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Gillbilly
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Joined: Sat Jan 04, 2020 1:46 pm

Inflight Oscillation at Cruise

Fri May 21, 2021 12:46 am

I was on a redeye A320 flight a few years back and at cruise the flight was quite smooth, however I eventually picked up on a regular and gentle oscillation of the plane slightly up and down every few seconds. It occurred for a pretty long time.

I'm not a pilot, but could this be some sort of auto-trimming that the A320 series have built in? Could it be due to a tailwind?

Thanks!
 
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Starlionblue
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Re: Inflight Oscillation at Cruise

Fri May 21, 2021 1:11 am

Porpoising like this could be due to shifting winds and/or slight density variations. The autopilot starts to chase the altitude, and you get this sort of cyclic overcompensation.

The A320 has autotrim in all phases of flight. I can't think of a modern jet airliner without auto-trim, but this being a.nut, someone will inevitably come up with an exception.
 
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DL_Mech
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Re: Inflight Oscillation at Cruise

Fri May 21, 2021 1:25 am

Could also be a surface position transmitter out of adjustment. 727s were notorious for porpoising in cruise, elevator position sensors or their associated mounting bracket were either worn or out of adjustment.
 
VMCA787
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Re: Inflight Oscillation at Cruise

Fri May 21, 2021 6:59 am

The 320 autopilot system allows for some altitude variance if the Vnav mode is engaged, Boeing has a similar approach. It's not a hard altitude hold like previous generation autopilots were. If you select FL350 on a previous generation autopilot, the FL was rock solid assuming no turbulence. While the Airbus would provide a soft hold and it would be common to see a variance of 20 feet or so in cruise. The net result is a more comfortable ride for the pax.
 
Flow2706
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Re: Inflight Oscillation at Cruise

Fri May 21, 2021 11:19 am

These oscillations can be caused by mountain wave activity. As mentioned by VMCA787 the ALT CRZ mode on the A320 allows small altitude variations to increase passenger comfort, therefore the reaction to slight altitude change due to mountain wave activity can be slightly delayed.
 
e38
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Re: Inflight Oscillation at Cruise

Fri May 21, 2021 11:20 am

VMCA787 wrote:
The net result is a more comfortable ride for the pax.


and it also reduces stress on the aircraft structure.

e38
 
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DaProf
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Re: Inflight Oscillation at Cruise

Fri May 21, 2021 2:29 pm

This seems to happen to me on about 80% of all 320 flights I've ever taken (probably 40+ segments on the 320, mostly over the southeast us). Personally I find it really annoying and now actively try to avoid flying on that plane when possible. I've yet to experience that on any other plane though I've heard it happening on others.
 
Redbellyguppy
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Re: Inflight Oscillation at Cruise

Fri May 21, 2021 6:16 pm

Happy times flying am autopilot deferred aircraft when I was at the regionals. The ambitious flight attendants were the worst, and especially when they had a lot of moment..
 
Gillbilly
Topic Author
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Re: Inflight Oscillation at Cruise

Fri May 21, 2021 9:32 pm

DaProf wrote:
This seems to happen to me on about 80% of all 320 flights I've ever taken (probably 40+ segments on the 320, mostly over the southeast us). Personally I find it really annoying and now actively try to avoid flying on that plane when possible. I've yet to experience that on any other plane though I've heard it happening on others.


Haha I guess I'm not the only one who notices it. I avoid A320s because they are so commonplace, although they do seem to be more comfortable than 737s in economy. At the time I thought it was kind of a cool novelty and somewhat relaxing but I do see how it would get annoying.
 
GalaxyFlyer
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Re: Inflight Oscillation at Cruise

Fri May 21, 2021 11:08 pm

If the plane is taking me where I need or want to go at an acceptable cost and schedule, I get on it, ride it out. Now, if it rolled over, I’d object.
 
Max Q
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Re: Inflight Oscillation at Cruise

Sat May 22, 2021 3:23 am

The MD80 used to allow +-300 feet from selected altitude in cruise (perf mode) to minimize autothrottle movement

Would not be permitted now with minimum RVSM
 
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tb727
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Re: Inflight Oscillation at Cruise

Mon May 24, 2021 2:18 pm

Yeah, sounds like a shifting tailwind in ALTCRZ. Doesn't take much to make it wander. Had a 319 yesterday with a tailwind that went from 45 to 25 to 40 knots over about 100 miles and you would have thought I was hand flying it with my eyes closed. Well maybe not that bad but you could tell it was wallowing around.
 
CRJockey
Posts: 354
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Re: Inflight Oscillation at Cruise

Mon May 24, 2021 7:53 pm

DaProf wrote:
This seems to happen to me on about 80% of all 320 flights I've ever taken (probably 40+ segments on the 320, mostly over the southeast us). Personally I find it really annoying and now actively try to avoid flying on that plane when possible. I've yet to experience that on any other plane though I've heard it happening on others.


Hahahaha, the most ridiculous of all the ridiculous reasons to avoid a plane I have ever heard.

What’s next? Avoiding 777s because you don’t like the looks?

By the way, I have yet to experience a plane that doesn’t do it. The CRJ tends to do it M0.8 and above if flying FL350 or above. If I do remember that correctly.
 
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DaProf
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Re: Inflight Oscillation at Cruise

Mon May 24, 2021 10:40 pm

CRJockey wrote:
Hahahaha, the most ridiculous of all the ridiculous reasons to avoid a plane I have ever heard.

What’s next? Avoiding 777s because you don’t like the looks?

By the way, I have yet to experience a plane that doesn’t do it. The CRJ tends to do it M0.8 and above if flying FL350 or above. If I do remember that correctly.


Why not? Previous negative experiences shape our decisions all the time on many many things. You may have gotten bad service at a restaurant and avoid it now, or gotten a skunky beer and won't even touch one from that brand now. Those things don't have to make sense to others. For example I also try not to fly through DTW since I've had some bad experiences there (.. Waits for the pitchforks), that's my choice and doesn't make any difference to anyone else.

The constant rocking back and forth on the 320s make me motion sick so there.
 
CRJockey
Posts: 354
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Re: Inflight Oscillation at Cruise

Tue May 25, 2021 7:13 am

DaProf wrote:
CRJockey wrote:
Hahahaha, the most ridiculous of all the ridiculous reasons to avoid a plane I have ever heard.

What’s next? Avoiding 777s because you don’t like the looks?

By the way, I have yet to experience a plane that doesn’t do it. The CRJ tends to do it M0.8 and above if flying FL350 or above. If I do remember that correctly.


Why not? Previous negative experiences shape our decisions all the time on many many things. You may have gotten bad service at a restaurant and avoid it now, or gotten a skunky beer and won't even touch one from that brand now. Those things don't have to make sense to others. For example I also try not to fly through DTW since I've had some bad experiences there (.. Waits for the pitchforks), that's my choice and doesn't make any difference to anyone else.

The constant rocking back and forth on the 320s make me motion sick so there.


I guess I am just amused by what people perceive, or think they perceive, when flying as a passenger. There was no offense meant and I see, that you thankfully have none taken.
I have hundreds and hundreds of segments on the A320 and never felt any rocking motion. But then again, I don't have any of those particular sensations others feel on any other plane in the sky.

DTW on the other hand, you are spot on ;)
 
a320fan
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Re: Inflight Oscillation at Cruise

Sat May 29, 2021 9:23 am

Never noticed any pitch oscillation on an A32x or any other airline flight. I do however remember one red eye Trans Tasman flight in an A320 where there was some slight roll oscillation. Could barely feel it but was obvious looking out the window with a bright full moon glinting off the sea far below and the aircrafts wing. The slight rocking was reminiscent of a baby’s cradle and the moonlit night, dark quiet cabin and steady hum of two V2500s made for quite a poetic atmosphere. Very relaxing.

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