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Old April 14th 07, 08:02 PM posted to rec.aviation.ifr
John R. Copeland
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Posts: 81
Default Step Down or Track Glide slope on GPS overlay approach

"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.