In reply to clipskipper:
I flied with Ryan Air last week and used a Lowe Alpine Alpine Attack 35-45 as hand luggage.
I checked the dimensions carefully and it just fit if I didn't overstuff it. Cinching the compression straps in made the pack narrower but fatter so meant it struggled to fit in the basket they use to assess size, loosening them off made it wider but with less depth which made it bang on.
Mine was the biggest pack on by a margin but drew no attention and fit the overhead locker fine.
On the way home, to compensate for the extra weight of wet ropes in our hold luggage I packed more in my rucksack so it was pretty full yet we had no problems again.
That said, we packed our helmets in the hold luggage on the way home with a view to carrying them separately as the second small bag if the pack was too big as was.