B747-400 Heavy Vibration After Take-Off
Join Date: Apr 2005
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WHBM,
Noted.
But if wheels/tyres are designed for random stop point to even out wear during operation/landing, stopping at same/similar point will lead to tyres being replaced early due to uneven wear.
This item costs lot of money and time, replace those altn anti skid vlvs...
Cheers...
Noted.
But if wheels/tyres are designed for random stop point to even out wear during operation/landing, stopping at same/similar point will lead to tyres being replaced early due to uneven wear.
This item costs lot of money and time, replace those altn anti skid vlvs...
Cheers...
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Hello all. This topic has been covered several times on the prune before but I can’t 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 is worn ( I can’t 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 “mod” program fixed the issue in early days but the aircraft are getting older and thing must be getting worn again.
This is the cause for the vibration on the 744, unnerving but no great drama
Hope this helps
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 is worn ( I can’t 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 “mod” program fixed the issue in early days but the aircraft are getting older and thing must be getting worn again.
This is the cause for the vibration on the 744, unnerving but no great drama
Hope this helps
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As Bolty says, all very good and well understood.
So aircraft will be ok with the vibs problem.
1, Vibs noticed by wheels not stopped spinning at correct time durin u/c retraction.
2. Vibs noticed even more as wheels will be way out of balance untill they stop spinning.
But if accountant likes changing wheels instead of changing anti-skid vlvs, he will keep changing wheels, he could poss even get a bonus payment for changing so many wheels ??? who knows....
So aircraft will be ok with the vibs problem.
1, Vibs noticed by wheels not stopped spinning at correct time durin u/c retraction.
2. Vibs noticed even more as wheels will be way out of balance untill they stop spinning.
But if accountant likes changing wheels instead of changing anti-skid vlvs, he will keep changing wheels, he could poss even get a bonus payment for changing so many wheels ??? who knows....
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The autobrake system applies the brakes on gear up selection on the mainwheels,the nosewheels are braked by a primitive arrangement of friction pads on leaf springs which contact the tires when they are retracted into the wheelwell.I suspect it is the nosewheel brakes which cause the vibration as it is usually more intense in the forward cabin.A 747 49x19 wheel assembly is an extremely heavy item - probably over 250lb,as I know it takes 3 people to get it into the back of a pickup! the amount of energy dissipated in stopping it rotating from Vr speeds must be immense.If you have a look inside the nosewheel bay you will see the black rubber dust ground off the tire during the spin braking.
After 20 years and 13k hours on the -400 I noticed that the after take-off autobrake system failed to operate on many occasions. This was evidenced by the vibration mentioned being felt even on the flight deck. My solution was a short application of the toe brakes which stopped it (and therefore the wheel rotation) within a couple of seconds. I always wrote the snag up on arrival at the next station for rectification. I don't know if this was an approved solution but it worked for me - if left to spin down unaided the ever diminishing vibration could last for a couple of minutes or so and caused some passengers alarm. If it was the nose wheel snubbers that were out of alignment then there was nothing to be done and you just had to sit and suffer until the wheels spun down.
To my amazement very few other pilots remarked on or even noticed the problem, which brings me to my bête-noir, those who stick so closely to the centre-line that they hit every centre-line light, on a 50 second take-off run this equates to hundreds of bangs, shakes and rattles. A foot or so either side and all is peaceful and quiet!
To my amazement very few other pilots remarked on or even noticed the problem, which brings me to my bête-noir, those who stick so closely to the centre-line that they hit every centre-line light, on a 50 second take-off run this equates to hundreds of bangs, shakes and rattles. A foot or so either side and all is peaceful and quiet!
This subject was covered in October last year in Spectator's Balcony in a thread titled "A380 Vibration". Various posters mentioned the same TO experience in 747, DC10/MD11 and A340 aircraft. The common feature is all these aircraft have one or two main landing gears mounted in the fuselage thus any vibration from a centre or body gear is transmitted straight in to the cabin floor and is fairly apparent to occupants around the main gear area. My own experience was it was nearly always out of balance tyre/wheel units often caused by uneven wear or flat spotting and was most apparent as the weight came off the wheels and the out of balance "rattled" the unloaded gear until the spindown brake system stopped the wheel rotation.
