failure of the copilot (pilot-rated cargo loader, who was flying the aircraft) to establish and maintain a positive rate of climb after taking off at night; and inadequate supervision by the pilot-in-command (PIC), by failing to ensure that proper altitude was obtained and maintained during the departure. Factors relating to the accident were: darkness; the lack of visual cues; and the resultant visual illusion, which the copilot failed to recognize during the night departure. Also, the lack of a restraint system (seat belt and/or shoulder harness) for the passenger was a possible related factor.