Geoffers,
I will have to challenge your maths on Chinook gearboxes.
I will happily accept there are three gear
boxes that if failed will end in tears...those being the Forward, Aft, and Combining gear boxes.
The Nose Boxes....Engine Gear box to some....will cause a loss of engine power and depending upon which end of the drive shaft that connects the Nose Box to the Combining Transmission should fail....it could cause some interesting damage....but a Nose box failure in itself is not that critical.
I have experienced two Nose Box failures and am still here. Those failures were treated as a single engine failure. Both were benign events mechanically although one was a bit sporty due to the phase of flight we were in at the time.
I know of only one person who survived a main gear box failure and that was the survivor of the 234 Crash in the UK.
To put it into perspective....I know of no one that survived a spindle failure on 61's.