Why higher LPV minimums?

dell30rb

dell30rb

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http://155.178.201.160/d-tpp/1204/09472R21.PDF

Why are the LPV minimums higher than the LNAV/VNAV here? I thought LPV was a more precise approach and was entitled to lower minimums than LNAV/VNAV ...

The airplane I normally fly and took the checkride in has no GPS so my knowledge is lacking in this department.
 
Per my understanding what you encountered is one of those super rare cases when LNAV/VNAV goes lower than LPV. This can apparently be caused by a different way how the obstacles are evaluated for these two approaches. And it is also my understanding that (if equipped with a certified WAAS/GPS) you could go ahead an fly the LNAV/VNAV in this case.
 
You need WAAS for the LPV. You do not need it for LNAV/VNAV. If you find the LPV mins higher just fly the LNAV/VNAV.

Sorry I can't answer your question as to the 'why' of it.
 
Captain said:
You do not need it for LNAV/VNAV.
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But you need a special baro setup that some lowly GA aircraft don't have. I think he asked from a perspective of a GA pilot in which case he needs WAAS/GPS to fly it.
 
Captain said:
You need WAAS for the LPV. You do not need it for LNAV/VNAV.
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...but if you don't have a certified Baro-VNAV system along with your non-WAAS IFR approach GPS (and not many planes below the bizjet level do, as the first thing you need for that is an air data computer), you need WAAS GPS to fly an LNAV/VNAV RNAV(GPS) approach.
 
Ron Levy said:
...but if you don't have a certified Baro-VNAV system along with your non-WAAS IFR approach GPS (and not many planes below the bizjet level do, as the first thing you need for that is an air data computer), you need WAAS GPS to fly an LNAV/VNAV RNAV(GPS) approach.
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I believe the WAAS navigator would automatically select LPV.
 
olasek said:
But you need a special baro setup that some lowly GA aircraft don't have. I think he asked from a perspective of a GA pilot in which case he needs WAAS/GPS to fly it.
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Ron Levy said:
...but if you don't have a certified Baro-VNAV system along with your non-WAAS IFR approach GPS (and not many planes below the bizjet level do, as the first thing you need for that is an air data computer), you need WAAS GPS to fly an LNAV/VNAV RNAV(GPS) approach.
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I did not know that. It's never applied as I've strictly flown RVSM capable planes since there was RVSM.

Thanks.

Btw, how would I know that? Is it printed on the chart someplace? 'Baro requirements in lieu of WAAS' or something?
 
Captain said:
I did not know that. It's never applied as I've strictly flown RVSM capable planes since there was RVSM.
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I don't know if RVSM capability necessarily means LNAV/VNAV capability or vice verrsa. LNAV/VNAV gives you vertical guidance and (I guess) FMS is involved, but really I am not an ATP I don't fly Citations, Falcons or Gulfstreams, isn't it stuff that you should know by heart?

Btw, how would I know that? Is it printed on the chart someplace? 'Baro requirements in lieu of WAAS' or something?
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Is it printed anywhere on an ILS chart that you need a ILS receiver? Captain with all due respect I am just a lowly PPL and frankly your questions surprised me.
 
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One more thing, visibility in the lpv is 1mi while the vnav is 7/8. A 1/8 mile difference. Seems ridiculous. I can't tell 1/8 mi difference in visibility.
 
olasek said:
I don't know if RVSM capability necessarily means LNAV/VNAV capability or vice verrsa. LNAV/VNAV gives you vertical guidance and (I guess) FMS is involved, but really I am not an ATP I don't fly Citations, Falcons or Gulfstreams, isn't it stuff that you should know by heart?

Is it printed anywhere on an ILS chart that you need a ILS receiver? Captain with all due respect I am just a lowly PPL and frankly your questions surprised me.
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I know what my plane is authorized to do. I can not fly an LPV but I can fly an LNAV/VNAV and I don't have WAAS. When I go to recurrent they don't ask what a Cessna is required to have to fly an approach. I also don't do BFRs or IPCs as such.
 
dell30rb said:
One more thing, visibility in the lpv is 1mi while the vnav is 7/8. A 1/8 mile difference. Seems ridiculous. I can't tell 1/8 mi difference in visibility.
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This was due to change 20 of TERPS which aligns the US standards closer with EU-OPS (according to Jeppesen). But yes... I know, and agree with you.
 
dell30rb said:
One more thing, visibility in the lpv is 1mi while the vnav is 7/8. A 1/8 mile difference. Seems ridiculous. I can't tell 1/8 mi difference in visibility.
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But this extra 1/8 mile makes sense here. If you are higher (524') this extra 1/8 mile is exactly what you need to spot approach lights/runway. If you compare 524 against 479 and work out the glideslope - you will get just about 1/8 mile diff. In practice it will mean if you can spot the approach lights and this is how you judge visibility (for Part 91 flying).
 
aterpster said:
I believe the WAAS navigator would automatically select LPV.
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You'd have to ask John Collins about that, because I don't know if the internal criteria for selection of LPV are more stringent than those for LNAV/VNAV or not, and he probably does.
 
Captain said:
I did not know that. It's never applied as I've strictly flown RVSM capable planes since there was RVSM.
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I doubt there's any way to get RVSM certified without an air data computer, and if you have one along with a non-WAAS approach GPS, it would make sense to hook it up for LNAV/VNAV. What avionics suite are you flying?

Btw, how would I know that? Is it printed on the chart someplace? 'Baro requirements in lieu of WAAS' or something?
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It's in the AIM and IPH. Just search Section 1 of the AIM and the Approach chapter of the IPH for "Baro-VNAV."
 
dell30rb said:
That's silly
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You can always round it up to the next 1/4 mile if precision of 1/8 mile bothers you but otherwise I see nothing silly here.... Both numbers ought to be different and the only question is by how much.
 
I suppose. I guess it's what you can see on the runway. Sir, when I looked up, I saw the numbers, touchdown markers and a rare bearded titmouse perched on the left most light of the VASI. I know the AWOS was reporting 3/4 mi, but it was definitely 7/8.
 
Ron Levy said:
I doubt there's any way to get RVSM certified without an air data computer, and if you have one along with a non-WAAS approach GPS, it would make sense to hook it up for LNAV/VNAV. What avionics suite are you flying?

It's in the AIM and IPH. Just search Section 1 of the AIM and the Approach chapter of the IPH for "Baro-VNAV."
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We do have an ADC (2 of them in fact). We have Collins Proline 21.

I think many PPLs would be surprised at what most ATPs don't know. All the WAAS stuff doesn't apply to 99% of airliners out there.

I try to stay abreast. I keep my CFI current and a major reason I do is I learn something every other year about what's new out there.
 
Captain said:
We do have an ADC (2 of them in fact). We have Collins Proline 21.
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You can get that with either the 4000A non-WAAS or 4000S WAAS GPS. That's the difference between sole source and LPV versus supplemental and Baro-VNAV.
 
HOW DO YOU KNOW THIS STUFF!!!???!!!


[picture Wayne and Garth on their knees]

I'm not worthy, I'm not worthy...
 
aterpster said:
I believe the WAAS navigator would automatically select LPV.
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That to me is the $60,000 question how WAAS navigator would handle it. Would you be allowed to force selection of LNAV/VNAV over LPV?. I do have a Garmin G1000 trainer but the database is very old and this particular airport shows no LPV. I will be monitoring this thread if somebody chimes in.
 
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HPNPilot1200 said:
This was due to change 20 of TERPS which aligns the US standards closer with EU-OPS (according to Jeppesen). But yes... I know, and agree with you.
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True for MDAs. But DAs are strictly the angle from the DA point to the threshold plus credt for approach lights, if any.
 
olasek said:
That to me is the $60,000 question how WAAS navigator would handle it. Would you be allowed to force selection of LNAV/VNAV over LPV?. I do have a Garmin G1000 trainer but the database is very old and this particular airport shows no LPV. I will be monitoring this thread if somebody chimes in.
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I just checked a current database. It defaults to LPV, which is the best service so far as the navigator is concerned.
 
aterpster said:
True for MDAs. But DAs are strictly the angle from the DA point to the threshold plus credt for approach lights, if any.
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Ah ok. I thought they used the table in 8260.3B (page 3-21). Do they use that for non precisions only?
 

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olasek said:
That to me is the $60,000 question how WAAS navigator would handle it. Would you be allowed to force selection of LNAV/VNAV over LPV?
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Not on any of the Garmin systems (400/500-series, 1000) -- it's done automatically with priority of LPV then LNAV/VNAV then LNAV+V then LNAV-only.
 
There are several paculiarities on these approaches if you really dig in and yes, there are times the LPV may not go as low as the LNAV/VNAV as has been said. The LPV must have the angle needed on the final approach segment, and that can influence DA. That's why one needs to look at the approach and brief it. The Garmin gives you the best approach available about one minute from the FAF. You may not want the LPV in this case and would just fly the LNV/VNAV.

There are times dive and drive works better from an MDA also if there is vertical guidance as the guidance can bring you down to MDA later than you would like when you come to the visual descent point. LNAV plus V can also lead to you busting a step down point with certain altimeter settings; so, one needs to pay attention.

At SIMCOM last week, the instructor discussed why there is now a DA. It would have taken the FAA a long time to meet all the requirements to make the LPV a 'precision' approach which could have delayed implementation. So, they created a new category (g). For all intents and purposes, it's a precision approach, but isn't categorized as such.

Have fun and fly safe!

Best,

Dave
 
dell30rb said:
http://155.178.201.160/d-tpp/1204/09472R21.PDF

Why are the LPV minimums higher than the LNAV/VNAV here? I thought LPV was a more precise approach and was entitled to lower minimums than LNAV/VNAV ...

The airplane I normally fly and took the checkride in has no GPS so my knowledge is lacking in this department.
Click to expand...

I'll go on record as saying that it's an oversight on the part of the folks designing and QAing the proceedure. There are cases where LNAV will beat LPV, but there should never be a case where LNAV/VNAV beats LPV.
 
One would think the order would go:

LNAV gets you low,

LNAV / VNAV gets you lower,

LPV gets you lowest.
 
Captain said:
One would think the order would go:

LNAV gets you low,

LNAV / VNAV gets you lower,

LPV gets you lowest.
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Logically that would make sense, but LNAV will often beat LNAV/VNAV in both visibility requirements (because the missed is over the threshold) and minima (generally due to the differences in the missed approach segment--LNAV/VNAV and LPV have straight segments whereas an LNAV missed can be a climbing turn).
 
Ron Levy said:
-- it's done automatically with priority of LPV then LNAV/VNAV then LNAV+V then LNAV-only.
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Yeah, but my understanding is you can still select individual approaches in Garmin, for example you can select LNAV rather than LPV or LNAV/VNAV over LPV. You still have this choice or are you suggesting otherwise?. I may have to try it out in my G1000 trainer tonight but have to pick the right airport.
 
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olasek said:
Yeah, but my understanding is you can still select individual approaches in Garmin, for example you can select LNAV rather than LPV or LNAV/VNAV over LPV. You still have this choice or are you suggesting otherwise?.
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Otherwise. I know of no manual override to the mode selection.
 
Brad Z said:
I'll go on record as saying that it's an oversight on the part of the folks designing and QAing the proceedure. There are cases where LNAV will beat LPV, but there should never be a case where LNAV/VNAV beats LPV.
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Except that in the case of the approach that started this thread, it does, and that's the issue. What do you do if you want to fly to the lower LNAV/VNAV mins but the system automatically takes you to LPV? I think you're stuck with the Garmin WAAS units.
 
I can think of one way to force LNAV in the 480, and that's by turning off WAAS (i.e. disable SBAS). You'll then get no GPS-based vertical guidance at all.

I'm not sure whether you can do that in the 430W or 530W.
 
azure said:
I can think of one way to force LNAV in the 480, and that's by turning off WAAS (i.e. disable SBAS). You'll then get no GPS-based vertical guidance at all.

I'm not sure whether you can do that in the 430W or 530W.
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Unless you also have an air data computer hooked up and certified for Baro-VNAV, that won't get you LNAV/VNAV.
 
I'm scratching my head a bit; why would flying to LNAV/VNAV minimums with the LPV selected be difficult? Just like flying an ILS to localizer minimums. I guess if you couple the AP in approach mode, it would be. Use heading mode and follow the step downs, or hand fly the approach. Watch the LNAV and descend to those minimums.

Best,

Dave
 
Dave Siciliano said:
I'm scratching my head a bit; why would flying to LNAV/VNAV minimums with the LPV selected be difficult? Just like flying an ILS to localizer minimums. I guess if you couple the AP in approach mode, it would be. Use heading mode and follow the step downs, or hand fly the approach. Watch the LNAV and descend to those minimums.

Best,

Dave
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Well, from what I understand flying LNAV/VNAV the localizer and glideslope are going to be computed differently with different sensitivity by the GPS than if it were in LPV mode. And flying an ILS to localizer minimums is not the same, as LNAV/VNAV does have vertical guidance (provided by waas or baro-aided equipment) where a localizer approach has no vertical guidance.
 
Dave Siciliano said:
I'm scratching my head a bit; why would flying to LNAV/VNAV minimums with the LPV selected be difficult? Just like flying an ILS to localizer minimums. I guess if you couple the AP in approach mode, it would be. Use heading mode and follow the step downs, or hand fly the approach. Watch the LNAV and descend to those minimums.

Best,

Dave
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Not difficult, Necessarily, just not legal, and possibly unsafe.
 
I guess I just didn't explain myself well; on a localizer approach, one doesn't have to follow the GS if working do they? Must one follow the Vert guidance on an LNAV if it's provided?

Dave
 
Dave Siciliano said:
I guess I just didn't explain myself well; on a localizer approach, one doesn't have to follow the GS if working do they? Must one follow the Vert guidance on an LNAV if it's provided?

Dave
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I understand... I am not sure if you could do that legally if your GPS is in LPV mode because the lateral guidance is calculated differently than it would be for LNAV. In practice it should be more sensitive and actually better, so I don't know, maybe it is allowed.
 
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