GPS approach question

CFII_ASC wrote:

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.

That is what I thought, but I couldn't figure out why it wouldn't sequence.


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.

Yes, obviously, I'm just not sure which one. I know that 1, 3, 4, 5, 6
and 7 were satisfied. I don't remember if I checked the ARM
annunciator. I had flown the LOC 28 prior to the GPS 28 and still had
that frequency tuned, however, it was in the #2 NAV as the #1 NAV is
removed for repairs. I didn't realize that the GPS interfaced to the
NAVs, so that may be it. I'll check the King manual shortly to see if
that might be the problem. I later flew a GPS at N38 with no problem,
but by then I'd tuned the NAV to a non-localizer frequency.


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

I didn't clear the flight plan, so that may have been the problem. I
flew a flight plan from ELM to N38, but simply did a DIRECT to ELZ and
therefore FP 0 would have still had ELM to N38. That may have been my
error.


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

Didn't have a NAV 1 that day!


Set the GPS/NAV switch to GPS

I don't recall our club plan having such a switch. Is this required for
all IFR installations? This wasn't mentioned during my ICC either.


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)

I believe I did the normal approach activation such that it asks you if
you want to insert the approach waypoints into the flight plan. I said
yes thinking that my "flight plan" was the direct to ELZ, however, I'll
bet it added the approach to my FP 0 which was ELM to N38. That very
well would confused the GPS royally, and me in rapid succession. :-)


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.

Yes, I did learn this during the ICC for both course reversals and holds
after a missed.


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!!!

I did this and that is when things went nuts. However, it sounds from
what you wrote above that I entered the approach into a flight plan to a
different airport. I didn't realize that entering a direct to fix
wouldn't replace flight plan 0.


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.

I was taught to hit direct to at the MAP and then fly in leg more
towards the holding fix and then switch to OBS mode to fly the hold.


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.

I've not heard of suspended mode before. I'll look this up. I only
read about LEG and OBS mode, and missed suspend.


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

Thanks, Gene, I appreciate the time you took to make this very detailed
approach. A few more flights and I'll get a handle on GPS approaches.
I still find them far more complicated than other approaches, even NDBs,
however, that is just my newness to GPS.


Matt