I suggest you contact the publishers
Their website addres is
http://www.airlifebooks.com/airlife/
I pulled out my old TT book 1.
It was first published in 1987
Second revised edition 1987
Reprinted 89,90
Third Ed 91
Reprinted 92, 93 (with rev), 94, 95
Fourth Ed 97
Reprinted 98,99 (with rev), and 99 again
So from the above, I can see no set revision pattern.
Reagrds
David