RNAV glideslope/ILS

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echelon

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echelon
I'm finishing up my CFII training in a C172 with the G1000. I was flying the RNAV 21L approach into KPRC and I noticed "NO GP" in the place of the glideslope after activating the approach. I checked through the AUX page on the MFD and didn't notice anything out of the ordinary. As we continued the approach, LNAV only, all of a sudden the glideslope came on.

The glideslope for the ILS to the same runway was notamed out of service, so just out of curiosity I switched CDI's... and now the ILS glideslope appeared to be working too, and showing what looked like identical deflection to the RNAV GP.

According to everything I've ever learned about RNAV/the G1000/GPS/etc the glidepath on a GPS/RNAV approach is entirely internally generated, but this strange coincidence got me thinking. I looked through my charts and sure enough, out of all the RNAV/GPS approaches I looked at, the only ones that have LPV minimums also have an ILS on the same runway.

So my question: Was this just a coincidence or is the RNAV glideslope somehow tied to a functioning ILS?
 
Isn't the CDI driven by the ILS (not GPS) inside the FAF? Don't you need to switch to VLOC (manually or automatically) at some point to active this? I know I have to on the 530W.
 
echelon said:
So my question: Was this just a coincidence or is the RNAV glideslope somehow tied to a functioning ILS?
Click to expand...
It has to be coincidence I should think. The RNAV (GPS) 24 at KIKW (Midland, MI) has LPV minimums but no ILS anywhere on the field. And there are lots of RNAV approaches for which some GPS receivers generate advisory glideslopes (though only the LNAV minimums are published) even though there is no ILS on the runway or on the field. The RNAV (GPS) 9 at my home base of VLL is an example.

Well I don't know anything about the G1000, so I suppose it could be a bug...
 
Ron Levy said:
Sounds like maybe you briefly lost WAAS, but then it came back.
Click to expand...

That's also what I'm thinking. I would guess you originally had LPV sensitivity and then it downgraded to LNAV when WAAS became unavailable, then upgraded back to LPV when WAAS became available again. What was the sensitivity annunciation on the HSI? When the GPS downgrades the approach, it should change the sensitivity annunciation on the HSI and give you a white advisory message accessed through the lower right softkey on the PFD.

echelon said:
So my question: Was this just a coincidence or is the RNAV glideslope somehow tied to a functioning ILS?
Click to expand...

I think it is just a coincidence -- I've flown quite a few RNAV/GPS approaches with LPV mins at airports without any ILS array, let alone one to the same runway.
 
LuvFlying said:
Isn't the CDI driven by the ILS (not GPS) inside the FAF? Don't you need to switch to VLOC (manually or automatically) at some point to active this? I know I have to on the 530W.
Click to expand...

You only use VLOC on an ILS, localizer, or VOR approach. If you are flying a LPV or any RNAV (GPS) approach with vertical guidance, you keep the 530W or G1000 CDI on GPS and the glidepath is generated by the GPS function.
 
echelon said:
According to everything I've ever learned about RNAV/the G1000/GPS/etc the glidepath on a GPS/RNAV approach is entirely internally generated, but this strange coincidence got me thinking. I looked through my charts and sure enough, out of all the RNAV/GPS approaches I looked at, the only ones that have LPV minimums also have an ILS on the same runway.

So my question: Was this just a coincidence or is the RNAV glideslope somehow tied to a functioning ILS?
Click to expand...

Coincidence. There are plenty of LPV approaches where there's no ILS on the same runway. However, at airports with both ILS and LPV, they're generally to the same runways - It's the same way at my home field. There are GPS approaches to five runways and ILS approaches to three, and the GPS approaches to the three ILS runways have LPV mins while the other two only have LNAV.

I was told that the reason for this is that the runways with ILS approaches have already been surveyed for obstacle clearance to the lower level, and thus it's easier to make an LPV approach to a runway that already has an ILS.
 
echelon said:
I'm finishing up my CFII training in a C172 with the G1000. I was flying the RNAV 21L approach into KPRC and I noticed "NO GP" in the place of the glideslope after activating the approach. I checked through the AUX page on the MFD and didn't notice anything out of the ordinary. As we continued the approach, LNAV only, all of a sudden the glideslope came on.

The glideslope for the ILS to the same runway was notamed out of service, so just out of curiosity I switched CDI's... and now the ILS glideslope appeared to be working too, and showing what looked like identical deflection to the RNAV GP.

According to everything I've ever learned about RNAV/the G1000/GPS/etc the glidepath on a GPS/RNAV approach is entirely internally generated, but this strange coincidence got me thinking. I looked through my charts and sure enough, out of all the RNAV/GPS approaches I looked at, the only ones that have LPV minimums also have an ILS on the same runway.

So my question: Was this just a coincidence or is the RNAV glideslope somehow tied to a functioning ILS?
Click to expand...

You are correct that the path is entirely generated internally by the GPS.

As of 10/21/2010 there were 890 LPV's that were published to runways that also had an ILS and there were 1437 LPV's to runways that did not have an ILS.

To answer your last question, No. The RNAV glidepath is in no way tied to a functioning ILS, although care will be taken in the design of the LPV to follow the same path.

I suspect that the "NO GP" indication is normal and remains so until passing the last fix prior to the FAF (in this case at PEVYU), at which time the GP indication comes on. Unlike an ILS, just because a LPV is activated, doesn't mean you are at a point where the GP will be displayed.
 
Ron Levy said:
Sounds like maybe you briefly lost WAAS, but then it came back.
Click to expand...

Ron,

I don't think this can happen, once you lose vertical, it is gone for the rest of the approach. I think the OP doesn't fully understand the conditions under which the GP shows up. If he was flying the full approach, he should not have expected to see a GP indication until passing the IF (PEVYU). If he was using VTF, then he should see the GP indication when he was within 45 degrees of the FAF, and prior to that, I believe he would see SUSP.
 
...is the RNAV glideslope somehow tied to a functioning ILS?
Click to expand...

I am not too familiar with GPS approaches, but I am pretty sure that RNAV is 100% unrelated to ILS equipment wise.

By that I mean, any airport can have a precision approach now thanks to GPS and WAAS without having to buy very expensive ILS ground equipment. In fact, many airports only have GPS approaches now. (perhaps didn't have any before)
 
LuvFlying said:
Isn't the CDI driven by the ILS (not GPS) inside the FAF? Don't you need to switch to VLOC (manually or automatically) at some point to active this? I know I have to on the 530W.
Click to expand...

John Collins said:
You only use VLOC on an ILS, localizer, or VOR approach. If you are flying a LPV or any RNAV (GPS) approach with vertical guidance, you keep the 530W or G1000 CDI on GPS and the glidepath is generated by the GPS function.
Click to expand...
Oops... thought we were talking about an ILS approach.
 
flyingcheesehead said:
I was told that the reason for this is that the runways with ILS approaches have already been surveyed for obstacle clearance to the lower level, and thus it's easier to make an LPV approach to a runway that already has an ILS.
Click to expand...

Nice, that would explain that.

I think my main question has been answered... but just to clarify the situation -
I was flying the approach via dct PEVYU and then the course reversal, so I activated the approach well before PEVYU, and like Mr. Collins mentioned, nothing showed up for the glidepath, because it isn't supposed to. Crossing PEVYU inbound from the course reversal is when the NO GP showed up. It remained there until several miles inside of RUBIW (LNAV FAF). The aux page showed "3D DIFF NAV" the whole time, which from my understanding indicates that WAAS is functioning. The HSI was indeed showing LNAV up until the glideslope came on, and unfortunately I didn't see if it changed after that.

Thanks for your input everyone.
 
flyingcheesehead said:
Coincidence. There are plenty of LPV approaches where there's no ILS on the same runway. However, at airports with both ILS and LPV, they're generally to the same runways - It's the same way at my home field. There are GPS approaches to five runways and ILS approaches to three, and the GPS approaches to the three ILS runways have LPV mins while the other two only have LNAV.

I was told that the reason for this is that the runways with ILS approaches have already been surveyed for obstacle clearance to the lower level, and thus it's easier to make an LPV approach to a runway that already has an ILS.
Click to expand...
For some reason they developed two LPV approaches to my home airport and neither serves the same runway as the single ILS. I suspect part of the reason has to do with the fact that the ILS runway (and the ILS of course) was shut down for most of a year. When that plan to close that runway and associated approaches was revealed I made some noises about getting an LPV to the just lengthened parallel runway and either they actually listened to me or someone with some actual clout voiced the same opinion because the two LPVs (one to each end) were published before the ILS was shut down. A couple months a LPV was commissioned to the runway with ILS and it has the same DA as the ILS (200 AGL) no doubt at least partly due to the ALS in place for the ILS. The DA for the other LPVs are 250 and 300 AGL.
 
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echelon said:
The HSI was indeed showing LNAV up until the glideslope came on, and unfortunately I didn't see if it changed after that.
Click to expand...

It sounds to me like the approach was downgraded to LNAV which is why the "NO GP" was displayed in the glidepath window. That said, I don't think vertical guidance (via WAAS) is restored inside the FAF once the approach is already downgraded so the fact that the glidepath reappeared inside the FAF is still boggling my mind.
 
HPNPilot1200 said:
It sounds to me like the approach was downgraded to LNAV which is why the "NO GP" was displayed in the glidepath window. That said, I don't think vertical guidance (via WAAS) is restored inside the FAF once the approach is already downgraded so the fact that the glidepath reappeared inside the FAF is still boggling my mind.
Click to expand...

Maybe it went into LNAV+V? :dunno:
 
echelon said:
Nice, that would explain that.

I think my main question has been answered... but just to clarify the situation -
I was flying the approach via dct PEVYU and then the course reversal, so I activated the approach well before PEVYU, and like Mr. Collins mentioned, nothing showed up for the glidepath, because it isn't supposed to. Crossing PEVYU inbound from the course reversal is when the NO GP showed up. It remained there until several miles inside of RUBIW (LNAV FAF). The aux page showed "3D DIFF NAV" the whole time, which from my understanding indicates that WAAS is functioning. The HSI was indeed showing LNAV up until the glideslope came on, and unfortunately I didn't see if it changed after that.

Thanks for your input everyone.
Click to expand...

Thanks for the additional information. This is an area that is not well documented in the Pilot Guide for the G1000. My guess is that the WAAS integrity did not meet the criteria for this approach but was on the edge.

It is my understanding that the integrity is checked one minute prior to the FAF and if it still does not meet the requirement, then the approach is downgraded to LNAV and no vertical guidance is provided for the remainder of the approach. If the approach does not get downgraded, then a loss of integrity inside the FAF will require the approach be missed.

If the loss of integrity occurs after the IF (PEVYU) inbound, but before the one minute mark to the FAF, then you can get the NO GS indication if the integrity does not meet the requirements. If it meets the requirements before reaching the one minute point, you can get your GS back. The NO GS indication is to notify the pilot that the integrity is unsatisfactory for the vertical guidance. If the integrity does not come back to meet requirement by the one minute point, the down grade will occur and will be to LNAV for the remainder of the approach. There are no downgrades to LNAV+V or LNAV/VNAV, it is all or nothing with respect to the vertical.

So in this approach, you have a relatively long intermediate segment of 8.6 NM, so there is a greater chance of seeing the condition you encountered. Also, because the minimums on the LPV approach have a DH of 200 feet, the integrity requirement is tighter than it would be for an LPV with a DH of 250+ feet. The VAL (Vertical Alarm Limit) is 35 meters for this approach and is 50 meters for an LPV with a DH of 250+ feet. These limits are for the integrity parameter VPL (Vertical Protection Limit) that WAAS provides.

If the satellite page displayed VPL, you could get an idea as to how close you were to the limit, but alas it does not. However, the G1000 displays another parameter, VFOM (Vertical Figure of Merit) that can be used as a rough proxy for VPL. If VFOM is under about 40 feet, you have a good chance of meeting the vertical integrity requirements. For a LPV with a 250+ feet DH, the equivalent VFOM value would be around 60 feet.

WAAS has a lower availability on the far West Coast and in the Arizona area, so my guess is that LPV 200 DH is more problematic than in most of the rest of the country.

It would be nice to get some reports from pilots in the West Coast as to the values they see for VFOM. The VFOM is also displayed on the GNS430W/530W satellite page. I typically see values in the 20 foot range here in the Carolina's.
 
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azure said:
I thought LNAV+V was also WAAS?
Click to expand...

LNAV+V is provided by WAAS, but just on approaches that only have LNAV minimums and the approach meets the straight in requirements.

When you select a WAAS approach, only guidance for the most precise level of service is provided. So if there are a co-located LPV, LNAV/VNAV, and LNAV set of minimums, then guidance is provided for the LPV. This doesn't prevent the pilot from using the glidepath and flying the approach to any of the minimums associated with the approach, but if the WAAS integrity doesn't meet the requirements for the most precise service, then the approach will down grade to LNAV minimums without any vertical guidance.

In cases where the integrity is between the LPV200 service level and the LPV250 service level, you may get vertical guidance to another runway or airport on an LNAV only approach (IOW LNAV+V) when you are not able to get vertical guidance on a LPV200 approach, but never on the same LPV200 approach.
 
flyingcheesehead said:
I was told that the reason for this is that the runways with ILS approaches have already been surveyed for obstacle clearance to the lower level, and thus it's easier to make an LPV approach to a runway that already has an ILS.
Click to expand...
The primary reason got to be that you need very similar CAT I runway lighting system to conduct either ILS Cat I or LPV approaches hence it is only natural that any runway with existing ILS is a perfect candidate for the LPV approach.
 
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olasek said:
The primary reason got to be that you need very similar CAT I runway lighting system to conduct either ILS Cat I or LPV approaches hence it is only natural that any runway with existing ILS is a perfect candidate for the LPV approach.
Click to expand...

A LPV approach doesn't require approach lights, but it helps. For example see KLKR RNAV (GPS) RWY 6. Approach lights will lower the minimum visibility requirement to 1/2 mile from 3/4 mil, but doesn't affect the DH. An existing survey for an ILS is one of the key elements that should aid in expediting the release of a LPV approach. Unfortunately, at my home drome, KUZA, there is an ILS to RWY 2, but the survey data was lost and we are waiting for a new survey to be accepted before we can get a LPV.
 
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