LPV vs LNAV/VNAV?

A sample approach that has both is KARR RNAV(GPS) RWY 9. LPV DA is
956, LNAV/VNAV DA is 990.

LPV requires a TSO 146 navigator. VNAV requires Baro VNAV (only found
in high end aircraft) or, now, a TSO 146 navigator.

With a TSO 146 navigator the VNAV glide-path is not as carefully
calculated as the LPV glide-path. It's as safe, but not as good for
minimums, etc.

Yes, and if WAAS is not available to the standards required by the specs
then the 146 box will not accept the WAAS solution, which means (using
Garmin as an example) LPV, L/VNAV, and LNAV+V will not be available;
only LNAV will be available.

And I can attest to that, Sam, because I've experienced having an
LNAV/VNAV approach "Downgraded to LNAV" only.
I couldn't take the time just then to check the SV reception page,
to see if the problem might have been explained there.

I don't think so. A TSO C129a GPS driving a FMS with baro VNAV should be able to fly RNAV(GPS)
approaches using LNAV/VNAV minimums. No WAAS involved.

Mike

Something I never understood is why LNAV approaches don't automatically
show a glideslope so that the airplane arrives at the MDA at the VDP.

Because L stands for lateral?

--
Transpose mxsmanic and gmail to reach me by e-mail.

Perhaps because it wasn't part of the certification at the time?

--
Peter

Perhaps my comment was not clearly stated. When you fly an LNAV
approach (or any nonprecision approach for that matter) instead of the
traditional dive and drive you can mentally calculate the vertical
speed required (VSR) to arrive at the VDP at a constant glide angle .
That mental calculation could be easily performed by the GPS and
displayed as a glideslope. But I have not seen any GPS do that.
Certification is irrelevant. We are not talking about a lower minimum
or anything new that we not already allowed to do.

Certification is most certainly relevant to your query, as that is most
likely what kept that feature out of the TSO C129a certified IFR GPS's.

A handheld Garmin 196 (I think that is the model a pilot-friend had with
him a couple of years ago) that we took up on a practice IFR flight did
just that. It displayed a glideslope for a non-precision approach. If the
cheaper handhelds can do it, then why don't their IFR-certified TSO C129a
big brothers do it? Because it wasn't part of the certification and
therefore, regardless of their ability to provide this feature, are
restricted from doing so due to the certification.

--
Peter

> displayed as a glideslope

Just took a "flight" using the GNS400W simulator (the GNS430 with WAAS)
becasue I wanted to see if the new WAAS upgrade for the 430/530 would do
this. I flew both a VOR approach and an RNAV/GPS approach that only had
LNAV minimums.

For the VOR approach the GPS did not provide a glideslope, but for the
RNAV/GPS approach into Weedsport, NY (B16 - RNAV/GPS 28):

http://www.myairplane.com/databases/approach/pdfs/09225R28.PDF

the GPS went into LNAV+V mode and a glideslope indicators on the HSI went
live, as you can see here:

http://img119.imageshack.us/img119/6793/01072007010417ev0.jpg

--
Peter

That has been true for a long time. The context of the recent
discussions in this forum have been about TSO C-146 (RTCA DO-229C).

With the 146 spec you will often have an advisory glideslope to MDA.
That is the LNAV+V mode and will be provided if Jeppesen has coded that
capability into the approach's database string. As Garmin says in the W
handbood, not all LNAV-only IAPs have been so coded.

LNAV+V

True, and that capability is permitted by the 146 spec. It does require
WAAS.

This is from the Garmin 500W handbook:

LNAV/VNAV and LNAV Approaches with Advisory
Vertical Guidance

GPS approaches with vertical guidance may be either LNAV/VNAV or LNAV
approaches with advisory vertical guidance. LNAV-only approaches with
advisory
vertical guidance only have LNAV minima listed on the bottom of the
approach plate. The glidepath is typically denoted by a light dashed
line on the vertical profile (Jeppesen only) with an associated
glidepath angle (usually in the 3.00° range). These approaches are
indicated with “LNAV+V”.

For approaches with LNAV/VNAV minimums, those will be controlling. For
LNAV approaches with advisory vertical guidance, the LNAV minimums will
be controlling. Approaches confirmed as “LNAV/VNAV” approaches in the
Jeppesen NavData are indicated

with an “L/VNAV” annunciation. At the time of this publication, not all
of the LNAV/VNAV approaches have been identified as such in the Jeppesen
NavData, therefore some LNAV/VNAV approaches may still be identified
with “LNAV+V” annunciation.

I misstated that somewhat.

If I read it correctly (assuming WAAS is available and passes muster)
you will always get LNAV+V on an LNAV-only IAP, thus you will have
vertical guidance to MDA.

If, on the other hand, it is an LNAV/VNAV approach you will get L/VNAV
if Jeppesen has that approach so coded, otherwise you will get LNAV+V.
You'll have vertical guidance in both cases, but I presume you can only
use the DA concept and VNAV minimums if you get the L/VNAV annuciation.

Can an RNAV approach have stepdown fixes on the final approach segment?

I am sure you are correct, but it doesn't seem to make a whole lot of
sense too keep a useful feature out of certification unless there is
something dangerous about it. I don't see anything unsafe about
providing a glideslope to a nonprecision approach.

The G/S has to be either Baro VNAV or WAAS "electronic" to be certified
for the approach phase of flight, primary or advisory. The VNAV path
provided by a receiver that doesn't have WAAS TSC146 certification would
be very unreliable.

Yes.

When VNAV first came in they could only be an an LNAV-only IAP. Then,
quite a few years ago that was changed to permit them for LNAV minimums
on an LNAV/VNAV IAP.

The CNX80/GNS480 with current SW will do that for many (not all) LNAV
approaches. It is called advisory vertical guidance and available when
published. I believe the presence of a VDP is one factor that usually
triggers this availability.
Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

But handhelds aren't certified, so there's no guarantee of correctness.

The basic requirement they don't meet is the Integrity requirement,
e.g. the guarantee a) that the error can be bounded and b) that
sufficient warning can be provided when Integrity can not be met.

129 boxes aren't certified for Vertical Guidance, so I suspect that,
even for an NPA (LNAV) approach the same would hold true.

The lack of certification is based on Standalone (Unaugmented) GPS not
being certified for Vertical guidance. This traces to the fact that the
dominant error (after SA was turned off) is the Ionospheric component
and the recevier's model (Klobuchar) is not certified to provide
sufficient Integrity for the Vertical component. With SBAS (e.g. WAAS
in the US), the Integrity requirement has been proven to be met with
sufficient Availability over the Service Volume, to approve approaches
with Vertical Guidance,.

Note that when even when the 145/6 boxes were deployed up in Alaska
(Capstone project), WAAS had yet to be commissioned, and thus the
published approaches were LNAV only.

Also note, there had been talk of building newer 129 boxes, but with
the 145/6 boxes now out, the manufs. apparently can't cost justify
upgrading a box that still wouldn't perform as well as the 145/6

> --
> Peter

Regards,
Jon

The cost of certification is probably an important factor.

An "advisory" guide slope could only be applied to a final approach segment
without stepdown fixes, or one in which the glide slope clears the
obstacle(s) by the required margin. I doubt that all LNAV-only approaches
have been TERPSed for that, especially those without VDPs.

Example:

http://tinyurl.com/y68phr

A very nice example it is, when comparing to the NACO chart
http://tinyurl.com/yfygkv .

NACO computes the descent from the FAWP to the stepdown fix. Jeppesen
computes it from the stepdown fix to the TCH at the runway.

I wonder how the GPS units present the glide slope: 3.67 deg from the FAWP
or 3.61 deg from 0.1 nm after the FAWP. There is only about 30 feet
difference at the stepdown fix between starting the two descent angles at
the FAWP.

NACO is not presenting a VNAV path, rather the descent angle from the
FAF to TDZ elevation.

Maybe so, but the angle seems to compute out closer to the descent angle
from FAWP to stepdown, limited by the precision in the numbers on the chart.

The question remains, though, where does the glideslope begin in the GPS
unit: from the FAWP or from the point 0.1 nm after that waypoint?

\
Beats me. Have you tried to find out in the Garmin Handbook and
couldn't find it?

Having said that, the advisory vertical path is provided to Garmin, and
other avionics vendors, by Jeppesen.

So, assuming no errors on the part of Jeppesen, it doesn't matter
whether you are using their chart or NACO's.

If my butt were on the line I would be keeping an eye on that stepdown fix.

I guess I would have more confidence with L/VNAV.

I would, however, only have complete confidence in LPV.

That's why I didn't even look in Garmin material. :-)

Whether it matters depends on how much you care about 30 feet. If the GPS
box has you begin the Jepp descent angle at the FAWP, then you wind up 30
feet low at the stepdown fix, if I did the numbers right.

For sure!

"paul kgyy" <phkm...@yahoo.com> wrote in message
news:1168012765....@s80g2000cwa.googlegroups.com...
> Can someone explain to me the difference between these two approach
> types, and how they relate to WAAS?
>
> A sample approach that has both is KARR RNAV(GPS) RWY 9. LPV DA is
> 956, LNAV/VNAV DA is 990.
>

Perhaps because that functionality is not at all desirable. The "dive
and drive" term is derisive, however, it is important to get to minimums
in order to be able to see the airport. Arriving right at the minimum
altitude just as you reach the place/time for going missed does not do
you any favors.

Many LNAV approaches have what is termed "advisory vertical guidance". ON
those approaches, a TSO146c box will output a GP signal that can be used
for a stabilized approach (versus the dive and drive approach). MDA does
not change. On the approaches with which I am familiar, the interception
of the advisory vertical guidance with the MDA may be prior to a VDP,
although they are designed to cross at that point. The guidance goes down
to the MAP or runway end.

Ron, do you have any idea why the actual intercept is not at the designed
point?

Stan

Stan,

I miswrote. Actually, at KEPM, the vertical path does intercept the MDA at
the VDP for both GPS approaches. What I was thinking about when I wrote,
was that the VDP is further from the MAP than the minimum visibility for
the approach. (VDP is 1.3 NM from AER; visibility minimums is 1.0 NM)

For an interesting GPS GP, look at the Jepp chart, if you have it, for the
RNAV (GPS) RWY 28 at KPQI. The GP starts down about 0.3 NM after the FAF.
This appears to be done in order to meet a crossing altitude restriction
insice the FAF, and still have a profile that brings you to an appropriate
height closer in to the runway.

I only have the NACO chart, and it doesn't show up that way. Shouldn't they
be the same if it is necessary to meet obstacle criteria?

I didn't think the NACO charts showed the GPS GP profile. Let me check at
KPQI ... Interesting difference.

On the Jepp chart it also shows a 3.25° GP, which intersects PESIC at
1200'.

The GP runs level from LISDE for 0.3 NM, with top of descent 4.4 NM from
HUMAX and ends at HUMAX but below the MDA.

The NACO chart also shows a 3.25° GP intersecting at PESIC at 1200', but it
appears to run smoothly from LISDE (which is 4.7 NM from HUMAX. Obviously
both cannot be correct.

Doing some trigonometry it appears the JEPP chart is correct:

PESIC--LISDE = 3.2 NM

PESIC--Top of Descent of GP = 2.9 NM (4.4-1.5)

The "triangle:

Height = 1000' (LISDE 2200- PESIC 1200)
Angle = 3.25°

Base = 1000 / tan (3.25°)

= 2.9 NM

Looking at it another way, also using trig and the same values, if you were
to start down at LISDE at 3.25°, by the time you got to PESIC (3.2 NM) you
would have descended 1104' instead of only 1000', so you would be 104' low
at PESIC.

I don't use NACO charts, but could it be that on the NACO chart, the 3.25°
notation only applies to the segment after PESIC?

It ought to apply to the final approach segment. This all sounds familiar,
like it has come up before. I don't remember how it was resolved, and it is
too late to dig through TERPS to try and figure it out. I'm going to bed.
:-) Maybe tomorrow will google the ng for KPQI.

Stan

This thread is why it sounded familiar: http://tinyurl.com/2uv32k

If the FAA calculates the angle from the FAF to the threshold crossing
altitude (or the TDZE), then the numbers on the KPQI chart don't work out.
If the distance is reduced by 0.3 nm per Jepp, the numbers seem OK. But
that's not shown on the NACO chart.

It works out pretty close using descent from PESIC to threshold crossing
altitude.

Stan

Well, the calculated slope from LISDE to PESIC is about 2.94°