I was practicing RNAV approaches today in the C172 using TDS GTNXi and the Bendix King auto pilot. I activated the approach in the GTNXi and changed the Bendix from Nav to APR. When i activated the approach the C172 turned towards the approach waypoint, but the 3rd approach waypoint had a lower altitude and the AP in the Bendix never adjusted the altitude lower. I did that manually via the Bendix. GS armed appeared on the Bendix and as i got closer to the runway “GS” was now on the top part of the Bendix display. And then the C172 started to descend while perfectly lined up with the runway. 

My question is, why didn’t the altitude drop automatically when the approach called for it on the third and 4th legs of the approach? Should it? The approach entered in the flight plan had it stepping down from 9,400 to 8,600 to 7800 feet, but it didn’t move even after passing TOD. I figure i am missing something, doing something wrong, or just not understanding how this really works. In any case seeing that runway perfectly lined up and descending spot on the GS was pretty cool. 

Edit: okay i think i know where i went wrong. The altitude in the AP was set to 9,400 and not the minimum of 7,800. Can anyone confirm what should be entered for ALT?

Edited January 21Jan 21 by RobJC

What is the exact approach?  I can try to replicate it.

And is this the marcfsx mod for the C172 found on the TDS website?

Typically support is done via the TDS discord:

https://discord.gg/xnNr6xVf

15 minutes ago, ryanbatc said:

What is the exact approach?  I can try to replicate it.

And is this the marcfsx mod for the C172 found on the TDS website?

Typically support is done via the TDS discord:

https://discord.gg/xnNr6xVf

Hey Ryan i am not sure this is a bug or i don’t fully understand how RNAV works. When i loaded the approach into the TDS it listed lower altitudes for NUTVE and JATOL and i was expecting the AP to automatically descend but nothing happened. Should that happen? If so, what should be entered for ALT in the Bendix? 

The approach i selected was for runway 31 at KCOS. I was flying from KAPA.  The approach was ADANE ZENER NUTVE JATOL. 

Yes it is the marcfsx mod.

Thanks for your help Ryan!

Edited January 21Jan 21 by RobJC

18 minutes ago, RobJC said:

Hey Ryan i am not sure this is a bug or i don’t fully understand how RNAV works. When i loaded the approach into the TDS it listed lower altitudes for NUTVE and JATOL and i was expecting the AP to automatically descend but nothing happened. Should that happen? If so, what should be entered for ALT in the Bendix? 

The approach i selected was for runway 31 at KCOS. I was flying from KAPA.  The approach was ADANE ZENER NUTVE JATOL. 

Yes it is the marcfsx mod.

Thanks for your help Ryan!

Well, the bendix default AP is a terrible for starters.  But, at the IAF, the APR mode should be working.  You could be as low as 9400 from ADANE to ZENER, and then pressing APR mode at ZENER should descend the aircraft.  

So I flew from KAPA to KCOS initially just loading that as the flight plan.  I flew at 11000 (that poor skyhawk haha).  Once airborne I activated the RNAV 31 via ADANE.  Let's assume ATC clears us for the approach at ADANE:  "Skyhawk 12345 cross ADANE at or above 9,400, cleared RNAV runway 31 approach, colorado springs airport"

Great we have our approach clearance, I cross ADANE at 11,000 (because we a) have a cross at or above and b) can easily descend to ZENER at 9400 with a 500fpm descent).  At this point I'm still on NAV and just using VS to descend.  I am not using APR mode yet because we've not reached the published IAF.  You can see the VNAV "banana" predicts us to reach 9400 before ZENER.

Upon crossing ZENER, I activate APR mode (you'll also note the LP+V glide path marker is high which means it's live and we're under the glide path (correct))

I was honestly not sure if this crappy AP would work.... but it does appear to be working!  Once I intercepted the glide path (during an RNAV approach the vertical guidance is called a glide PATH, during an ILS its called a glide SLOPE), the AP commands a descent to follow and we're descending properly as shown.  So it appears to work fine here but don't press APR mode till you're passing the IAF.  The fix ADANE is a transition to the IAP (instrument approach procedure), but ATC will sometimes still give an approach clearance there.  There's actually some debate about if ATC can legally do that - it'll vary from facility to facility in real life.  If you don't get the clearance at ADANE you'll hear it before ZENER with something like "Skyhawk 12345, cleared direct ZENER, cross ZENER at or above 8,600 (or ATC's Minimum Vectoring Altitude MVA or Minimum IFR Altitude MIA, which may be HIGHER), cleared RNAV RY 31 approach, colorado springs airport"

 

 

 

Thanks Ryan! You are awesome. So i messed up by hitting APR too soon? I will try it again. Thanks again for your efforts. I really appreciate it! 

1 minute ago, RobJC said:

Thanks Ryan! You are awesome. So i messed up by hitting APR too soon? I will try it again. Thanks again for your efforts. I really appreciate it! 

Possibly...  sometimes it's a little quirky.  I don't honestly the know the answer to:  Can you press APR after ADANE?  I think the technique used by pilots with an AP is not to engage APR until the IAF (or maybe just as you cross it)

I think you would need to see the LP or LPV glide path (like in my pic 2 there) or on an HSI the glide path markers are pinned to the top of the scale... meaning the GPS is now receiving vertical guidance, I believe the GS flag should also disappear if your instrument has one.  (I think that little up and down black arrow with red background is the GS flag... so imo it should be removed - but instruments vary so I'm not exactly sure)

Also Bruceair's site has some great info on this stuff - it's kinda scattered around though.  I found one of his videos into KCLM, where he shoots an RNAV 26 approach.  His plane is better equipped with a GFC605 and VNAV.  The TDS isn't doing coupled VNAV (autopilot VNAV rather) at the moment.  It does kind of work if you fly a sim addon using the G3X and GTNxi though.  But in his video, I skimmed through it, he seems to engage VNAV from cruise down to the IAF and then I think he uses APR mode after the IAF.  I didn't watch the entire thing though.  That's usually what I do in the sim.  Or I just wing it and do my own math on when to descend, and then engage APR mode basically as I cross the IAF or IF.  And you want to be below the glide path when you do this.  You should be unless ATC left you high or you didn't read the chart correctly.

https://www.youtube.com/watch?v=TsyWaZbHCL4

 

Thanks again Ryan. I will give it another go and see if pressing APR until i reach IAF. What am i looking for in picture 2? I have googled LP+V glidepath marker but i am not getting where it is. Btw i started watching that video. Good stuff. 

11 hours ago, RobJC said:

My question is, why didn’t the altitude drop automatically when the approach called for it on the third and 4th legs of the approach? Should it? The approach entered in the flight plan had it stepping down from 9,400 to 8,600 to 7800 feet, but it didn’t move even after passing TOD. I figure i am missing something, doing something wrong, or just not understanding how this really works.

I think the problem is one of understanding.  With this AP/GPS combo, you will not see any descents until you're at whatever point you intercept the approach glidepath - assuming, of course, that you properly intercept it from below.

The way I would normally fly this approach (assuming I'm assigned the full approach) would be to descend in VS mode at each subsequent fix - down to 9400 once I've passed ADANE, then VS to 8600 once past ZENER, then 7800 once past NUTVE.  Unlike Ryan, I typically don't engage APR mode on the AP until the IF (NUTVE in this case).  Now, you could just stay at 8600, engage APR a bit earlier as Ryan does, and intercept and capture the glidepath somewhere around NUTVE at which point the AP/GPS will handle your descent, but I don't like to do that simply because the approach as charted shows the intercept at about JATOL if at 7800.  While I'm sure the glidepath is reliable much further out, and it's not "wrong" in any legal sense as the altitudes are "at or above", I feel better flying the approach as charted, which calls for a stepdown to 7800 at NUTVE and glidepath intercept at JATOL.

Bottom line, though, is that the only vertical control you'll get flying IAPs with this plane/avionics combo comes at glidepath intercept (or glideslope intercept on an ILS) - wherever that occurs.  The rest is up to you.  Hope that helps.

 

Scott

A quick clarifying edit - I said "wherever that occurs", but the approach plate does help you here.  It is expected to occur at JATOL, assuming you're at 7800'.  This is noted on the plate's profile view with an "x"  As I've noted before, I think Jepp plates make this more obvious as they show the profile as step-downs, rather than continuous slopes, but both will indicate the expected intercept with an "x".  While you may intercept before this if higher - and that's OK - if at the proper altitude and capture has not occurred by JATOL, you'll then know that something's amiss.

S 

Edited January 21Jan 21 by tttocs

Thanks Scott. My understanding was incorrect. Makes sense. Appreciate yours and Ryan’s help! 

9 hours ago, tttocs said:

 

A quick clarifying edit - I said "wherever that occurs", but the approach plate does help you here.  It is expected to occur at JATOL, assuming you're at 7800'.  This is noted on the plate's profile view with an "x"  As I've noted before, I think Jepp plates make this more obvious as they show the profile as step-downs, rather than continuous slopes, but both will indicate the expected intercept with an "x".  While you may intercept before this if higher - and that's OK - if at the proper altitude and capture has not occurred by JATOL, you'll then know that something's amiss.

 

Yeah I figured you'd show up here.  I think I've heard others do that as well...  I'm just a lazy sim pilot haha

I learned a lot from this discussion. Thanks guys.