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-   -   noise and vibrations on take-off 747-800 (https://www.pprune.org/tech-log/620552-noise-vibrations-take-off-747-800-a.html)

BRE 15th Apr 2019 12:00

noise and vibrations on take-off 747-800
I flew from JFK this Saturday in a 748 and was sitting pretty much above the center of the wingbox. There was a very loud rattling noise and strong shaking / vibration for about 30 s before lift-off that stopped a few seconds later at what felt like a height of only a few meters. After that, the engines were immediately surprisingly quiet for such a full plane in climb.

The take-off was towards the South, and the plane was about 98% full. I have taken off in a 748 from JFK before and don't remember this noise/vibration but was sitting further in the back. I believe I have experienced a similar noise, but not as loud and without the vibration sometimes in A330 and A340s.

There were two flight attendants in backwards facing crew seats who acted completely unconcerned so I decided there wasn't any need for concern :)

What was that noise? Poor runway surface? But then why did it seem to stop only at a height of a few meters?

Same for damaged or poorly balanced tire.

Could it be the turbines? But it is normal to throttle back immediately after lift-off?

KriVa 15th Apr 2019 12:57

An out of balance tyre, perhaps in combination with some slight wear on the wheel bearings, sounds like the likely culprit.
The reason why the rattling would only stop some time after lift-off, is because the wheel doesnít stop rotating immediately after it leaves the surface, but still spins down for a while.
Once the gear is selected up, wheel brakes are applied and the wheels stop spinning.

B2N2 15th Apr 2019 13:35

Keep in mind also that engine noise is now no longer reflected by the runway surface so they may appear to have been throttled back but that is not the case.
If climb obstacles or other climb profile requirements allow it a thrust reduction is done at Flaps 5 or 3000í.
As far as the rumbling noise I can only guess:
Runway surface
New tire and worn tire on the same gear truck
Flaps 20 take off and some play in flap mechanism transmitted through the wing.
Floor boards and seat tracks causing a little harmonic vibration.
Youíre sitting pretty much right above the Air Conditioning packs.

Could be any and all of the above and maybe another dozen I didnít think about.
The airplane is fully alive at that time, youíve got bleedair going allover the place, hydraulic pumps running, electric motors the whole kit and kaboodle.
No doubt under certain conditions it will generate a little harmonic vibration somewhere down there.

LeadSled 15th Apr 2019 23:54

Out of balance wheels combined with a structural resonance ---- sound like it does the same thing as the -400, where it could be a significant problem, really upsetting pax and cabin crew sitting anywhere in the area.
Tootle pip!!

PS: In the -400, it was in the category of a "known problem".

scifi 16th Apr 2019 10:31

Not too sure about 'Structural Resonance'. The speed is increasing, so any driving vibrations are increasing in frequency. A 'resonance' would only appear at one frequency.(= one speed.)

Back in the pre MOT days, when 1/4 a turn of steering wheel play was the norm, out of balance wheels would vibrate at 45 mph. Two mph faster or slower would be Ok, the resonant speed was 45, which was about the speed of the traffic those days..

Uplinker 16th Apr 2019 10:44

Twin wheel nose gear can sometimes have a speed related vibration caused by the tyres not being worn to the same extent, (e.g. one new tyre fitted next to an older one), and/or excessive wear in the torque link joints. This vibration builds up towards a speed that induces maximum resonance somewhere on the airframe, then dies away as the aircraft accelerates.

BalusKaptan 17th Apr 2019 09:40

You say you were in an -8 then thrust reduction would most likely occured at 1000' agl. I've operated the -8 out of JFK, both short and long haul and at MTOW, under most ambient conditions, this would be normal. Sometimes, more frequent than many would think, when relatively light weight (think short haul) the minimum climb thrust is higher than the take-off thrust especially when using TO2 with max ATM. This actually causes a thrust increase when going to climb thrust.
As mentioned above most likely reason is tyre imbalance which can occur for a multitude of reasons and gets written up by the Tech crew for checking at the next stop.

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