Question regarding the cruise in an A321

[trustno1]

I've flown quite a lot lately back home to Ireland with EI and I 've noticed the following. During the cruise the aircraft at times seems to rock very gently along the horizontal axis. Please am I talking bollix and the reason for the rocking is the JD's I consumed before the flight or does this aircraft actually do this. I've never noticed it on the B734/5. :confused:

[MarkD]

Maybe I just don't notice, my recent EI 321 legs were in rows 7-14, were you in the back?

[Baggy]

No you arent going mad, the EI 321's were fitted with a mod to the flight control system, which introduces a mild rocking during flight through smooth air. This improves cruise fuel efficency by up to 0.5 %

[Spearing Britney]

:D ha ha, very funny baggy...

Only idea is that you may have been flying whilst there was a variable stregnth cross wind at cruise levels giving a little rocking. Or if it was during descent could be locking onto the ILS localiser (left right guidance radio beam) a long way out (happens a lot at Dub) which is a bit wobbly till you get in a bit...

Certainly isnt a regular occurance on 321's, EI's or not...

[Young Paul]

Sounds a bit like mild dutch roll to me. Not likely to be the localiser in the cruise.

[PAXboy]

I seem to recall this very topic being raised in Tech log some months ago. There were reports about fine tuning to the software program to prevent this, it being a problem in the early release of the software for this machine. Some crew reported that the rocking was distinct and upset a number of pax.

Trustno1, you might want to ask the original question again but in Tech Log.

[Nihontraveller]

I've felt this. Can't remember if it was on a airbus though. Slight forward-aft rocking that lasts not more than 30 secs or so. Cycles around 1-2 secs. I seem to remember it being at the top of climb or just before decent.

[gofer]

Have commented on the A321's distinct longtitudinal instability while the A319 has noticeable lateral instability in the same conditions.

Think personally its something to do with the first having a very long fuselage and the second a very short fuselage compared to the wings - while the first version the A320 got it almost right.

But I'm sure somebody will come up with the real reason.:D

[Propsareforkids]

Enough! Here is the fact. First, the A321 (Read any A3XX) is equipted with Auto-thrust. Furthermore, the Auto Flight System uses a "Soft Ride" mode during cruise which is supposed to prevent the aircraft from chasing minor deviations in altitude and airspeed, but the auto-thrust mode operates at a different reaction speed.

My experience (+12000 hours; A320 Commander) with the A3XX is this. Normally, it is fine on a short leg, but on longer trips the aircraft will begin to ossilate a little. This is typically where I turn the Auto-thrust off, stabilize the engines, then re-engage the auto-thrust.

I hope this answers your question....Finally!

[trustno1]

Hi Propsareforkids

Thanks for your reply. I guess it is safe to say that the rocking was not just solely due to my JD's. Just wondered - your answer states that it is normally fine on a short leg. The 1 hour hop between EIDW and EGLL would hardly be considered anything other than a short hop. Does your answer cover this duration ?.

Regards

[Propsareforkids]

It could. And, at lower altitudes the issue is only compounded. On the issue of safety, it is ENTIRELY safe and should not cause you any concern.

This might help...At the very least it will provide interesting reading...

Straight from the Airbus Flight Crew Operating Manual...


Altitude Acquire/Hold Modes (Continued)
Soft Altitude
The soft altitude hold mode allows the aircraft altitude to vary slightly when at the Hold desired FMS cruise (CRZ) flight level. When the FCU altitude is selected to the same
altitude as the FMS CRZ flight level, and the aircraft reaches that altitude, the soft altitude hold mode engages. The altitude can vary as much as 50 feet either side of the selected altitude. This minimizes thrust variations and improves fuel efficiency.
Once engaged, the FMS CRZ FL automatically follows the FCU altitude.

If the initial FCU cruise altitude is selected to an altitude lower than the MCDU CRZ altitude, the pilot must change the MCDU CRZ altitude if a long term level off is anticipated. The FMS flight phase stays in the climb phase until the aircraft reaches the MCDU CRZ altitude. For example, FL370 is the planned CRZ altitude. The aircraft is leveled off at FL330 due to turbulence. The MCDU CRZ altitude must be changed to FL330 to sequence the FMS to the cruise phase and engage the soft altitude hold mode.
When the soft altitude hold mode engages “ALT CRZ” is displayed on the FMA.

[Spearing Britney]

And where in the fcoms do you come up with disconnecting the autothrust propsareforkids? This doesnt affect the soft alt hold you refer to, you should know that!! It will only affect the speed it can mainatin at the altitude it is still soft holding!!!

I have flown IAE and CFM 19's 20's and 21's and have never had autothrust induced rocking nor heard of anyone in any on the three bus flying comaines I have worked for mention it!!

Would love to know more if this suspected phenomenon is documented but I cant find anything about it!!!!

[trustno1]

Can I just state that the amount of rocking I experienced is quite small indeed.....in fact you would have to be a "spotter" to notice it and only by looking out the window at some external reference point.

[gofer]

Confirm - its not strong movement at all, and its not always there - but when its there, a sensitive backside can feel it.

Must admit that on the day I noticed it most, I was on AF flights - a 319 first (TLS-ORY) and the a 321 (ORY-MLH) and we were just at, on, in or around a very very thin layer of hazy cloud 1 to 2 meters thick with a strong and seemingly uneven wind coming from due west. Assume that the AF Buses have CFM engines ?

Does that help or confuse anybody.:( :eek: :(

[Spearing Britney]

Well I tried my hardest for eight hours today and couldnt feel a darn thing!!! Still havent found anyone aware of it either...

Re autothrust oscillation, I just don't buy it, the speed is allowed to vary enough that it doesnt bounce around a speed. Soft alt hold doesnt cause it except in turbulence. There isnt enough of a pitch moment on the bus (unlike the 73) to blame the thrust either...

:confused: :rolleyes:

[Offset]

Shall we all just admit that we don't know what causes it, rather than come out with garbage that exposes our lack of understanding in areas that some of us are claiming a degree of expertise?