On the Boeing 767, the control surfaces are so large that the pilots cannot move them with muscle power alone. Instead, hydraulic systems are used to multiply the forces applied by the pilots. Since the engines supply power for the hydraulic systems, the aircraft was designed with a ram air turbine, a backup generator that swings out from a compartment and converts the air flowing past the airplane into rotational movement. There are numerous designs of ram air turbines. The version on the B767 resembles a 2-blade propeller similar in size to that of an ultralight aircraft and it directly drives a hydraulic pump. As the Gimli pilots performed a partial side-slip manoeuvre (forward slip) to reduce altitude prior to landing, the disrupted airflow past the ram air turbine also meant a decrease in the hydraulic power available, and they were surprised to find the aircraft slow to respond when straightening after the forward-slip.