Originally Posted by ryane946
I think a major thought when purchasing an aircraft is not only how it flies, but passenger comfort.
So I have a question for anyone who has ever flown a A320? What the heck is that annoying noise!!! It sounds like the flaps are coming down, but it takes way too long (a minute or so), and it is way too loud (eeeeeeerrrrrrrrr). You hear it on the ground before push back, and you hear it around engine shut down. It starts to fade out, and it progressivley gets slower (eeeer,eeeer,...eeer,eeer,.........eer,eer). Sorry, I know thats not real descriptive, but if you've ever heard you, you'd know what I'm talking about. I have heard it on every A320 flight I have taken. It is real annoying. The passengers always ask the flight attendent what the heck it is, and I have no idea what it is.
Any ideas?
Ryanne,
The noise you hear is called the PTU or power transfer unit and is basically a backup hydraulic pump. The A320 has three hydraulic systems Green, Blue, and Yellow. The Blue is run entirely by electric pumps or a ram air turbine in an emergency. The Green and Yellow are normally driven by engine driven pumps, although the yellow system has an electric and a hand pump as well. The green and yellow systems share a common two-way pump called a PTU. If the pump senses a certain differential in the pressure between the green and yellow hydraulic systems, it pumps pressure into the weaker system to bring it back up to 3000 psi. The whole idea is that unless you lose fluid, you should be able to keep the all of the major hydraulic systems operational on the airplane in the event of engine or engine driven pump failure. The pump action is kind of like an mechanical accordian flexing back and forth, which is what makes the barking dog sound. You hear it often on the ground because the system does a self test during the engine start process(there are a series of things that will inhibit that aren't worth getting into). You will also hear it on single engine taxis, if the crew elects to taxi on the number 2 because the yellow system will be at 3000 psi and the green at 0 because the engine isn't running. If you single engine taxi with the number 1 engine and turn on the yellow system electric pump, both systems will be pressurized and you won't hear the barking dog, but you will hear a slight whine from the yellow electric pump until the 2nd engine is started and the electric pump is shut off. Hydraulic pumps on all airplanes get worn out like everything else, so sometimes you'll hear the PTU momentarily when the gear or flaps move causing a high temporary draw on one hydraulic system kicking in the PTU. You'll also hear it on shut down if the crew taxiied in on one engine and forgot to turn off the yellow system electric pump at the gate.