What is the PTU on the Airbus A320, and what does it do?
The Airbus A320 PTU (Power Transfer Unit) lets the Green and Yellow hydraulic systems share mechanical power without mixing or transferring hydraulic fluid. If one system has sufficient pressure and the other does not, the bidirectional PTU uses a hydraulic motor and pump to pressurise the weaker side automatically.
In real-world A320 operation—and in a systems-detailed flight simulator—the crucial distinction is that the PTU transfers energy, not fluid. Each hydraulic circuit retains its own fluid and reservoir, protecting the healthy system if the other develops a leak.
How does the A320 PTU work?
The PTU connects the Green and Yellow systems through mechanically linked hydraulic machines. Pressure from the stronger system turns one side of the unit, which mechanically drives a pump in the lower-pressure system.
| Hydraulic system | Normal power source | PTU relationship |
|---|---|---|
| Green | Engine 1-driven pump | Can receive mechanical power from Yellow |
| Yellow | Engine 2-driven pump and Yellow electric pump | Can receive mechanical power from Green |
| Blue | Electric pump, with RAT support in an emergency | Not connected to the PTU |
The arrangement is bidirectional: Green can power Yellow or Yellow can power Green. The PTU does not determine which system should be the donor; the pressure difference does that.
When does the Airbus A320 PTU operate?
With the overhead HYD PTU pushbutton in its normal AUTO position, the PTU operates when Green and Yellow pressure differ by roughly 500 psi. A failed engine-driven pump, a stopped engine or substantial demand on one system can create that condition.
The PTU is subject to automatic ground inhibitions. Depending on the aircraft standard, these include portions of the first engine start, cargo-door operation, certain one-engine states with the parking brake set, and towing configuration. It may also run briefly as part of the start sequence or an automatic test.
Normal crew practice is not to switch the PTU on manually—it has no conventional ON selection. It is left in AUTO unless an ECAM procedure, abnormal checklist or maintenance requirement directs otherwise. Our explanation of where system checks fit into the normal A320 flow provides the operating context.
Why does the A320 PTU make a barking-dog sound?
The familiar barking or sawing noise is vibration from the PTU loading, unloading and responding to hydraulic pressure differences. It is often most noticeable around engine starts, shutdowns or ground hydraulic activity because the cabin is quiet and only one side may be fully pressurised.
The sound alone does not indicate a fault. A repeated noise accompanied by a hydraulic warning, low quantity or pressure that fails to recover is different; the crew should follow the displayed procedure. In a simulator, use the HYD system page and warnings rather than judging the system only by sound—our guide to interpreting A320 ECAM indications explains that workflow.
Which failures can the PTU handle?
The PTU can restore or maintain pressure when a Green or Yellow system has lost its normal power source but still contains usable hydraulic fluid. Typical examples include an unavailable engine-driven pump or an engine that is not running.
It cannot fix every low-pressure condition:
- A major leak or empty reservoir: the receiving system lacks the fluid needed to build pressure. The PTU deliberately does not transfer fluid from the healthy side.
- Loss of both useful pressure sources: the PTU creates no power of its own; one side must be able to drive it.
- A Blue-system problem: Blue is independent and has no PTU connection.
- An electrical failure: the PTU is not a generator and is separate from the RAT and emergency electrical system. See how A320 hydraulic power, the RAT and emergency generation interact.
Why might the PTU appear not to work in a simulator?
An apparently inactive PTU usually has a straightforward cause: the overhead pushbutton is OFF, automatic ground logic is inhibiting it, neither system has enough pressure to act as the donor, or the affected circuit has lost fluid rather than pump power.
Aircraft add-ons also model the system at different depths. A simplified A320 may reproduce the barking sound without complete pressure logic, while a detailed model may enforce the real ground inhibitions. Check the Green and Yellow pressures, reservoir quantities, pump states and ECAM messages before assuming the simulation is faulty.