Ben Flewett wrote:

A number of times I have switched on my transponder
and called for a clearance but ATC have not been able
to see me. I can see my transponder being interogated,
I have good battery power, everything seems to be working
but ATC still can't see me. This has happened to a
number of pilots I know.

Their have been similar cases to this in our club. It turns out that ATC
at Cape Town International had set their radar to filter out anything
moving slower than 40kt (ground speed). This I believe to filter out
stationery aircraft on the ground taxiing etc.

Gliders in wave simply disappeared off the radar screens, which did not
help anybody ...


Ian