Dear friends and followers, today I'll be answering a great question: What is behind this sound? [PTU barking sound] I'm sure many of you have heard this before, especially when you have been flying on an Airbus A320. So let's take a close look at this mysterious barking dog below the cabin floor. [Intro] The noise is usually heard briefly as the second engine is being started, but most passengers more commonly notice it after shutdown at the gate. I must admit it really does sound like a large, angry dog which is stuck in a cage, so what is it then? The noise comes from the hydraulic system's Power Transfer Unit, or better known as the PTU. So what is it good for? Now, the Airbus A320 hydraulics consist of three independent systems, named in colours Green, Blue and Yellow, and one of their highest priorities is to pressure the actuators to move the flight controls. Now, the green system is powered by an engine-driven hydraulic pump (engine number one, so, in flying direction, the engine to your left-hand side), and the yellow system is powered by the engine number two (the right hand side), 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 at all. It does, however, have its own unique backup mechanism, the Ram Air Turbine, but that's a whole 'nother video. So imagine the PTU as a reversible motor pump located between the green and yellow system. Hydraulic fluid from either system can drive the pump to pressurize the other hydraulic system, but the fluids only drive the pump and so remain isolated from each other, meaning they don't mix. Now, each hydraulic system normally operates at 3000 PSI, and if the difference between the green and the yellow system exceeds 500 PSI, the system energizes the PTU to pressurize the low system. So, during engine start, engine number two is not yet running 'cause you can't start the engines simultaneously. Therefore, one hydraulic system isn't pressurized, and during that time, the PTU runs a self-test. It's most likely to be heard during pushback or as the second engine is spooling up. Okay, now let's look at the situation during arrival. Most airlines have a so-called "one engine taxi procedure", especially to reduce the fuel consumption on long taxi routes after landing. So, as you follow the checklist, you've given the engine some cooling-down time and started the APU in the meantime, And then, you can shut down engine number two and switch on the yellow electric pump to maintain the pressure on the system. And as you've reached your parking position, it states: "Yellow Electric Pump Off, Shutdown Engine One" So as soon as you switch off the electric pump, the yellow pressure drops, while the green stays pressurized as the left engine spools down, and then the PTU kicks in until the system equalizes, and therefore it cycles on and off, on and off, on and off, due to the pressure fluctuations, and that's when you can hear this sound: [PTU sound] Now you might ask: Joe, why do you need the yellow system to be pressurized on the ground anyway? You said, they primarily move the flight controls? That's correct, but one other big part being energized by the yellow system is the Alternate Braking and Parking Brake System. But even more importantly, it pressures the Nose Wheel Steering, so if that ain't working, you couldn't steer the aircraft on ground. And by the way, a passenger recently came up to me saying "Sorry, Sir, I heard this very strange noise coming from below the cabin floor, it sounded like one of the loaders was trying to start his chainsaw in the cargo compartment after we've reached the gate." That for sure was very funny. I hope you enjoyed this video about another Airbus myth. Make sure to subscribe to my channel and check out my Instagram account @CaptainJoe. And spread the word with your friends! All the best, see you next Thursday, your Captain Joe. [Outro]
