Nut Loose,
Possibly true in later years, but, the Tech Apps retained the original Halton Apps entry number system when they were introduced and passed out as Cpls after 3 years.
The Craft Apps started the 2xx entry numbers and did 2 years, leaving as a J/T.
There were, for a short while, Mech Apps starting with a 3xx entry number who did 1 year and left as an S.A.C ( mech ).
Thereafter, it was 3 yrs to Sgt and 4 to C/T for the tech.apps..or, for the craft.apps 3 years to Cpl and then as per the techs.
All subject to passing exams ( the easy bit ) and then the wonderful assessment system the R.A.F had in place in those days, which ensured you were at the whim of the woefully incompetent Eng.O cadre. I only met two who were actually capable of being, and understanding, engineers and both of them left the R.A.F prematurely to get a decent role in the civilian world.
The time promotion resulted in a serious promotion stagnation, due to the surplus, of engineers in the late 70's / early 80's, and a high PVR rate at one time.
I have never forgotten the complete look of shock and inability to understand on the face of a Flt.Lt from Innsworth who briefed (if you could call it a brief, more a hectoring lecture) us at Bruggen as to why Cpl was the rank to be in the future....the cheers and clapping from the front rows, the old cooks, admin, MT trade groups were in stark contrast to the wall of ice and " far from acquiescent " vocal response from the rear.....all engineers.
The poor dear had probably never met engineers in person before...but he did then.