Very recently I made a couple flights (for real) in an Airbus A319.  My question is what is the weird noise the plane is making as it is being pushed back and starting to taxi?  It is a rhythmic muffled "barking" noise which seemed to last a couple minutes. Maybe hydraulics?

It is indeed the PTU, or Power Transfer Unit. There are three hydraulic systems in the aircraft. One is powered by an engine-driven pump and an electric pump, one an engine-driven pump only, and one an electric pump only (and a Ram Air Turbine, or RAT, in the event of a total electrical failure). The PTU provides pressure to the system with only the engine drive pump from one that is driven by an electric pump when ever the engine is not operating, such as on the ground before engine start or after shutdown, or during an engine failure in flight.

You won't hear that in a Boeing, because each system has both an engine-driven and an electric pump. But the DC-10 had a system very similar to the Airbus which was retrofit to all aircraft after the Sioux City United crash to preclude the total loss of hydraulics in the event of a catastrophic failure.

Probably more info than you really wanted to know. Class is in session 

Thanks for the information! Fortunately I didn't hear that in the Boeing 737 we flew in earlier.

Quote:


That's why we affectionately refer to them as a "ScareBus"

Dave