Airbus question

[Mahesh]

I have a weird question about a sound I hear whenever I fly on the narrow body Airbuses (A320/319/318). When the plane pushes back from the gate and is being removed from the tow or has just started taxiing, I hear this loud noise like a hydraulic ratchet used to remove a race car tire. It's pretty loud and coming from the wing area.

Is this from the flaps coming down for takeoff settings? What is this sound?

 

[QX DEN]

Are you talking about the "barking dog" like sound? If IIRC its a power transfer unit that balances the pressure between hydraulic systems. Could be wrong though.

 

[Minuteman]

Probably the hydraulic "Power Transfer Unit" ... its a hydraulically-driven pump that pumps fluid on a separate hydraulic system.

Power Transfer Unit (PTU)
A bidirectional power transfer unit enables the yellow system to pressurize the green system and vice-versa.

The power transfer units comes into action automatically when the differential pressure between the green and yellow systems is greater than 500 PSI.

The PTU therefore allows the green system to be pressureized on the ground when the engines are stopped [or, as you say, near the gate at startup].

 

[Mahesh]

Thanks guys! That makes sense! I definitely only hear it on the ground and it's these little bursts of sound (as if someone is pressing a drill) that lasts only about 20 seconds or so..

 

[FlyerJosh]

It is the PTU... but it's much more fun to tell passengers that it's the mechanical guard dog that's protecting the checked luggage...