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You are on a plane and suddenly there is a sound like a dog barking or a screeching noise when the engines start, for what is this?
Post written by: Captain Rudolph Star
Twitter: @rodo_estrella
Many times when we've flown on a family plane Airbus A320, We have heard that when it starts its engines, a very loud sound appears that seems to be the barking of a puppy that is in the lower part of the plane., This sound is coming from an aircraft hydraulic power transfer pump (PTU Power Transfer Unit).
"The little dog that barks"
To understand the barking sound we must first know a little about the hydraulic system of the aircraft to which it is related., why it sounds and under what conditions the pump can be heard working.
The airplanes Airbus of the A320 series have three separate and independent hydraulic systems that feed the systems that require hydraulic actuation of the aircraft, for example control surfaces, undercarriage, spoilers, among others. These three systems are differentiated by the manufacturer as a green system, yellow system and blue system.
the green system (left side of the plane) It is powered by a hydraulic pump actuated by the motor number 1, and the yellow system (right side of the plane) by number engine hydraulic pump 2 and additionally has a reserve electric hydraulic pump and a manual pump to operate cargo doors from the outside.
If you never heard this sound, we share it here:
The blue system is actuated solely by an electric pump or in an emergency by an emergency air shock generator or RAT, this system is not associated with the “barking dog”
Now that we are clear about the three independent hydraulic systems, let's focus on the green and yellow. These systems normally operate with a hydraulic pressure of 3000 PSI that must be the same for both systems.
If this pressure during operation for any reason or due to engine loss, decreases 500 PSI, The PTU pump comes into action, which, as we mentioned, is a hydraulic pump located between the two systems and acts to equalize the pressure of both systems or supply pressure to the system in failure due to engine loss and keep both in operation. 3000 PSI, using system pressure that is normal to equalize system pressure with low pressure.
It is very important to mention that this pump uses hydraulic pressure from each system to equalize them in pressure., the PTU does not mix hydraulic fluids or transfer fluid from one system to another. That is why, if the fault stems from a loss of hydraulic fluid in one of the systems, the PTU pump should not be activated and the hydraulic system losing fluid should be declared lost.
When the aircraft is on the ground with its engines off, systems have no hydraulic pressure, but when you start the engine 1, the green system gets the 3000 Normal Operating PSI, BUT the yellow system associated with the second motor has no pressure, since this is a condition where the PTU should kick in to equalize the pressure between the two systems, BUT it is in the ignition stage, it enters a SELF TEST stage where it starts to work as long as inhibiting conditions for the PTU are met, such as that the parking brake is activated. It is because of this situation that the "barking dog" is usually heard more clearly on the ground during the ignition of the plane..
When both engines have started and both systems reach their normal pressure of 3000 PSI, the pump stops working and the sound ends. Nevertheless, Normal aircraft operation requires hydraulic power for its different systems and this pressure often fluctuates, so the system comes into operation every time it has to restore the necessary pressure of the system that requires it..
Since the PTU pump is located in the central part of the aircraft, passengers directly above it, especially seats that are in the positions on the wings are those that tend to hear the sound with more intensity while the passengers who are further forward do not usually perceive it too much.
We now know that this famous "dog barking" sound is part of the normal operation of the aircraft's systems and we need not worry that someone locked a poor little animal in the cargo hold or is there any plane problem.
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T I congratulate very good post.
I would like you to make a post explaining how I know the A/C works on airplanes.
a very interesting topic, let's investigate it.
so that was it!
Thank you very much for the clarification, I recently flew in an A 320 in a central seat but the strange thing was that throughout the flight he did, hour and 20 minutes, The flight attendant told me that it was a failure of the hydraulic system without any problem., when mentioning hydraulic I thought landing gear, when arriving at the airport, I expected to see the firemen and ambulances, What a scare!
Sometimes happens, very little, but it happens, in the end there is no problem.
excellent report, I thought they were the landing gear doors..
nice louis! Thank you for reading.
The sound is more like the co-pilot sawing the pilot's floor haha
He looks like a DJ scratching a record lol
The truth is I always wanted to know what that sound was but I could never find a good answer thank you very much for explaining that sound, now when I go by plane I'll be calmer. Another doubt I have is that at the Quito airport, at one of the runway heads on the last exit, there is like an "ear" right between the taxiway and the curve to enter the runway and I always wondered what it was and I was wondering if you could look into that because it has me a little confused.. A very good job in this post and in all, I've been following you for more than a year and I admire you a lot, keep it up.
Hi Joaquin, It's good that your question was resolved.. About your question, that ear is just that, an ear where planes can park for testing.
Thank you very much I always wondered what it was and now I finally know. I love your blog and I see it every day I hope you continue posting like this for a long time, I wish you the best. Successes!
Thanks!