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scruggs
11th Dec 2006, 16:31
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.

BOAC
11th Dec 2006, 17:43
No explanation as I do not fly them, but a common experience for me as a 747 pax.

Carnage Matey!
11th Dec 2006, 18:07
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.:)

BOAC
11th Dec 2006, 18:50
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.

Intruder
11th Dec 2006, 19:36
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.

scruggs
11th Dec 2006, 19:38
Hey guys, thanks very much indeed for your replies.

Much appreciated!

eP

Rainboe
11th Dec 2006, 20:06
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!

BOAC
11th Dec 2006, 20:11
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!:)

Rick Storm
11th Dec 2006, 20:55
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.

Joetom
11th Dec 2006, 21:13
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.

Rainboe
11th Dec 2006, 22:08
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!

TotalBeginner
12th Dec 2006, 01:46
Is this the noise that you're referring to?

http://www.flightlevel350.com/Aircraft_Boeing_747-400-Airline_Virgin_Atlantic_Airways_Aviation_Video-5653.html

csd
12th Dec 2006, 01:59
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

Superpilot
12th Dec 2006, 07:56
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!!!!" :rolleyes:

EDDNHopper
13th Dec 2006, 20:28
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!

glhcarl
14th Dec 2006, 00:01
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?

robin747
27th Dec 2006, 19:01
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.

Bolty McBolt
28th Dec 2006, 07:27
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 :ok:

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

Joetom
28th Dec 2006, 10:14
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

chemical alli
29th Dec 2006, 06:07
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

Mode7
29th Dec 2006, 12:53
so that debris is not thrown around inside the undercarriage bay.
Erm.... not quite. := Each wheel on the 747 weighs 160kgs. So 16 wheels weighing 160kgs doing 180knots is a serious gyroscopic force - 2.56 tonnes of gyro (half that actually as it is only the wing gear that retracts sideways, body and nose retract forward along the direction of gyro rotation) - which makes it extremely difficult to move the legs, so stopping the rotation removes the force and the legs move with 'ease'....:D applying the brakes prevents a very slow deceleration of rotation speed with added vibration as they will all slow at different speeds.
Certain MEL brake deficiencies require the gear to be left down for up to 5 minutes after takeoff to allow the wheels to stop rotating prior to retraction.

scruggs
29th Dec 2006, 13:10
Thanks again for all the replies :ok:

Piper19
29th Dec 2006, 16:25
I have some doubts about some explanations here. First, it are the wheels that are balanced, not the wheel/tyre assembly. Because the two wheel halves are not perfect symmetrical you have to balance that. But tyres wear, and some inbalance can occur. We've had that many times on nosewheels, when the left is worn to limits and the right is perfect, you get shimmy. But only in the case of unmatched wear. And mechanics will normaly change nosewheeltyres that are not matching. So if someone says here that you feel it every takeoff, I doubt it had to do with tyre wear. Second, the antiskid will activate the brakes before gear retraction, but this is to PREVENT high stress and vibrations. so I cannot see why they CAUSE vibrations.
I also don't believe that it can be worn wheel bearings, as bearings get changed together with wheel change, and this is quite often.
My guess the vibrations are just due to changing aerodynamic flow around the wing/body/empennage during rotation, just as extending flaps/gear/airbrakes cause more turbulent flight.

Piper19
29th Dec 2006, 18:04
I just imagined this...similar to the gyroscopic effects of rotating wheels, wouldn't a take off with APU running cause a lot of gyroscopic stress/vibrations. Take an APU running at a few thousand revolutions a minute, at the very end of the aircraft, making a swing of a few meters during rotation.

Rainboe
29th Dec 2006, 18:46
Why do you doubt the explanations? I think the vibration talked about here is before automatic braking and gear retraction. I think it is worst when the wheels are unloaded off the ground and rotating very fast in unloaded bearings. Whilst the noise can be alarming when seated near to the undercarriage legs, with the general noise of the engines, just a few rows away you cannot hear it above the rest of the noise, and that explains the unconcern from the crew away from the wheels. If you run pushchair wheels very fast, then lift them off the ground, they can suddenly vibrate quite badly when unloaded from the ground. that is what is happening to the aeroplane wheels.

As for gyroscopic forces on the APU/engines, not a factor. They are designed to take it. Indeed, these forces were looked on as one possible cause of the Comet accidents in the 1950s and shown to be irrelevant. Aeroplanes are designed to manoeuvre with the engines at high power and the APU running. The actual swing has no effect on gyroscopic forces anyway, it is the angular change and rate that counts, not sideways or vertical movement.

With flap out, you can see marked vibration and shaking on the flaps, but at take-off, lower settings are used than on landing, and the shaking is in excess of the shaking on mid-flap settings. Again, not a factor I believe.

Piper19
29th Dec 2006, 19:23
I still doubt it, but I can see the logic in it. (it's good to doubt even if there's no reason too, I've learned not to take everything for granted). Maybe if I encounter this in real it will be obvious.
But then, why is the vibration only on the 744? More wheels more vibrations?

Rainboe
29th Dec 2006, 20:04
It's there on all jets I've flown.

Mode7
Originally Posted by Rainboe View Post
'so that debris is not thrown around inside the undercarriage bay.'
Erm.... not quite. Each wheel on the 747 weighs 160kgs. So 16 wheels weighing 160kgs doing 180knots is a serious gyroscopic force - 2.56 tonnes of gyro (half that actually as it is only the wing gear that retracts sideways, body and nose retract forward along the direction of gyro rotation)

If you disagree with my statement, perhaps you could explain why the 747 body gear (which retracts in a fore/aft plane) also automatically brakes during the retraction sequence? Your last sentence doesn't follow your initial line. You only have to look inside the nose bay to see how much debris gets thrown around.

chemical alli
30th Dec 2006, 00:10
this thread has been done to death bolty mcbolts answer is correct when it comes to how despin braking works as for wheel and tyre balancing(they all wear and yes could be a small factor)worn bearings i say blah blah blah what a crock. so lets just finish it by saying yes there is vibration but nothing to be alarmed about