What's that noise?!

[monchavo]

I flew from Budapest to Gatwick recently on a BA bird, don't recall the type, sorry folks.
2 engined airbus of some sort.

When we landed there was an extended and rather interesting sound that I can only
say sounded very much like sawing. It happened as we were preparing to disembark the aircraft
and lasted 30-40 seconds. Clearly, nobody had taken a many-toothed instrument
to the aeroplane, but I am interested in what causes this phenomenon.

Smart arse comments stick to off-topic please.

[aardvark2zz]

Passenger tunnel moving towards plane.
.

[flyboy2548m]

When we landed there was an extended and rather interesting sound that I can only
say sounded very much like sawing. It happened as we were preparing to disembark the aircraft
and lasted 30-40 seconds. Clearly, nobody had taken a many-toothed instrument
to the aeroplane, but I am interested in what causes this phenomenon.

Smart arse comments stick to off-topic please.

You were on a 319/320 and what you heard was the famous/infamous Airbus PTU. I bet you also heard it during startup for a few seconds.

[Dmmoore]

To expand on flyboy's post slightly. A PTU (Power Transfer Unit) is a device that uses hydraulic motor in a system with pressure drive a pump in a system without a source of pressure.
Many aircraft use a similar device but none are as noticeable as the A320 series.

[monchavo]

"The PTU is mechanical pump that links the yellow and the green hydraulic systems. It is energized when either system has been turned off( ex: one engine off) it produces a loud high frequency noise when it turns and produces 3000 psi hydraulic pressure for oppsosite system."

OK. Thank you for the information. What is the practical application of such a system? What would I need that hydraulic pressure for?

[monchavo]

Amusing Salon.com article about this phenomena

http://www.salon.com/tech/col/smith/200 ... epilot170/

[Giles]

OK. Thank you for the information. What is the practical application of such a system? What would I need that hydraulic pressure for?

from the article you referenced;

"The A320 has two main hydraulic systems that operate flaps, landing gear, flight controls, cargo doors and brakes."

[flyboy2548m]

OK. Thank you for the information. What is the practical application of such a system? What would I need that hydraulic pressure for?

from the article you referenced;

"The A320 has two main hydraulic systems that operate flaps, landing gear, flight controls, cargo doors and brakes."

Yeah, you know, little stuff like that. To add, the A320 has three hydraulic systems, not two.