Passengers sitting around the wing of an Airbus A320-family aircraft can sometimes hear a distinctive mechanical sound on the ground that is popularly compared with a dog barking. The sound is associated with operation of the aircraft’s hydraulic Power Transfer Unit, or PTU. The important point is that the PTU is not an alarm and it is not an engine component. It is part of the A320 family’s hydraulic architecture, whose job is to make hydraulic power available to flight-control, landing-gear, braking and other aircraft systems. Airbus describes the A320 as having three independent hydraulic systems — Green, Yellow and Blue — and identifies the PTU as an auxiliary source of pressure for the Green and Yellow systems. [1]
Airbus’s own technical material describes the PTU as a hydraulic motor-pump that transfers hydraulic power between the Green and Yellow systems without transferring hydraulic fluid between them. That distinction explains much of what the unit does. One side of the PTU can be driven by pressure in one hydraulic system, while the mechanically coupled pump on the other side supplies hydraulic power to the other system. The fluid in the two hydraulic circuits remains separate. In the Airbus Safety first description, the PTU operates automatically when the pressure differential between Green and Yellow reaches the specified threshold for the configuration discussed in that publication. [1]
Why an A320 needs several hydraulic systems
A transport aircraft cannot sensibly depend on one pump and one fluid circuit for every hydraulically powered function. The A320 family therefore distributes hydraulic power across three systems. Airbus documentation shows Green, Yellow and Blue circuits supplying different combinations of primary and secondary flight controls and other consumers, so loss of a single source does not automatically remove every hydraulically powered function. Airbus identifies engine-driven pumps as the normal pressure source for the Green and Yellow systems, while the Blue system has its own architecture. The precise assignment of consumers and backup sources is defined in approved aircraft documentation and can vary with aircraft standard, so operational decisions must always be based on the applicable manuals rather than a general article. [2]
The redundancy is not simply “three copies of the same system”. Different pumps, reservoirs, pipes and actuators are arranged so that essential functions can retain hydraulic capability after certain failures. The PTU is one element of that architecture because it allows hydraulic power available in Green or Yellow to be used to pressurise the other circuit without mixing their fluid. This is a power-transfer function, not a cross-feed valve joining two reservoirs. Airbus’s wording is explicit that the PTU transfers hydraulic power without fluid transfer. [1]
Motor on one side, pump on the other
A useful way to visualise the PTU is as two hydraulic machines mechanically linked together. Pressurised fluid from one system drives the motor section. The resulting shaft torque drives the pump section connected to the other system. Energy crosses the mechanical shaft, while the two hydraulic fluids remain in their respective circuits. This arrangement means the PTU does not create energy: it redistributes available hydraulic power. Airbus calls it a hydraulic motor-pump and places it between the Green and Yellow systems in its system descriptions. [1]
That also explains why the PTU can become active during perfectly normal ground operations. If one of the Green or Yellow systems is pressurised while the other has significantly lower pressure, the conditions for automatic PTU operation can be met. The unit then has real hydraulic work to do, and mechanical rotation plus changing hydraulic loads can be audible through the airframe. Describing the sound as a “bark” is a passenger-friendly nickname rather than an Airbus technical term; the technically supportable statement is that the PTU is an automatically operating hydraulic motor-pump whose operation can be heard during changes in hydraulic-system pressure. [1]
Why the PTU may be heard during engine start or shutdown
Engine-driven pumps are tied to engine operation, so the hydraulic state of an aircraft changes as engines are started or shut down and as other hydraulic sources are selected. If one of the two PTU-connected systems has normal pressure and the other does not, the PTU can respond automatically when its control logic sees the required differential. The exact sequence heard by a passenger therefore depends on the aircraft configuration, the ground operation being performed and which pressure sources are available at that moment. Airbus’s hydraulic description is the appropriate basis for the mechanism; it would be inaccurate to claim that every bark-like sound occurs at one fixed point in every turnaround. [1]
The PTU is not a replacement for an engine-driven pump in the sense of being an independent prime mover. If pressure on the driving side is unavailable, the PTU cannot manufacture hydraulic energy from nothing. Its usefulness comes from allowing available hydraulic power on one side to support pressure generation on the other. That distinction is why Airbus calls it a power transfer unit rather than a fluid transfer system. [1]
What “without transfer of fluid” really means
Keeping Green and Yellow fluid separated is technically important. A conventional fluid connection between two hydraulic systems could provide a path by which a leak in one circuit depleted the other. The A320 PTU instead transfers mechanical energy across the motor-pump assembly. Airbus’s published system description specifically states that the power transfer occurs without fluid transfer. That does not mean the PTU makes the aircraft immune to every multiple-system problem; it means that its normal operating principle does not deliberately combine the Green and Yellow fluid inventories. [1]
This separation also helps explain why abnormal low-fluid conditions require specific logic and crew procedures. Airbus has documented scenarios in which a low fluid level in one system can cause the PTU to run at high demand while attempting to restore pressure, with the potential for heating effects in the remaining serviceable system. The Airbus Safety first article describes ECAM cautions and PTU/engine-driven-pump actions applicable to the aircraft standard addressed by that publication. These are operational details, so they should not be converted into generic passenger advice; crews use the approved procedure for their aircraft. [1]
Why Airbus and EASA changed PTU inhibition logic on some aircraft
Hydraulic-system design continues to be refined after an aircraft enters service. EASA Safety Information Bulletin 2012-16R1 discussed an Airbus A320-family hydraulic-system improvement involving PTU inhibition logic. EASA explained that the modification was intended to improve robustness of the hydraulic architecture following certain fluid-loss scenarios and to help assure continued availability of two hydraulic systems. The bulletin described recommendations and applicability thresholds rather than declaring the PTU itself unsafe. [3]
Airbus’s later A320-family enhancement documentation likewise identifies an automatic PTU-inhibition function intended to address situations involving loss of Green or Yellow hydraulic fluid. Airbus describes the enhancement as automatically preventing PTU operation under defined conditions instead of relying only on crew action after an alert. Because that statement comes from Airbus, it should be read as the manufacturer’s description of its design enhancement; the regulatory status of any individual aircraft depends on the applicable certification, modification and operator records. [4] [3]
The PTU is only one layer of hydraulic redundancy
The A320 hydraulic architecture includes more than the PTU. Airbus diagrams show multiple pressure sources and a distribution of aircraft functions among Green, Yellow and Blue. The practical engineering objective is not that every consumer remains powered by every possible source; it is that the aircraft retains the combinations of control and system capability required by its certification basis after defined failures. The PTU contributes by allowing Green and Yellow power to support one another in defined circumstances. [2] [1]
Hydraulic actuators are used because they can develop substantial force from compact equipment, which is valuable when moving flight-control surfaces or other heavily loaded mechanisms. On the A320, however, “hydraulic flight controls” does not mean the pilot’s sidestick is mechanically pumping fluid to the control surface. The aircraft’s flight-control computers process control inputs and command hydraulically powered actuators. The hydraulic system supplies the energy that enables the commanded surface movement. Airbus flight-control and hydraulic-system publications should therefore be read together when explaining the complete chain from pilot input to surface motion. [2]
Why the noise is intermittent rather than continuous
Because PTU operation responds to the relationship between Green and Yellow hydraulic pressure, it need not run continuously whenever the aircraft is on the ground. When system pressures are sufficiently balanced, the operating trigger is not present. When the differential grows enough under the applicable control logic, the PTU can run. Once hydraulic conditions change again, operation changes accordingly. Airbus’s published description gives the automatic pressure-differential principle; exact switching thresholds and inhibition conditions must be taken from documentation applicable to the specific aircraft standard. [1] [4]
This is why a passenger may hear several short pulses or changing tones rather than a steady pump noise. The acoustic character is a consequence of a mechanical hydraulic unit working under changing pressure and load conditions, but neither Airbus nor EASA defines a particular passenger-heard “bark pattern” as a diagnostic indication. It would therefore be wrong to infer system health from the number or timing of sounds heard in the cabin. Maintenance crews diagnose hydraulic systems using approved indications, tests and maintenance data, not a passenger description of the noise. [1] [3]
What pilots actually see
Flight crews do not need to identify the PTU by sound to operate the aircraft. The A320’s electronic monitoring and alerting architecture presents hydraulic-system status and applicable cautions to the crew. Airbus’s hydraulic-loss publication describes an amber ECAM caution in the low-fluid scenario it discusses and explains the associated actions for the aircraft standard concerned. That is a very different proposition from the popular idea that the “barking” itself is a warning. The sound is a by-product of equipment operation; cockpit indications and approved procedures are the operational information. [1]
In normal service, the PTU can therefore be doing precisely what its design intends: reacting automatically to a hydraulic-pressure imbalance between Green and Yellow. Hearing it while an aircraft is parked, being prepared for departure or shutting down is not, by itself, evidence of a fault. Equally, this article cannot determine the condition of a particular aircraft from a sound recording or passenger observation. Aircraft serviceability is established through approved maintenance and operational procedures. [1] [3]
Why the PTU is a clever piece of systems engineering
The engineering value of the PTU is easiest to see when its limits are understood. It links two hydraulic power networks without deliberately joining their fluid supplies. It can automatically exploit pressure available on one side to support the other. It operates as one component within a wider three-system hydraulic architecture rather than as the aircraft’s sole backup. And, as EASA and Airbus publications show, its control logic can itself be refined to improve system robustness for particular failure cases. [1] [3] [4]
So the famous A320 “bark” is interesting not because it reveals a quirky fault, but because it makes an otherwise hidden aircraft system audible. Behind the noise is a straightforward physical idea: hydraulic pressure on one side turns a motor, mechanical torque crosses a shaft, and a pump on the other side converts that torque back into hydraulic power. The Green and Yellow fluids remain separate while power is transferred between them. That is the core function documented by Airbus and the reason the PTU occupies such an important place in the A320-family hydraulic architecture. [1]
Verified Sources / References
- Airbus Safety first — A320 Dual Hydraulic Loss. Airbus technical discussion of Green, Yellow and Blue hydraulic systems, normal pressure sources, PTU function and hydraulic-loss logic.
- Airbus FAST 51. Airbus technical material illustrating A320-family hydraulic-system architecture and distribution.
- EASA Safety Information Bulletin 2012-16R1 — Airbus SA Hydraulic System Improvement. Regulatory information concerning A320-family PTU inhibition logic and hydraulic-system robustness.
- Airbus A320 Family Digest of Available Enhancements, Version 12. Manufacturer description of later hydraulic-system/PTU inhibition enhancements.
Editorial Notice
Editorial Notice: This article was prepared using information considered reliable and publicly available at the time of publication. Every reasonable effort has been made to ensure accuracy; however, aviation requirements, technical standards and operational guidance may change as further information or revised regulation becomes available. This article is for general aviation education and reporting and is not a substitute for approved aircraft manuals, operator procedures, regulatory material or professional training. Cockpit King does not allege fault or responsibility against any person or organisation unless confirmed by an authoritative source. If you believe any material is inaccurate, misleading, improperly attributed or should be reviewed for amendment or removal, please contact us with the article title, the specific passage concerned and supporting evidence. We will assess legitimate requests promptly and, where appropriate, correct, clarify, update or remove the material.