Interesting one. It's always surprised me how many GA pilots and instructors in the UK routinely leave encoding switched off on Mode C equipped transponders. It surprises me even more that this regularly happens under the London TMA where I would have thought it causes the most problems. The instructors at least should know better and be passing on the habit of using Mode C. I believe it comes from the fear of being caught on a temporary incursion of a couple of hundred feet into controlled airspace from below, something which frequently happens in that section of tight airspace.
Maybe it is time to make some mandatory mode C areas, say, under the London TMA?
[This message has been edited by ALKIN Hold (edited 09 January 2001).]