Strange airplane noises concern people. If you fly often, you’ve probably learned the common noises. Most people recognize the engines spooling up or coming back towards idle. Landing gear raising and lowering is also pretty discernible. Very experienced fliers are also familiar with the rumbling and slight shaking associated with the speed brakes.
However, day after day, year after year, I continue to hear comments and questions about the strange noise from the belly of the Airbus A320. If I’m riding in the back, I eavesdrop on the comments:
“What is that?”
“That’s one loud dog. Maybe the owner should have sedated him before the flight.”
When I’m saying goodbye to guests at the end of the flight, at least one will ask about the strange noise.
The more enthusiastic people don’t even ask: “Call your mechanic.” they say. “There’s something seriously screwed up below the floor of the jet.”
The noise is “usually” heard briefly as the second engine is being started. People more commonly notice it after shutdown at the gate. It really does sound like a large angry dog stuck in a cage. So, what is it?
The noise comes from the hydraulic system’s Power Transfer Unit (PTU). That’s all the knowledge you need to impress your fellow passengers at the end of the flight. But, if you want to know more, keep reading.
The A320 has three separate hydraulic systems which actuate the flight controls and other hydraulic systems on the aircraft. Airbus, just to be different from Boeing, named them the green, blue, and yellow systems.
The green system is powered by an engine driven hydraulic pump. The yellow system is powered by the other engine’s pump but can also be powered by an electric backup pump. (It can also be partially pressurized by a hand pump to operate the cargo doors… but, that’s not too relevant to this conversation.) The blue system is powered by an electric pump… but, not associated with the PTU. (It does, however, have its own unique backup mechanism… again, beyond the scope of this post.)
The PTU is 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. Since the fluid drives the pump, the fluids from each system remain isolated. They do not mix.
Each hydraulic system normally operates at 3000psi. If the difference between the yellow and green system exceeds 500psi, 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.
There’s a whole list of conditions that will inhibit the PTU from running…. but, here’s when you’ll “normally” hear it:
- When starting the second engine, the PTU runs a self-test. You will hear it either just before taxi or when we start the second engine on the way to the runway.
- At the gate, especially if we taxi in on one engine. During the taxi, the left engine is running and pressurizing the green system. The yellow system is pressurized by the electric pump. When the pump is switched off at the gate, yellow pressure drops while the green stays pressurized as the left engine spools down. The PTU runs until the systems equalize.
When it runs, that’s when you’ll think there’s a dog trapped below. Or, if you’re into logging, you’ll think someone is sawing a big tree. Either way, it is definitely a noise unique to the Airbus.
So, go now… book your next flight on an A320. When you hear the noise, impress your seatmates. Or, grin and say “did you see the size of that dog they loaded into the pit?”
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