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744 rotation vibration

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744 rotation vibration

Old 11th Dec 2006, 15:31
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744 rotation vibration

Hi,

Last month I made a few trips on Qantas 744's. This only happened on one of the flights. Departing Singapore for Sydney, immediately after rotation and lift-off the aircraft began to vibrate. Not violently, but it was a notable vibration and a few over-head bins opened due to it. A few passengers gasped, one women screamed "oh my god" (there's always a screamer!). It lasted about 10-15 seconds. After that, the flight smoothed out the CC were soon after released to start the service.

No explanation from the flight Crew. I didn't think it warranted explanation but I heard lots of passengers asking the CC what had happened - they didn't know.

I was sitting over the wing, so my assumption was - it was the wheels braking as they rotated into the undercarriage bay. I couldn't think of anything else (Iím not a pilot). The only thing is I've flown on many 744's, and sat in similar locations on the plane, and have never experienced this before.

Can anyone else offer alternative explanations?

Cheers,
eP.
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Old 11th Dec 2006, 16:43
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No explanation as I do not fly them, but a common experience for me as a 747 pax.
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Old 11th Dec 2006, 17:07
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At a guess some sort of shimmy from the gear. The nose wheel makes a hell of a noise when it hits the rub strip in the gear bay. It's absolutely the worst thing about sitting in first class.
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Old 11th Dec 2006, 17:50
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Well, if we are talking the same 'thing', to me it is just at lift-off, well before retract - and I do get the odd 'anxious moment' I will admit - it is that sort of event.
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Old 11th Dec 2006, 18:36
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Happens frequently on the 747. Unbalanced nosewheels, usually precipitated by changing 1 at a time instead of the pair. Gyroscopic effects as the gear swing up and the wheels spin down make it worse.
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Old 11th Dec 2006, 18:38
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Hey guys, thanks very much indeed for your replies.

Much appreciated!

eP
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Old 11th Dec 2006, 19:06
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The 747 uses pretty fast airborne speeds. What you are hearing and feeling is the wheels all spinning at high speed. During the retraction sequence of the main undercarriage, brakes are applied automatically so that debris is not thrown around inside the undercarriage bay. You should only hear this near the wheels- in just a short distance away in the cabin, it will be inaudible- that is why other people and the crew are unconcerned- they didn't hear it!
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Old 11th Dec 2006, 19:11
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well before retract?
Methinks Intruder has it. Odd, though, that it has happened on every rotate I have been on. It really does feel like a stall burble, and before anyone jumps on me, I am NOT saying it is!
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Old 11th Dec 2006, 19:55
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Me thinks it's worn (but safe) wheel bearings. Eg....(for pilots) Doing 85-90mph down M6 in car with worn bearings. At this speed and if possible one could lift the car from road surface turn 45deg wow would you gets some vibs in the body work. MD80 bearings are well know for this vibration. Some ladies of a certain age enjoy the effect.
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Old 11th Dec 2006, 20:13
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Talking

Reason is often main wheels outer balance, they can shake a little before spin-down brakes stop them as gear is stowed away after T/O.
.
In the perfect world, balance is maintained by wheels being stopped at a random point during gear up operation and so touch-down wear and tear is spread over many points of the tyre, so the wheels/tyres are kept in balance during the time fitted to the aircraft.
.
However random is random and tyres can get outer balance, plus if spin-down system is not perfect, wheels can tend to stop in a similar location after gear up operation, means touch-down wear and tear may often be the same place, this will get them well outer balance and can get heavy vibs until wheels are stopped during gear up.
.
Seem to remember wheels are braked by alt-anti-skid during gear up ops, if system a bit crook, some wheels tend to stop at common places and lose the random effect, mix that with number of cycles and vibs can follow, all fixed when new wheels/tyres fitted, however apart the vibs/noise no need to worry, the 744 gear is most strong.
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Old 11th Dec 2006, 21:08
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I don't know if it's worn wheel bearings or out of balance wheels in an unloaded condition rotating rapidly after lift off and possibly at various speeds for different wheels hitting their resonant rotatin speed. My father's Ford Anglia used to do it something horrible above 55mph, but modern cars never do it now (well, not BMWs!). Definitely no stall burble!
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Old 12th Dec 2006, 00:46
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Is this the noise that you're referring to?

http://www.flightlevel350.com/Aircra...ideo-5653.html
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Old 12th Dec 2006, 00:59
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It is often the nosewheel that starts shimmying (sp?) at rotate. I've always suspected that it can be caused by contacting a runway centreline light just as the nosewheel lifts off.
Regards
csd
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Old 12th Dec 2006, 06:56
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I experienced this as a pax on a rainy BA departure from DXB. Was accompanied by some amazing wing-flex. Not that they were related but naturally everyone was on the seat of their pants and 'omagodding'. Not helped when my little daughter shouted out "It's raining! yeah!!!!"
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Old 13th Dec 2006, 19:28
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Noticed that many times on the 744 and been wondering about it for many years. Short and fast vib immediately after lift-off, noticeable in the entire cabin. Finally Joetom and others have come up with an explanation that seems pretty plausible to me. Thanks!
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Old 13th Dec 2006, 23:01
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How and why are large aircraft tires balanced? I have yet to find a place that balances large aircraft tires. Why would you balance them, after the first landing the balance would be gone?
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Old 27th Dec 2006, 18:01
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There are a lot of explanations to this 'problem'... one of them being out of balance wheels / under inflation / new- old tyres. Happens even before gear retraction, esp on heavy weight t/o - higher rotation speeds.

Need to mention though, during the gear retraction phase, anti-skid and all are deactivated. Mostly the sound is from the nose gear and as no braking available there, there is a rub pad to do the job which further accentuates the problem.
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Old 28th Dec 2006, 06:27
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744 Vibration

Hello all. This topic has been covered several times on the prune before but I canít find it with the search engine so here goes.

On the 744 when the gear is selected up the gear doors open. When the gear doors are open a hydraulic sequence valve positioned by the right hand doors ports hydraulics to an actuator (de-spin actuator) which actuates the alternate brake metering valve in the r/h wheel well which is meant to stop the wheels spinning before the gear is swung into the wheel bay. The braking pressure on retraction is sufficient to stop the wheels turning within a revolution. I have seen this when trouble shooting the defect, white stripes were painted on the tyres and high speed film on take off.

Either the sequence valve or de-spin actuator wears ( I canít remember which) and insufficient pressure is supplied to stop the wheels turning before retraction which results in some fairly violent vibration being felt in the cabin. It has been happing since the first aircraft arrived and a ďmodĒ program fixed the issue in early days but the aircraft are getting older and thing must be getting worn again.

This is what causes the vibration, unnerving but no great drama

glhcarl How and why are large aircraft tires balanced? I have yet to find a place that balances large aircraft tires. Why would you balance them, after the first landing the balance would be gone?
Large wheels are balance statically you will see balance weights on the hubs.

In the perfect world, balance is maintained by wheels being stopped at a random point during gear up operation and so touch-down wear and tear is spread over many points of the tyre, so the wheels/tyres are kept in balance during the time fitted to the aircraft.
.
However random is random and tyres can get outer balance, plus if spin-down system is not perfect, wheels can tend to stop in a similar location after gear up operation, means touch-down wear and tear may often be the same place, this will get them well outer balance and can get heavy vibs until wheels are stopped during gear up.
When trouble shooting this problem a full set of brand new "balanced wheels were fitted and the result was still the same cabin vibration and someone mentioned worn wheel bearings. I have never changed a wheel assy without a fresh set of bearings being installed with it.

Hope this helps
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Old 28th Dec 2006, 09:14
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Talking

Seem to remember, if you want to stop the vibs after T/O, inspect all the main gear tyres, those that show uneven wear(low spots), replace and vibs will be better, replace the offending altn brake valve/s and problem should not come back.
.
The wheels loose the random effect when said altn brake valve/s do not operate in correct fashion, often providing a lower pressure than is reqd and so lets the wheel stop at a hi-spot on the brake unit.
.
Happy flying to all in 2007
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Old 29th Dec 2006, 05:07
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Devil

Originally Posted by Joetom
Seem to remember, if you want to stop the vibs after T/O, inspect all the main gear tyres, those that show uneven wear(low spots), replace and vibs will be better, replace the offending altn brake valve/s and problem should not come back.
.
The wheels loose the random effect when said altn brake valve/s do not operate in correct fashion, often providing a lower pressure than is reqd and so lets the wheel stop at a hi-spot on the brake unit.
.
Happy flying to all in 2007

most flat spots ,if all on one bogy are caused by low accumulator pressure in the wing and body gear wheel wells on 744
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