Abstract: Employee #1 was working for a company that annually inspected and maintained the fire sprinkler system of a major airline's hangar at San Francisco Airport (SFO). By agreement, the employer used an aerial platform owned and maintained by the airline to carry out the inspection. An airline mechanic inspected and tested the aerial platform before turning it over for use by Employee #1. The mechanic also watched for about two hours to make sure that Employee #1 was using the lift properly. Later, while Employee #1 was up on the platform with the boom fully extended to approximately 60 ft, the center section suddenly lost hydraulic pressure and retracted. The bucket/case, with Employee #1 in it, fell freely about 20 ft. He suffered fractured tibias and fibulas in both legs due to the impact. Employee #1 was wearing a body harness and did not fall off the case. The mechanic suspected possible mechanical failure in the hydraulic cylinder.