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PostPosted: Fri Sep 01, 2017 12:09 pm 
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Hello guys,

Just a quick one: should our KAP 140 autopilot installation be capable of following vertical guidance provided by an LPV or LNAV+V approach?

In my case, the GPS (RXP GNS 530W V2) is driving the glideslope pointers on the HSI as you'd expect, but the autopilot doesn't follow this glideslope deviation info.

I was initially under the mistaken impression that the autopilot was following vertical guidance from the GPS, but it turned out it was just intercepting the ILS glideslope which I had tuned into NAV1, even though the GNS unit was in GPS mode! :?

Just trying to distinguish between real limitation of this setup, and possible issues with the RXP GNS 530W V2 integration. (I know it's not officially supported yet...)

Thanks,
Nick

P.S. You'll have to excuse me: it's my first experience with a 'proper' GPS in the sim! :mrgreen:


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PostPosted: Fri Sep 01, 2017 2:21 pm 
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Location: Typically hanging around N07, 12N, KLDJ, KCDW
Yes the KAP 140 can follow this type of guidance as it really doesn't understand what the source of the navigation signal is from. As long as the signal and deviation is provided it will happily follow along.

I use a KAP140 to fly ILS and LPV approaches.

I do not know if the GPS would provide a vertical deviation signal and if it therefore appropriate for the autopilot to follow LNAV+V guidance as that is not guaranteed to provide obstacle clearance.

All said as an IFR student, not an IFR pilot!


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PostPosted: Fri Sep 01, 2017 2:47 pm 
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Thanks Oracle - I thought that was the case but wanted to make sure. From some of Esa's posts, I vaguely understand that the autopilot is just a slave to voltages from the CDI and glideslope needles, so it seemed logical that it should follow them regardless of whether they're being driven by ground-based navaids or the GPS. However, I wondered if in GPS mode, glideslope deviation info to the KAP 140 comes directly from the GTN.

Anyway, it looks like integration of the new RXP GTN isn't as smooth sailing as I'd first thought! :| I've had one LPV approach where the glideslope pointers never even appeared, and where they do, the autopilot doesn't follow them.

Thanks again and hope the instrument rating is going smoothly still. :)

Nick

P.S. Yeah, I appreciate that for LNAV+V approaches the vertical guidance is "advisory" only, but I understand the glideslope deviation is still presented via the HSI etc in the same way. That's certainly how it appears in the sim.


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PostPosted: Fri Sep 01, 2017 3:12 pm 
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Location: Typically hanging around N07, 12N, KLDJ, KCDW
Good point on the HSI vertical guidance on LNAV+V.

I'm using a G1000 and it has a different presentation between LPV and LNAV+V. I have never tried to couple it with the guidance on LNAV+V because you'd have to keep switching modes to halt descent.

I have very little experience with LPV approaches because there aren't many around here and mostly at the airports I would not want to fly to.

I guess it would likely follow the vertical guidance all the way down on a mechanical HSI unit.

In the end what really matters is that you obey the crossing altitudes and properly identify the intersections. That is regardless of how you actually manage the descent, provided that it is a stable descent.


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PostPosted: Fri Sep 01, 2017 3:39 pm 
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Oracle427 wrote:
I'm using a G1000 and it has a different presentation between LPV and LNAV+V.
Though I could never bring myself to put modern avionics (i.e. touchscreen GTNs) into the A2A GA products, I'd love to get my hands on an A2A aircraft with native support for the G1000 PC trainer... In the (probably) unlikely event this ever happens... :)

Nick


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PostPosted: Fri Sep 01, 2017 7:50 pm 
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One of the instructors at my flying club did a presentation about it a few years ago.

Maybe this has some helpful info for you.

http://flywithhoward.com/wp-content/upl ... proach.pdf


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PostPosted: Sat Sep 02, 2017 3:08 am 
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Thanks Brian - that presentation does indeed contain some very useful info. :) Reading about the "APR guidance available" message and the need to manually "enable A/P APR outputs" for each approach with vertical guidance made me wonder if the same is true of the GNS series.

And, sure enough, it is. (Page 114 of the pilot's guide if only I'd read that far!) However, I'm 99% certain that this message didn't appear during the approaches I've flown so I suspect this behaviour isn't active in the RXP product. I will double check my procedure though, and at least it gives me some extra info to get back to RXP with.

Cheers,
Nick


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PostPosted: Sat Sep 02, 2017 8:46 am 
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Nick,

To generalize: If the black box is able to drive the G/S needle, the autopilot is capable of coupling on to it. Exactly as Oracle mentioned in the first reply, it doesn't know what's driving that needle, it only follows it. :)

Now, there are exceptions. An obvious one would be if the box was coupled to a NAV display but was not connected to the autopilot. Your wiring may vary! RNAV, NAV, indicators, and the autopilot are all independent boxes, and may or may not be interconnected in various ways. A case study of this would be the GPSS simulation. ;)

Edit: To be on it, the G/S and the GPS-based vertical share the signal if connected to the autopilot. If jumped from an indicator, the bets are off.

Image

GNS systems are more or less similar.

-Esa


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PostPosted: Sat Sep 02, 2017 11:11 am 
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Thanks Esa - I love a good system interface diagram! :mrgreen:

This sort of thing does bring home the complexities of simulating this kind of set-up in the way A2A have chosen to do (creating their own autopilot simulation I mean), especially when products from different developers are brought into the mix.

I'm curious as to why the GNS/GTN + KAP 140/KFC 225 combination needs manual intervention by the pilot for each approach with vertical guidance, as discussed above. I mean, I appreciate the point in the presentation that the KAP 140 was designed before the advent of WAAS approaches, but if the firmware has been updated to take account of this, why not just automatically enable the vertical guidance to the autopilot on approaching the FAF?

Anyway, I think this is irrelevant to troubleshooting the issue with the RXP GNS. Presumably—because it's autopilot-specific—this feature isn't included in the Garmin trainer at all.

Nick


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PostPosted: Sat Sep 02, 2017 12:13 pm 
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Nick M wrote:
I'm curious as to why the GNS/GTN + KAP 140/KFC 225 combination needs manual intervention by the pilot for each approach with vertical guidance, as discussed above. I mean, I appreciate the point in the presentation that the KAP 140 was designed before the advent of WAAS approaches, but if the firmware has been updated to take account of this, why not just automatically enable the vertical guidance to the autopilot on approaching the FAF?
Hmm, I've had a pause since last using the Cessnas (been on the Comanche as a GA and busy with FSL for a while), but I recall I did some vertical guidance with autopilot, using F1 GTN650. Interesting.

IRL, KAP 140 & GNS/GTN are not 1-to-1 compatible, for instance the proprietary GPSS input for KAP 140 would not accept any of the GNS/GTN outputs, so we'd be on the needles. Anything that goes to the "MAIN CDI/HSI" from the connector P1001 should be standard volt-or-not signals, and the autopilot should not know the difference.

I should check out the KAP 140 data as well, how one should properly interface these two. There are a few options from the above. Maybe tomorrow. :)

Edit: Re. all the developers making their addons into the mix, I'd love them to provide outputs in regular formats used in real things. It should not be too much to simulate today. For voltage signals, just give a byte or two.

-Esa


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PostPosted: Sun Sep 03, 2017 5:56 am 
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AKar wrote:
Hmm, I've had a pause since last using the Cessnas (been on the Comanche as a GA and busy with FSL for a while), but I recall I did some vertical guidance with autopilot, using F1 GTN650. Interesting.
Yes, could be down to some differences in coding when it comes to the Flight 1 vs. Reality XP approach.

For anyone considering the RXP GNS units, my advice would be to hold off for the moment if you plan to install them in your A2A aircraft. You can see the support thread I opened on the RXP forum on Avsim here; I'm still hopeful compatibility between the two products can be improved.

Nick


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PostPosted: Sun Sep 03, 2017 12:23 pm 
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Yeah, tbh all this addon avionics market is going towards a mess like faster than the Korean peninsula.

What should have been realized from the starters is that the signals utilized in the real world are fairly standard voltage signals, discrete or analog, OR, either standardized ARINC 429 or RS 232, OR, in cases something like MIL-STD-1553, OR, if else then proprietary.

The protocols and analog signals are mostly well-defined and public (or easily reversible). In principle, if one referred into these when doing a simulation, a simple table of inputs referring against the 'standard' would be required for good compatibility and it would remain somewhat future proof.

-Esa


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PostPosted: Tue Sep 12, 2017 7:18 am 
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Just to revisit this topic, can any users of the RXP GNS V1 (or indeed Flight 1 GNS units) confirm if this functionality (the KAP 140 following vertical guidance for a RNAV approach) works OK?

Thanks,
Nick


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