Power Transfer Unit transfers hydraulic power from one of an aircraft's hydraulic systems to another. The PTU onboard the A320 is a hydraulically powered unit that exists to restore a minimum pressure across two of the aircraft's hydraulic systems; it is a bidirectional unit used between the yellow and green hydraulic systems. A motor and a pump, mechanically connected by a shaft, form the unit, which operates automatically when it detects a pressure difference of more than 500 psi between the green and yellow systems, thereby guaranteeing that power is automatically sensed and transferred.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is a PTU on an aircraft?
The power transfer unit (PTU) is a device that transfers hydraulic power from one of an aircraft's hydraulic systems to another. It is designed to allow the right hand hydraulic system to assist the left hand hydraulic system when no left hydraulic system pressure exists.
On an Airbus aircraft the Power Transfer Unit is a hydraulically driven motor-pump that bridges the green and yellow hydraulic circuits. Because these two circuits power flight controls, landing-gear, brakes and other vital services, PTU exists to restore minimum 3,000 psi pressure to either side the instant a 500-psi drop is detected, thereby preserving full control authority for take-off and landing without ever transferring fluid between the systems.
PTU is a safety device, a cornerstone of Airbus' emphasis on redundancy and fail-safety, but it is also bi-directional: the same unit can pressurise the yellow system from the green or the green from the yellow, depending on which has lost pressure. Self-start occurs through pure mechanical influence once the differential exceeds the threshold, so the mechanism is automatically armed even on one-engine taxis.
What is the purpose of a power transfer unit?
The purpose of a power transfer unit (PTU) is to transfer rotational power from an engine or electric motor to another unit or system. In aerospace, the PTU acts as a bridge that connects the green and yellow hydraulic systems with each other. When a pressure differential between the two systems equals 500 psi, the PTU becomes responsible for sending pressure to the opposite system. The bidirectional unit enables the green system to pressurize the yellow system and vice-versa, restoring pressure in the deficient circuit without transferring fluid from the working circuit. This safety system ensures that hydraulic pressure is available at takeoff even if one hydraulic circuit has failed, guaranteeing hydraulic pressure is available even if one or two hydraulic circuits have failed.
Improvements in flight safety arise because the PTU can restore pressure in a deficient circuit automatically. Check-valves and a back-pressure stall line protect the unit, allowing the right-hand hydraulic system to assist the left-hand system and the left-hand auxiliary motor pump to retract the landing gear during a port engine failure. An electric pump operated by airport ground personnel opens the cargo hold doors when the aircraft is on land, while the engine-driven pump remains the main source for each hydraulic system.
Does Boeing have a power transfer unit?
The Boeing 757 is equipped with a Power Transfer Unit (PTU). This aircraft relies on three fully independent hydraulic systems, and the PTU supports them by transferring power between the left and right circuits when pressure differential is detected.
How does a hydraulic power transfer unit work?
A hydraulic power transfer unit works on the principle of pressure differential. A hydraulic power transfer unit is essentially a hydraulic motor coupled to a hydraulic pump via a single shaft; no transfer of hydraulic fluid occurs. The motor is driven by one hydraulic system, and the pump pressurizes the second hydraulic system. The motor and pump are mechanically connected, so PTU works mechanically, not through hydraulic fluid transmission. PTU self starts by pure mechanical influence alone resulting from the delta pressure between the two hydraulic systems. A piston on the PTU control valve starts working when it senses a pressure difference of 500 psi, allowing the unit to surge into action and create the required pressure.
