Why does a hydraulic system failure increase the landing distance?
Any hydraulic failure will affect the landing distance as most of aircraft controls are operated by hydraulics.


If the high lift devices (flaps) become inoperative (or become degraded) due to hydraulic failure, the aircraft will have a higher approach speed, which will increase the landing distance.
Hydraulics are also used for operating a number of other systems like brakes, ground spoilers and thrust reversers. Any degradation in their operation will increase landing distance.