What happens if an aircraft's hydraulic system starts leaking during a flight? What will happen if hydraulic fluid starts leaking during a flight? What will happen to the overall performance of the flight, and how will the pilot compensate for the loss of the system?
That depends on the aeroplane. On the B737 for instance, there are three independent hydraulic systems: System A, System B, and Standby System. Normally Systems A and B are operating and Standy is off, only switched ON when necessary from a malfunction.

The flight systems are powered redundantly, with more than one hydraulic system powering the essential aircraft systems. The following systems are powered by the hydraulics:



The hydraulic systems are each redundantly powered as well. For instance hydraulic system A is powered by an engine driven pump and an electrical AC pump. System A is protected from leaks in the engine driven pump lines by a stand-pipe: should a leak occur there, fluid leaks away until the fluid level reaches the stand-pipe, at which point the fluid level is approx. 20% and the engine driven pump circuitry is empty. System pressure is then maintained by the electric pump. Should a leak develop in the electric pump circuitry, the fluid quantity in system A reser- voir, and therefore system pressure, steadily drop to zero. 



System B has similar anti-leak features, with a stand-pipe that allows for the Power Transfer Unit (PTU) to operate on the remaining fluid. The PTU is a dual hydraulic pump/motor: one end is driven by System A, the other end can pressurise the fluid in System B.