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As highlighted by this answer and by this page the PTU (power transfer unit) is an hydraulic driven pump. It gets energy from one hydraulic system and uses it to pressurize another hydraulic system.
What triggers the functioning of this pump? Is it automatic or manual? Is this pump available even if a leak is detected in the hydraulic system it tries to restore pressurisation to?
(A320 PTU (Vickers), source)
The Power Transfer Unit is well known on Airbus due to its barking-like sound (refresher: Video of a PTU in action). The sound comes from a pump which creates a 3,000 psi pressure being quickly switched ON and OFF.
(Source: Youtube)
The PTU is a safety system to ensure hydraulic pressure is available, specially at takeoff, even if one or two hydraulic circuits have failed.
The PTU can restore pressure in a deficient circuit without actually transferring fluid from the working circuit to the deficient one. This prevents throwing hydraulic fluid from the working circuit overboard if the deficient circuit is leaking.
The PTU is always activated, but is actually triggered based on a pressure differential.
When is it activated?
On Airbus A320 series, the PTU is actually the second source of pressure for the yellow and green hydraulic systems, after the EDP. It's always ON, ready for the pump activation if there is a pressure drop on the yellow or green hydraulic system and the differential pressure is more than 500 psi.
Airbus A318, A319, A320 and A321 aeroplanes are equipped with three hydraulic systems and they can be safely controlled with only one system operative. A bi-directional PTU enables the yellow system to pressurize the green system, and vice versa, without the transfer of fluid from one system to the other.
In flight, the PTU operates automatically if a pressure drop in one of the systems is detected. The PTU ensures optimal hydraulic system availability for take-off.
(Source: EASA safety bulletin)
Automatic inhibition below 1,500 ft in case of leakage
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, eventually overheat. The crew receives a failure amber caution on the ECAM display and, up to recently, had to switch OFF (inhibit) the PTU manually to prevent damage.
In case of loss of hydraulic fluid of the yellow or green system, the PTU cannot pressurize the failed system and must be switched OFF as required by ECAM procedure to avoid a PTU overheat which may occur two minutes later.
(Source: EASA safety bulletin)
A320 PTU switch (source: Baltic Aviation Academy which, by the way, has plenty of quality training videos)
To avoid distraction from non-essential warnings during takeoff and landing, the ECAM message is not displayed when airborne below 1,500 ft height. There were several incidents (e.g. G-EZDM) where overheating became harmful before the amber ECAM message appeared at 1,500 ft, causing the temporary loss of a second system. Airbus added logic for automatic PTU inhibition when the aircraft is airborne, but still below 1,500 ft.
The previous quotes are from an EASA bulletin related to this modification (also detailed in Safety First, the Airbus safety magazine), with a failure scenario that had to be prevented:
Principle
A PTU is connected to two independent hydraulic systems: The first to drive an hydraulic motor (or turbine) mechanically connected to a pump. The second hydraulic system is pressurized by the pump.
PTU principle, source
PTU can be reversible (each hydraulic system can drive the motor or be pressurized by the pump) or not (the same system always drive the motor).
(Actual PTU schematics, source)
Airbus A320 (also A319) has three hydraulic systems — green, yellow and blue. Green is driven by engine 1, while the yellow is driven by engine 2 (there are electric pumps for yellow and blue, but that's beside the point). The PTU is a bidirectional unit which enables the yellow system to pressurize the green and vice versa.
The PTU runs automatically when the differential pressure between the green and yellow systems is more than 500 psi (normally, the system pressure is 3000 psi). The PTU (hydraulic) control/indicators in the overhead panel is shown in the following image:
Image from A320 hydraulic manual
The PTU is inhibited during the first engine start and self tested when the second engine starts. During flight, the flight crew receive warnings for the following situations:
Overheat in green/yellow reservoir.
Low air pressure in green/yellow reservoir.
Low fluid level in green/yellow reservoir (due to leak).
In these cases, the PTU will be inhibited automatically and will show a fault if it is still in AUTO mode, while the crew can take appropriate action.
References: A320 hydraulic Manual
PTU Logic requires:
BOTH ENG MASTERS ON POSITION
DIFF PRESSURE > 500 PSI BETWEEN GREEN AND YELLOW HYDRAULIC SYSTEM
NOSE GEAR COMPRESSED
PTU SIWTCH AUTO POSITION
PARKING BRAKE SET TO OFF
PTU Inhibited below 1500 and airborne, otherwise Operating Engineering Bulletin for the aircraft with instruction to switch off when encountering hydraulic overheat or leakage.
Also dúring engine start, the PTU is tested by the Flight Warning Computers --> barking sound.
Cheers
The A320 has a yellow hydraulic system pressure indication on the ECAM which shows xx =even the press by edp or by yellow elect pump = the press txd replaced -the power to txd measured ok 28 v=xx indication on yellow sys is always with press applied or without.
