If you have 12 hours left in your sick bank and are sick for an 18 hour trip, you need to contact your ACP (I emailed my FOS in a similar situation), might/might not need to provide a Doctors note.
In my situation, they took all the hours out of my disability account versus splitting the hours between my regular sick account and my disability account.
If you don't contact them, they might just remove you from the trip w/out pay----which is also an option for you if you want to save some sick leave.

14.C.6.a
14.D.2

https://pilot.fedex.com/contract/cba/sec14/sec14c.shtml