Have a look at Airline pilots: industry wide FRE for 2006/07 onwards: summary
and the links from that page, especially Airline pilots: industry wide FRE for 2006/07 onwards: employer contributions
These pages specifically refer to employed pilots but the details of what you can claim and not claim for apply equally to s/emp'd pilots - it's just that as a s/emp'd person you will have to claim the items on your return rather than just ask for the Flat Rate Expense in your code.
Golden rule - keep receipts.
For something to be eligible as an expense it must be 'wholly, necessarily and exclusively incurred in the performance of your duties'. At the end of the day if you're not sure but you think it applies, bung it down but be prepared to pull if they turn it down.