GPS approach question

I was flying this weekend to maintain my instrument currency and came
across a GPS approach unlike any I'd seen thus far. Then again, I'm
pretty new to GPS approaches so I'm still finding new stuff fairly often.

I was flying approaches into Wellsvill, NY (ELZ) and was setting up for
the GPS RWY 28. I hadn't studied the approaches in advance, which is my
norm for instrument practice as I like to simulate having to divert to
an unplanned destination and thus find and brief the approach while also
flying the airplane.

I saw the two "waypoints" as I scanned the plan view and then selected
the GPS 28 approach on the King KLN 89B. I don't remember the details
exactly now, but the GPS didn't act as I expected in leg mode. It
wouldn't sequence the approach so I put it in OBS mode and flew the
approach in that manner. Looking at the plate later on the ground, I'm
pretty sure I understand what was wrong.

I mistakenly thought that RW28 was the MAP waypoint, but it appears that
this isn't the case. I don't have the GPS handy at home so I can't pull
up the approach again to see what waypoints it lists, but there is no
MAP WP symbol beside RW28. It looks like the MAP is simply the point
2.9 NM from HALOS and isn't a GPS waypoint at all. I'm also thus
assuming that this type of overlay approach can only be flown in OBS
mode with no automatic sequencing. Is this correct?

I also flew the VOR-A approach, but not its overlaid GPS-A approach. I
see that the chart shows what appears to be a MAP wapoint, MAGNV. This
is listed in parens at the location of the runway. I don't find this in
the terminal procedures legend, so can someone confirm that (MAGNV)
really is intended to indicate a GPS waypoint? However, again it has no
MAP WP symbol beside it, but if it isn't the MAP waypoint I can't figure
out what it is.

I also checked the GPS approaches into ELM and they do have the same
waypoint names RW06, RW28, etc., but they DO have the symbol for the MAP
WP as shown in the legend. I'd only flown GPS approaches into ELM
previously and thus I guess that is why when I saw RW28 at ELZ I assumed
it was the MAP waypoint.

It seems odd that there would be such subtle variations in the symbology
of various GPS approaches. Can anyone explain this briefly or refer me
to a good reference for GPS approaches? It is clear that reading the
King manual and my last ICC didn't yet cover all of the bases. I've not
yet flown a GPS approach in actual as I still don't feel comfortable
that I understand both the approaches and the 89B sufficiently to be
safe. However, I would like to get to that point before warm weather
returns to the northeast. :-)


Matt

Matt Whiting wrote:
I also flew the VOR-A approach, but not its overlaid GPS-A approach. I
see that the chart shows what appears to be a MAP wapoint, MAGNV. This
is listed in parens at the location of the runway. I don't find this in
the terminal procedures legend, so can someone confirm that (MAGNV)
really is intended to indicate a GPS waypoint? However, again it has no
MAP WP symbol beside it, but if it isn't the MAP waypoint I can't figure
out what it is.

Waypoints in parens, like (MAGNV) are Computer Navigation Fixes (CNFs).
They're waypoints which only exist in your GPS database (and on the SIAP
charts). They are not marked on the controller's radar display. If you
told the controller that you were "2 miles east of MAGNV", he wouldn't
know where that was. That's the explanation of what they are; as for
*why* they exist, I have no idea. See the AIM 1-1-19-k-2 for more
details.

MAGNV sure looks like the MAP to me. Although I can't find the
reference now, I vaguely remember reading that CNFs used as MAPs all
have names starting in MA.

Roy Smith wrote:

Matt Whiting wrote:

I also flew the VOR-A approach, but not its overlaid GPS-A approach. I
see that the chart shows what appears to be a MAP wapoint, MAGNV. This
is listed in parens at the location of the runway. I don't find this in
the terminal procedures legend, so can someone confirm that (MAGNV)
really is intended to indicate a GPS waypoint? However, again it has no
MAP WP symbol beside it, but if it isn't the MAP waypoint I can't figure
out what it is.


Waypoints in parens, like (MAGNV) are Computer Navigation Fixes (CNFs).
They're waypoints which only exist in your GPS database (and on the SIAP
charts). They are not marked on the controller's radar display. If you
told the controller that you were "2 miles east of MAGNV", he wouldn't
know where that was. That's the explanation of what they are; as for
*why* they exist, I have no idea. See the AIM 1-1-19-k-2 for more
details.

Thanks, the AIM was the next stop on my journey. I read the GPS section
back in the spring, but obviously didn't remember all of the requisite
details!


MAGNV sure looks like the MAP to me. Although I can't find the
reference now, I vaguely remember reading that CNFs used as MAPs all
have names starting in MA.

Yes, it appears to be the MAP, but it doesn't have the map symbol beside
it so I'm not sure.


Matt

Matt,

There are *no* GPS approaches that are authorized to be flown in the
OBS mode. In order for you to fly a GPS approach, it is required that
you approach the final approach fix in the LEG mode, and the TERM or
ARM annunciator light lit on your external control panel. As you hit
2nm outside the FAF, the annunciators should change from ARM to ACTV
(known as the approach mode on some other mfgs boxes). IT IS ILLEGAL
TO CONTINUE (DESCEND) inside the FAF if you do not get the ACTV
light--and you will never get it in the OBS mode.

The common problem with getting a GPS approach to work is that you miss
one of the nine or so conditions that are required to be true at the
FAF-2nm point to allow the box to switch into ACTV. Look in the manual
on your KLN89B, and you will see a list of those required by your box,
but they are all pretty similar since they are written into the TSO
under which all of the approach-certified boxes are approved.

Typically, to get it to go into the ACTV mode, these things must be
true:
1. In LEG mode (not OBS)
2. In ARM mode (this comes up when you have a valid approach loaded,
and you are within 30nm)
3. Valid RAIM now, and predicted at FAF and MAP
4. Outside FAF (110 degree arc each side of FAF)
5. Distance to FAF decreasing
6. Within 2nm of FAF, but not past (inside) FAF
7. NAV/GPS switch must be in the GPS position
8. No localizer frequency may be loaded into NAV 1 (some boxes)

You probably had one of these conditions not met.

The approach plate shows a procedure turn type approach. Assuming you
did this approach without ATC vectors to final, it should run something
like this:

Clear the flight plan

Check Nav 1, and make sure no valid localizer freq is in that radio.

Set the GPS/NAV switch to GPS

Load the approach (APT 8 page); select the version of the approach that
uses the LOM as the IAF. On this approach it is also the FAF, and in
the 89B they are the same waypoint, which can be a point of confusion.

Activate the approach, or put the cursor on the FAF waypoint in the
flight plan and hit direct-enter-enter (proceed direct to the IAF/FAF)

Now, heres the trick: push the OBS button on the external control
panel; this puts your system into the OBS mode and DISABLES AUTOMATIC
WAYPOINT SEQUENCING. We have to do this (temporarily), because we are
going to cross the waypoint twice (once outbound to the PT, and then
again inbound established on final) and we do NOT want the box to
sequence down the flight plan to the next waypoint (in this case the
MAP) at the first crossing; we need the box to keep the FAF as the
active waypoint until we are established inbound from the PT.

At 30 nm out, check that you get an ARM light on your external display.

Cross the FAF, and turn outbound towards the PT; set the obs on your #1
vor indicator to the outbound course (098). Intercept it and track
outbound until you are ready to do the PT (1 min, 2min or so)

Once you start the PT (left turn to 053) reset the obs on #1 indicator
to the inbound intermediate course (from the PT to the FAF), in this
case 278.

Turn inbound in the PT and then intercept and track the 278 course
towards the FAF.

DANGER!!! DO NOT MISS THIS NEXT STEP OR ALL IS LOST!!!
Once established inbound, but prior to 2nm from FAF, push the OBS
button again on the external GPS display and VERIFY THAT THE OBS LIGHT
GOES OUT!!!

The box will now go back into the LEG mode (you should see LEG
displayed in the left portion of the GPS display), which is necessary
to get to ACTV mode.

Watch the distance tick down; at 2nm from the FAF, the ARM light should
go out, and the ACTV light should come on. If this does not happen,
you have about 1:20 (2 nm at 90 kts) to figure out which of the nine
items in the list you missed and correct it prior to crossing the FAF.
If you do not get an ACTV light and pass the FAF, nothing you can do
will get it from there on. You have to abandon the approach, go back
out and start over.

At the FAF the box should sequence to the MAP, and then automatically
go into the suspend mode (waypoint sequencing disabled). At 0 dist to
MAP, you must either have the runway in sight and land visuallly, or
execute the missed.

If you go missed, hit the OBS button again; the box will go back into
the LEG mode just long enough for it to sequence to the MAHP, and then
it will go back into suspend mode again. This is so you can do
multiple turns in the hold at the MAHP without the box autosequencing
to some other fix.

If you want to go back and do another approach, you have to hit the OBS
button again (get it out of suspend mode and back into leg mode), and
set it up to proceed direct to the IAF and start the procedure over as
before.

Somewhere in there I think you will find a piece you missed.

When you get back in front of the GPS again, try loading the approach
and then scrolling down through the waypoints. There should be 3 for
this approach: the first one should be XXXXXf (database name of the
LOM, with a small f after it), the second one should be YYYYYm
(database name of the MAP, probably RW28 or some such, with a little m
after it), and then ZZZZZh (database name of the MAHP, with a little h
after it.)

If you dont have at least those three waypoints in the approach, time
to contact your database provider and ask some questions.

If at first you do not get an ACTV light, try, try again.
Gene
CFII, ASC

"CFII_ASC" wrote:

Typically, to get it to go into the ACTV mode, these things must be
true:
1. In LEG mode (not OBS)
2. In ARM mode (this comes up when you have a valid approach loaded,
and you are within 30nm)

Within 30 nm of what? The IAF? The destination?

3. Valid RAIM now, and predicted at FAF and MAP
4. Outside FAF (110 degree arc each side of FAF)
5. Distance to FAF decreasing
6. Within 2nm of FAF, but not past (inside) FAF
7. NAV/GPS switch must be in the GPS position
8. No localizer frequency may be loaded into NAV 1 (some boxes)

Wow, that last one is pretty strange. I often have an ILS frequency
loaded up, even if I'm not using it. The most frequent use I make of
the NAV part of the GPS/NAV/COM is to fly an ILS, so the last ILS flown
is usually what's left there. What is the logic behind requiring me to
change it to something else?

I don't have the book here, but I *believe* the 30nm is referenced to
the Airport Reference Point, which is the physical location from which
they began the survey of the land to lay out the airport. Mostly this
is somewhere near the center of the airport, but not always. sometimes
it is not even *on* the airport.

The last issue, #8, is implemented on some boxes and not others. I do
not remember if the 89B had that feature or not. But on some machines
it was wired to automatically switch the NAV/GPS switch back to NAV if
you had a valid Localizer freq in Nav 1. The theory was that if you
were trying to fly an ILS and fogot your NAV/GPS switch was left in the
GPS position, it would automatically default back to the NAV position
so that the data displayed on the needle you were tracking was from the
ILS and not the GPS (that you were presumably not using.)

Of course, this created the opposite problem; if you are trying to do a
GPS approach, and forget to get the ILS freq out of Nav 1, it would not
allow you to *load* the approach, let alone fly it. Infuriating.

I have not seen this feature implemented on any of the more recent
boxes.

Bottom line, you have to know what your box does and doesn't do
automatically, and when it does it, and why. An put a check of that
NAV/GPS switch in your set-up procedure for *all* your approaches, not
just the GPS ones.

Regards,

Gene

"CFII_ASC" wrote:
Bottom line, you have to know what your box does and doesn't do
automatically, and when it does it, and why. An put a check of that
NAV/GPS switch in your set-up procedure for *all* your approaches, not
just the GPS ones.

Yeah, I've been burned by that one, and it's one of the most common
mistakes I see students make.

The other common one (at least on the CNX-80) is to forget to hit the
EXEC button after modifying a flight plan or selecting an approach.

"Roy Smith" wrote in message
...
Wow, that last one is pretty strange. I often have an ILS frequency
loaded up, even if I'm not using it. The most frequent use I make of
the NAV part of the GPS/NAV/COM is to fly an ILS, so the last ILS flown
is usually what's left there. What is the logic behind requiring me to
change it to something else?

Many, if not all, IFR GPS units and or switching relay/annunicators have an
option (often wiring) to force the switching relay to NAV if there is a LOC
frequency selected. In the newer integrated units, this "relay" is inside
the box.

Mike
MU-2

Roy Smith wrote:
"CFII_ASC" wrote:


Typically, to get it to go into the ACTV mode, these things must be
true:
1. In LEG mode (not OBS)
2. In ARM mode (this comes up when you have a valid approach loaded,
and you are within 30nm)


Within 30 nm of what? The IAF? The destination?

The FAF, I think, but someone else will give a more authoritative answer.



3. Valid RAIM now, and predicted at FAF and MAP
4. Outside FAF (110 degree arc each side of FAF)
5. Distance to FAF decreasing
6. Within 2nm of FAF, but not past (inside) FAF
7. NAV/GPS switch must be in the GPS position
8. No localizer frequency may be loaded into NAV 1 (some boxes)


Wow, that last one is pretty strange. I often have an ILS frequency
loaded up, even if I'm not using it. The most frequent use I make of
the NAV part of the GPS/NAV/COM is to fly an ILS, so the last ILS flown
is usually what's left there. What is the logic behind requiring me to
change it to something else?

The GX50 I had installed in my old Archer worked that way. I've also seen a GX50
that didn't work that way. It must be an installation option.

In the plane that wasn't installed that way, I've been safety pilot and seen a
confused pilot when he tuned the localizer without switching the GPS/NAV switch
to NAV, and couldn't figure out why the OBS wasn't behaving the way he thought
it should. OTOH, I can imagine the inverse kind of confusion occurring if the
OBS switched to NAV mode on its own when a localizer is tuned. You have to know
your installation.

What is the logic? Dunno.

On Tue, 04 Jan 2005 12:01:19 -0500, Dave Butler wrote:

Within 30 nm of what? The IAF? The destination?

The FAF, I think, but someone else will give a more authoritative answer.



the airport, if I am not mistaken.