If you have heard that 'Weird Barking Dog Noise' in the Airbus A320 during an Engine Start. Here's Why.
Image Description - An A320 in its Original Airbus Livery.
The "weird sound" you hear during the engine start of an Airbus A320, particularly from the Power Transfer Unit (PTU).
It is a normal operational characteristic of the aircraft. Here’s a breakdown of why this happens:
Image - Boeing 737 PTU / Airbus A320 PTU
Function of the PTU: The PTU is designed to maintain hydraulic pressure (3000 Psi) and assist in transferring hydraulic power between the Green and Yellow hydraulic systems of the aircraft. During engine start and in situations of differential pressure, it helps ensure that there is adequate hydraulic pressure for all required systems. Note: Here, differential pressure between the systems (Green and Yellow) needs to be over 750 Psi.
Hydraulic Fluid Movement: The noise is primarily due to the movement of hydraulic fluid through the PTU and associated valves. As the PTU engages, it can create a distinct sound as fluid is pumped, and pressure is built up. BUT, no fluid is exchanged between the green and yellow systems, only Power.
Normal Operational Noise: This sound is a normal part of the aircraft's operation and is often described as the BARKING DOG. It typically dissipates once the engine start process is complete and the systems are pressurized appropriately. The PTU is located in the Main Landing Gear Assembly of the Aircraft.
Overall, while the sound may seem unusual, it is a normal part of the A320’s operational procedures and indicates that the hydraulic systems are functioning correctly.