Gps as DME for non published waypoint during ILS

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Rich
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Re: Gps as DME for non published waypoint during ILS

Post by Rich »

ultraturtle wrote:Most places I fly, the ILS gets you lower minimums than the RNAV LPV to the same runway... but not always (KFFC LPV to RWY 31 gets you a whole foot lower than the ILS to RWY 31).
Not something I can recall running into. Usually a higher LPV than 200 ft. is because of lack of the approach light infrastructure. Around here that shortcoming seems to be worth about 50 ft. (i.e., 250 ft AGL instead of 200 ft.)

Conversely, most of the places I fly to that have an LPV (and various other WAAS approaches) lack an ILS altogether. The only airport within 100 nm I can find with an ILS and LPV to the same runway where the DA for the LPV DA > ILS is Klamath Falls.
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Re: Gps as DME for non published waypoint during ILS

Post by ultraturtle »

TimS wrote:
ultraturtle wrote:Most places I fly, the ILS gets you lower minimums than the RNAV LPV to the same runway... but not always (KFFC LPV to RWY 31 gets you a whole foot lower than the ILS to RWY 31).
Curious, where?
Lots of places around here:
KCTJ RWY 35 ILS DH is 50 ft lower than LPV
KCCO RWY 31 ILS DH is 40 ft lower than LPV
KLGC RWY 31 ILS DH is 19 ft lower than LPV
TimS wrote:...And if possible what TERPS aspect allows the lower altitude...
I'm no expert on TERPS, but surmise that the difference lies not within the approach itself, but with the missed approach procedure.
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Re: Gps as DME for non published waypoint during ILS

Post by TimS »

ultraturtle wrote: Lots of places around here:
KCTJ RWY 35 ILS DH is 50 ft lower than LPV
KCCO RWY 31 ILS DH is 40 ft lower than LPV
KLGC RWY 31 ILS DH is 19 ft lower than LPV

I'm no expert on TERPS, but surmise that the difference lies not within the approach itself, but with the missed approach procedure.
My knowledge is all second hand. So....
Looking at the airports, missed approach is not likely a factor. Reason is the approach path through the air between ILS and LPV are generally identical on the final approach segment. So the missed approach performance is usually the same.
From what I have read, obstructions and lighting are generally the two most common reasons.
I still do not have a good handle on these two aspects when comparing the approaches.

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Re: Gps as DME for non published waypoint during ILS

Post by ultraturtle »

Missed approach for all of the above LPV procedures differ from the ILS to the same runway. The pathway through the air differs significantly.
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Re: Gps as DME for non published waypoint during ILS

Post by TimS »

ultraturtle wrote:Missed approach for all of the above LPV procedures differ from the ILS to the same runway. The pathway through the air differs significantly.
Maybe I could have explained it better.
WAAS has tighter lateral and vertical guidance then VOR/NDB or other ground based navigation systems. Therefore, LPV generally has a smaller restriction area on the missed than ILS does since you continue with WAAS based navigation while with ILS you traditionally switch to VOR (or rarely NDB). e.g. one degree of a VOR at one mile has an error of approximately 1000ft. While WAAS will be 50ft.
Therefore, unless the VOR is on the field, and you are within 500ft or so of it, WAAS will generally be more accurate. Because of this accuracy, LPV has more flexibility on the missed than ILS; which as a general rule means missed approaches do not factor into why an ILS has a lower minimum than LPV, since the missed starts from the same vector path.
However, with that stated. I will post this list over on BeechTalk. There are a few FAA chart guys on there who answer such technical questions.

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Re: Gps as DME for non published waypoint during ILS

Post by Rich »

FWIW, I analyzed the LPV vs. ILS approaches for RWY 35 at KCTJ. Despite what it says on both charts (GP 3.00 degrees) I conclude that the LPV approach is marginally steeper, 3.5 degrees. This doesn't sound like much, but at 1 nm from a given point (say the rwy threshold), the LPV glidepath is 50ft. higher above said point. Voila!

Now why the RNAV approach is steeper? I have some thoughts but I'm not a terpster.
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Re: Gps as DME for non published waypoint during ILS

Post by Chris B »

TimS wrote:Because of this accuracy, LPV has more flexibility on the missed than ILS; which as a general rule means missed approaches do not factor into why an ILS has a lower minimum than LPV, since the missed starts from the same vector path.
FYI, I submitted a request in 2015 to change the RNAV approach at SBA, which currently has LPV minimums 1.8x the ILS. In that case the discrepancy was completely due to the MAP obstacles, according to the FAA. The revised procedure is due in November 2018, when the ILS & LPV minimums should match.

LPV approach + ILS lighting is the best! :thumbsup:

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Re: Gps as DME for non published waypoint during ILS

Post by Wall-e »

ILS can have lower mins than LPV due to approach light systems among others. Remember that RNAV approaches are technically non-precision in the eyes of the FAA regardless of vertical guidance. Only ILS or others MLS are considered precision approaches. My airport is uncontrolled & has no ILS but we have an RNAV down to 200' HAT but the required vis is 1 mi vs 1/2 mi for the typical ILS. We have no approach lights at all. The main reason that approach goes that low is due to no obstacles on the approach over the water.
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Re: Gps as DME for non published waypoint during ILS

Post by Chris B »

Wall-e wrote:Remember that RNAV approaches are technically non-precision in the eyes of the FAA regardless of vertical guidance. Only ILS or others MLS are considered precision approaches.
This is an ICAO distinction that is only relevant for RNP ("Authorization Required") approaches. At an airport with ILS lighting, ILS Cat I & LPV are functionally equivalent. IME, LPV is actually *more* precise due to better stability. ILS uses analog signals that wobble. This is clearly visible with synthetic vision.

Per the FAA: "LPV approaches are operationally equivalent to the
legacy instrument landing systems (ILS) but are more economical because no navigation infrastructure has to be installed at the runway."

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Re: Gps as DME for non published waypoint during ILS

Post by Rich »

The only places I've found where the LPV has the same minima as an ILS is where it is coincident with an existing ILS. That's because it can leverage the approach lights installed long ago for said ILS. Absent that, and where there aren't obstacle/terrain problems, minima will usually be 250-1.

A great example would be RDM, where the RNAV 23 has the same minima as ILS 35. And two of the other 3 provide 250-(3/4 or 7/8) and the remaining one is 286-1. (One does have to choose carefully, among RNAV Y vs Z)

I know of no airport runway lacking an ILS where approach lights have been installed specifically to support an LPV. I have seen cases where the GS is out of service for some reason but everything else is still up and you still have a good LPV available.

I've also seen LOC BC approaches de-published where an RNAV has supplanted it. Good riddance :thumbsup:
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