If you HAVE to shoot an approach, would you choose an ILS or an LPV?

RotorDude said:
As I understand it, both the ILS and LPV GS are supposed to bring you to a touchdown on the thousand foot markers (TFMs). However, that depends on where the receiver antenna is installed on the aircraft relative to the bottom of the wheels. I also heard that ILS GS typically "flares" a bit over the runway, whereas LPV is a laser-straight line from the FAF to the TFMs. Both are supposed to cross the threshold at around 50' (not sure if that's antenna height or wheel height), which is how the (typically 3 degree) descent angle is defined.
But as always, I stand to be corrected on all these points by the experts.
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I've followed the LPV down on numerous occasions at numerous airports and you are correct - Thousand-footers on the nose.
 
Captain said:
Has anyone ever rode both to the asphalt on the same runway? It'd be interesting to compare actual data. I still suspect the ILS would do better but am open to evidence otherwise.

WAAS/ non-WAAS should be stipulated. Also needle deflection at 10' should be noted (or some representative altitude prior to flare)
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Yes, actually I do it regularly in practice. The needles don't typically diverge much at all until well inside the missed.
 
MAKG1 said:
With a 200 AGL minimum it damn well had better be pointed at the TDZ.

LPVs are supposed to be "just like" ILSs. Though the FAA considers them non precision.
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MAKG, where does the FAA say specifically that LPV or LNAV/VNAV approaches are "non-precision"?

I think they actually put it into another category called "Approaches with Vertical Guidance" (APV) see the AIM 5-4-5.

That approach type is maybe not "precision", but it's certainly not considered "non-precision". You can in fact use an LPV approach as a substitute to demonstrate the requirements of a precision approach on a checkride so I would definitely not lump this into the non-precision category. It's one of those weird hybrids, but given the choice between a non-precision and an LPV, I'm definitely taking the LPV if it's available.
 
Caramon13 said:
MAKG, where does the FAA say specifically that LPV or LNAV/VNAV approaches are "non-precision"?

I think they actually put it into another category called "Approaches with Vertical Guidance" (APV) see the AIM 5-4-5.

That approach type is maybe not "precision", but it's certainly not considered "non-precision". You can in fact use an LPV approach as a substitute to demonstrate the requirements of a precision approach on a checkride so I would definitely not lump this into the non-precision category. It's one of those weird hybrids, but given the choice between a non-precision and an LPV, I'm definitely taking the LPV if it's available.
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There was a lot of confusion within the FAA in an attempt to keep ICAO happy. In a poor attempt to comply with ICAO specifications pertaining to precision instrument approaches the FAA classified LPV and LNAV/VNAV as NPAs. That went over like a lead balloon so they eventually dropped that absurdity for the far more logical APV.
 
Ummmmm, I might be close to converting.
 
Jaybird180 said:
You have to choose a Religion first before you convert.:)
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Moving from the faith of the localizer and glide slope to a child of the magenta is indeed a conversion...
 
Clark1961 said:
Moving from the faith of the localizer and glide slope to a child of the magenta is indeed a conversion...
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While I agree with you, it does require him to confess his faith first which is exactly my point.:lol:
 
I know that the TCH is the same for the ILS and the GPS approach to one runway I'm thinking of. The GS is 3 degrees on both... So both should put you on the pavement at the same place, theoretically. I know I've stayed on the glideslope well below DA with either type of approach, but once close to the threshold I forget about looking at the panel. Obviously in the crazy scenario painted in the OP, eyes on it the whole way down. I'm really curious to what it would show. I've got dual glideslopes so I could compare them.
 
Jaybird180 said:
While I agree with you, it does require him to confess his faith first which is exactly my point.:lol:
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I have actually, in real life, flown a GPS and an LNAV/VNAV. I need not confess faith in either since I have personally observed both...alot.

The "conversion" I refer to is my opinion that one would be better than the other in the OP's scenario. I still think I'd take the ILS but at least now I don't think someone would be crazy to take the LPV. That's me converting...
 
I'd take the LPV and I'd turn on my PFD's synthetic vision for good measure regardless of the approach (it's usually on anyways, but just in case I had turned it off for some reason.)
 
Interesting conversation. Purely from a technical standpoint the LPV is likely going to give a steadier signal all the way to the 1000' markers since the ability of the aircraft of measure it's position doesn't really change along the flight path (since the satellites are in space the whole time).

The GS of an ILS does got a bit wonky sometimes right before touchdown (if you follow it all the way down) since at that point you are literally just feet from the antenna. My understanding is that true CAT-IIIc aircraft that fly these things to the ground in "zero" vis switch over to ground proximity radar altitude for the final "GS" adjustments to the ground for the autopilot inputs.

In a real scenario, use what ya got. If you have both use both. If you have synthetic vision on top of all that even better.
 
I don't often fly instrument approaches to the ground, but when I do, I fly LPV....
 
rocketflyer84 said:
The GS of an ILS does got a bit wonky sometimes right before touchdown (if you follow it all the way down) since at that point you are literally just feet from the antenna. My understanding is that true CAT-IIIc aircraft that fly these things to the ground in "zero" vis switch over to ground proximity radar altitude for the final "GS" adjustments to the ground for the autopilot inputs.
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On a CAT III approach the flight computer/autoflight begin to blend the GS with the radar altimeter at 150 feet. This blending progresses in favor of the radar altimeter until it is 100% radar altitude as the flare begin.

Also, a CAT III ILS is rock-solid unlike many CAT I ILSes.
 
aterpster said:
On a CAT III approach the flight computer/autoflight begin to blend the GS with the radar altimeter at 150 feet. This blending progresses in favor of the radar altimeter until it is 100% radar altitude as the flare begin.

Also, a CAT III ILS is rock-solid unlike many CAT I ILSes.
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In theory, or in practice?
 
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