One of the biggest precipitators of helicopter gearbox failures is the use of the same oil that is used to lubricate the gearboxes and bearings of gas turbine engines. The US Army wanted to standardize on the lubrication oils used in transmissions and engines and decided that it would cut down on operational and supply costs to go with a single oil and that oil was that used in the engines.
The problem was that the film strength relative to high point contact loading of gear meshes was at the ragged end of the spectrum when used in helicopter transmissions. These gearboxes were originally designed to operate using mineral-based oils as the lubricant and when the oil specs were changed they started to have problems. Later design transmissions were designed to use engine oils (synthetic) as the primary lubricant with a winter and summer grade the same as used in the engines. Even though the gearboxes were designed to use the synthetic oil, the problem of high point contact loading was not solved. And, under certain conditions the film strength will deteriorate and the gears and bearings will run metal-to-metal and that is when the wear and deterioration and subsequent failure will occur.
Once again I reiterate Spectrometric Oil Analysis will detect the impending breakdown of the oil as well as to detect the incipient wear of the lubricated parts.