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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:
Function of the PTU: The PTU is designed to maintain hydraulic pressure and assist in transferring hydraulic power between the two hydraulic systems of the aircraft. During engine start, it helps ensure that there is adequate hydraulic pressure for various systems.
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.
Air Compressors: Additionally, the auxiliary power unit (APU) or engine bleed air systems can also contribute to sounds during start-up, as they pressurize the aircraft's systems and bleed air for engine start.
Normal Operational Noise: This sound is a normal part of the aircraft's operation and is often described as a whirring or buzzing noise. It typically dissipates once the engine start process is complete and the systems stabilize.
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.