I just ran across an example of a plane which lost all 3 of its redundant hydraulic systems.
United Airlines Flight 232 - Wikipedia
Reading this article, one can see that loss of hydraulics meant total loss of flight control
surfaces, despite the fact that mechanical controls were still connected as before.
Excerpted....
The various gauges for all three hydraulic systems were registering zero.
[4] The three hydraulic systems were separate, so that failure of any one of them would leave the crew with full control, but lines for all three systems shared the same narrow passage through the tail where the engine debris had penetrated, and thus
control surfaces were inoperative.
[1] The crew contacted United maintenance personnel via radio, but were told that, as a total loss of hydraulics on the DC-10 was considered "virtually impossible", there were no established procedures for such an event.
[1]
The plane was tending to pull right, and slowly oscillated vertically in a
phugoid cycle – characteristic of planes in which control surface command is lost. With each iteration of the cycle, the aircraft lost approximately 1,500 feet (460 m) of altitude. On learning that Fitch, an experienced United Airlines
captain and DC-10
flight instructor, was among the passengers, the crew called him into the cockpit for assistance.
[4]
Haynes asked Fitch to observe the ailerons through the passenger cabin windows to see if control inputs were having any effect.
[4] Fitch reported back that the ailerons were not moving at all. Nonetheless, the crew continued to manipulate their control columns for the remainder of the flight, hoping for at least some effect. Haynes then asked Fitch to take over control of the throttles so that Haynes could concentrate on his control column. With one throttle in each hand, Fitch was able to mitigate the phugoid cycle and make rough steering adjustments.
