On the flight from Athens to Thessaloniki yesterday the AEGEAN A320 developed a strange vibration, that was actually bad enough to make me think if maybe an engine would explode To me it felt like something rotating beeing out of balance, very much the kind of vibration we know from many of our piston airplanes, comparable to an O-320 in idle … (just to give you an idea what it felt like).
The interesting thing is that nobody around me seemed to notice it, although it was pretty strong. I asked the Australian guy next to me and he went “now that you say it i can feel it” …
It stayed like that for about ten minutes and went away when we descended. It must have been some kind of strange turbulence, but I have never had that before. A light “rattle” with a pretty high frequency.
Any ideas?
I stopped worrying about this kind of stuff long ago. Both flying as passenger and as pilot. If it is really bad, a warning light in the cockpit will come on and someone is going to do something about it. You will notice that soon enough… If I would listen to every strange noise that even a small Cessna jet is capable of making I would long be a nervous wreck 
;-)
Yes, I know, but I thought maybe one of you had an idea.
Flyer59 wrote:
maybe one of you had an idea.
One idea which would explain this kind of sensation with “our” small Pratt&Whitney engines is a sudden change in bleed-air demand. Usually caused by selecting anti-ice on or off. Bleed air is taken from the high pressure core engine (N2 in our engines) which will then turn faster or slower than before, while the low pressure part (N1 / fan) will remain unchanged as it’s speed is mainly dependent from the airspeed. The sudden change in the N1-N2 relation will excite different vibaration/oscillation modes in the engines (and support structures).
We have one passenger who is very sensitive about such things. We have the choice of synchronising either N1 or N2 between the engines. The usual setting is N1, as the large fans are capable of producing stronger out-of-sync beatings than the core engines. This passenger however is sensitive to core-engine unsynchonisity and will complain about “strange engine humming” if we don’t switch the synchroniser to N2… BTW: Turning the engine synchroniser off in flight (which is required for major power changes and during the approach phase) will also result in increased vibration due to out-of-sync oscillations of the engines.
You think this works similar in the A320?
Very interesting!
Flyer59 wrote:
You think this works similar in the A320?
Yes, physics is the same across the whole universe.
;-), Ah, thanks for that. Another important lesson, yesterday I learned that water freezes at 0° C !!
No, i mean, does the engine synchronization in an A320 work in a similar way?
what_next wrote:
Turning the engine synchroniser off in flight (which is required for major power changes and during the approach phase) will also result in increased vibration due to out-of-sync oscillations of the engines.
You beat me to it, that is exactly what I thought it could have been, particularly as it stopped when they went into descent.
Engines out of sync can cause this kind of sensation and it can be rather uncomfortable if you are a nervous flyer. Particularly in the tail mounted engines of an MD80 or worse TU154, unsynced engines were quite bad, the latter also did not have any automatic syncing. It was an interesting task to get those 3 Solovievs into sync… at least if they were not, toilets in the rear were never occupied long time…
Flyer59 wrote:
No, i mean, does the engine synchronization in an A320 work in a similar way?
They have a synchronizer for sure. I’ve experienced a slight unsynchronisation once on a Swiss A321 that it did not work properly and asked the PIC about it, he was surprised that it could be felt in the cabin, they did not notice anything up front. Usually however it can be “reset” by shortly changing the power of one engine and then returning the power lever to the CLB detent, where it resides during AT operation. At least that is what they told me.
What that would sound like is like a harmonic like “hum-hum-hum”, difficult to describe. I was thoroughly taught to sync the engines on the Seneca I did my MEP on and unsynced engines catch my attention ever since.
Flyer59 wrote:
No, i mean, does the engine synchronization in an A320 work in a similar way?
They are FADEC engines, so there is probably more computer control involved, but basically yes. Here is a thread from a “forbidden” forum about this issue: http://www.pprune.org/tech-log/447120-warning-slf-query-re-engine-sync.html
Thank you, very interesting read!
After having read the first ten posts I am pretty sure that what I felt is about the engines not beeing in sync. Having not flown twins for more than ten years (lost the rating too)…i even forgot (and now remember) what it was like to sync two engines.
