Abstract: Employees of the final operations department were testing the various hydraulic systems on a P-21 plane. At the same time, Employee #1, of the paint department, was on the call board and had been assigned by the final operations manager to strip paint from the leading of the aileron. The manager did not tell Employee #1 that tests were to be conducted on the plane. A coworker was assigned to be lookout on the right wing. Another worker was assigned to run the CRT to raise and lower the flaps and the aileron on the plane. These two workers did not have visual contact with one another; however, they did have radio contact. Employee #1 was taping the leading edge of the aileron. As Employee #1 was prepping the area, the worker who was the lookout asked to borrow a screwdriver. Shortly after this exchange, hydraulic functions were started. The lookout worker gave clearance to the coworker that the area was clear. System #4 was tested but was not registering any pressure or flow. System #1 had already been tested and was found to be inoperative. The coworker put pressure into System #3 to see if the entire system was inoperative. System #3 operated. The counterbalance on the aileron caught Employee #1's right arm against the counterbalance housing. Employee #1 sustained fractures to his ulna and radius and required hospitalization. Causal factors in this accident include the employer's failure to block or otherwise secure the aileron counterbalance from inadvertent movement while the equipment was being repaired, and the employer's failure to train and instruct Employee #1 of the hazards of his new job assignment.