GPS approach requests

genna

genna

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So, up until now, I've only had access to non-WAAS GPS(LNAV) and ILS in the planes I fly. Soon, I will be doing more WAAS flying and this will obviously open up LPV approaches for me.

This brings me to what is maybe a silly question. The approach plates for ILS/LOC are always called ILS with LOC being a "fallback option" and typically you request ILS approach if you want to fly it or LOC if you want to do the LOC. The approach plates for LPV are typically named RNAV and LPV is an "option". I've always requested them as RNAV or GPS. So when I want to fly an LPV approach, do I request RNAV/GPS as always and simply fly LPV(does ATC/Tower even care?)? Or do I request LPV(or tell them that this is my intention)?

Also, side note. What's the vertical guidance of WAAS based on? Baro? GPS? Both? Something else? I can't seem to find a clear answer.
 
First, the vertical guidance with WAAS is GPS.

When flying a GPS approach - just request the GPS approach. Whether it is an LNAV or LNAV+V or LPV is a function of your aircraft's equipment, and ATC/Tower doesn't know or care really (they will find out if you have to miss because you can't use low enough minimums :eek: )
Pretty much the same for an ILS/LOC. You can fly either one depending on your equipment.

On checkrides in LPV equipped airplanes, sometimes the examiner requests that the approach be flown as an LNAV (using the appropriate stepdowns and altitudes instead of the GS) - since you can't turn off the GS.
In the 'old days' most planes had a LOC and and ILS head so you just used the one for the approach the examiner wanted you to do.
 
I have a WAAS GPS in my plane. When I request an RNAV, I'll fly to the lowest minimums.

Sometimes that's LPV, but could be LP or LNAV/VNAV or just LNAV.

You'll know what you can fly to as the annunciator on the GPS itself should tell you what "mode" it's in. If it says LPV, that's what you can fly down to. LNAV is LNAV, or you get a LNAV+V for the LNAV/VNAV, etc. Dennis is correct that if you are TOLD what it says (LNAV/LP for example) you must fly the step down fixes for example, in a checkride.

Vertical guidance is based on two different things in a GPS approach (from what I've read).

If you're flying an LNAV+V (which is a LNAV/VNAV approach): The Vertical Navigation (VNAV) utilizes an internally generated glideslope based on WAAS or baro-VNAV systems. Minimums are published as a DA. If baro-VNAV is used instead of WAAS, the pilot may have approach restrictions as a result of temperature limitations and must check predictive RAIM.

If you're flying an LPV: LPV approaches take advantage of the refined accuracy of WAAS
lateral and vertical guidance to provide an approach very similar to a Category I ILS. Like an ILS, an LPV has vertical guidance and is flown to a Decision Altitude (DA). The design of an LPV approach incorporates angular guidance with increasing sensitivity
as an aircraft gets closer to the runway (or point in space (PinS) type approaches for helicopters). Sensitivities are nearly identical to those of the ILS at similar distances. This is intentional to aid pilots in transferring their ILS flying skills to LPV approaches.

Both vertical descriptions come from the FAA material here.
 
genna said:
So, up until now, I've only had access to non-WAAS GPS(LNAV) and ILS in the planes I fly. Soon, I will be doing more WAAS flying and this will obviously open up LPV approaches for me.

This brings me to what is maybe a silly question. The approach plates for ILS/LOC are always called ILS with LOC being a "fallback option" and typically you request ILS approach if you want to fly it or LOC if you want to do the LOC. The approach plates for LPV are typically named RNAV and LPV is an "option". I've always requested them as RNAV or GPS. So when I want to fly an LPV approach, do I request RNAV/GPS as always and simply fly LPV(does ATC/Tower even care?)? Or do I request LPV(or tell them that this is my intention)?

Also, side note. What's the vertical guidance of WAAS based on? Baro? GPS? Both? Something else? I can't seem to find a clear answer.
Click to expand...

I'm just guessing, but is it possible that you have not read the new Instrument Procedures Handbook? Available on the faa.gov website.

Bob Gardner
 
Good info, Caramon.

I like pie. (Bookmarking this thread)
 
If the approach has LPV minimums and you have an LPV annunciation on that WAAS GPS (you don't always), you can fly it as LPV. You will only be cleared for the RNAV approach if it is combined as LPV, LNAV (etc). If the GPS is only showing LNAV mode, you're stuck with that.

Some airports have different approaches with and without vertical guidance for the same runway, generally associated with terrain or traffic. LPV approaches can't tolerate as much turning. In that case, you'll be cleared for the RNAV Y or RNAV Z (etc), and you have to follow that. That's what you ask for as well.

For instance, flying into KRHV, there is terrain on the right and the San Jose conga line on the left. For this reason, the non precision RNAV Y 31R is usually in use. It off alignment somewhat (only about 5 deg) and has a turn at the FAF. If you want the LPV approach, you ask for RNAV Z 31R. It's closer to San Jose traffic, so you might get an extra delay. But, there is no turn at the FAF and it's straight in.
 
My GPS is the CNX80/GNA480. When an RNAV approach is loaded, executed and being flown, an annunciation for LPV or LNAV+V or LNAV will appear just above the bottom row of information. I'm developing the habit of calling out, "I have ____ on the GPS, which means my minimums are now ______". And I have a simple bug suction cupped to the altimeter and I set that to the minimum altitude.

In addition to the book that Bob recommends, read up on the flight manual for your GPS unit to see 1) where does it tell you what lateral and vertical guidance parameters it is using, and 2) how to perform a RAIM check.
 
A note on LNAV+V: If I'm reading correctly (links below), that is an LNAV approach for all intents and purposes and should be flown as such. The +V is vertical guidance calculated by the GPS unit and is only advisory. It helps avoid dive and drive. It is not LNAV/VNAV and all step down altitudes are still mandatory.

https://www.faa.gov/about/office_or...nss/library/factsheets/media/RNAV_QFSheet.pdf

http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC 90-107.pdf

LPV: use LPV DA (flown like ILS)
LP: use LP MDA (flown like LOC)
LNAV: use LNAV MDA (flown like LOC)
LNAV+V: use LNAV mins (again, +V is advisory only)
 
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Guys, the dude just asked what to call the approach when talking to ATC.

Just call it whatever the approach is called in your GPS or on the TOP of the plate.

RNAV 23, etc.

You shoot it LPV or not doesn't matter.
 
Caramon13 said:
I have a WAAS GPS in my plane. When I request an RNAV, I'll fly to the lowest minimums.

Sometimes that's LPV, but could be LP or LNAV/VNAV or just LNAV.

You'll know what you can fly to as the annunciator on the GPS itself should tell you what "mode" it's in. If it says LPV, that's what you can fly down to. LNAV is LNAV, or you get a LNAV+V for the LNAV/VNAV, etc. Dennis is correct that if you are TOLD what it says (LNAV/LP for example) you must fly the step down fixes for example, in a checkride.

Vertical guidance is based on two different things in a GPS approach (from what I've read).

If you're flying an LNAV+V (which is a LNAV/VNAV approach): The Vertical Navigation (VNAV) utilizes an internally generated glideslope based on WAAS or baro-VNAV systems. Minimums are published as a DA. If baro-VNAV is used instead of WAAS, the pilot may have approach restrictions as a result of temperature limitations and must check predictive RAIM.

If you're flying an LPV: LPV approaches take advantage of the refined accuracy of WAAS
lateral and vertical guidance to provide an approach very similar to a Category I ILS. Like an ILS, an LPV has vertical guidance and is flown to a Decision Altitude (DA). The design of an LPV approach incorporates angular guidance with increasing sensitivity
as an aircraft gets closer to the runway (or point in space (PinS) type approaches for helicopters). Sensitivities are nearly identical to those of the ILS at similar distances. This is intentional to aid pilots in transferring their ILS flying skills to LPV approaches.

Both vertical descriptions come from the FAA material here.
Click to expand...
Confession: I get lost in the alphabet discussions of GPS approaches.
 
Jaybird180 said:
Confession: I get lost in the alphabet discussions of GPS approaches.
Click to expand...

And it's really annoying, since most GPSs are WAAS these days, so you really only see LPV and LNAV.

I'm sure LP approaches exist somewhere, but I've never seen one. I've seen a lot of LNAV/VNAV minimums, but I haven't seen an airplane with Baro-aided VNAV, and with WAAS, you can just use LPV.
 
So. To recap this discussion:
1. To answer my main question: Request RNAV, fly what you can(GPS receiver will tell you what you can fly). ATC doesn't care
2. I don't care what they do on checkrides as I don't have to do one anymore :)
3. The WAAS question: It's not GPS that gives you the vertical guidance. At least not only GPS. That FAA statement(which I have read before) about WAAS capability does not answer my question on what this capability is technically based on. However, the new instrument procedures handbook DOES. Thank you Bob. And no, I have not looked at it(LPV/WAAS part) before, since I had no use for it. The answer is this:

"WAAS is a navigation service using a combination of GPS satellites and the WAAS geostationary satellites to improve the navigational service provided by GPS."

p.s. Garmin 750 is the unit
 
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I have asked for the GPS or RNAV approach if the chart shows both. Either works. Then fly to the service level annunciated on the GPS.
 
Pretty sure ATC doesnt need to know which minimums you are going to use.
 
MAKG1 said:
And it's really annoying, since most GPSs are WAAS these days, so you really only see LPV and LNAV.

I'm sure LP approaches exist somewhere, but I've never seen one. I've seen a lot of LNAV/VNAV minimums, but I haven't seen an airplane with Baro-aided VNAV, and with WAAS, you can just use LPV.
Click to expand...
Baro-VNAV is pretty much an airline system.

The difference between LPV and LP is kind of like the difference between an ILS and a LOC-only or offset LDA approach. For similar reasons - terrain makes a protected straight-in electronic glidepath difficult or impossible. And with similar relative numbers; I recall hearing a few years ago that the number of LPs are obly about 10% of the number of LPVs.
 
coloradobluesky said:
Pretty sure ATC doesnt need to know which minimums you are going to use.
Click to expand...
It's not about the minimums. I think it used to be a fairly common practice to mention it if you were flying an ILS approach LOC-only because you might "dive and drive" substantially below the ILS glidepath and set off a concern or advisory.

The issue theoretically exists for precsion vs non-precision GPS approaches but there are a few things that mitigate it. One is that there are still plenty of non-WAAS units out there. Another is that the extra situational awareness we have with everything from on-board GPS to tablet makes it easier to avoid feeling a need to "dive and drive," even on a VOR approach.
 
Mark, is an LPV approach still considered non-precision? I recall reading that it was, despite being nearly as accurate as an ILS.. We just had the debate, don't need to go there, just wondering if the terminology has changed.
 
fiveoboy01 said:
Mark, is an LPV approach still considered non-precision? I recall reading that it was, despite being nearly as accurate as an ILS.. We just had the debate, don't need to go there, just wondering if the terminology has changed.
Click to expand...
An LPV approach is still considered non precision.
 
Thank you. I didn't figure it had changed, but if it did I probably wouldn't know about it:)
 
midlifeflyer said:
Baro-VNAV is pretty much an airline system.
Click to expand...
Quite a few corporate aircraft with pre-WAAS GPS Approach-capable FMSs have Baro VNAV capability as well.
fiveoboy01 said:
Mark, is an LPV approach still considered non-precision? I recall reading that it was, despite being nearly as accurate as an ILS.. We just had the debate, don't need to go there, just wondering if the terminology has changed.
Click to expand...
I'm not sure what it's considered for the purposes of selecting an alternate airport, but, per the PTS "an LPV approach can be flown to demonstrate precision approach proficiency if the LPV DA is equal to or less than 300 feet HAT."
 
fiveoboy01 said:
A RAIM check isn't required with a WAAS equipped GPS unit unless there is an outage.
Click to expand...

I do it anyhow as a simple check. Belt, suspenders, and safety pin idea. Maybe not regulatory, but peace of mind for me.
 
midlifeflyer said:
Baro-VNAV is pretty much an airline system.

The difference between LPV and LP is kind of like the difference between an ILS and a LOC-only or offset LDA approach. For similar reasons - terrain makes a protected straight-in electronic glidepath difficult or impossible. And with similar relative numbers; I recall hearing a few years ago that the number of LPs are obly about 10% of the number of LPVs.
Click to expand...
My GPS yells at me about not having baro data if I have my transponder off. Not sure what it's doing with it, though. It's a 430W.
 
My CNX80 also will ask for the baro setting. But I'm not able to find what it does with the info.
 
AggieMike88 said:
My CNX80 also will ask for the baro setting. But I'm not able to find what it does with the info.
Click to expand...

Searching for "barometric" in the 430W manual didn't help. I did find this tidbit in the 400W series installer manual:

Note: To take full advantage of the 400W Series unit capabilities, an optional barometric altitude source is recommended for automatic sequencing of fix-to-altitude (FA) and hold-to-altitude (HA) leg types. If no barometric altitude data is provided to the 400W Series unit, FA and HA legs must be manually sequenced
Click to expand...
 
mkosmo said:
Searching for "barometric" in the 430W manual didn't help. I did find this tidbit in the 400W series installer manual:
Click to expand...

Thanks, but the FA and HA are not a feature of the CNX80/480. I also searched online for a clue on how my box uses it, and only found pdf's for the install manual that said some baro units could be connected. But nothing found yet on how this is used. I am guessing for the display of TAS if you have an air data computer hooked to it.
 
jordane93 said:
An LPV approach is still considered non precision.
Click to expand...
The argument within the FAA on that one is endless. It distorts the concept of DA vs. MDA.

If it quacks like a duck and walks like a duck....
 
AggieMike88 said:
Thanks, but the FA and HA are not a feature of the CNX80/480. I also searched online for a clue on how my box uses it, and only found pdf's for the install manual that said some baro units could be connected. But nothing found yet on how this is used. I am guessing for the display of TAS if you have an air data computer hooked to it.
Click to expand...

KLN-94 asks for Baro also
 
fiveoboy01 said:
Mark, is an LPV approach still considered non-precision? I recall reading that it was, despite being nearly as accurate as an ILS.. We just had the debate, don't need to go there, just wondering if the terminology has changed.
Click to expand...
In reality it's a precision approach and the FAA recognizes it as one for many purposes - the precision approach required by the Instrument PTS can be an LPV. It even fits the FAR 1.1 definition of "precision approach" although it doesn't specifically mention it:
Precision approach procedure means a standard instrument approach procedure in which an electronic glide slope is provided, such as ILS and PAR.
​
The problem is that the ICAO still technically uses the term to refer only to an ILS glideslope, so the FAA retains the technical distinction for international consistency. As the AIM tells us:
A class of approach procedures which provide vertical guidance, but which do not meet the ICAO Annex 10 requirements for precision approaches has been developed to support satellite navigation use for aviation applications worldwide. These procedures are not precision and are referred to as Approach with Vertical Guidance (APV), are defined in ICAO Annex 6, and include approaches such as the LNAV/VNAV and localizer performance with vertical guidance (LPV). These approaches provide vertical guidance, but do not meet the more stringent standards of a precision approach.
​
 
The reason some of these GPS's ask you for barometer is the transponder isnt warmed up yet. They get baro from the transponder for some accuracy calculations. Just ignore it.
 
midlifeflyer said:
In reality it's a precision approach and the FAA recognizes it as one for many purposes - the precision approach required by the Instrument PTS can be an LPV. It even fits the FAR 1.1 definition of "precision approach" although it doesn't specifically mention it....
Click to expand...
Note, though, the wording in the PTS..."an LPV approach can be flown to demonstrate precision approach proficiency..." with proper minimums criteria. It's not saying "LPV is considered a precision approach."

Taken to an extreme, the ILS 16R at KRNO, with its 2000+ foot DA is still a precision approach, where an LPV with a 301 foot DA can't be used to demonstrate precision approach proficiency.:eek:

Which makes sense in light of your further posting...
[/INDENT]The problem is that the ICAO still technically uses the term to refer only to an ILS glideslope, so the FAA retains the technical distinction for international consistency. As the AIM tells us:
A class of approach procedures which provide vertical guidance, but which do not meet the ICAO Annex 10 requirements for precision approaches has been developed to support satellite navigation use for aviation applications worldwide. These procedures are not precision and are referred to as Approach with Vertical Guidance (APV), are defined in ICAO Annex 6, and include approaches such as the LNAV/VNAV and localizer performance with vertical guidance (LPV). These approaches provide vertical guidance, but do not meet the more stringent standards of a precision approach.
​
Click to expand...
 
I'm confused... On one hand a precision approach is one with an electronic glideslope, but then it goes on to say the LPV doesn't meet the standards of a precision approach.
 
fiveoboy01 said:
I'm confused... On one hand a precision approach is one with an electronic glideslope, but then it goes on to say the LPV doesn't meet the standards of a precision approach.
Click to expand...
You'd have to study the current definition of what ICAO considers a precision approach to figure out that one.
 
midlifeflyer said:
You'd have to study the current definition of what ICAO considers a precision approach to figure out that one.
Click to expand...

Now that LPV is coming into significant use in Europe, I suspect there will be a lot more pressure put on ICAO to back off their stance on LPV.

It's easy to forget that ICAO is part of the United Nations. :eek:
 
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