In July 1989, the United Flight 232 crashed in Sioux City, Iowa, while the pilots were trying to land on the runway without any yoke control. Today, I was reading the PHAK and found this phrase: "In the event of complete (hydraulic or electrical) power unit failure, movement of the control surface can be effected by manually controlling the control tabs" (4-46).
Why didn't the pilots of the United Flight 232 have a backup manual control when their hydraulic system failed? Is it because the aircraft (DC-10) was equipped with three independent hydraulic systems, which were considered to provide adequate redundancy for emergencies? Then, what about other modern passenger jets like Boeing and Airbus models? Do they have a backup manual control system in case of such a hydraulic or electrical control system failure?