If you dropped the bit about "fixed costs are covered so a contribution towards fuel will be fine" and just said:
"I'm going flying on Saturday, anyone want to come along?" or "every weekend I go flying and I know some of you are interested. If you'd like to come along, give me a bell"
and then when they phone you say "sure the flight is free but you can contribute towards the fuel if you like, which will be about £50"
then that would be fine.