Power Transfer Unit
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The purpose of the Power Transfer Unit (PTU) is to produce hydraulic pressure in the left hydraulic system in the event of a left hydraulic system engine-driven pump failure.
The PTU is used when there is right hydraulic system pressure but no left hydraulic system pressure. It transfers hydraulic power from the right hydraulic system to the left hydraulic system. 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.
On the Airbus A320 family of aircraft, during engine startup or shutdown, a distinctive noise (sometimes known as the "barking" noise, because the noise resembles a dog barking) can be heard inside the aircraft when the PTU is used.[1]
