![]() To prevent this scenario from becoming a reality Airbus engineers split most hydrulic services between the various hydraulic systems on the aircraft. For example, on the wings the five spoilers are split in the following order Green, Yellow, Blue, Yellow, Green. If that hydraulic system was to lose all of it's fluid, or had a pressure loss etc then they'd once again be inoperable. The same could be said about the rudder on the tail too. Taking that redundancy a step further it would also have been incredibly short-sighed to allow all of the flight control surfaces on a single wing to be powered by one hydraulic system. Hence the colouring in the above graphic. ![]() These systems are known as the Green, Blue and Yellow systems. They didn't just add one additional hydraulic system though. I think you and I can agree that in the modern world that's impractical and so Airbus's designers added some redundancy. It would have been very easy to design the aircraft with one hydraulic system, one source of pressure and a single tank of hydraulic fluid, however if something was ever to happen to said system then none of the above listed items could function and the plane would essentially be inoperable. * Map of A320 hydraulic systems to flight controls. Thankfully the A320 achieves this in two ways. You can see that with the aircraft being so dependent on hydraulics that it becomes paramount there be redundancy in place. Opening and closing reverse thrust doors.Extending / retracting the landing gear.To help you picture just how many aircraft services require hydraulics let me list some: The hydraulic system provides 3000psi of pressure to it's respective services. The Airbus A320 is heavily dependent on the presence of hydraulics to power a wide variety of things on the aircraft. The answer will then make a lot more sense. So bear with me with the rest of the post. The saying is "knowledge is power" after all. Note - My aim isn't to overcomplicate things but to provide enough detail that a nervous flyer can piece the respective parts of the puzzle together and begin to understand why noises occur when they do etc. To do this though I need to go into a little more detail. From my writing above we already know the noise comes from a pump, but why do we need that pump in the first place and why does it make the noise at the times it does? That's what I am wanting to answer with this blog post. I could literally jump in and say "Oh the PTU does this." but I feel that doesn't actually answer the question properly. If you're not sure what sound i'm talking about, I've captured it during a recent trip. In this blog post I aim to explain exactly what the PTU is to provide some context to the noise you hear on your travels. Unlike traditional motors/pumps in our lives - which are likely electrically powered - the PTU onboard the A320 is a hydraulically powered unit that exists to restore a minimum pressure across two of the aircrafts' hydraulic systems. ![]() Due to the PTU's location on the aircraft you'd be most likely to hear this sound should you sit in close proximity to the wings of the aircraft. Those intermittent barking or sawing noises you can hear are actually coming from a hydraulic motor/pump known as the "Power Transfer Unit", or PTU for short. In fact, it's probably up there among the most commonly asked question from both passengers and members of the public alike - especially those individuals who have a fear of flying and/or suffer from anxiety or heightened-nerves in the flying environment because, granted, it's a peculiar sound. If you've ever flown as a passenger aboard an Airbus A320 aircraft then there's a high chance you've heard a rather distinct "barking dog" sound which has also been likened to a workman with a saw. If you've ever wondered exactly what that sound is then you'd not be alone. * A Power Transfer Unit - Source: YouTube *
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