What happens if an aircraft's hydraulic system starts leaking during a flight?
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.