LPV Approach Glide Path Intercept?

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Clorix
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LPV Approach Glide Path Intercept?

Post by Clorix »

First of all, I can't even begin to explain how excited I have been to fly this thing, and now that it's finally in my hands I have to say I am absolutely impressed in every way. This is EASILY the best GA aircraft in my hangar now, and will be flying it probably way too much (if that's even possible!).

Now on to my question :)

My first flight was a couple hand-flown circuits just to familiarize myself with this aircraft's dynamics and handling characteristics (which will take lots of getting used to!). My second flight was a quick cross-country one where I tried to use the autopilot a bit to familiarize myself with it. I finally figured out how to make it climb and descend, which was more difficult than I expected as this is the first time I've used this particular autopilot. Who needs flight manuals anyway? I did actually read the manual eventually, though! :D Well, my first approach was down the ILS, which went without trouble.

Then I decided to try an RNAV LPV approach. I set it up in the GTN 750 as I would normally. As soon as LPV is annunciated on the GTN, I press the APPR button on the autopilot control panel expecting the aircraft to descend once it intercepts the LPV glide path, to no avail. And yes, the CDI source was set to the GTN, not VLOC.

Am I missing something, or does the autopilot not couple vertically to GPS approaches?

Thanks for such an amazing addon!

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Clorix
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Re: LPV Approach Glide Path Intercept?

Post by Clorix »

I just re-read the manual again and noticed something I missed the first time. It says "During VOR or RNAV approaches, Glideslope capture will not occur because the NAV receiver is channelled to a VOR station, not an ILS, and this locks out the Glideslope function."

I know VOR approaches don't have vertical guidance, but many RNAVs do, whether it's LPV, LNAV+V, or LNAV+VNAV. As far as I'm aware, LPV RNAV approaches are augmented through ground-based stations (at least I think that's what WAAS does), which is what enables such precision. Surely this doesn't require the use of the nav radios like the above quote from the manual says. Even the A2A C182 can do fully-coupled LPVs just fine last I checked.

This has me a bit confused. Can anyone clarify or set me straight?

Thanks again!

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AKar
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Re: LPV Approach Glide Path Intercept?

Post by AKar »

Clorix wrote:I just re-read the manual again and noticed something I missed the first time. It says "During VOR or RNAV approaches, Glideslope capture will not occur because the NAV receiver is channelled to a VOR station, not an ILS, and this locks out the Glideslope function."

I know VOR approaches don't have vertical guidance, but many RNAVs do, whether it's LPV, LNAV+V, or LNAV+VNAV. As far as I'm aware, LPV RNAV approaches are augmented through ground-based stations (at least I think that's what WAAS does), which is what enables such precision. Surely this doesn't require the use of the nav radios like the above quote from the manual says. Even the A2A C182 can do fully-coupled LPVs just fine last I checked.

This has me a bit confused. Can anyone clarify or set me straight?

Thanks again!
Hey,

I commented this on another topic discussing the same thing. I must say I am a little bit confused myself on how this "should be". First, I think we need to read the manual in its context. RNAV approaches with vertical guidance are fairly new addition to the game, and likely not considered in the original text. Generally speaking, an autopilot is usually wired to the NAV1 deviation signals and flags as appropriate. Now, the manual states the obvious that should you have a VOR tuned, or the NAV1 driven by GPS, no vertical deviation signal is received and therefore no capture will occur. However, if the NAV1 is driven by a GPS source that includes the vertical guidance, it should be the same thing for the autopilot. Further, the autopilot most likely has no clue of the selected frequency, this is not something that is usually shared in between the boxes in any way. Something called flag signals are used instead, simply indicating a presence of a valid indication. This is the same signal that biases the needles of the instrument to the default position when not present and pops open the flags, hence the name. Typically (not always) the combined GPS/NAV1 receiver is wired to use the same flag signals for valid GPS guidance which it would use for regular ground-based sources.

In principle, it could be the case that the autopilot receives its flags only from the valid glideslope indication. I think remembering the GNS/GTN series are equipped to provide discrete signals for these but would need to check to confirm. Why to wire it so, I don't know, as just about the only reason would be to purposely disable the GPS vertical guidance.

-Esa

wothan
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Re: LPV Approach Glide Path Intercept?

Post by wothan »

The RNAV described for the KFC200 in the manual is based on VORTAC bearing and distance and not inputs from a GPS.
Early RNAV navigation systens used the network of VOR, DME and VORTAC's.

So I guess using a GPS with LPV / LNAV+V should work and APPR mode aldo working in these GPS modes.

FinnJ
When I like to do basic flying, I turn to A2A Aircraft, cause A2A "basic" flying means "complex" procedures.

diseased
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Re: LPV Approach Glide Path Intercept?

Post by diseased »

The autopilot doesn't care where the vertical guidance is coming from, it knows just to follow it. Many of these autopilots were designed well before such fancy things as LPV approaches were created. Fortunately the autopilot itself doesn't receive a glideslope signal....it only follows what the flight director/internal guidance is commanding. The flight director/internal guidance can receive the vertical commands via external glideslope or internal guidance.

So basically, yes....I hope A2A fixes this :) :) :)

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CAPFlyer
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Re: LPV Approach Glide Path Intercept?

Post by CAPFlyer »

A2A may not fix it because it may not be an error. According the the documentation, the KFC500 for RNAV/GPS operations is running through the VOR system, meaning it doesn't receive vertical inputs. So, it wouldn't work for LPV approaches. You would simply have to manually set the vertical speed using the pitch switch or flying the approach by hand.

I also refer you to the following thread in the Support Forum - https://a2asimulations.com/forum/viewto ... 39&t=65298
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wothan
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Re: LPV Approach Glide Path Intercept?

Post by wothan »

CAPFlyer wrote:A2A may not fix it because it may not be an error. According the the documentation, the KFC500 for RNAV/GPS operations is running through the VOR system, meaning it doesn't receive vertical inputs. So, it wouldn't work for LPV approaches. You would simply have to manually set the vertical speed using the pitch switch or flying the approach by hand.

I also refer you to the following thread in the Support Forum - https://a2asimulations.com/forum/viewto ... 39&t=65298
How can the glideslope on the HSI then be shown for the LPV / LNAV+V approaches ?

Normally the Autopilot uses the signals from the FD to control the aircraft, thats why the FD must be on for the AP to function.

Again, if You read my post above, the RNAV system described for the autopilot in the manual, is not the same as the GPS systems we have available.
In the time before GPS's there where RNAV systems based on VOR, DME and VORTAC's, where the description makes sense, where the localiser / glideslope signals are passed from the GPS to the autopilot / flight director.

FinnJ
When I like to do basic flying, I turn to A2A Aircraft, cause A2A "basic" flying means "complex" procedures.

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CAPFlyer
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Re: LPV Approach Glide Path Intercept?

Post by CAPFlyer »

It has to do with where the signal is coming from. Just because the GPS can drive the HSI glideslope doesn't mean it can drive the autopilot or the FD. Again, just because it doesn't do what you want it to doesn't necessarily make it a bug.

Here's an example of where a real world Autopilot doesn't play well with GPS. I had a mentor who has a C310. He got an S-Tec 55 installed in it about 4 years ago. He has a GNS530W as well. When they first started trying to couple the GPS to the S-Tec, they kept running into problems. Why? Because apparently the signal to the HSI wasn't strong enough to drive the S-Tec. It was plenty to get the HSI needles to move properly (in vertical and horizontal) but not the A/P. They replaced the HSI with a newer unit that had a stronger signal generator. They put a signal booster on the GPS. And what did it give him? An autopilot that would put you 1/4 mile RIGHT of the final approach every time you tried to fly a GPS approach and wouldn't drive the vertical for the LPV. It was perfectly 1/4 mile right every time and would even guide perfectly in heavy winds to hold that 1/4 mile offset, but it wouldn't line up that needle. I don't remember what they ended up having to do in the end, but it meant that there were modifications made to the S-Tec STC for the 310 because of what was discovered.
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AKar
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Re: LPV Approach Glide Path Intercept?

Post by AKar »

Speaking RL, the specifications of avionics list their signal levels, either in amps or in (milli)volts as applicable. Also, possible interconnections are listed. Compatibility should be easily determined by avionics technicians with some experience of the game. Generally speaking, if avionics don't talk too well with the autopilot in real life, the issue most often is poor installation, either in design or quality, bad configuration or bad box. With GPS installations it can get complicated because roll steering converters are often used. These essentially force to use the AP in heading mode when flying GPS NAV (as the GPS drives the FD/AP via heading error signal). Not many autopilots couple to the glideslope when in heading mode, so if this is the method used in GPS-to-autopilot interfacing, then the autopilot won't do glides by default, except via regular NAV mode, following the lateral deviation signal.

If the GPS does provide vertical guidance to the HSI and the autopilot is capable of coupling to the ILS fed into the HSI, it can be said almost as a fact without seeing that it can be made to couple to the GPS vertical guidance as well. For instance, GTN 6XX/7XX series use the same vertical and lateral displacement signals and flags regardless of whether ILS or GPS is used. This applies both to the navigation indicator interface and the flight control system interface. Obviously, an individual installation can be configured in curious ways, and for various reasons.

-Esa

wothan
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Re: LPV Approach Glide Path Intercept?

Post by wothan »

In the case of using the Garmin GNS 430/530 or GTN 650/75,0 the Nav radio is included in these and thus the output from the GPS is the same for the Autopilot.
To say it differently... The AP does not know if the signal driving it, is from the NAV radio or from the GPS LPV / LNAV+ V, and would simply be treated as a Localiser / Glideslope signal, which in our case also is depicted by the indication on the HSI with indications for both the Localiser and Glideslope, and the AP should follow these in APPR mode, simply because the signal is present for both.


Also, as AKar (Esa) says above..
Looking on the wiring diagram for the GTN 750 (page 19): https://static.garmincdn.com/pumac/190- ... _04Web.pdf, there are no seperate wires from the GTN for ILS/GS and LPV/LNAV+V, so the KFC200 simply don´t know which source the signals comes from, it simply get the approach, lateral and vertical error signals (named "ILS/GPS Approach", "Lat deviation & Flags", "Vert deviation & Flags").

Also read this thread from real-life use of the KFC 200 with Garmin GPS:

https://www.bonanza.org/community/membe ... autopilot/

Quote from the thread:

When I fly a GPS LPV approach with my KFC200, I just leave the AP/FD in NAV mode and watch it happen! When the aircraft turns on final outside of the IAF I select the "approach" mode and just watch. The GS will show on the G500 and the backup HSI. AutoPilot display will show AutoPilot Coupled. When the GS is one dot above I put in approach flaps and lower the gear. GS will light up on the display as the AP starts to descend on the GS. Power in my P Baron is set at 18"MPx 2300 RPM and approach flaps. Leave Mixtures and full flaps as set until runway is secured. When runway is secure, advance Mixture to full rich, props to full climb power, and mentally be ready for a "go around". Very easy stable approach has worked for years.

FinnJ
When I like to do basic flying, I turn to A2A Aircraft, cause A2A "basic" flying means "complex" procedures.

Bert Pieke
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Re: LPV Approach Glide Path Intercept?

Post by Bert Pieke »

As I wrote in the other topic on this subject, I would think that a Bonanza owner who pays to have a GTN 750 installed in his airplane would also get the autopilot wired so that it will follow both ILS and RNAV/LPV approaches.. :)
Bert

wothan
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Re: LPV Approach Glide Path Intercept?

Post by wothan »

Bert Pieke wrote:As I wrote in the other topic on this subject, I would think that a Bonanza owner who pays to have a GTN 750 installed in his airplane would also get the autopilot wired so that it will follow both ILS and RNAV/LPV approaches.. :)
According to the wiring diagram for the GTN 750 (650), additional wires would not even be necessary.
If the GTN 750 is connected to the KFC200 and can perform normal ILS approaches, then LPV/LNAV+V will also work.

FinnJ
When I like to do basic flying, I turn to A2A Aircraft, cause A2A "basic" flying means "complex" procedures.

Bert Pieke
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Re: LPV Approach Glide Path Intercept?

Post by Bert Pieke »

wothan wrote:
Bert Pieke wrote:As I wrote in the other topic on this subject, I would think that a Bonanza owner who pays to have a GTN 750 installed in his airplane would also get the autopilot wired so that it will follow both ILS and RNAV/LPV approaches.. :)
According to the wiring diagram for the GTN 750 (650), additional wires would not even be necessary.
If the GTN 750 is connected to the KFC200 and can perform normal ILS approaches, then LPV/LNAV+V will also work.

FinnJ
Even better :D
Bert

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some1 - A2A
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Re: LPV Approach Glide Path Intercept?

Post by some1 - A2A »

We will add that in a future patch :)
Michael Krawczyk

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Jocko Flocko
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Re: LPV Approach Glide Path Intercept?

Post by Jocko Flocko »

some1 - A2A wrote:We will add that in a future patch :)
Thank god...

It was kind of disappointing when I tried an RNAV into Juneau tonight in low visibility and rain, hitting the approach button did nothing, however I was able to follow the glide-slope down via the GS indicators on the HSI.

Looking forward to it being an option or corrected... This post answered the question I was about to ask as I don't fly VFR in sim that much anymore.

Cheers
Fantastic aircraft!

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