While a properly qualified transmission 30-minute loss of lube capability is a good idea, it doesn't completely eliminate lube system related failures in the transmission. Here's a fatal crash that occurred in 1982 with a CH-47.
The crash of Boeing's CH-47C Chinook 74-22292.
The final catastrophic structural failure was preceded by a sequence of failure events, which were initiated by a loss of lube oil flow to one single point in the input nose gearbox module (an oil jet clogged by debris). As I understand it, the rest of the lube system was still functioning right up until the structural failure. Just a few moments prior to the crash the crew received some sort of warning indication and were attempting to land. I'm not sure of the failed gearbox's maintenance history, but it is quite possible the aircraft had been operating for some time with this condition (ie. longer than 30 minutes), and it didn't manifest itself until the drivetrain was loaded sufficiently.
The point of the story is that there is no direct method to monitor for this type of failure in transmissions. There are only indirect detection methods such as chip detectors or local thermocouples. Having said that, a clogged oil jet is no longer likely to occur because all modern gearbox designs typically use "last chance" filter screens on every oil jet.
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