At Heli-Expo at least one company is showing the robustness of the MGBs they make after an oil loss
zalt,
Thanks for the link. That particular AH-64 MRGB lube system situation was a bit different than what happened with the Cougar S-92 MRGB, but it does provide a perfect example of how a gearbox can achieve a loss-of-lube operational capability.
Most MRGB's have high lube circuit flow rates because they use lube oil for cooling. To keep weight down, the MRGB lube system oil volume is kept to a minimum, and thus the turn-over rates tend to be quite high. I don't know the specifics behind the S-92 MRGB, but if I had to guess, I'd say its lube oil circuit turn-over rate is probably around 15 or 20 seconds at most. And if there's a serious leak in an external hose, filter housing, heat exchanger, etc. that contains 100% of the system flow rate, all of the MRGB oil volume can be discharged very quickly.
While the noted AH-64 MRGB has a qualified loss-of-lube capability, what really helped it survive was that the oil leak situation was probably not completely catastrophic in nature, and possibly some small amount of oil remained circulating. Even a fine mist of oil sprayed on the gears and bearings is enough to reduce friction and limit thermal failure of these parts.
Supplementary total loss back-up lube circuits are a very effective way to provide an MRGB loss-of-lube operational capability. But such systems tend to be complex, expensive, and add weight, so they are not widely used. If there is a push to qualify MRGB's with loss-of-lube capabilities beyond 30 minutes, they may become more common.
riff_raff