by Captain Joe
What is that barking sound coming from below the cabin floor as soon as we reach the gate position?
Have you ever wondered what is going on in the cargo compartment where this very strange noise is coming from?
Many people refer to this as the “Airbus Barking Dog Sound”.
This strange barking sound emits from the PTU, the Power Transfer Unit which is a substantial part of the Airbus hydraulic system.
I explain in further detail when the PTU is being operated and it´s purpose.
Now the Airbus A320 hydraulics consist of three independent system, named in colours, green, blue and yellow and one of their highest priorities is to pressure the actuators to move the flight controls.
The green system is powered by an engine driven hydraulic pump, engine number 1, so in flying direction the engine to your left hand side and the yellow system is powered by engine number 2 but can also be powered by an electric backup pump.
The blue system is powered by an electric pump only but isn’t associated with the PTU.
It does, however, have its own unique backup mechanism, the Ram Air Turbine, but that´s a whole another video.
So imagine the PTU as a reversible motor pump located between the green and yellow systems.
Hydraulic fluid from either system can drive the pump to pressurize the other hydraulic system.
But the fluids only drive the pump so the remain isolated from each other, meaning they don’t mix.
Each hydraulic system normally operates at 3000 psi and if the difference between the green and yellow system exceeds 500 psi, the system energizes the PTU to pressurize the low system.
In other words, the green or yellow hydraulic pump has failed and is being pressurized from the other system through the PTU.
ONE MISTAKE IN VIDEO, you preferably start engine number 2 first and then engine number 1, in that sequence! Sorry about that!