Airbus 320 question

[Pax-man]

Is it just me, or is the Airbus 320 / 321 particularly 'wobbly' on approach??

I've flown MAN - TFS about a dozen times or so in the past year, with different airlines, and the first couple of times I put the wobbles down to conditions etc. Yet it seems now that every approach in a MiniBus is a wobbly one!

I've been lucky enough to fly on a good number of different aircraft types, and finals on the 320 / 321 never fails to shut the rest of the pax up! Not quite as good as a Saab into Inverness in winter winds though! Flying a 'carrier' approach on a 146 is fun too. Nose down, deep breath...

Any pilots of said types care to comment?

[square leg]

Hello Pax-man,

When steering the A/C with the elevator, one effectively moves the tail up and down. Eventually the whole A/C flight path (vector) will change. Then the ailerons and roll spoilers play a role too. When these are combined, it does in fact cause the rear end of the A/C to move around quite a bit.

In fact, it's like holding a nail against a wall with the sharp end touching the wall and your index finger on the head of the nail. The pointy end is the cockpit, where your finger is, that's the tail. Now move your index finger around in circles and imagine that your index finger is the tail end. That is the motion the tail end does, while the "cockpit" is relatively still.

The Avro, without slats will fly a typical 3° glide with quite a bit of nose down (-5°- -7°). If the trim on the Avro is not in the "autoland" position (ANU) and one flies it by hand, then one has to exercise quite a bit of stick force (forward or aft, depending on high orlow speed) as the A/C has not yet settled into its optimum pitch attitude with the centre of pressure in the optimum position. If the A/C is therefore not trimmed properly, the pilot has to work that bit extra and as a pax you'll feel every movement on the stick, thus the wobbly effect perhaps.

Now imagine EGLC in 15G20kts X-winds and the effect is even worse.

On the A320, turbulence plays quite a role. In still air, and perfectly stabilised one shouldn't feel too much. The trick is to leave the stick alone when all parameters are correct. It could be so-called PIO (pilot induced oscillations), whereby the pilot overcorrects and thus this to-and-fro swaying occurs. And the further aft you sit, the more pronounced the feeling of wobbling will come across to you. The rudder is seldomly used in the air on the A320, except in strong X-winds (during landing) and here also only partially. Of course the rudder will be used during engine out operations.

Wobble-dy wob. I guess there's no one answer for your question, as every approach is different with different variables.

Hope this helps

[Pax-man]

Thanks for the detailed response, square leg - you've certainly cleared up the physics of the matter!

However, I'm still curious about the WobblyBus. It's not something I've noticed much on 737s, but it's a common phenomenon on the 320 / 321.

Is is simply a case of the smaller the aircraft the more susceptible it is to buffering? Or are the directional controls on the Bus a touch over-responsive?

[square leg]

The other day, before descending, I took walk to the rear toilet and did notice that in cruise the bus moves around quite a bit. The air was still, but the tail was moving around ever so slightly as opposed to a very stable feeling in the cockpit.

I also asked a hostie who was sitting in the back, what she thought of the approach and she mentioned that the bus does tend to be quite "unstable" in the back. This confirmed what you have observed.

But as I said, the tail has to move to make the whole A/C move. The longer, the more leverage (i.e. A321)

[flyer4life]

In fact, it's like holding a nail against a wall with the sharp end touching the wall and your index finger on the head of the nail. The pointy end is the cockpit, where your finger is, that's the tail. Now move your index finger around in circles and imagine that your index finger is the tail end. That is the motion the tail end does, while the "cockpit" is relatively still.

As an A320/321 pilot I have to disagree. The aircraft moves about its centre of gravity which is pretty much in the middle of the cabin. I think we feel less movement in the cockpit because we're either used to it or because we're creating the movements ourselves, a bit like a car driver not realising the forces he's subjecting his passengers to. Also, window seat passengers would feel the roll movements more so since they sit further away from the roll axis than the pilots.

As for wobbly approaches on the A320/321, I'd put it down to pilot induced oscillations. We're told the fly-by-wire computers will correct for gusts and keep the wings level but it doesn't do a perfect job. So you have the situation of the flight computers and the pilot putting in corrective actions, which results in overcontrolling. Also, with the sidestick, a pitch input could accidentally invoke a slight roll input if the stick isn't moved exactly along the pitch axis.

Interesting thread though. I've done landings which the Captain and I both thought were nice, only to hear the cabin crew at the rear galley whinge about it later on the crew bus.

[square leg]

flyer4life,

Naturally you are correct about the CG thing.

I just used a simple method of explaining the question at hand in layman terms.

[square leg]

Wobbliness during approach => PIO (AP off), wind, turbulence, Yaw damper etc.

Wobbliness in cruise (still air, AP on) => Yaw damper