A320 noise

[1oztoffee1]

Hello all! this is my first post......a question!
Took a Jetstar A320 LST-SYD recently. As the ship left the gate there was a tremendous noise similar to flap extension that went on for at least a minute. All pax looked nervous (including myself and I fly A320 fairly regularly), however the cabin crew were as calm as... The noise was so loud all must have heard it including the flight crew. I had never heard that noise before and I can say that the noise was not flap extension as I could see that they were not! The flight was uneventful (flight crew must not have been perturbed by the noise) but I wondered what the noise could have been? Perhaps pressurisation pumps?
Do aircraft pressurise immediately on departure or is it done while on climb-out?
I thought there may have been a thread on this subject many moons ago, but I can't find it. My apologies if I'm repeating old stuff.

Anyone have any ideas?

[Double Hydco]

It's probably the PTU (power transfer unit), which can transfer hydraulic power between the aircraft's hydraulic systems when required.

The PTU does a self test as the second engine is started (on the A320 series, anyway).

http://www.youtube.com/watch?v=9ffJgk1-tCA

DH

[Wirbelsturm]

As DH stated above it's most likely the PTU.

It exists to be able to power both the main Yellow and Green hydraulic systems from either engine driven pump in the event of an engine failure in flight.

I seem to remember that it is a variable flow pump thus the Woof-Woof noise during the self test after the number two engine is started. Once the number one engine is up and running it stops.

The PTU running ECAM is inhibited below 1500 feet during the take off phase as not to cause distractions if an engine fails during take-off.

This is possibly the most commonly asked question for passengers seated above the main spar! :}

[1oztoffee1]

Thanks for that info. I wonder why that particular ship was so loud? As I said, I'd never heard it before on A320s.

What is the situation for cabin pressurisation? Whilst taxiing or during climbout?........and why at that time?

[HEATHROW DIRECTOR]

<<ship>>

Do you mean aeroplane?

[WHBM]

This is possibly the most commonly asked question for passengers seated above the main spar!
Have Airbus never developed a modification to deal with this PTU noise issue ? A pump/valve assembly doesn't HAVE to make that noise.

I believe IAE did a mod to deal with the buzz-saw noise from the engine inlet at high power/low speed, which was the other A320 noise regularly questioned.

[Wirbelsturm]

WHBM, I'm not so sure a mod is needed to be honest.

The PTU deals with the failure of an engine and it's associated engine driven pump. Subsequently it is running at 3000psi and having to shift alot of energy to pressurise the second system. The system is designed in such a way that the Hydraulic systems remain totally seperate from each other. PTU overheat can be a problem in the event of a leak as one side of the pump has no resistence due to no fluid braking effect and the driven side (i.e. the one with fluid in it) goes into overdrive and overheats the remaining system!!!! To be honest, in a situation requiring the use of the PTU in flight I would be a little more concerned about getting the aircraft down quickly than a large barking dog noise in the cabin! :}

The CFM 56 always sounded like a chainsaw! IAE changed the sound damping material in the engine intake to reduce the main fan noise on the v2500.

What is the situation for cabin pressurisation? Whilst taxiing or during climbout?........and why at that time? I don't understand what you are asking? Hydraulics and pressurisation are totally seperate systems.

[1oztoffee1]

A little confusion?
Thanks for the info on the PTU.
Just asking as a seperate item, at what point does cabin pressurisation occur - and why at that time? (Ascending and descending).

Thanks in advance.

[Wirbelsturm]

The cruise altitude is programmed into the FMGC during initialisation. The pressurisation computer then looks at the cruise altitude and the rate of climb immediately after take off and calculates the climb pressure differential to give a nice smooth cabin pressure rise during the flight whilst taking into account step climbs.

The FMGC also contains a database of company destinations which, when the pressure mode selector on the overhead panel is set to auto, it uses to determine the height of the destination so that it can reverse the pressurisation and ensure that the outflow valve is fully open upon touch down.

Thus it gives a nice, gradual pressure change that isn't too nasty on the ears. Especially good for the kiddies!

In the little window in the passenger doors are two lights, a white one for slide armed warning and a red one for excess pressurisation in the cabin on the ground. At that point you would take the pressurisation system into manual and motor the outflow valve fully open.

Hope that answers your question.

[1oztoffee1]

Absolutely, many thanks! :)