747 Yaw oscillation

[ChrisVJ]

After forty odd years of SLF flying I finally flew in a 747 last week, Vancouver/Tokyo by Japan Air Lines 747-400. I expected a smooth ride but was hugely disappointed. All the way, apart from a couple of ten minute spells the tail, (we were in row 62) oscillated in yaw, from almost nothing up to about an inch or two and about 100 cycles a minute, a jerky and tiring ride.

Is this typical of 747 of does someone need to have a serious word with the yaw damper? Would the pilot feel it at the front?

[FCS Explorer]

up to about an inch or two and about 100 cycles a minute

less booze for you next time!

[BOAC73]

Chris....
at what point did the tail fall off?
B73

[SincoTC]

I flew from Heathrow to Singapore a few years ago in an SIA 747-400 (the one that was destroyed in a take off accident in Tapei a month later). I too was seated in row 62 and yes, I did notice some oscillations, mainly vertical rather than yaw, I say vertical rather than pitch because when I "sighted" up the aisle, I got the impression that it was due to structural flexing. It looked and felt like it was wagging its tail rather than the whole plane, so maybe the flight-crew wouldn't feel it. Although, I do recall reading somewhere, that on testing a new large aircaft, the test crew commented that you could feel the fuselage whip when it was given a "bootful of rudder". I noticed vertical motions mainly at rotation, flare and touchdown and laterals during descent through turbulance.

Being an engineer and realising that things of that size are never totally rigid, it didn't really concern me that much (but it was quite a bit slower than your description), so I just sat back and enjoyed the trip; it was great, plenty of legroom and nobody behind to complain about leaving the blind up for fourteen hours so I could see half a world go by my window, hell you can watch a video any day!

[fdgolf]

I noticed similar movements (specially in yaw, but not uncomfortable either) a few years ago flying in the last row on a BA 744 from LHR to SEA... Didn't bother much I have to say.

Cheers

[NSEU]

Yes, the ride up the back is always less smooth than up the front.

The 747 does bend, and the Yaw Damper system is designed to reduce the effects of this. There are accelerometers at either ends of the fuselage to measure the bending. This is called Structural Modal Oscillation.

Perhaps the system was partially inoperative? I think you can fly with one of the two Yaw Damper Systems inop. One system controls the upper rudder, and the other the lower rudder (The rudder is split into two) I assume the effectivity of the system is halved by doing this (at least at the extremes of control).

Rgds.
NSEU

[muduckace]

The aircraft bends...

yeah, not much. The longer the aircraft the more you will experience YAW, the aft cabin seems worse but none the less being seated over the wing will lessen the experience. More modern aircraft have better YAW dampening abilities but none the less as the rudder kicks over ... If you are in the aft cabin of a 747 you are going to feel it.

[point8six]

......just the "Queen of the Skies" shaking her booty.

[WHBM]

Originally Posted by muduckace

More modern aircraft have better YAW dampening abilities but none the less as the rudder kicks over ... If you are in the aft cabin of a 747 you are going to feel it.

If you are in the back row, aisle seat of an A346, and looking forward, then the longitudinal bending appears even more pronounced !

[scaredflyer]

Yeah I experienced this for the first time on a flight from Munich To LHR, on a Lufthansa Airbus about 5 years ago.

Terrible sensation, but like someone said things of that size are seldon rigid, I never ride in the back any more though.

Doesn't help my fear of flying any bit at all, when I'm having a bad day.

[Mr Optimistic]

isn't it a residue of dutch roll ?

I noticed the same thing years ago on smooth night flights. A coupled roll/yawing, very smooth and hardly noticable. Took it to be a very sudued dutch roll (like the autopilot had almost conquered the oscillation but a bit remained). Perhaps someone can explain. Nothing to worry about eg like climbing out of Washington in heavy turbulence, being thumped against the 747 cabin wall sitting next to a nun who continuously prayed and counted her rosary beads (aren't these people supposed to have faith ? - I do wish they wouldn't do that)

[MarkerInbound]

If I recall correctly, both yaw dampers can be deferred with no performance penalty on a 747-400.

[AtoBsafely]

In our company MEL we require at least one yaw damper for dispatch of the B744.

I've never noticed oscillations in the front, and the turbulence is less noticeable than any other type I've been in.

I agree with .86; it's just the queen shimmying a little!

Joe

[Loose rivets]

I posted a while back on the unpleasant ride I had in the back of a 777. Held as still as I could, my coffee was spilling from an inch below the rim as it slopped side to side. The slightest chop set it going again. Practically the whole flight was like this.


I had come to the conclusion that the flexing seemed to be out of phase with the correction. The statement above describing remote sensors seems to belie this however.

I don't know, but as a paying guest, it was bad enough, but as someone with a lifetime in aviation, it made me wonder just how far we haven't come in all these years of design 'improvement'.

The 'man made' stresses on that fuselage seemed completely unacceptable.

[CR2]

The 747's tail does "cork-screw" for want of a better word. Some people are sensitive to it, others aren't, its basically motion sickness. I've never had a problem, don't even notice it; on all versions, 1-2-3-400 and SP. -F/SF versions (barring the SP) too.
I've been told the 777 is worse (I've never been on a 777, so can't comment).

[muduckace]

Looking at the aft fucelage of a seasoned 747, in my experience the classics, the "beer canning" as we called it or bent skin visual inbetween longerons is probably a result of this. I have not noticed skin distortions in the 744,777,A340.

[WHBM]

Originally Posted by CR2

I've been told the 777 is worse (I've never been on a 777, so can't comment).

I agree. The 777 has both the worst ride aft of the wings, and back there is also the noisiest in the cruise, of all the current generation of long-haul types.

[Jorge46]

For what it's worth------ I think when this occurs that the airplane is out of trim and the autopilot is working against that. Pilots tend to 'plug' in the autopilot early and leave it engaged without ever checking to see if the airplane can fly 'straight and level'. Try hand flying at cruise for a few minutes and see if there is any 'turning tendency' while holding it level. You'll notice that by having to put pressure on the wheel. If there is, take out the tendency by using rudder trim. When the a/c will remain straight and level without any pilot effort on the wheel, plug in the autopilot and the continuous yawing will have disappeared.
I believe this is true for any swept wing jet.

[PETTIFOGGER]

lateral shakes

I thought that it was well known that any aircraft with the thrust distributed across the wings hundreds of feet apart, can be prone to minor lateral accelerations, or “the shakes”, most keenly felt near the tail. I have heard flight crew announce many times over the years to expect some sideways movement at the back because of a tail wind. It was most noticeable on 707s for some reason, but is also evident on 747s and 340s. I think it was referred to as ‘tugging’ or ‘snatching’, or something like that.
Like the OP said, it can be very jerky and tiring at the back, but it is hardly noticeable around the centre of the aircraft, and by the time you get near the front you can feel the effect but the severity is well damped. I am not a pilot nor have I worked aloft in any capacity; it would be interesting to hear about this from someone who has.

[edsilkworth]

Structural modal oscillations

Structural modal oscillations are an undesired effect created by turbulence, which causes bending of the fuselage around the wing area. The Boeing 747-400 uses modal accelerometers in the yaw damper system to suppress the oscillations.
Ref: Avionics:Systems and Troubleshooting, Eismin ISBN:0-9708109-1-1