Programmed to deceive

The amazing technology that directs you to wherever you want to go will just as happily direct you to where you don’t want to go, as this commercial pilot discovered in a close call first published in 2011

Name withheld by request

The flight started as a routine transit from Coober Pedy to Port Augusta in South Australia. I had planned via the standard IFR route to the west of Woomera’s restricted areas, that had been active earlier that day, and all was well with the aircraft as I departed Coober Pedy and called Melbourne Centre with my estimates.

I was advised that Woomera had ceased operations for the day, and a direct route would be available to Port Augusta. This I happily accepted, as it would save about 10 minutes of flight time and fuel. Since I was flying a pressurised turboprop, the saving was worth considering. The clearance was issued, I made the track adjustment, climbed to 27,000 feet and settled in for the one-hour trip.

My aircraft was equipped with an IFR GPS, as well as a separate moving map with its own GPS database. Although independent systems, they talk to each other, and the moving map displays fixes and tracks from the other unit.

All was well, and the weather was perfect up there, as it generally is above the clouds. We were in bright sunshine, with everything going to plan and an early return home expected because my passengers had finished their business early. I even noticed that the GPS was indicating we were making very good time and had a good tailwind. I couldn’t have been happier.

Top of descent arrived, and I called Melbourne Centre to obtain clearance and traffic, and started down. By this time, the cloud below had built to a solid layer with the tops around 18,000ft, but this was not unusual for the time of year, and I had ascertained that Port Augusta was clear, expecting a simple visual arrival. As we popped out of cloud at 12,000ft, I was surprised to note that the terrain looked unlike what I had been expecting for this stage of the flight. We were over a large salt lake that should have been some miles behind us, and I couldn’t see the top of Spencer Gulf, as I would have expected to from that height. I suddenly realised that I wasn’t where I thought I was, and I was getting there at nearly 300kt!

I stopped the descent and started a troubleshooting cycle to see where I was, and why I was there. Part of my cycle included zooming in on the moving map to see what it had to say. In fact it told me a couple of things. Firstly, it showed the aircraft to be over the southern end of Lake Torrens but, more interestingly, it showed a waypoint for the Port Augusta airport to be there as well! That of course couldn’t be accurate, so the error had to be related to the GPS programming. I didn’t have time to delve any further into the GPS at that stage, because we were still some distance from Port Augusta, and I busied myself with setting up the ADF to track to the aid, making certain I could stay visual for the remainder of the trip, and amending my ETA with Melbourne Centre.

The rest of the trip went without incident, but after landing I had to get to the bottom of my GPS error.

Firstly, I checked the accuracy of the current GPS position against the ADF indication and the airport chart. All looked as it should. This was getting frustrating. Then, I entered the airport reference on the GPS, and was shocked to find that it was 56nm away! I had found the error , but why was the GPS telling me this? It turned out to be a very simple solution.

With this particular GPS unit, there were a large number of waypoints pre-programmed into the certified database, and updated from time to time by the manufacturer. In addition, there were user waypoints that could be set by the pilot. These took the form of Lat/Long, Brg/Dist, or Rad/Dist, depending on how you wanted to set it up. It became evident that at some point previously a pilot had entered a waypoint into the GPS, some 56nm to the north of Port Augusta, based on the Leigh Creek VOR, and called it YPAG! This was significant for a couple of reasons. Firstly, you should never use a name that indicates the airport position in this manner as it can be confused with the standard identifiers. Secondly, when this is done, it overrides the GPS programming, and the unit then displays this new ‘user waypoint’ whenever the airport is selected on the GPS. Lastly, if you set up a user waypoint in a GPS, and you are not the only pilot of that aircraft, you should have some kind of naming convention, or you should delete the waypoint after you’ve used it so as not to confuse anybody else, or have numerous similar waypoints in the system with different names.

Now that I had discovered the error in the GPS, I had to work out why it had caused my difficulty; that was actually the easy part.

As I departed Coober Pedy and obtained a direct clearance, I had inadvertently set the GPS up to use the airport waypoint instead of the NDB. This, I can only rationalise, was because I so often go to airfields that have no aid, and therefore are entered into the GPS with the four letter identifier. In reality, it shouldn’t have made any significant difference, as the Port Augusta NDB is located very close to the airport, but the procedure is valid and in place nevertheless. Secondly, while I normally would do a confidence check by comparing my flight plan distances, bearings and times with the GPS data I had just entered, this check wasn’t available once I had selected ‘direct to’. My flight plan was of little assistance to confirm the new data. In fact, only the distance to go would have been obviously different anyway, as the new waypoint was almost on the direct track. Combined with the strong tailwind, the shorter distance gave me a very good time interval, but instead of simply accepting it, I should have investigated further. I should also have zoomed in on the moving map much earlier, when I would have been able to see the variation in waypoint locations.

It was a timely reminder that when relying on the GPS for remote area navigation, you should also check with other sources of information and not simply accept that the GPS data is correct. Normally it will be, but every now and then, it will surprise you. I’m just thankful that this occurred in daytime and good weather.. I can live with the embarrassment, but the situation could have been very different at night or in bad weather!