Abstract: At 10:15 a.m. on May 18, 2020, an employee was working for a firm that provided fixed base operator (FBO) services for general aviation. He was working in the facility's structure shop. In the shop, there was a Tennsmith T-52 foot shear, with serial number 18445. The shear was used daily to cut custom parts. The shear was a human-powered or jump shear. The shear's blade came down when the operator stood on the treadle or foot bar and put his weight on it. The shear came from the manufacturer with a front guard and a back gauge, but by now, the shear was missing the back gauge. The front of the shear was guarded, but the shear's point of operation was not guarded from behind. In this incident, a coworker of the employee placed a 4-in. by 12-in. (100-mm by 300-mm) piece of 0.060-in. (1.52-mm) piece of aluminum stock into the far-right side of the shear. When the coworker activated the shear, it only bent the metal. The employee, who used the shear daily, knew it was the wrong side for cutting metal. He offered to assist, and the coworker accepted. The employee grabbed the piece of metal and inserted it into the far-left side of the shear, where the blade came down faster and with more force. The piece of metal, bent from the earlier attempt, hung up in the shear. The employee reached behind the shear with his left hand to bend the metal down while still holding the section in the front of the shear with his right hand. The employee heard the spring initiate and immediately pulled his hand out as the shear blade came down. The coworker had stepped on the treadle. The employee immediately asked the coworker why he would do that. The coworker was in shock and didn't have an answer. The tip of the employee's left thumb had been amputated. He was treated without hospitalization. The employee stated he had never had to reach behind the shear and that he would normally never do that. He usually operated the shear by himself. The coworker had used the shear about three times.