What causes the Airbus A320 'barking dog' sound?
The Airbus A320’s “barking dog” sound comes from the hydraulic power transfer unit (PTU). It runs when pressure differs significantly between the Green and Yellow systems, transferring hydraulic power—but not fluid—from the pressurised side to the low-pressure side. It is usually a normal start-up or shutdown noise, not an engine fault.
What does the A320 hydraulic PTU do?
The PTU allows either the Green or Yellow hydraulic system to help pressurise the other while keeping their hydraulic fluid separate. It consists of a hydraulic motor mechanically connected to a pump, so power crosses between the systems without the fluid itself crossing.
On the A320, the Green system is normally pressurised by the Engine 1 pump and the Yellow system by the Engine 2 pump. When their pressure difference reaches roughly 500 psi, the PTU can start automatically. The independent Blue system is not connected to it.
The characteristic bark is produced as the PTU’s speed and load change while pressure builds, stabilises and drops again. Repeated starting and stopping creates the pulsating groan heard through the cabin floor. It is not combustion noise and does not come from either engine.
When is the barking sound normally heard?
The PTU is most noticeable while hydraulic pumps are changing state and one system is temporarily pressurised more than the other. Common occasions include:
- during engine start as the engine-driven pumps come online;
- during engine shutdown as hydraulic pressure decays;
- during some single-engine ground operations;
- when a hydraulic pump is selected on or off for testing or maintenance.
The aircraft also inhibits PTU operation under certain ground conditions, which may include parts of the first-engine-start sequence and cargo-door operation. The exact timing depends on the aircraft standard, switch configuration and pressure state. An A320 may therefore produce several barks, one short burst or no audible bark at all during an otherwise normal turnaround.
Does the A320 barking noise indicate a fault?
The sound by itself does not indicate a fault; the hydraulic indications and ECAM messages determine whether a problem exists.
| Usually normal | Requires investigation |
|---|---|
| Short or intermittent noise during start-up or shutdown | Persistent operation accompanied by a hydraulic warning |
| Green and Yellow pressures stabilise normally | One system remains at low pressure |
| No PTU or hydraulic fault indication appears | ECAM reports a pump, reservoir, overheat or pressure problem |
A continuously running PTU is not automatically defective. It may be responding correctly to a sustained pressure imbalance caused by an unavailable engine-driven pump or another hydraulic problem. Because it transfers power rather than fluid, it cannot replace fluid lost through a leak.
Flight crews use the hydraulic system page and ECAM messages rather than diagnosing the aircraft by sound. Our guide to interpreting and responding to A320 ECAM information explains how those indications are presented.
Why does the PTU sound different in a flight simulator?
Simulator implementations range from a timed sound sample to a PTU driven by modelled Green and Yellow system pressures. A detailed aircraft may reproduce individual cycles, inhibition logic and pressure changes, while a simpler model may play the sound once during start-up or omit it entirely.
The same hydraulic principle applies across A320-family variants, but recordings, sound insulation and add-on logic can change what the simmer hears. If accurate systems behaviour matters, compare the hydraulic and audio modelling discussed in our assessment of deeper A320neo add-ons for Microsoft Flight Simulator.
To hear the PTU under normal conditions, begin from a cold-and-dark state and follow the aircraft’s standard engine-start sequence rather than forcing the PTU switch. Our practical A320 start-up and flight sequence for Microsoft Flight Simulator places the sound in its proper operational context.
What should you check if the sound loops continuously in a sim?
- Open the hydraulic system page. Check whether Green and Yellow pressures are actually unequal or whether the sound is playing despite normal indications.
- Check the overhead controls. The PTU is normally left in its automatic state; engine pump and Yellow electric pump selections should match the procedure being flown.
- Inspect active failures. A failed pump or hydraulic-system fault can cause legitimate prolonged PTU operation.
- Reset the aircraft state if indications are normal. If the bark continues with balanced pressures and no fault messages, the add-on may have a looping audio or state-initialisation problem.
A steady electric-pump whine, pack airflow and engine-start sounds are sometimes mistaken for the PTU. The genuine A320 “barking dog” is recognisably rhythmic and usually appears while Green and Yellow hydraulic pressure is being balanced.