I fly frequently on AirAsia A320. I observed there is a strange sound right a few moments after the doors are closed and armed. It sounds very loud like some motor whining. What is that sound?
I know it’s not the flaps lowering as it doesn’t sound that loud.
On the Boeing 737s, there is no such sound.
If you have not read the earlier articles in the Travel 3Sixty in-flight magazine (would soon be compiled into a book), I will reproduce the answer below:-
Is there a poor dog looking for its owner?
Once you’ve settled yourself comfortably down in an Airbus A320, you may hear the sound of a “dog” barking below at the cargo bay as soon as one of the engines starts. Is that the sound of someone’s lost pet? No, it is the sound of the hydraulic power transfer unit (PTU) coming into operation or doing a self-test.
It arises when the PTU is transferring hydraulic pressures from one side to the other. This automatic transfer provides the power required as a backup for the many safety functions that are necessary in an emergency. If one engine fails, this ensures that there is sufficient hydraulics power to stop the plane or assist the lowering of the landing gears and flaps for a safe landing.
So the whining sound you hear after all, is due to the hydraulic pressures transferring (from yellow to green system or vice versa when the difference is less than 500 psi)
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If you like what you read, more stories are found in my book LIFE IN THE SKIES (Preview here) and you can purchase a copy here. To check for any latest updates or postings, you can follow my Twitter at @CaptKHLim or Facebook here