Step Down or Track Glide slope on GPS overlay approach

On 14 Apr 2007 06:04:57 -0700, "Mooney" wrote:

On Apr 13, 11:27 pm, "Viperdoc" wrote:
I have a 530W/430W combo recently, and in fact did some RNAV/GPS approaches
tonight. I find it easier to let the autopilot track the GS in LNAV/VNAV
approaches rather than dive and drive. It's identical to an ILS- when the DH
is reached, then either land or go around.

It's probably the same reason why precision approaches with GS are easier to
fly than non precision approaches.

By the way, tracking the VNAV or LPV glideslope and GPS "localizer" were
much smoother than a traditinional ILS or localizer approach.

I agree that this is easier to just fly the GPS glideslope and my
temptation is to just do it that way, but I'm trying to figure out why
my instructor is adamant that I use the stepdown process.

Not sure. If you're flying the GPS overlay then the GPS, WAAS or not,
is just as valid as the VOR radials for identifying fixes.

Can you be
confident if you fly the glideslope that you will not violate the
minimum altitudes at each stepdown fix?

Not unless it's an LPV approach, which none of the approaches to LWM
are. The manual says "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 if
typically denoted by a light dashed line on the vertical profile
(Jeppesen only) with an associated glidepath angle (usually in the
3.00deg range). These approaches are indicated with "LNAV+V".

So, if there is a dashed line on your Jepp chart, then you should be
above or at the altitudes at each fix.

If you are below these BUT ON
THE GS are you legal or does the GS have no legal status? More
importantly is there any safety issue of just flying the GS?

If you are below the listed altitude for any segment of the approach
it's not legal, regardless of how you got there.

"Peter Clark" wrote in message ...
On 14 Apr 2007 06:04:57 -0700, "Mooney" wrote:

Can you be
confident if you fly the glideslope that you will not violate the
minimum altitudes at each stepdown fix?

Not unless it's an LPV approach, which none of the approaches to LWM
are. The manual says "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 if
typically denoted by a light dashed line on the vertical profile
(Jeppesen only) with an associated glidepath angle (usually in the
3.00deg range). These approaches are indicated with "LNAV+V".

So, if there is a dashed line on your Jepp chart, then you should be
above or at the altitudes at each fix.

If you are below these BUT ON
THE GS are you legal or does the GS have no legal status? More
importantly is there any safety issue of just flying the GS?

If you are below the listed altitude for any segment of the approach
it's not legal, regardless of how you got there.

Jeppesen says their VNAV depiction meets all altitude restrictions.
I don't see a specific restriction to LPV approaches.

Here is a small extract from a 2002 Jeppesen Briefing Bulletin at:
http://www.jeppesen.com/download/bri...ulletin02C.pdf

"VNAV path information illustrates the geometric descent path with a descent
angle from the Final Approach Fix (FAF) to the Threshold Crossing Height
(TCH). The inclusion of VNAV angles on non-precision approach charts was
done on a limited basis.

For those non-precision approach procedures for which the State authority
did not specify a descent gradient or did not provide a recommended DME/
Altitude table, a descent angle derived from the Jeppesen NavData database
is to be shown on the corresponding approach chart.

This angle, if used by certified VNAV-capable avionics equipment, will ensure
a stable, constant rate of descent clearing all intervening altitude restrictions
(step-down fixes) established by the State authority."

For the O.P. "Mooney", that says your altitudes will be OK because your
430W is computing vertical guidance from its NavData database.
If you find an example where you'd be below the stepdown altitudes,
yet "on the glide slope", please post more details here.
Jeppesen doesn't seem to expect that to happen.