B777-200 lateral oscillation in turbulence

[rgsaero]

Although I trained (during the late ‘50s) as an a/c engineer, my experience of commercial aircraft has been as a passenger since 1965.
I raise this matter following a return trip to Auckland in an AirNZ 772. I have done this trip about 20 times since 1983, via both LA and HK in 747s and latterly 777s. On this trip, for the first time, I sat in the rearmost two rows of the aft cabin of a 777-200.

On the outbound two legs there was little turbulence, but what there was promoted a rapid ( 1+cycle/sec) lateral oscillation of between 2 and 6 cm. On the return trip, particularly on the AKL – HK leg the high altitude turbulance was more severe. This caused, at the rear of the aft compartment, a continuing oscillation as described above which effectively prevented any chance of rest. It was so severe at times that one’s rib-cage was hitting each arm rest every second! The aircraft was quite literally wagging it’s tail!

Being interested in this beyond self comfort I went forward ahead of row 53 – the front of the rear cabin, and into the mid cabin. The lateral movement was almost un-noticeable. From there, observing the passengers in the rear cabin was interesting; heads were rocking on relaxed necks from side to side, and the lower trunks of the heavier passengers were visibly wobbling back and forth. This phenomenon arose in anything but the mildest turbulence.

I made enquiries of the cabin crew and was told that ALL their 777s were the same, demonstrating the same oscillation in anything but still air. They acknowledged that the rearmost 14 rows were extremely uncomfortable in any turbulence for this reason.

The movement appears to be a rapid yaw and reverse about a point somewhere near the wing centreline, or perhaps the c of g.

While I am sure that Boeing’s stressmen are more than competent, I was given to wonder at the moments being created at the base of the vertical stabiliser; I estimate that this lateral oscillation was present for more that seven ours of the twelve hour leg.

Apart from resolving never again to occupy a seat in the rear cabin of a 777 I would be interested to hear any engineering comment about the phenomenon, and a confirmation that this problem is not unique to AirNZ’s fleet.

[boguing]

Intriguing.

I'd like to know more about how you derived the distance measurement.

I'd also like to hear "they all do that" from more than one source.

Given that this was in the cruise phase, probably on ap, it infers that the ap and/or yaw damper are over reacting, trying too hard to remedy a natural oscillation.

If the fuselage structure is compliant/flexible/damping enough not to be doing the same to the front I would certainly be inviting the very front row down to the back to experience it for themselves.

[Spooky 2]

Perhaps the gust suppresion system was not working to it's fullest? It provides a better ride through a combination of yaw damper and roll commands. Just a guess as I have heard of this condition being s speciific
777 issue?

[rgsaero]

The movement measurement was necessarily approximate and to some degree subjective, but with my backside on the seat and a five to six cm gap between my left and right sides, to the arm rest, that gap was being closed by the oscillation on each side!

I, too, wondered about the "they all do that" comment; interestingly enough I had my airmedical (for my PPL) today and my very experienced man who looks after countless commercials etc and flies as a passenger long-haul often (including 777s) says he's experienced it and "it's usual".

There was, incidentally, no roll apparent when this was happening, but then it was happening for most of the flight!

[flightleader]

In some turbulence conditions, the gust suppression is working but not able to so call 'predict' what's ahead. therefore,minor over corrections or under corrections. Thus results in many minor lateral 'jolts'. This is a character of the B777. In the A330, it fish tail. Imagine pin the cockpit on a dart board and let the tail sway. The A330 tail will oscillate in a 'U' shape manner. Making many first time travellers at the back end feel sick.....crew too.

Not here to judge which a/c is better. They are build definitely less tough than the B747 but that dinosaur burns nearly 5000kgs more per hour.

[barit1]

I too have sensed a rapid yaw oscillation - twenty years ago, in a RJ L15, during an early AM reduced thrust TO from DXB. It only lasted 10 sec. or so, and was a high frequency (~2 hz). The crew felt it was engine caused, which I doubted.

But they invited me to observe the gages on the next TO which was at rated thrust. No gage indication, and no sensed yaw oscillation this time.

But there may be some relationship to the 777 situation of rgsaero.

[Poire]

The 777 also incorporates a "modal suppression" function of the rudder.
It tries to synchronize the torsion movements between the rear and front fuselage ...

[Bungfai]

Concerning this matter the B777-300 is worse.

[rgsaero]

"....Concerning this matter the B777-300 is worse...."

This may be a consequence of the extra 5 m fuselage length behind the wing.

I've asked Air NZ about this phenomenon and await their response with interest.

[TURIN]

It is very common to see an active fault with the suppression system when looking at the Existing faults list on the MAT. It doesn't create an EICAS message so there are no dispatch problems to worry about. Maybe this is why many of the crew are of the opinion that 'they all do it'.

[halas]

All normal on a 777, as stated before, it's the gust suppression system at work.

My first line training flight l brought this up with the instructor: The only aircraft l know that has turbulence sideways!

The system goes into overdrive on short final. On a calm day it appears to actually replicate gusty conditions just before landing.

halas

[rgsaero]

I have received a full and very detailed explanation of what goes on at the rear of the 777 and its various systems from a senior engineering manager at Air NZ (NOT a Customer Relations person!)

He confirms that the fault appears to be in a failure in the Lateral Gust Suppression System. Such failures do not directly affect airworthiness and the system is not on the MEL.

He also says that NZ aircrew are incorrect in saying "they all do that" as aircraft are not regularly despatched with such problems existing.

A full and detailed reply from the right person at AirNZ - well done!

[Heading180]

I only fly the bus but I think I read in Flight once that Boeing / FAA had done alot of research into this.

[Volume]

Funny to read this here. I "complained" about this uncomfortable feature of the 777 in another forum, and all the others tried to convince me that I´m crazy and that the 777 flies smooth as silk...
I would support the 2 Hz statement in favour of the around 1 Hz, I think the amplitude of about 5 cm is quite well matching my estimation.
However, I was seated close to C/G, so I would rule out the rudder (or yaw damper) to be the reason, I believe this is an eigenmode (normal mode) of the aircrafts elastic structure involving the engines swinging sideways on their pylon and the rest of the aircraft against it (to keep the C/G steady). It surely is excited by lateral gusts, but I believe the phenomenon does not include areodynamics.
And they don´t do it all, it is especially those with the heavy engines on soft pylons which do it. The difference between A340-200/300, A330 and A340-500/600 is remarkable (with the effect increasing in that order). 747-400 does it. 777-300ER is worst. Never felt it on the smaller aircraft (frequency too high to be excited by lateral gusts) or on the MD-11. Never felt it on aircraft with tail mounted engines. For some smaller aircraft you can clearly feel the yaw damper working (e.g. CRJ 200). On some you can feel worn rudder / rudder actuator bearings, but the frequency is more in the 3-4 Hz range then, and you only feel it in the rear cabin.

[Turbine D]

I recall sitting in extreme rear seating on more than one occasion on a DC-8-73 stretch. As you go through high altitude turbulence, you could actually see the "torquing" motion, looking up the aisle towards the front of the aircraft, as you were being shifted from side to side. I always thought it to be the fact they added fuselage plugs to lengthen the aircraft to add more seats, without changing anything else from the shorter length version...

TD

[EEngr]

The 777 also incorporates a "modal suppression" function of the rudder.I think, given the increased flexibility of composite structures (albeit just as strong as metal), we age going to see a lot more active mode suppression algorithms used. Given the opportunity to get some of the software parameters wrong, or just dispatching with these systems inop, its going to make for some interesting rides.:yuk:

[richard III]

not going to elaborate, but the remark they "all seem to do that" is accurate...I have 15000 hrs. in other boeings and when I came to the 777 I was a bit surprised by the way it takes turbulence obviously due to the gust suppression system, I much rather take it on a 767....overall a great plane, just not a very good dancer

[FowlPilot]

On my recent 777-300ER flight LHR-YVR we had long periods (hours) of very light turbulence (until we passed Hudson Bay) and a very short period of light-medium turbulence going through heavy cloud leaving LHR (20 seconds.)
I was sitting directly over the wing and the very light turbulence always seemed to resolve into exactly the same 2hz movement roughly 5cm in amplitude from top-left to bottom-right.

It seems strange that what is essentially a random input should cause the fuselage to move so predictably (and annoyingly.)

It felt like an older open-top car that has been driven for years over potholes.

Surely this can't be good for metal fatigue reasons?

Does the 748 or 744 do this? I don't recall other planes shaking in similar ways.

[stilton]

Best aircraft in turbulence was the B727, like a hot knife through butter, no tail wagging, there again it had a very generously sized vertical stabilizer.


767 is almost as good, the 757 is not good at all.


The fin on the Triple 7 always looked way too small to me, compare it to the 747, I think it's as simple as that.

[JammedStab]

Best aircraft in turbulence was the B727, like a hot knife through butter, no tail wagging, there again it had a very generously sized vertical stabilizer.




It seemed that on the 727, you would sometimes get some wind gust noise in the cockpit in turbulence.

As for the triple, I am frequently a pax in the forward section and have never felt anything unusual.

[underfire]

Stilton,

That is an interesting observation. Here is a quick overlay of the tails from the respective manufacturers CAD drawings.
Was surprised to see almost identical tails from 788 and 767-4. There was no difference in -ER variants from the non -ER...A350 and A333 were identical.

http://i60.tinypic.com/w18g78.jpg

[stilton]

Interesting graphic, I think it does illustrate my point, the vertical fin
on the triple 7 is quite small for the size of the aircraft for minimal drag
i'm sure.


But it doesn't provide much area and I believe this is a large part of it's 'tail
wagging' tendencies.

[underfire]

This was an interesting little experiement to look at. I did note that all of the Boeing aircraft have the same angle on the back of the their tails, and the Airbus have the same angle on theirs, albeit different between the manufacturers...

The angle on the front of the tails is the same for both manufacturers, with the exception of the 748, which is odd to all (but same beginning point as the A388...)

Perhaps I will try the same with the wings!

[TShan1]

I don't know if anyone has had the pleasure of the back row of a Q400 Dash-8, but they suffer this problem quite a bit during turbulence.


Felt like oversteer in a rear wheel drive car.

[FullWings]

It’s annoying at the front end too. The oscillation in turbulence is just the right frequency to be resonant with the fluid in our chinaware. Many a time I’ve flown through very light ripples, nowhere near enough to put the seatbelt on, then experienced my tea making a bid for freedom.

First thing I do on getting to the FD (FCC in EASA-land) is stuff the cupholders with towels to soak up the inevitable spillages.

[TShan1]

FullWings, it's a very hard life mate. It really is.

[tdracer]

Stilton, quads will always have a bigger vertical tail than twins. It's that outboard engine effect.


At least on the Boeing side, the aircraft with the minimalistic vertical tail is the 787, not the 777.

[FCeng84]

The mass of all three engines goes a long way to reduce the turbulence response of the aft end of the 727.

As for vertical fin leading edge angle, I believe that this is similar to wing sweep angle with respect to the aerodynamics of flight a Mach numbers closer to 1. The 747 has a higher cruise Mach number than most in part enabled by the greater tail sweep angle.

[FCeng84]

I am very curious as to the frequency of the lateral oscillation that is felt in the aft end of the 777. If the frequency is 1 Hz or less the associated mode probably does not involve flexing of structure. If it is 2 Hz or higher it very likely does involve structural flexing.

The 777 control laws involve functions designed to damp out oscillations at various frequencies, both lower frequency "rigid body" motions and higher frequency "structural mode" motions.

[stilton]

Yes the 787 vertical tail looks small but not out of proportion like the 777 whose fin really looks 'chopped off'


Wonder what its like in the back of a 787 in turbulence ?

[underfire]

Interesting NASA article on tail tech

NASA Tail Technology (http://www.nasa.gov/image-feature/nasa-tail-technology-could-someday-reduce-airplane-fuel-use)

[Amadis of Gaul]

Wonder what its like in the back of a 787 in turbulence ?

I hear it's just bumpy enough to distract you from the smell of burning wires.

[underfire]

Flew the 788 from MEL to AKL...its rattles and creaks quite a bit in the back, well, through the whole ac in turbulence. The center overhead compartment is attached in the center, and was wagging like a yodelers uvula in turbulence.

What is really disturbing was on rotation, the wings really flex up and down until enough speed has settled them down, like an albatros taking off....there was a very apparent up/down motion in the rear of the ac....

the dragon painted down the side seemed very appropriate....

[c100driver]

Smaug is on the T7-300