Eduardo Fonseca writes:

I have a Flytec Element Track. There is firmware version 2.00.00
and I have asked the Naviter people if that firmware version includes the WGS84
model. Standing by for a response.

He asks Joerg Ewald:

What is the Earth model used in the Flytec Element Track? FAI Cat
1 comps require the WGS84 model. However, if the Element doesn’t have that
model, I would like to calculate what the error would be and adjust the tasks
accordingly. Essentially, I want to determine the maximum circle diameter from
the Element model that would fit inside the WGS84 circle, thus ensuring that I
always hit the waypoints no matter what model is actually used.

Joerg Ewald responds:

Indeed, the Element Track was developed at a time when the FAI
sphere was still the earth model used by CIVL in all of its competitions, and
that’s what is used there. Unfortunately, I doubt the processor would be able
perform the WGS84 calculations. That said, I also doubt that any pilot who flies
in an FAI Category 1 event would rely on an Element Track for navigation.

The error you are looking for is well known: For many years, we have been using
a turnpoint radius tolerance of 0.5% to accommodate both devices that use WGS84
and those that use the FAI sphere. So essentially, by going in 0.5% deeper than
the required radius, you will always be on the safe side. The actual difference
varies depending on the direction from which you approach a turnpoint. At the
extremes, from East/West, with WGS84 you reach the cylinder before reaching it
on the FAI sphere. On the other hand, from North/South you reach the turnpoint
first on the FAI sphere, and have to fly further in to reach it on the WGS84
ellipsoid.

You can reduce the turnpoint radius by 0.5% from the values given
by the task committee. For example, 10 km turnpoint radius should be set at 9.95
km. And thinking about a recent example, at 53 km turnpoint radius should be set
at 52.735 km, or 265 meters smaller. Flying at 50 km/hr it will take you 19
seconds to go the extra 265 meters and then an additional 19 seconds to get back
to the edge of the 53 km cylinder (ignoring the time it took to turn around).

The 2019 Big Spring Nationals is the pre-Pan-Americans and therefore uses as
much as possible Category 1 rules which includes the WGS84 earth model and the
0.1% error band.