Garmin 430 and ILS

The Garmin 430 documentation says that you can use the GPS for course
guidance on an ILS/LOC up until the final approach fix. Where exactly
does the approval for this come from? My inclination would have been
that in absence of an overlay, you could not use GPS anywhere after
the IAF.


Also, assuming this approval is legit, shouldn't this extend to a
non-overlay VOR approach?

Greg Esres wrote:

The Garmin 430 documentation says that you can use the GPS for course
guidance on an ILS/LOC up until the final approach fix. Where exactly
does the approval for this come from? My inclination would have been
that in absence of an overlay, you could not use GPS anywhere after
the IAF.


Garmin is correct. You don't need approach RAIM or approach sensitivity
until the beginning of a final approach segment for either a stand-alone
RNAV approach or an approved overlay.

So, you do not need localizer guidance until the P-FAF for an ILS approach
or the NPA FAF for a localizer approach. Because overlay is not approved
for a localizer, the localizer itself has to be used for the ILS or
localizer final approach segment.

The "flip-side" is also true, which is what Garmin states.


Also, assuming this approval is legit, shouldn't this extend to a
non-overlay VOR approach?

True, it could, but the vendors haven't seen that as a real-world issue.

Garmin is correct. You don't need approach RAIM or approach
sensitivity until the beginning of a final approach segment

I was sure they were, but I'm concerned about how to demonstrate this
correctness to others. The AIM says nothing about this.

I accept that flying the localizer outbound for the PT does not
require approach sensitivity and that the protected area is wide on
the initial segment, but I shouldn't have to know these things in
order to know what I can do with this box.

I'm mostly concerned with checkrides. Is there anything other than
Garmin documentation that can demonstrate this is a legal thing to do?

" non-overlay VOR approach?" True, it could, but the vendors
haven't seen that as a real-world issue.

I haven't done this in the real airplane, but on the simulator, the
box will advise changing to Vloc once established on the inbound
course of a VOR, but will not switch over. In fact, it goes down to
approach sensitivity, but doesn't indicate approach RAIM.

Would you argue that using the GPS for course guidance up to this
point is legal?


Thanks!

Greg Esres wrote:

Garmin is correct. You don't need approach RAIM or approach
sensitivity until the beginning of a final approach segment

I was sure they were, but I'm concerned about how to demonstrate this
correctness to others. The AIM says nothing about this.

I accept that flying the localizer outbound for the PT does not
require approach sensitivity and that the protected area is wide on
the initial segment, but I shouldn't have to know these things in
order to know what I can do with this box.

I'm mostly concerned with checkrides. Is there anything other than
Garmin documentation that can demonstrate this is a legal thing to do?

" non-overlay VOR approach?" True, it could, but the vendors
haven't seen that as a real-world issue.

I haven't done this in the real airplane, but on the simulator, the
box will advise changing to Vloc once established on the inbound
course of a VOR, but will not switch over. In fact, it goes down to
approach sensitivity, but doesn't indicate approach RAIM.

Would you argue that using the GPS for course guidance up to this
point is legal?

If you can retrieve it from a current database with an IFR
approach-approved GPS, it is legal to use. You will note the warning
message Garmin provides when you pull up and ILS approach.

You will note the warning message Garmin provides when you pull up
and ILS approach.

Yes, and I consider it to contradict what the manual says. The note
says for "monitoring" only, insinuating that approach guidance needs
to come from the underlying navaid at all times.

That's part of the reason I asked the question. I have difficulty
explaining away the inconsistency between the note and the
permissiveness of the manual.

You can "use" the GPS anywhere you like for guidance.

Not really on target.

The issue is using GPS as your primary source of navigation on an
instument approach.

There is no appraoch RAIM on an ILS.

My understanding is that an IFR GPS always has RAIM, if the geometry
of the satellites permits. Approach RAIM just has narrower
tolerances.

On Sun, 13 Feb 2005 19:21:07 GMT, Greg Esres
wrote:

You will note the warning message Garmin provides when you pull up
and ILS approach.

Yes, and I consider it to contradict what the manual says. The note
says for "monitoring" only, insinuating that approach guidance needs
to come from the underlying navaid at all times.

That's part of the reason I asked the question. I have difficulty
explaining away the inconsistency between the note and the
permissiveness of the manual.

Same issues I'm attempting to work through in my thread "Yet more GPS
substitution questions", cept I was looking at a VOR/DME. I'll just
merge into this one - thanks.

Greg Esres wrote:

You will note the warning message Garmin provides when you pull up
and ILS approach.

Yes, and I consider it to contradict what the manual says. The note
says for "monitoring" only, insinuating that approach guidance needs
to come from the underlying navaid at all times.

That's part of the reason I asked the question. I have difficulty
explaining away the inconsistency between the note and the
permissiveness of the manual.

You should discuss that with Garmin. The restriction is meant to apply
to the final approach segment only. It doesn't bother me because I know
it's the same for FMS/LNAV in a 777 or a Garmin 430 in a Cessna 182.
You are using terminal mode except for the final approach segment.

What exactly does "primary source" mean, anyway?

[...] If the VOR happens
to be off a degree or two, and indicates a turn in one direction, and
the GPS indicates a turn in then other direction, that the pilot must
follow the "primary" device, even though it's indicating the wrong
thing?

Yes, I think so. If you have two devices with contrary indications, you
need to decide which to believe. If an accident occurs because you
believed the GPS (and it was wrong) the FAA busts you for not following
rules. IF an accident occurs because you believed the ADF, the FAA
busts you for being careless and reckless.

Jose