Thanks for that. I was wondering about the use of the numerals 8 and 9 because when I'm doing Delivery I sometimes get a pilot giving a wrong readback of a squawk, which sometimes contains an 8 or a 9!
So am I correct in thinking that generally if an aircraft has TCAS II then it has Mode S?
On another but related subject, if you're on approach at, say, 3000 feet, and a helicopter crosses underneath you squawking, but no Mode C, you only get a 'Traffic Traffic' call, correct? And what is your procedure then? Look for the traffic and ask ATC it's level?
Gonzo.