 Such flights are generically referred to as Compassion Flights.Most PBF group members are volunteer pilots, thus many PBFs are also called Volunteer Pilot Organizations (VPOs).The ACA website's Directory of Groups is the only place where all such groups as are known are listed for the benefit of patients, social workers, medical referral groups, agencies needing assistance, or the volunteer pilots who wish to find all groups they might want to join.