I'm no expert in the light aircraft industry so I don't know what the common practice is here, but I do know a bit about fractional ownership. I would always advise people to discuss the sharing arrangment carefully before committing to buy and to have a written agreement covering the main issues.
There is lots of guidance on issues to consider on the
yours2share website. We are currently working on a new fractional ownership trust deed/sharing agreement for light aircraft which should be available within about a month, but we have templates for boat shares which are fundamentally similar.
I hope the arrangement works well for you, with a little care at the outset, it should do.