A power transfer unit (PTU) is used in some aircraft hydraulic systems to transfer hydraulic power from one hydraulic system to another without mixing the hydraulic fluid between the systems. The PTU is able to transfer power, but not fluid, so the two hydraulic systems remain completely separate. This arrangement improves system redundancy because one hydraulic system can assist another if pressure or pump output is lost.
Different types of PTUs are used in aircraft. Some PTUs transfer power in only one direction, while others are capable of bidirectional operation. Some PTUs use fixed-displacement units, while others incorporate a variable-displacement hydraulic pump to better match system demand.
A typical PTU consists of a hydraulic motor connected directly to a hydraulic pump by a common drive shaft. The motor side is connected to one hydraulic system, while the pump side is connected to another. When one hydraulic system has a higher pressure than the other, hydraulic fluid drives the motor. The motor rotates the shaft, which in turn drives the pump and supplies pressure to the second hydraulic system. Depending on the direction of power transfer, each unit may operate alternately as either a motor or a pump.
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| Power transfer unit |
PTUs are commonly installed between independent hydraulic systems on transport-category aircraft. For example, a PTU may allow the hydraulic system powered by one engine to operate landing gear, brakes, or flight controls if another hydraulic system loses pressure. Because no hydraulic fluid passes between the systems, contamination or leakage in one system is less likely to affect the other.
During operation, a PTU may produce a noticeable whining or barking sound as hydraulic power is transferred. This sound is normal and often occurs when there is a large pressure difference between the two hydraulic systems, such as during landing gear extension or when an engine-driven pump is not operating.