Originally Posted by
SASless
You miss the point....using Glycol is not the same as a genuine Run Dry capability just as assuming there shall always be some trapped Oil in the Gearbox (such as in the S-92 original assumptions). The goal would be to have a gearbox that can be run at Cruise Power for 30 Minutes with no oil, glycol, or fluid of any kind. The question I would pose is what does the Crew do if there is no landing site within that 30 Minute Range and the Sea State exceeds the Certification Limit of the Emergency Floatation System particularly in very cold air and water temperatures and rough seas? Why have a 30 minute run dry time if a safe landing (Ditching) can be made and SAR resources are available to retrieve the Crew and Passengers in a timely manner? Are we perhaps chasing the wrong Horse?
These are all valid points. And there is currently a big effort underway to improve loss of lube capability of rotorcraft gearboxes.
First, let's consider what the real problem is when an MRGB operates under loss of lube conditions. The primary functions of the lube oil are to reduce friction and provide cooling of the bearings and gears. What causes failure of gears/bearings with loss of lube is the combination of increased friction heat generation and a greatly diminished cooling capability. If the gear/bearing materials are heated much beyond their tempering temperature, they begin to lose strength. The reason for a loss of lube back-up system using glycol injection rather than oil is because glycol provides better cooling. There are also new gear materials (like C64) that can operate at very high temperatures (~900degF). This will significantly improve loss of lube capability as these materials become more widely used in new designs.
Regulatory agencies such as the FAA have also recognized that their certification requirements need improvement. I think the loss of lube qual test procedure will become far more rigorous, including increasing the time requirement beyond 30 minutes.
Lastly, one change I would like to see the FAA implement is for every certified gearbox design in production to undergo a formal design review every few years using the most current analysis tools/techniques available. This analysis technology improves at a very rapid pace, and this proactive approach could likely reveal many potential problems that were missed in the original design analysis.