Sunday, June 24, 2007

The Case of the Disappearing Waypoint

T. is one of many customers who does a monthly sim session with me. He flies a Mooney, equipped with a Garmin 430W and is proficient on its use. His June sim session brought up an interesting scenario which is well worth sharing with you.

In the sim session, T. was flying an IFR flight from Anoka (KANE) to Rochester, Minnesota (KRST). For this particular flight, I had selected strong winds from the southeast. Playing the part of ATC, I directed T. to fly directly to CORDY intersection to start the GPS-13/RST.

T. loaded the approach into the Garmin 430 in the sim and activated it, with CORDY as the IAF. Unfortunately none of the Garmin 400/500 series downloadable simulators have a database current enough to contain the GPS-13/RST approach, so I will have to instead describe as best I can what happened. After T. loaded and activated the approach, FPL0 looked like this.

Approach GPS 13
RWY 13 (MA)

I had thoughtfully set the ceiling so he would be forced to fly the missed approach, holding at the Rochester VOR (RST). As usual, T. flew it perfectly, executing the missed approach and entering the hold at the Rochester VOR, using the SUSP feature of the Garmin 430 to bring up the holding waypoint, RST in this case.

What happened next caught both of us by surprise. I told him to anticipate the ILS-13 approach into Rochester and was planning on having him do the transition from the VOR to ELLIE.

While still in the hold at the Rochester VOR, T. decided to load the ILS-13/RST approach. So he loaded it, using ELLIE as the IAF, but did not activate it since he was still holding around RST. As soon as he did this, RST disappeared as the active waypoint. This left him in a hold without an active holding waypoint.

In discussing it later, he explained his logic was to simply load the ILS approach, the logic being that he would not activate it since he was still in the hold at RST. However, as soon as he loaded the ILS approach in the Garmin, all of the waypoints from the previous approach promptly disappeared. Since RST was not a waypoint in the loaded approach, T. was left without his holding waypoint. Had he chosen RST as the starting point when he loaded the approach, he would have still had RST in the set of FPL waypoints.

It was an unexpected scenario to both of us. The bottom line is that you should not replace an approach in the Garmin 430 if you are depending on a waypoint in the current approach for navigation. Unless you are very careful, doing so can cause you to suddenly be navigating to an unintended waypoint. I think this would be a case of being up the well known creek without a proper waypoint!


john said...

I hope you don't mind but I pointed this entry out to Aviation mentor
and asked his opinion here's his comment:

I read the post and my only comment is I'd recommend against reprograming a GPS while flying a hold over a GPS waypoint UNLESS I had a way to fly that hold using the VOR receivers. In the example cited, the missed approach holding waypoint was in fact a VOR! Apparently it never occurred to the pilot (or the instructor never suggested) that it might be a good idea to tune the RST VOR on the 430's VOR receiver.

Many pilots I fly with get stuck in GPS mode and have a hard time changing gears and going back to VORs. Though it is more work, I recommend that the VOR receivers be tuned with something that could be helpful in the unlikely event the GPS goes Tango Uniform.

In a plane with dual 430 or 530 units with automatic crossfill enabled only from the TOP unit to the BOTTOM unit, I would consider loading and activating a new approach on the BOTTOM unit. This would leave the TOP unit undisturbed and let me continue to fly the hold using GPS alone. When given clearance to leave the hold and proceed to the next approach, I'd manually crossfill from the BOTTOM unit to the TOP unit. That's the only way I know to get ahead of the button pushing and knob twisting without shooting yourself in the foot.

Linda said...

Hello John,

I would have had the RST VOR tuned in as a backup, but I didn't suggest it as I wanted to see what he would do. It certainly does drive home the point that once you select an approach, whether it is activated or just loaded, all of the waypoints in the previous approach are going to disappear. The student was correct in his logic not to activate the approach, but had not extended it to realize that all previous waypoints would be replaced.

Having something like this happen makes a much more vivid impression as opposed to simply discussing it. And of course the sim is the perfect place to create this type of learning.

Thanks for writing. It's always nice to know someone is actually reading the posts.