LPV vs LP+V

[RobertGary1]

Been enjoying my new Garmin cockpit and played around with some unless common approaches we have in the mountains. One was an LP approach. (Not LPV). Surprisingly I did get a glide slope and the indication was LP+V. So that brings up the question of what the difference is. I guess it’s a non standard slope??

 

-Robert 

[y2kiah]

the +V is an advisory glideslope provided by the WAAS GPS receiver, but the approach is still a non-precision type with step down altitudes and an MDA, and will have higher minimums than an LPV approach would. You can fly it just like an ILS, just realize the underlying approach has an MDA so you do not fly the approach to minimums and go missed upon arriving at a DA, you would ride the MDA and go missed at the MAP.

Oddly LPV is not classified as a precision approach, but has a DA and is flown like a precision approach. An LPV approach can be flown to demonstrate precision approach proficiency if the LPV DA is equal to or less than 300 feet HAT.

[RobertGary1]

So I think the legal difference is that the minimums and waypoints on an LP+V are still barometric so you must gross check them on the altimeter. Not so with the LPV. Both are legally non precision which mostly just affects their use as an alternate for ifr filing. 
 

-Robert 

[y2kiah]

well I think I know what you're getting at but minimums on LPV approaches are still referencing barometric altitude and absolutely should be cross-checked with the altimeter, so I would not phrase it that way exactly

[RobertGary1]

I have two legal primary altimeters and it’s not uncommon for them to be off more than 50’. Shop says that’s within tolerances. Makes you wonder if WAAS is more accurate. 
 

 -Robert

[carusoam]

WAAS has the ability to be more accurate... Then an old pressure sensor connected to a needle by tiny gears...

To see the requirements of waas accuracy as outlined by the FAA for our use would require us looking that up somewhere...

Lets see if Mark @midlifeflyer is around...  for IFR trivia questions...   

 

Best regards,

-a-

[RobertGary1]

1 minute ago, carusoam said:

WAAS has the ability to be more accurate... Then an old pressure sensor connected to a needle by tiny gears...

 

Yea the shop thought the altimeter that is driven off the ADC is probably more accurate than the mechanical one. But it’s legal for shooting an ILS to 200 feet. Or is that 150 feet   

 

-Robert 

[carusoam]

Close enough works for...
 

Horse shoes, hand grenades, and IFR approaches..?

PP attempt at humor only...

Best regards,

-a-

[jaylw314]

IIRC, you can only use the glideslope on a +V approach if you have GPS equipment that has a certified barometric sensor input, which I don't think anyone has in a GA aircraft.

Wasn't there a recent conversation about a +V approach that gives you a glideslope that busts the actual non-precision minimums?

Legally, you can fly a +V approach as a glideslope (and I recall in my readings some materials encourage this), but you still need to make sure you know where, when and what your minimum altitudes are so you don't bust them

Edit: correction, it's the VNAV glideslope that requires the certified barometric sensor to use.  +V is always advisory

Edited October 10, 2020 by jaylw314

[RobertGary1]

4 hours ago, jaylw314 said:

.

Wasn't there a recent conversation about a +V approach that gives you a glideslope that busts the actual non-precision minimums?

 

All the +v approaches I’ve seen on garmin go right down to the ground just like an ils glideslope. The +v is something invented by the manufacturer as guidance and not by the FAA 

 

-Robert 

[takair]

10 hours ago, RobertGary1 said:

I have two legal primary altimeters and it’s not uncommon for them to be off more than 50’. Shop says that’s within tolerances. Makes you wonder if WAAS is more accurate. 
 

 -Robert

Interesting.  At sea level, each individual altimeter must be +-20’ for it to pass an IFR inspection.  So, if they are 50’ apart in a static condition, I would argue that at least one is out of calibration.  As a pilot, we do a gross check of accuracy by assuring that they are within 70’ of your location, but that does eat into your margins.  If you add up all of the errors (altimeter, glideslope, pilot) it can leave you with precious little room on a low ILS approach.

 

[Vance Harral]

Here's a good article on LP+V.  Note the "smoking hole" illustration: https://www.planeandpilotmag.com/article/tripped-up-by-stepdowns/

[jaylw314]

3 hours ago, Vance Harral said:

Here's a good article on LP+V.  Note the "smoking hole" illustration: https://www.planeandpilotmag.com/article/tripped-up-by-stepdowns/

Thanks, I had seen that illustration before somewhere

[David Lloyd]

Vance, good reading, thanks for posting that.

[jlunseth]

This is really good stuff. GPS approaches are still new, and from what I have seen, actual approaches are being constantly tweaked by the FAA.  Who is going to be the moderator for our new forum?

[midlifeflyer]

how how it works is above my pay grade but @y2kiahhad what upstairs is absolutely right.

LPV and LNAV/VNAV glidepaths (and ILS glideslopes) are TERPS based and provide guaranteed obstacle clearance and no need to stop and level off for stepdowns on the final approach segment. 

LNAV+V is nothing more than a GPS manufacturer calculation of a standard glidepath. It is only advisory, guarantees nothing vertically, and you will see no mention of it in an approach chart. 

[midlifeflyer]

On 10/10/2020 at 3:19 AM, jaylw314 said:

 

Edit: correction, it's the VNAV glideslope that requires the certified barometric sensor to use.  

Actually it doesn't. Most WAAS+based GPS systems are authorized to fly LNAV/VNAV. It's mentioned in the manuals and in FAA guidance material, including  AIM 1-1-18.b.1.. If you load an approach with LNAV/VNAV mins but no LPV mins (there are some) you will see LNAV/VNAV annunciated as available for the approach.

[Bob - S50]

Here is the bottom line:

An LPV can be flown just like an ILS even though it is technically a non-precision approach.  Follow the glidepath (GP) to a decision altitude (DA) and either land, or if you don't see the runway, go around.  The GP gives you obstacle clearance all the way to minimums.

An LP+V or LNAV+V is a true non-precision approach.  You must comply with all stepdown restrictions and you can only go to the minimum descent altitude (MDA).  The +V gives you an unofficial GP.  While it USUALLY keeps you above stepdown restrictions, it isn't a guarantee.  It also does not guarantee obstacle clearance.  Only complying with stepdown restrictions will do that.  It just provides you with an easy way to arrive at a VDP in a stabilized descent.  If you get to the MDA you must level off just like you would if you didn't have the +V.

[lamont337]

On 10/10/2020 at 9:53 AM, Vance Harral said:

Here's a good article on LP+V.  Note the "smoking hole" illustration: https://www.planeandpilotmag.com/article/tripped-up-by-stepdowns/

I give this article to every one of my instrument students. I also emphasize that if you have +V instead of LVP there is more than likely an obstacle somewhere on the approach that is preventing the establishment of an LPV glidepath and it's minimums. Lookout below.

[jaylw314]

3 hours ago, midlifeflyer said:

Actually it doesn't. Most WAAS+based GPS systems are authorized to fly LNAV/VNAV. It's mentioned in the manuals and in FAA guidance material, including  AIM 1-1-18.b.1.. If you load an approach with LNAV/VNAV mins but no LPV mins (there are some) you will see LNAV/VNAV annunciated as available for the approach.

I stand corrected again.  Huh!  so that implies VNAV is determined by TERPS criteria for terrain clearance, e.g. no smoking hole, right?

Edit: Oops, saw your subsequent post, thanks!

Edited October 11, 2020 by jaylw314

[donkaye]

4 hours ago, midlifeflyer said:

Actually it doesn't. Most WAAS+based GPS systems are authorized to fly LNAV/VNAV. It's mentioned in the manuals and in FAA guidance material, including  AIM 1-1-18.b.1.. If you load an approach with LNAV/VNAV mins but no LPV mins (there are some) you will see LNAV/VNAV annunciated as available for the approach.

Talking about VNAV not associated with approaches.

 

[RobertGary1]

The gps is pretty good at understanding terrain and making approaches. I have guidance to all runways without approaches too. If there aren’t obstacles I get what looks like an LPV approach. If there are I get what looks like LP approach. This even though the FAA has charted nothing for these airports. They say it’s good for awareness but knowing I could shoot an approach right down to the runway at a vfr field in an emergency is very nice. 
 

-Robert 

[midlifeflyer]

30 minutes ago, donkaye said:

Talking about VNAV not associated with approaches.

 

I don't think so, Don (my emphasis below)

AIM 1-1-18.b.1.  Properly certified WAAS receivers will be able to fly to LPV minima and LNAV/VNAV minima, using a WAAS electronic glide path, which eliminates the errors that can be introduced by using Barometric altimetry.  

AIM 1-1-18.b.4. "WAAS provides a level of service that supports all phases of flight, including RNAV (GPS) approaches to LNAV, LP, LNAV/VNAV, and LPV lines of minima, within system coverage."  

GNS user manuals: "The GPS 500W has a WAAS GPS engine and is TSO C146a certified for primary domestic, oceanic, and remote navigation including en route, terminal, and non-precision approaches, and approaches with vertical guidance, such as LPV and LNAV/VNAV"

(Anyone here in the Atlanta area and want to give it a try in the air?)

[midlifeflyer]

59 minutes ago, jaylw314 said:

I stand corrected again.  Huh!  so that implies VNAV is determined by TERPS criteria for terrain clearance, e.g. no smoking hole, right?

Edit: Oops, saw your subsequent post, thanks!

We have been talking about two completely different things, so let's make sure we are not talking about apples and oranges.

LPV - localizer performance with vertical guidance. FAA TERPS standards used for vertical guidance.

LNAV/VNAV - an approach with vertical guidance using FAA TERPS standards. It was originally designed for airlines with Baro-VNAV equipment but our WAAS boxes are designed to be capable of flying them without the old airline equipment. 

LNAV+V - From the FAA standpoint a pure lateral GPS approach. The +V only means a GPS manufacturer added a glidepath usually based on the standard 3° stabilized approach. Use at your own option and your own risk. No one flight tested it for obstacles.

[donkaye]

Again, I said a Baro-Corrected Altimeter Source was required for VNAV enroute descents on the GTNs and AP coupled VNAV descents for the GFC 500.  This is independent of the AIM.  It is dependent on what Garmin says works for their units.