Odd sound after landing...

Hey guys…

Just a quick q
I recently got back from holiday and I noticed an odd sound after touchdown.

The sound began after braking and reverse thrust when we were at a steady 20knots approaching the terminal.

The sound persisted even when the aircraft was stationary.

The sound was almost hydraulic and sounded as if a massive drill was on coming from beneath the fusilage. (It also fluctuated in frequency and pitch occasionally once parked)

I was postioned over the wing on an airbus A321

I don’t know if it was the auxilry power unit kicking in but doubt it.

Look forward to your response to a very Vaughn question.
No worries if no one knows it is a bit difficult to surprise.


Sometimes upon landing I hear a kind of squeaking noise…I think what you’re hearing could be the same thing, and is just the plane “shaking” and “shuddering” when touching down and slowing down. Although you should have someone with higher authority than me (like a mod or dev or real world aviationalist) take a look at this to confirm.


Hey :)

I guess this was due to some hydraulic systems turning themselves off.

Anyways please wait until your trust level’s high enough to post in #real-world-aviation, thanks


was this on a real flight or an “Infinite Flight” flight… ? if real then poss need to move real world aviation instead!

Wasn’t the APU kicking in but the three different hydraulic systems levelling them selves out. Capt Joe on You tube has a far better description on what this is, called “why is there a dog in the hold”!


Was the sound somewhat similar to a barking dog?


Might be the PTU, power transfer unit.


It’s usually the PTU (Power Transfer Unit) it transfers power between the two primary hydraulic systems allowing one pump to operate both systems if necessary. It will also kick in if there is an unexpectedly high demand on one system or the other leading to a drop in pressure.

It kicks into life for a short while during engine startup and shutdown.

We often shutdown an engine during the taxy to save fuel.


I didn’t know that. Shut down an engine during taxi…thanks man :) I learned something today.

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Heard the same while on a TK A321 a few yrs ago

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I heard that sound to when I traveled on an airbus for the first time and it kinda freaked me out at first. Pretty tripy to listen to

Whilst @Yuan_Tugo has given a great explanation. Found this you tube vid which gives a visual explanation on the “dog below decks” i.e. PTU.


I’m new to this and am not sure what I’m doing wrong.

Admittedly I do use this forum to ask people passionate about aviation obscure questions that the average Joe or even google doesn’t have the answers to.

I do this to assist myself in the becoming of hopefully a pilot one day.

Apologies if this is wasting time and space on your software and will stop if you say so.

Just drop me a message if it is (I’d also appreciate some ground rules on what’s okay and what’s not)

Thanks guys

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@Yuan_Tugo so when we were flying American Airlines 319 I want to say it was, while taxing out and pushback as well every once in awhile I would here abrattling sound like a strut or something was broke. What would that be if you understand what I’m trying to explain and I also heard it on the 321

Lol Noot