Originally Posted by
Someone Somewhere
N2 Overspeed (and various equivalents) are designed to prevent a rotor burst by shutting the engine down if a particular rotor overspeeds. The Trent 900 on QF32 didn't have overspeed protection on its N2 rotor because it was thought that it was impossible for that rotor to overspeed without also overspeeding N3, which was protected. I expect that's fixed now.
Rolls "TOS" system (Turbine Over Speed) is rather interesting - it checks for speed differences between the 'front' (compressor) and 'aft' (turbine) part of the shaft, and will shut the engine down if they differ (by more than some small tolerance). One issue with the 3 spool architecture is that bearing lubrication becomes tricky, and if the lube fails, the shaft can shear which can allow the turbine to accelerate rapidly to burst speed - faster than the normal fuel metering valve can react. This was first manifested on the RB211 - where an N1 shaft failure released the fan on an L1011 center engine - which then proceeded to try to cut the aircraft in half (fortunately it failed and they were able to land safely). This lead to the adoption of a 'fan catcher' - which worked to keep the fan pretty much in place, but then the unloaded turbine oversped and burst (happened at least once on a 747, fortunately without serious aircraft damage). This in turn lead to an N1 "TOS" - which could shutdown the engine (via the fuel control shutoff valve) in milliseconds. TOS was added to the N3 shaft when Rolls introduced the Trent series of engines, and then eventually to the N2 shaft after the QF32 event.
I don't recall now which shaft, but Rolls did an unplanned test of the TOS system on a Trent 1000 TEN engine when a shaft failed during an engineering test
(it apparently worked as advertised).
BTW, GE uses a different design philosophy for a shaft failure - the turbine shaft is designed to move aft if the shaft fails, causing a clash with the fixed vanes in the turbine which slows the turbine preventing an overspeed (and the turbine case is designed to contain the resultant debris). There has been the odd shaft failure on GE engines over the years and none have resulted in a burst turbine - so apparently it works (granted, the CF6 series has had some burst turbines, but those were not the result of overspeed events).
No firsthand knowledge, but I believe the Pratt philosophy is similar to GE.
Originally Posted by
Someone Somewhere
I'm pretty sure you're BugBear under a different name, so I suggest you re-read and re-interpret this post again:
That would explain a lot...