Step Down or Track Glide slope on GPS overlay approach

On Apr 13, 7:26 pm, "Mooney" wrote:
I have just recently acquired a Garmin 430/WAAS for my Mooney 201. In
practicing approaches to familiarize myself with the unit, I went up
with an instructor to get some advice/tips. I had flown a GPS overlay
approach (NDB/GPS Runway 5 KLWM, Lawrence MA) previously in VFR
conditions and tracked the vertical guidance provided by the GPS and
loved it ... very stable approach and no need to dive/level/dive etc.

Then I went up with the instructor and did what I thought was a great
approach (also NDP/GPS 5 KLWM) and he was upset I didn't fly it more
like the "original" non-precision approach by identifying fixes with
cross radials and doing the stepdowns.

So that is the question. Which technique should be used and why? If I
give up the "track the GPS glideslope" approach I feel I'm giving up
the advantage of a very stable/controlled approach configuration and
not sure what I'm gaining in return.

Comments from the experts??

Final questions: With the WAAS GPS on this approach, can I descend to
the lower minimum based on identifying the final stepdown fix if I am
just flying the GPS's vertical guidance? Where are the answers to
these questions provided?

There is no reason you need to use the minimum altitude unless the
altitude on the plate has a bar abouve it (meaning max altitude)

-robert

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.

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

On Apr 14, 6:04 am, "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. Can you be
confident if you fly the glideslope that you will not violate the
minimum altitudes at each stepdown 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?

I guess I've done that same with students. Perhaps what he was trying
to do was to see how well you can follow the steps (which is much
harder/more critical) and you made it too easy by following the GS.

-Robert, CFII

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.

On Apr 14, 9:04 am, "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. Can you be
confident if you fly the glideslope that you will not violate the
minimum altitudes at each stepdown 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?


Probably because GPS 5@LWM does not have a LPV or VNAV minimum
published. Even in that case there is nothing wrong with flying the
GPS GS, but it becomes your responsibility to ensure that you cross
HAGET and KRIED intersections above the specified altitudes.

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

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.

You'll have to ask him. It may be due to lack of familiarity with the
stabilized approach concept, and its advantages.

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

Yes. However, in the words of a former politician, "Trust, but Verify".

If you are below these BUT ON
THE GS are you legal or does the GS have no legal status?

The GP does not supercede published minimums.

More
importantly is there any safety issue of just flying the GS?

I've not seen an approach with advisory vertical guidance that violates a
stepdown fix. The GP in these instances is, on Jepp charts, represented by
a light dashed line, and a GP angle notation. So, if you are using Jepp
charts, you can verify this.

The only "issue", and it is not unusual in the NE (ME and NH, at least) is
that the marginal WAAS coverage occasionally leads to loss of the WAAS-GP
signal. So then you'll be flying without the GP signal. Not a big deal if
you're ready for it. And, if you are properly set up, you can still
continue down to MDA at the same rate, or perhaps slightly faster.

Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

On Sun, 15 Apr 2007 07:25:50 -0400, Ron Rosenfeld
wrote:

I've not seen an approach with advisory vertical guidance that violates a
stepdown fix. The GP in these instances is, on Jepp charts, represented by
a light dashed line, and a GP angle notation. So, if you are using Jepp
charts, you can verify this.

Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

Ron, are you able to provide us with some examples? Stan

On Sat, 28 Apr 2007 12:29:12 GMT, wrote:

On Sun, 15 Apr 2007 07:25:50 -0400, Ron Rosenfeld
wrote:

I've not seen an approach with advisory vertical guidance that violates a
stepdown fix. The GP in these instances is, on Jepp charts, represented by
a light dashed line, and a GP angle notation. So, if you are using Jepp
charts, you can verify this.

Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

Ron, are you able to provide us with some examples? Stan

Sorry Ron, I mis read your post. Everyone please disregard. Stan