I suppose having a PPL (A) would pretty much rule out being posted on a heli?!
If anything a PPL causes more problems than it solves. The RAF and the RN (can't speak for the green slime) probably prefer to start from scratch and teach their way. A PPL holder might think he/she is God's gift and need to re-learn, which can be hard.
I had 40 hours when I joined, and I can honestly say it was a waste of money.
Just as well it was the RAF who gave me that flying scholarship
