Step Down or Track Glide slope on GPS overlay approach

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 13 Apr 2007 19:26:50 -0700, "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.

Use the stabilized approach because, as you have discovered, it is easier
to fly! Aren't ILS's easier to fly than dive & drive non-precision
approaches? Just don't forget to level off at the MDA. "You" cannot treat
MDA as a DA without special authorization.

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?

The requirement to identify KRIED is not dependent on how you are flying
the approach. However, if KRIED is in your DB, you can use the GPS to
identify it.

Where are the answers to these questions provided?

Which question?

The issue of stabilized versus D&D approaches is discussed in airline
safety material.

Rules for flying approaches are in the FAR's and AIM.

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

On 14 Apr 2007 08:16:05 -0700, "paul kgyy" wrote:

Sometimes step-downs are to clear obstacles. I haven't tried the 430W
yet, but how does the "stabilized approach" deal with this? Is there
a variable glideslope?

The GP angle can vary by approach. Not all LNAV approaches will have
advisory vertical guidance.
Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

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)

I think what we are saying is that even though Jeppesen says the VNAV
GS ensures being above the step-down fixes and GARMIN wrote approved
software to make sure the GS needle properly indicates the Jeppesen-
provided vertical guidance, the pilot needs to explicitly note whether
the step-down fixes have been passed and not follow the GS unless it
is consistent with these restrictions. So, it is ok to fly the
glideslope, but you must perform these checks as the approach
progresses.

Seems like a good double check yet lets me keep the stablized
approach. The other issues of MDA vs DA and the possible advantage of
entering VFR earlier if you dive & drive are noted. Thanks to everyone
who responded.

Mooney wrote:
I think what we are saying is that even though Jeppesen says the VNAV
GS ensures being above the step-down fixes and GARMIN wrote approved
software to make sure the GS needle properly indicates the Jeppesen-
provided vertical guidance, the pilot needs to explicitly note whether
the step-down fixes have been passed and not follow the GS unless it
is consistent with these restrictions. So, it is ok to fly the
glideslope, but you must perform these checks as the approach
progresses.

Seems like a good double check yet lets me keep the stablized
approach. The other issues of MDA vs DA and the possible advantage of
entering VFR earlier if you dive & drive are noted. Thanks to everyone
who responded.

Google for "constant angle non-precision approach" for lots of
discussion about the pros and cons.

Dave

Stable controlled descent approaches are both easier and safer than
stepdown approaches, which was what the airlines found. There is no
reason to expect different from light airplanes.

I would find an instructor more amenable to new technologies.

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?

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