GPS Question

"Mark Hansen" wrote in message
...
On 03/09/06 11:35, rps wrote:
I thought I understood this, but maybe not. GPS systems were certified
for approaches after I got my instrument ticket, so my knowledge is
more anecdotal than from actual training.

Can an approach-certified GPS be used to substitute for NDB and DME
during an approach (or even en route)? What if the approach says ADF
Required or DME Required? My understanding of the AIM is that you can
only use the GPS as a substitute when there is a published GPS overlay
for the approach.

(My question is about the legality and not about whether you'd be close
enough.)


The AIM has a section devoted to the use of GPS in lieu of ADF and DME,
which will answer your question, and much more ;-)

Go to section 1-1-19, and look for "Use of GPS in lieu of ADF and DME".

The short answer is yes, you can use GPS as a replacement for ADF and DME
unless the navigational station is the primary for the approach (which
could be true for ADF, but not DME).

So, if you're flying an NDB approach, you could not use the GPS as a
replacement for the NDB ground station.

However, if you're flying an approach that requires DME, or an NDB to
identify the OM, you can use the GPS to provide that information.


--
Mark Hansen, PP-ASEL, Instrument Airplane
Cal Aggie Flying Farmers
Sacramento, CA

Except for flying an approach at the alternate:

Ref: AIM 1-1-19, paragraph f.1.(b)(8)
Restrictions

"For TSO-C129/129A users, any required alternate airport must still have an
approved instrument approach procedure other than GPS that is anticipated to
be operational and available at the estimated time of arrival, and which the
aircraft is equipped to fly. If the non-GPS approaches on which the pilot
must rely require DME or ADF, the aircraft must be equipped with DME or ADF
avionics as appropriate."

Except for flying an approach at the alternate:

Nope. Except for determining a legal alternate. Once you are in the
air, if you need to go to =an= alternate, it doesn't have to be your
filed one, and even if you decide to go to your filed alternate, there
are no special GPS rules. You can use the approach-certified GPS as a
substitute for NDB and DME, except for an NDB approach, just like your
original destination.

Jose
--
Money: what you need when you run out of brains.
for Email, make the obvious change in the address.

Except for flying an approach at the alternate:

Ref: AIM 1-1-19, paragraph f.1.(b)(8)
Restrictions

"For TSO-C129/129A users, any required alternate airport must still have
an approved instrument approach procedure other than GPS that is
anticipated to be operational and available at the estimated time of
arrival, and which the aircraft is equipped to fly. If the non-GPS
approaches on which the pilot must rely require DME or ADF, the aircraft
must be equipped with DME or ADF avionics as appropriate."


Not so. One can still fly a GPS approach at the alternate. You just need the
other equipment installed. If you have approved WAAS avionics, you may plan
to use any instrument approach authorized for use with WAAS avionics at a
required alternate.

Karl
"Curator" N185KG

"kgruber" wrote in message
...
Except for flying an approach at the alternate:

Ref: AIM 1-1-19, paragraph f.1.(b)(8)
Restrictions

"For TSO-C129/129A users, any required alternate airport must still have
an approved instrument approach procedure other than GPS that is
anticipated to be operational and available at the estimated time of
arrival, and which the aircraft is equipped to fly. If the non-GPS
approaches on which the pilot must rely require DME or ADF, the aircraft
must be equipped with DME or ADF avionics as appropriate."


Not so. One can still fly a GPS approach at the alternate. You just need
the other equipment installed. If you have approved WAAS avionics, you may
plan to use any instrument approach authorized for use with WAAS avionics
at a required alternate.

Karl
"Curator" N185KG


"Not so" on which point?

The AIM clearly states that the FILED alternate must "still have an approved
instrument approach procedure other than GPS..." That's word for word out
of the book. But it doesn't SAY you can't FLY a GPS approach at said
alternate, so I agree that you can fly a GPS approach at your filed
alternate. It just can't be the ONLY approach available at that alternate.

There are several airports around that have only a GPS approach or only
approaches (such as an ILS or VOR) that require DME or NDB for position
fixing and/or executing the missed. These airports are, therefore, not
LEGAL alternates for the purpose of filing an alternate. The motivation
here is to have an executable Plan B in case of RAIM failure or GPS
interference or jamming. If GPS keeps humming, no harm, no foul but if it
lets you down for any reason, you'll need the *real* DME or ADF installed to
navigate the approach and that's the point of the AIM reference.

I also agree with your second point about WAAS equipment but your point is
outside the scope of the AIM paragraph in that it addresses TSO-C129/129A
installations, not TSO-145A which applies to WAAS augmentation of GPS.

Therefore, if your new Cirrus doesn't have bona fide NDB, DME, or WAAS, your
FILED alternate would need to have an available approach without the
requirement for any of those.

That being said, what you file and what you fly will likely be based on why
you miss the primary approach in the first place.

It just can't be the ONLY approach available at that alternate.

Actually, it can... just not at the time of filing. If your alternate
has another approach (say an ILS), making it legal for filing as an
alternate, and when things go south, their ILS also dies, you can still
legally FLY their GPS approach, or an approach requring NDB or DME
(except an NDB approach). To fly their NDB approach you'd need the
emergency authority.

Jose
--
Money: what you need when you run out of brains.
for Email, make the obvious change in the address.

Can an approach-certified GPS be used to substitute for NDB and DME
during an approach (or even en route)? What if the approach says ADF
Required or DME Required? My understanding of the AIM is that you can
only use the GPS as a substitute when there is a published GPS overlay
for the approach.

My understanding is that it can, except for an NDB approach. If the NDB
approach has a GPS overlay, then it is a GPS approach anyway.

Jose
--
Money: what you need when you run out of brains.
for Email, make the obvious change in the address.