777 vibration on take off?

[ATC Watcher]

Last week as a pax, just after rotation , the a/c nose was raised on take off then followed by strong vibrations for 4-5 seconds . Climb angle remained constant. Did not think much of it , but the same occurred on the return leg ( different crew, diff tail number but same airline )

Occasionally you do have this buffet but not as strong and not as long as these 2 times. Are certain 777 models known to have this ? or is this a new "flex take off" procedure ? or this particular airline training ?
Just curious .
On both cases runway length was not an issue , nor was obstacle to clear or temperature ( One of the legs was out of SIN early morning)
On all aircraft I do fly myself, I ( and was trained to) slightly lower the nose after rotation to gain a few knots just to avoid this. Or is the margin between buffet and actual stall so big on the 777 wing that this is a non-issue ?

[STBYRUD]

Buffet after rotation should never be felt on any modern transport aircraft, out of curiousity - where along the fuselage did you sit? I'm thinking you felt the wheels rotate for a brief amount of time until they were stopped...

[wiggy]

"777 pre-stall buffet on take off" :rolleyes:

Before the newspapers get hold of this I'd very much suggest it wasn't any such thing.

I'd very politely and very much suggest :ok: it's nothing aerodynamic at all and much much much much more likely to be nose wheel "shimmy" (a wobble/vibration as the wheel(s) spin downs) or something else gear/tyre related.


It's not that uncommon on many types, even the biggies, and yes, you do get it sometimes on the 777. If it happens it will be much more pronounced at higher rotation speeds.

[JammedStab]

If you saw the slats extend from the retracted position to the extended position at this time, that would be part of the stall recovery procedure. Unlikely that this situation happened though.

[wiggy]

Unlikely that this situation happened though.

Blimey do I hope so, especially since the OP said the same occurred on the return leg and our QRH says in the event of needing a stall recovery "During liftoff if flaps are up, call for flap 1". (AFAIK that part of the procedure is to cater for an accidental flapless liftoff. FWIW we don't do flapless takeoffs or Flap 1 take-offs and I'm not sure if any 777 operators do).

....So I'll very much stick with the noise/rumble being shimmy or of similar origin.....

(Edit to add: A couple of points:

The OP mentions SIN. Without wishing to offend the locals 02C/20C at SIN isn't the smoothest runway in the world and it can certainly give the nose gear a good work out at higher speeds, and often makes for a noisy first few seconds after liftoff

Secondly many operators use or are moving to the use of "improved climb" for take-off performance, which may well mean increased ground speeds before rotate than under previous methods. That in turn may well lead to an increase in gear noise levels during retraction).

[casablanca]

Definitely going with wheel shimmy.....as mentioned using improved climb performance often results in rotation speeds in excess of 190 it's so it is noticeable

[ATC Watcher]

Hey, I was not fishing for sensationalism! Nose wheel shimmy could indeed explain it. Did not think of that possibility . I was seating up front in C, well before the wings so indeed that could be it.
The vibrations were quite strong and really feeling like buffet, so that was what came up to my mind first since the angle of climb after rotation was steep.

[wiggy]

Hey, I was not fishing for sensationalism!

Yep I know. To be fair maybe a slightly more open original post and even a "?" in the title might have avoided any misunderstanding.

Anyhow hopefully we've got to the bottom of it all.

[ATC Watcher]

Yes I realized that already and wanted to remove the words "pre stall buffet by :' vibrations " on the header but apparently once posted you cannot edit it it. Only the Mods can , so if one read this , you can do it for me .Thanks

[Volume]

Probably more a poorly balanced main wheel tire, than nose wheel shimmy.
Had it on two following legs on a Qantas Jumbo (out of FRA, out of SIN). You could see the galleys/toilets above the wheel wells moving in resonance... And it clearly matched the landing gear retraction cycle, which includes braking the wheels at a certain pont, when all the vibration suddenly stopped. Slight vibration could be felt during take-off and landing run at higher speed, but it clearly went extreme just with the wheels off the ground.
Also felt it (to a much lower extent) in the nose of a KLM 747 with a more gradual spindown of the wheels and end of the vibration (also including a fading of the vibartion frequency) after retraction.

[CCA]

Vibes from the nose gear tyres contacting the snubbers during gear retraction can vary widely depending on tyres and/or snubber condition vs speed.

[NSEU]

Vibes from the nose gear tyres contacting the snubbers during gear retraction can vary widely depending on tyres and/or snubber condition vs speed.

On a 747-400, it can be loud and sound like a band saw cutting through wood :uhoh: On retraction, main gear wheels are braked hydraulically, nose wheels are braked by two heat resistant pads rubbing against the tyres.

[mary meagher]

ATC watcher, you can remove your posts yourself, or the entire thread that you have started, don't have to ask the mods to do it. I had to do it once to one of mine!

PS. Did you fly a pegasus glider? I did 3 diamonds in mine!

[underfire]

Isnt the tail flutter from the auto gust response system on the 777 a known issue? Perhaps it was that.

[maui]

Underfire. 10,000+ hours on type and 16 years, never heard of it!

I'll go with the nosewheel shimmy/braking vibration scenario's.

[underfire]

maui,
There are plenty of articles about the issue, and some fixes from Boeing. This issue was on the 777, especially the 300, but others have noted the same issue.
The auto gust modal suppression system on the tail causes a flutter that was so bad it was making crew and passengers sick in the back...
I believe it was new ANZ aircraft that were in the news...

Older thread on the issue...

B777-200 lateral oscillation in turbulence [Archive] - PPRuNe Forums (http://www.pprune.org/archive/index.php/t-480350.html)

[LlamaFarmer]

If you saw the slats extend from the retracted position to the extended position at this time, that would be part of the stall recovery procedure. Unlikely that this situation happened though.

Wouldn't slats be extended anyway?


I'm not familiar with the 777 flavour, but slats tend to come before or with flaps, and I can't imagine performing a clean departure.

[maui]

Underfire.


OK I have heard of some hypersensitivity to some minor gust suppression induced oscillation. But I do believe you referred to "tail flutter", which is an entirely different kettle of fish.


Still haven't heard of "tail flutter"


M

[wiggy]

maui

OK I have heard of some hypersensitivity to some minor gust suppression induced oscillation.

Yep likewise.

There were stories from some down the back that the 777 did something "odd" in the cruise that made them feel ill ("hypersensitive"). I believe that may well have been blamed on gust suppression, but I've never heard of or experienced it causing anything like the symptoms described by the OP.


LlamaFarmer

Correct.

[heavy_landing]

Although I don't think it's relevant to this situation, I think the auto slats extend the slats from the mid position (which they'd be at with take off flap settings) to the fully extended position if an impending stall is detected