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 wears ( 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 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.
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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