A Random Collection of Items
The Questions
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 parentheses 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
An Answer
Waypoints in parentheses 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.
Response
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.
Matt,
Reply
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 annunciator should change from ARM to ACTV (known as the approach mode on some other named 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, here's 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 don't 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
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
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