The 7 passengers had been on holiday @ Langeoog. They got there by car, but since the driver did not feel fit to drive back home, the family booked the Piper @ Dortmund to firstly bring a driver from Arnsberg to the island and secondly get them back home. How much of their baggage, if any, was loaded into the Saratoga @ Langeoog is yet unknown. As little as we know about the pilotīs fuel planning.
Fact is the tank wasnīt full @ Dortmund and had not been refilled until the incident.
Annex14:
"A fuelgauge is never as accurate as some might believe it is"
Agree, but would you take off in a chartered aircraft without having visually checked the fuel level in the tanks before?