In aviation, a power transfer unit (PTU) is a device that transfers hydraulic power from one of an aircraft's hydraulic systems to another in the event that second system has failed or been turned off.
Modern airliners have separate, independent hydraulic systems powered by engine-driven pumps. The PTU is used when, for example, there is right hydraulic system pressure but no left hydraulic system pressure. In this example, the PTU transfers hydraulic power from the right hydraulic system to the left hydraulic system. A PTU consists of a hydraulic motor that powers a hydraulic pump. As the connection is purely mechanical, there is no intermixing of hydraulic fluid between the left and right hydraulic systems during PTU operation.
The PTU is able to operate the hydraulic motor generator during all phases of flight. It cycles on whenever an engine is started, and may repeatedly cycle on and off whenever pressure in one system drops below a certain point. While it would engage in the event of a failure, in day-to-day operations it is commonly used during taxiing when one engine is shut down to conserve fuel.