Could be anything. It is just a non commissoned rank. My uncle was a Flt Sgt in WW2 and was a pilot on Wellingtons, but you could be any of the flight crew or ground based. Even office staff could be Flt Sgt's.
Wikipedia have quite a good explanation (sorry this machine wont let me do links)
It depends on what "trade" they were in as it is a rank rather than a job title or description. My father was in the RAF for over 30years and at one time was a Flight Seargent before being promoted to Warrant Officer. He was in the Supply side, so as I said it depends on their job, not whether they were flight crew.