What does the stripe on the cap mean - RAF

[KatPez]

These two photos are of my grandfather John Alexander Linnell at various stages during his RAF career.  I don't know much about when he was in the RAF apart from that he was enlisted during WW2 (not sure if this was by choice or conscription). 

Anyway I was wondering what the white band meant on his cap.  I'm guessing it's some type of promotion/ranking but any help would be great

[KGarrad]

The white band may indicate he was an officer cadet, or at least flight crew under training.

[Regorian]

RAF College Cranwell (and elsewhere) officer cadets to 1939, aircrew training thereafter. However, not seen it 'thereafter'. 

[Rena]

There's a whole group of cadets wearing that white band o this url

http://www.preloved.co.uk/adverts/show/112982786/ww2-raf-air-crew-training-class-cranwell-picture.html

[KatPez]

I have a small piece of paper which was with his medals (which were unfortunately stolen in a burglary) which says he was an ac2

[Regorian]

AC 2 was probably just the start. Do you know what he ended up as. He looks like officer material.

[Rena]

AC 2 was probably just the start. Do you know what he ended up as. He looks like officer material.

That's the lowest rank and must have been his starting point:-

aircraftman 2nd class (AC2; also colloquially known as an "AC plonk"

he then had to pass various exams to get to other ranks such as

Leading Air Craftsman LAC, and then
Senior Air Craftsman SAC

[KatPez]

I don't. I think my mum wants to order his military records. I was just intrigued by the hat

[Brian1949]

The other replies hint at it - the white stripe on the forage cap indicates air crew under training.
I would recommend obtaining his RAF records which give his dates and times of training and where he was posted after training.
Regards
Brian