Only small planes can fly without power actuation. The largest size airliner that can be flown manually would be about a B737, which actually has manual back-up for the elevator & ailerons as in this answer. The B737 uses a balance tab to assist in lowering the hinge moments during manual actuation.
In a much larger aeroplane like the DC-10, there is no practical amount of reducing hinge moments with servo tabs or other means, and flight control surfaces are deflected using hydraulic power only. This is schematically as follows:
When the pilot moves the stick, they deflect an input linkage which opens the servo valve of one or more hydraulic actuators. These then deflect, moving the output end, thereby closing the servo valve again. As long as there is enough total hydraulic pressure to overcome the aeroforce hinge moments, the actuator moves the control surface without any effort from the pilot required at all. In fact this would be so very light that artificial feel springs are mounted.
However, in some circumstances the actuators cannot overcome the control hinge moments, for instance due to:
- Having to fight one of the other actuators, for instance because of a hardcover servo valve failure;
- Hydraulic pressure loss in one or more of the actuators;
- Mis-trim and high airspeed cause surface blow-down, as in several recent crashes.
Note that in this case, when the pilot pulls on the stick, they first deflect the servo valve until it hits the stop, and this now becomes the fixed hinge point of the input linkage. Further stick deflection results in direct manual surface deflection, against the hinge moments caused by the aeroforces and against oil still slushing around in the hydraulic system.
Like mentioned, in large aircraft with no measures to lighten hinge moments, the flight crew cannot practically control the flight path using manual force. A DC-10 did manage to actually land after total hydraulic failure by using differential engine thrust, not manual actuation.