a VOR approach though it appears the APR is used to activate the approach.

It sounds like you are using it correctly. Just like you, other than the management of the glideslope, I have no idea why the APR button exists.


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The APR button (also) increases the sensitivity of the AP. I.e., it make corrections more frequently. Try it someday while cruising. dial in a VOR to fly to...fly it with NAV for a few minutes, then press APR. You'll notice much more jerking around after the APR press.

I have always pressed the APR key when closing in on the localizer but before I get on the localizer. Don't recall it behaving in a counterintuitive way.

Robert

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5 hours ago, Urs_Wildermuth said:

Ok, this I understand perfectly. However, let's say you are on vectors to final. "Fly Heading 120 to intercept the localizer".

For me, in this situation, you'd have to press NAV and HDG simultaneously (plus whatever vertical mode you need) and not APR/HDG. Correct?

 

For some folks who are used to other AP's like the Kings or even Airbus/MDC/Boeing types this is very confusing as APR is usually the mode for both loc and gs.

If you’re on VTF, then you are already in heading mode (assuming you’re using the heading bug to follow ATC guidance). Once you get established on the localizer, you should just be able to press the APR button and it will determine the best angle to use, capture the localizer and then the glideslope. If you’re hand-flying the vectors, then yes, perhaps you’ll need to press NAV first and then APR. I don’t hand fly vectors cuz there are so many other things I’m taking care of in this phase of flight.

I’ve never actually used the feature, but as I understand it, you would press HDG and NAV simultaneously if you wanted the system to keep you on your current heading until intercepting the nav course, which I guess would be useful if you were way beyond full-scale deflection and needed to keep that same heading for quite awhile before getting back on Nav course. I guess that situation has just not come up in my flying.

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I've just re-read the Stec 55x manual, and it implies that if you are on a localizer/ILS that the system will automatically go into APR mode.  (I was used to hitting the APR button)

Checking the wiring diagrams, there is a LOC (enable) line, where the nav radio tells the AP that the NAV is sending a LOC signal, so the 55X goes into APR  mode automatically when sensing LOC.    (This answers your "how" question) 

I have the G1000 installation, and I don't remember if it does that sequence automatically,  I will see the next time I'm up.    The G1000 wiring diagram does indicate this line is also connected.  If your installation is not going directly into APR mode from NAV we would have to consult the wiring diagram for your radio. 

For the VTF -> Localizer intercept (outside of 50% deviation) holding HDG while pressing either NAV or APR should keep the current intercept angle and arm NAV capture.  

I'll also check to see if the "LOC" line is enabled for RNAV approaches.    That would be another place where you want to tell the AP to go into the APR sensitivity +GS capture mode. 

Edited by PaulM
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9 hours ago, Robert C. said:

I flew 3 LOC and 2 ILS approaches today as I was calibrating my WAAS installation.

Paul is correct that it automatically goes into APR mode when in NAV mode both for ILS and for LOC. Like Paul I have the G1000 so YMMV.

Robert

Robert, I'm curious what "calibrating my WAAS installation" means in this context. I don't remember doing anything like that when I had my WAAS update done this year, nor any instructions for it. Is that something you just did for fun?

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Robert and I had the 55X sent back to Genesys for the latest hardware mods, and when it comes back the roll centering zero will be moved by their bench testing.  So you just need to adjust the roll centering to match your gyro.   So it wasn't the WAAS part.. it was the AP part of the work. 

Robert's was further out than mine, but I still have to do a flight and fine-tune the roll centering. 

 

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Robert and I had the 55X sent back to Genesys for the latest hardware mods, and when it comes back the roll centering zero will be moved by their bench testing.  So you just need to adjust the roll centering to match your gyro.   So it wasn't the WAAS part.. it was the AP part of the work. 
Robert's was further out than mine, but I still have to do a flight and fine-tune the roll centering. 
 


Can you explain more about this process? I never heard the term “roll centering zero”. Curious what you are actually zeroing.


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51 minutes ago, Marauder said:

Can you explain more about this process? I never heard the term “roll centering zero”. Curious what you are actually zeroing.

See the STEC Service manual section 2 "Roll Centering"

http://www.aeroelectric.com/Installation_Data/S-Tec/S-TecServiceManual.pdf

If the autopilot is not tracking the center of the heading bug or nav deviation line there is an adjustment via the front panel.

From my experience  the rate based turn stabilization from the Turn Coordinator can have a little bias in the signal, without the roll center adjustment the PID loop fights between the TC's biased signal (turn, turn) and the NAV deviation (go the other way) and the two signals cancel each other out at some level of NAV deviation.  So it flies straight and level 1/2 a dot out (for example) 

The roll center adjustment is an attempt to remove the TC bias.  In my installation Genesys said that my RC adjustment was at max... and that meant that the TC was out of spec not the AP.  The Avionics shop was able to bring the TC output within bench spec, but it is still just a little off center in flight, so I need to tweak that. 

 

The full history?  When I bought the M20M 4 years ago I noticed on the flight home that the AP tracked the nav signal (NAV NAV GPSS) a bit to the left(1/2 dot).  It would obviously overshoot right turns... and then PID loop back to the 1/2 dot out..   I tweaked the RC adjustment and it tracked everything straight. 
One time I decided to run the entire preflight check (I was having pitch oscillation issues) , and when you test the CWS the yoke should stay still, but mine would go to  full lock (left, right I don't remember which)     Now I don't normally use CWS.. HDG +NAV tracking seemed perfect, but since the plane would be down for this WAAS update and the display backlight was broken we sent it out for repair.    Seems that the RC adjustment isn't used in CWS mode... I don't quite get why... but that showed that the signal from the TC was out of spec & that my RC adjustment was at full limit.  

So,   Hit the CWS button on the ground (55X preflight item #11-#14) , and if the yoke doesn't remain steady, you may also have an out of adjustment TC. 

 

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See the STEC Service manual section 2 "Roll Centering"
http://www.aeroelectric.com/Installation_Data/S-Tec/S-TecServiceManual.pdf
If the autopilot is not tracking the center of the heading bug or nav deviation line there is an adjustment via the front panel.
From my experience  the rate based turn stabilization from the Turn Coordinator can have a little bias in the signal, without the roll center adjustment the PID loop fights between the TC's biased signal (turn, turn) and the NAV deviation (go the other way) and the two signals cancel each other out at some level of NAV deviation.  So it flies straight and level 1/2 a dot out (for example) 
The roll center adjustment is an attempt to remove the TC bias.  In my installation Genesys said that my RC adjustment was at max... and that meant that the TC was out of spec not the AP.  The Avionics shop was able to bring the TC output within bench spec, but it is still just a little off center in flight, so I need to tweak that. 
 
The full history?  When I bought the M20M 4 years ago I noticed on the flight home that the AP tracked the nav signal (NAV NAV GPSS) a bit to the left(1/2 dot).  It would obviously overshoot right turns... and then PID loop back to the 1/2 dot out..   I tweaked the RC adjustment and it tracked everything straight. 
One time I decided to run the entire preflight check (I was having pitch oscillation issues) , and when you test the CWS the yoke should stay still, but mine would go to  full lock (left, right I don't remember which)     Now I don't normally use CWS.. HDG +NAV tracking seemed perfect, but since the plane would be down for this WAAS update and the display backlight was broken we sent it out for repair.    Seems that the RC adjustment isn't used in CWS mode... I don't quite get why... but that showed that the signal from the TC was out of spec & that my RC adjustment was at full limit.  
So,   Hit the CWS button on the ground (55X preflight item #11-#14) , and if the yoke doesn't remain steady, you may also have an out of adjustment TC. 
 


Thanks for providing a detailed response. Guess I have been fortunate that I never needed to have this done either when I had steam gauges or with my glass panel.

You bring up a good point on the preflight checks that STEC has in their pilot’s manual. If I know I’ll be in the soup for a flight and intend on using the AP, I’ll walk though the preflight tests before the flight. They are a good indicator of the health of the AP.


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18 hours ago, PaulM said:

See the STEC Service manual section 2 "Roll Centering"

http://www.aeroelectric.com/Installation_Data/S-Tec/S-TecServiceManual.pdf

If the autopilot is not tracking the center of the heading bug or nav deviation line there is an adjustment via the front panel.

From my experience  the rate based turn stabilization from the Turn Coordinator can have a little bias in the signal, without the roll center adjustment the PID loop fights between the TC's biased signal (turn, turn) and the NAV deviation (go the other way) and the two signals cancel each other out at some level of NAV deviation.  So it flies straight and level 1/2 a dot out (for example) 

The roll center adjustment is an attempt to remove the TC bias.  In my installation Genesys said that my RC adjustment was at max... and that meant that the TC was out of spec not the AP.  The Avionics shop was able to bring the TC output within bench spec, but it is still just a little off center in flight, so I need to tweak that. 

 

The full history?  When I bought the M20M 4 years ago I noticed on the flight home that the AP tracked the nav signal (NAV NAV GPSS) a bit to the left(1/2 dot).  It would obviously overshoot right turns... and then PID loop back to the 1/2 dot out..   I tweaked the RC adjustment and it tracked everything straight. 
One time I decided to run the entire preflight check (I was having pitch oscillation issues) , and when you test the CWS the yoke should stay still, but mine would go to  full lock (left, right I don't remember which)     Now I don't normally use CWS.. HDG +NAV tracking seemed perfect, but since the plane would be down for this WAAS update and the display backlight was broken we sent it out for repair.    Seems that the RC adjustment isn't used in CWS mode... I don't quite get why... but that showed that the signal from the TC was out of spec & that my RC adjustment was at full limit.  

So,   Hit the CWS button on the ground (55X preflight item #11-#14) , and if the yoke doesn't remain steady, you may also have an out of adjustment TC. 

 

So Paul, you sound very knowledgeable in this regard, let me post another question and I'll do it in this thread instead of via PM in case it interests others. My A/P has no trouble tracking the centerline in cruise flight. But when in GPSS mode, I've always noticed (even before the WAAS upgrade) that it is slow to begin the leading turn to stay exactly on course when making turns. This is especially noticeable at cruise speeds, less so when you have slowed to do an approach, but it is ALWAYS outside the turn path and then has to correct back in. However, if I just use the heading bug it commands a standard-rate turn immediately and I can easily get it to stay on course even at cruise speeds.

My avionics shop showed me how to get to the G1000 system adjustment and we massaged that a little bit to see if it would help, but it hasn't really.  It's not a big deal and I never get more than a dot or two of deflection, but I've been wondering if this is something that can be fixed and what the cost would be.  I'm willing to live with a one-dot deflection if the cure is too many $AUs!  I'd appreciate your thoughts. Thanks!

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I am not sure if this is the same issue but when I got my STEC back from overhaul after the WAAS upgrade I noticed GPSS was tracking to the left of intended path whereas NAV was true.  With SVT it was obvious.  I made a tool that could reach the potentiometer and dialed it in.

Same issue?

Russ

32FD891C-F2C8-47A2-B5B9-3CBC1D9D90A9.jpeg

646EF9D7-FC9D-48C6-9804-3D2EA5B50C5B.jpeg

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Hello Paul and Robert,

 

On ‎29‎.‎11‎.‎2017 at 12:04 AM, PaulM said:

Checking the wiring diagrams, there is a LOC (enable) line, where the nav radio tells the AP that the NAV is sending a LOC signal, so the 55X goes into APR  mode automatically when sensing LOC.

thanks. This confirms what I read out of the manual.

So the sequence is NAV / ALT Buttons for the LOC intercept which automatically arms APR. That is what I thought the manual said but it's highly non standard as opposed to a bit less pedestrian AP's. That is why some people get it wrong when they use it.

Thanks again, very helpful.

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On 11/30/2017 at 8:33 PM, Txbyker said:

I am not sure if this is the same issue but when I got my STEC back from overhaul after the WAAS upgrade I noticed GPSS was tracking to the left of intended path whereas NAV was true.  With SVT it was obvious.  I made a tool that could reach the potentiometer and dialed it in.

Same issue?

Russ

32FD891C-F2C8-47A2-B5B9-3CBC1D9D90A9.jpeg

646EF9D7-FC9D-48C6-9804-3D2EA5B50C5B.jpeg

Russ, did you do that while in GPS mode, or during an ILS approach? Am asking because my avionics shop said to do it during an ILS or LOC appr where I could visually track the runway.

Going out again tomorrow to try and adjust out the remaining offset.

Robert

 

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1 minute ago, Robert C. said:

Russ, did you do that while in GPS mode, or during an ILS approach? Am asking because my avionics shop said to do it during an ILS or LOC appr where I could visually track the runway.

Looks like 8000'.. Magenta indicator with "GPS ENR" ..   My recommendation is to zoom in on the magenta line while adjusting. (same tech as the SVT boxes) .  The issue isn't the G1000.. it knows you aren't on course.. its the 55X holding an offset.   

 I'll be over in ABE tomorrow morning. 

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10 hours ago, Robert C. said:

Russ, did you do that while in GPS mode, or during an ILS approach? Am asking because my avionics shop said to do it during an ILS or LOC appr where I could visually track the runway.

Going out again tomorrow to try and adjust out the remaining offset.

Robert

 

Robert,

NAV tracked in the SVT hit boxes whereas GPSS flew offset to the left in my case 250 ft if I recall. I corrected it in level flight, smooth air in GPSS.  The screw is deep behind the faceplate and small flat.  You can take a binder clip or something and sharpen the end.  Very slight touch and wait for correction, then repeat.  The correction takes a few minutes.  Not sure there is enough time to do on approach unless perhaps you corrected a lot on a cruise flight.  If no SVT perhaps zoomed in Forefight or SVT on it. Just my opinion.  Not an A&P.

Russ (Macgyver)

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9 hours ago, Robert C. said:

Russ, did you do that while in GPS mode, or during an ILS approach? Am asking because my avionics shop said to do it during an ILS or LOC appr where I could visually track the runway.

Going out again tomorrow to try and adjust out the remaining offset.

Robert

 

Mine is exactly like Ty's, I also made a tool to set or dial it in, the most effective was is to set a vor signal while in smooth air and a little at a time dial it in until the SVT boxes are centered. Trying it in the gps mode is not as accurate. Mine if off a little right now. Since it's not a big deal this thread will encourage me to set it again.

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40 minutes ago, Danb said:

Mine is exactly like Ty's, I also made a tool to set or dial it in, the most effective was is to set a vor signal while in smooth air and a little at a time dial it in until the SVT boxes are centered. Trying it in the gps mode is not as accurate. Mine if off a little right now. Since it's not a big deal this thread will encourage me to set it again.

Dan, great, I will try VOR.

Russ

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On 11/28/2017 at 6:04 PM, PaulM said:

I'll also check to see if the "LOC" line is enabled for RNAV approaches.    That would be another place where you want to tell the AP to go into the APR sensitivity +GS capture mode. 

I flew a few approaches today.  The 1st RNAV did not switch to APR mode, but I may have not been on a published segment to the FAF.

The second RNAV approach (LPV) enabled APR+GS mode as soon as I hit the NAV button from HDG.

Checked an ILS and it also immediately went into APR+GS when hitting the NAV button.
So, the APR button is only required for VOR approaches and perhaps GPS non Localizer precision.  

I also adjusted the roll centering on the way home. 

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