Actually I didn't choose it Whirly did
It was at the Sywell fly-in when we were all introducing ourselves. I'd arrived with Wycombe, and was introduced as a "PPruNer with no name". Whirly took one look at the sweatshirt I was wearing and christened me Brooklands.
I decided that I liked it, as its the birthplace of both British motor racing and aviation so I registered with it (and was quite surprised to find no one else had beaten me to it).
I've actually been Ppruning for a long time though - I was going to register a few years ago as the 9999th member (nickname All_the _nines). I stayed late a work one night waiting for the magic number to come up, but gave up at about midnight about four short of the target. I'm pretty certain that I can remember reading PPrune when there were LESS THAN 5000 registered users, and long before there was a Private Flying forum
