Originally Posted by
BTC8183
Is not the 189 back up oil supply,to the inputs only?.The rest of it has no back up.
Exactly.
Since the most critical areas of the gearbox are the two high speed inputs, a clever "passive" method to continue to feed oil in case of loss of lubrication has been designed, that does not need pilot's activation, sensors, pumps, external coolants or whatever.
Basically, the working main lubrication system keeps a small reservoir always filled, the reservoir drains very very slowly, thus guaranteeing a minimum oil drip that keeps input lubricated and cooled in case of loss of oil pressure because of pump failures or loss of oil.
The rest of the gearbox has to run dry.