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As noted in this answer and on this page, the PTU (power transfer unit) is a hydraulically driven pump. It gets power from one hydraulic system and uses another hydraulic system to pressurize.
Aircraft Ptu
What makes this pump work? Is it automatic or manual? Does this pump work despite detecting a leak in the hydraulic system trying to restore pressure?
Power Transfer Unit (ptu)
The power transfer unit is known for its bark-like sound in Airbus (refreshing: video of a PTU in action). The sound comes from a pump that generates 3,000 psi of pressure, which turns on and off quickly.
The PTU is a safety system to ensure that hydraulic pressure is available, especially during take-off, even if one or both hydraulic circuits fail.
The PTU can restore pressure in a deficient circuit without transferring fluid from the working circuit to the deficient circuit. This prevents hydraulic fluid from the working circuit from being thrown overboard if the faulty circuit leaks.
On the Airbus A320 series, the PTU is the second source of pressure for the yellow and green hydraulic systems, after the EDP. It is always on, ready to activate the pump if there is a pressure drop in the yellow or green hydraulic system and the differential pressure is greater than 500 psi.
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Airbus A318, A319, A320 and A321 aircraft are equipped with three hydraulic systems and can be safely controlled with only one operating system. A two-way PTU allows the yellow system to pressurize the green system, and vice versa, without transferring fluid from one system to the other. In flight, the PTU operates automatically if a pressure drop is detected in any of the systems. The PTU guarantees optimum availability of the hydraulic system for take-off. (Source: EASA safety bulletin)
The PTU cannot restore the correct pressure in a circuit with a permanent leak. In this case, the PTU and possibly the working hydraulic system overheats. The crew receives a malfunction
On the ECAM screen and until recently I had to manually turn off (inhibit) the PTU to prevent damage.
If the yellow or green system loses hydraulic fluid, the PTU cannot pressurize the failed system and must be shut down as required by the ECAM procedure to prevent the PTU from overheating, which can occur two minutes later. (Source: EASA safety bulletin)
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To avoid unnecessary warnings during takeoff and landing, the ECAM message is not displayed when flying below 1,500 feet. There were several incidents (eg G-EZDM) where overheating was detrimental before the amber ECAM message appeared at 1,500 feet, resulting in the temporary loss of a second system. Airbus added logic to automatically inhibit the PTU when the aircraft is airborne, but still below 1,500 feet.
A PTU is connected to two independent hydraulic systems: The first drives a hydraulic motor (or turbine) mechanically connected to a pump. The second hydraulic system is pressurized by the pump.
The PTU can be reversible (each hydraulic system can drive the motor or pressurize the pump) or not (the same system always drives the motor).
The Airbus A320 (also A319) has three hydraulic systems: green, yellow and blue. Green is driven by motor 1, while yellow is driven by motor 2 (there are electric pumps for yellow and blue, but that's not important). The PTU is a two-way unit, with the yellow system pushing the green and vice versa.
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The PTU automatically activates when the differential pressure between the green and yellow system is greater than 500 psi (normal system pressure is 3000 psi). The PTU (hydraulic) controls/indicators on the top panel are shown in the figure below:
The PTU is inhibited when the first engine starts and self-tests when the second engine starts. During the flight, the flight crew receives warnings of the following situations:
In such cases, the PTU will automatically inhibit and display a fault if still in AUTO mode, while the crew can take appropriate action.
Inhibited below PTU 1500 and airborne, otherwise Operational Engineering Bulletin for aircraft with a stop order when hydraulic overheating or leakage is detected.
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A320 has a yellow hydraulic system pressure indication on ECAM which means xx = press also via edp or yellow selector pump = press txd replaced -power to txd well metered 28 v=xx on yellow system is always on press. applied or not
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