BVN GPS approach minimums

Briar Rabbit

Briar Rabbit

Line Up and Wait
Joined
Mar 22, 2017
Messages
626
Location
Albion, Nebraska
Display Name
Display name:
Rob
GNSS approaches were not part of my instrument training 45 years ago. They certainly have simplified approaches at rural airports! I have not seen exact definitions for the abbreviations but anticipate the following apply:
LPV = lateral precision vertical
LNAV/VNAV = lateral navigation & vertical navigation
LNAV = lateral navigation only

Two questions?

At KBVN RNAV (GPS) rwy 15 the DA for the LNAV/VNAV is higher than MDA for the LNAV, why?

LPV DA 2130
LNAV/VNAV DA 2269
LNAV MDA 2140

What is my Avidyne 540 receiving differently for the first two and how will I know which to use? My personal minimum for shooting the approach is 500 feet so has never been an issue but will appreciate any help understanding this please.
 
@citizen5000 , thanks for the link to my blog!

@Briar Rabbit - it's a situation that doesn't make intuitive sense, but it does happen more often than you'd think. (And obviously this question comes up often enough that I decided to write about it.) I think I cover the situation sufficiently in the article, but if you still have questions, let me know and I'll see if I can answer them (and might update the blog post if it isn't clear enough).

I am not very familiar with the Avidyne 540, but it should be annunciating which line of minima it is capable of flying - which should usually be the LPV. There would usually be no reason for you to fly the LNAV/VNAV or LNAV lines of minima with the equipment you have. In fact, if it's anything like the Garmin receivers, you can't easily get it to fly the other lines of minima without going deep into the menus and changing some unusual settings.
 
Thank you for linking to the explanation article. That clears up question #1. And then for the second question -since the Avidyne unit does not have any ties to baro-VNAV it will never see information pertaining to a LNAV/VNAV approach. If the glide slope is operating correctly it has to be information for a LPV approach? Correct?

Thanks for clearing this up for me!
 
Briar Rabbit said:
And then for the second question -since the Avidyne unit does not have any ties to baro-VNAV it will never see information pertaining to a LNAV/VNAV approach. If the glide slope is operating correctly it has to be information for a LPV approach? Correct?
Click to expand...
Check your AFM/Supplement, but WAAS (required for LPV) can generally replace the Baro requirement for LNAV/VNAV.
 
Yes, WAAS can be used to fly LNAV/VNAV (replacing the baro component), but if the approach also has LPV minimums, it will default to that. On the Garmin series, I am not aware of any way to force the unit to fly LNAV/VNAV if LPV is available. I imagine the Avidyne is similar.

However, if the approach procedure only has LNAV and LNAV/VNAV minimums, then the unit would default to the LNAV/VNAV minimums and should annunciate that.

Example: 2O8 RNAV (GPS) RWY 35. (Not that the LNAV/VNAV gets much benefit though.)

https://skyvector.com/files/tpp/1705/pdf/09415R35.PDF
 
Briar Rabbit said:
GNSS approaches were not part of my instrument training 45 years ago. They certainly have simplified approaches at rural airports! I have not seen exact definitions for the abbreviations but anticipate the following apply:
LPV = lateral precision vertical
LNAV/VNAV = lateral navigation & vertical navigation
LNAV = lateral navigation only

Two questions?

At KBVN RNAV (GPS) rwy 15 the DA for the LNAV/VNAV is higher than MDA for the LNAV, why?

LPV DA 2130
LNAV/VNAV DA 2269
LNAV MDA 2140

What is my Avidyne 540 receiving differently for the first two and how will I know which to use? My personal minimum for shooting the approach is 500 feet so has never been an issue but will appreciate any help understanding this please.
Click to expand...
AIM 5-4-5 f., FIG 5-4-12 tells you all about it.
 
RussR said:
Yes, WAAS can be used to fly LNAV/VNAV (replacing the baro component), but if the approach also has LPV minimums, it will default to that. On the Garmin series, I am not aware of any way to force the unit to fly LNAV/VNAV if LPV is available. I imagine the Avidyne is similar.
Click to expand...
The unit will display the LPV glide path, but you can fly to any minimums the unit is certified for.
 
MauleSkinner said:
The unit will display the LPV glide path, but you can fly to any minimums the unit is certified for.
Click to expand...

You can fly an LPV glidepath to whatever minimums you want (above the LPV DA), but why would you fly them to a higher DA? Sure, you could just fly them to the LNAV/VNAV DA, but that's not "quite" the same as intentionally dumbing down your system to fly the actual LNAV/VNAV minimums. This is for somewhat more esoteric reasons such as lateral course width and even glidepath accuracy and integrity, but that's well beyond the OP's question.

Undoubtedly, there are some semantics at play here. If you fly the LPV to the LNAV/VNAV minimums, I wouldn't call it "flying the LNAV/VNAV", I'd call it "flying the LPV to an arbitrary higher DA". If the box is annunciating LNAV/VNAV, then of course I'd fly to the LNAV/VNAV DA and call it "flying the LNAV/VNAV".

With the Garmin 430W/530W, you can force it to fly to LNAV-only minimums on a LPV procedure by going in the menus and turning off WAAS. I do this sometimes for training. It will then annunciate "LNAV" and the trainee will be forced to fly it without vertical guidance, leveling off at the MDA and flying to the MAP. However, I am unfamiliar with a way to force it to annunciate LNAV/VNAV. (But I would love to learn a way to do that.)
 
Last edited:
RussR said:
You can fly an LPV glidepath to whatever minimums you want (above the LPV DA), but why would you fly them to a higher DA? Sure, you could just fly them to the LNAV/VNAV DA, but that's not "quite" the same as intentionally dumbing down your system to fly the actual LNAV/VNAV minimums. This is for somewhat more esoteric reasons such as lateral course width and even glidepath accuracy and integrity, but that's well beyond the OP's question.
Click to expand...
It's actually probably very relevant to the OP's question, since one of the questions he was asking regarded LNAV/VNAV DA being lower than the LPV DA.
 
MauleSkinner said:
It's actually probably very relevant to the OP's question, since one of the questions he was asking regarded LNAV/VNAV DA being lower than the LPV DA.
Click to expand...

That question was answered in Post #2 of the thread, hopefully to the OP's satisfaction.
 
A factor in DA's being higher than MDA's is that MDA's are a hard bottom, you cant be flying below them. DA's are not a hard bottom. You have begin the missed approach there but inertia will take you a little lower before the climb begins. And the glide path is protected for being a little low, maybe a dot or two. Sometimes an obstruction sticks up at just the right place to force the DA higher than the MDA.
 
MauleSkinner said:
doesnt appear that way to me, based on Post #4.
Click to expand...

You're right. But I think you answered that question in Post 5.

I think we're talking past each other a bit here. WAAS can be used in lieu of Baro for and LNAV/VNAV, yes. But if the approach has LPV minimums, then there is no likely no way to force the unit to annunciate LNAV/VNAV minimums. You'd have to find an approach where LNAV/VNAV is the "best" line of minimums in order to "use" that WAAS-for-Baro substitution. There are many examples of these types of procedures.
 
I thought I had this figured out but your latest posts are putting doubts in my mind. The LPV DA is the lowest. The LNAV MDA is next, and the highest is the LNAV/VNAV DA in this example. I understand now that due to the criteria of the formula why the LNAV/VNAV is the highest. OK so far so good.

But to my knowledge the Avidyne unit has no barometric inputs, not sure I totally understand the baro-VNAV source of information but I am thinking it is substituting the higher quality WAAS input internally in the Avidyne 540 unless it is getting a baro signal from the associated mode C transponder input? Is there a situation where a LNAV/VNAV true baro-VNAV source will be more accurate than a WAAS related LPV signal?

Thanks again for your help!
 
Sorry to confuse you...LPV will always be the most accurate. If an approach doesn't have LPV minimums, the vertical guidance available is the equivalent of Baro VNAV, and in fact is probably more accurate, as it's not affected by nonstandard temperatures or mis-set altimeters.
 
RussR said:
Yes, WAAS can be used to fly LNAV/VNAV (replacing the baro component), but if the approach also has LPV minimums, it will default to that. On the Garmin series, I am not aware of any way to force the unit to fly LNAV/VNAV if LPV is available.
Click to expand...
The way it was explained to me by a Garmin avionics engineer: the WAAS receiver has downgrade logic in the event the highest level of service doesn't pass the internal integrity checks; i.e., LPV drops to LNAV/VNAV, and if that doesn't pass muster then to LNAV only. At LNAV-only level of service it is then in TSO-C129 mode. He added, although this is the way the avionics are engineered, the chances of seeing LPV drop to LNAV/VNAV is about 1 or 2 percent. Typically, it would drop to LNAV only (TSO-C129 specs). In the case of an LP approach, it would drop to LNAV.
 
Briar Rabbit said:
GNSS approaches were not part of my instrument training 45 years ago. They certainly have simplified approaches at rural airports! I have not seen exact definitions for the abbreviations but anticipate the following apply:
LPV = lateral precision vertical
LNAV/VNAV = lateral navigation & vertical navigation
LNAV = lateral navigation only

Two questions?

At KBVN RNAV (GPS) rwy 15 the DA for the LNAV/VNAV is higher than MDA for the LNAV, why?

LPV DA 2130
LNAV/VNAV DA 2269
LNAV MDA 2140

What is my Avidyne 540 receiving differently for the first two and how will I know which to use? My personal minimum for shooting the approach is 500 feet so has never been an issue but will appreciate any help understanding this please.
Click to expand...

You don't get to pick your poison. The approach plate gives you the minima for all, but the Box tells you which minima you are using. However, if you have a non-WAAS Box, then it's always the LNAV.
 
SbestCFII said:
You don't get to pick your poison. The approach plate gives you the minima for all, but the Box tells you which minima you are using. However, if you have a non-WAAS Box, then it's always the LNAV.
Click to expand...
No, the box tells you what's available to you...if it says LPV, you have the choice of LPV, LNAV/VNAV, or LNAV if all of those minima are on the approach plate.

If it says LNAV/VNAV, you can also choose LNAV if it's published on the plate.
 
I'm wondering about the "baro-nav." I think I get it that pressure is used in "crunching the numbers" to calculate and display a "faux" glideslope. But are there GPS units where you can read when your at DH right on the GPS unit without having to look at the altimeter?
 
You must log in or register to reply here.
Back
Top