The evidence did not state that there is greater proportional risk from gearboxes in a multi engined helicopter.
Jarvis:
During the course of his evidence, I asked Mr Jarvis if he thought there was something inherently more dangerous in helicopters compared to other aircraft. He replied as follows[48]:
"It's a function of the design of helicopters, my Lord, in that rotory wing aircraft, helicopters, essentially have a critical load path through the gearbox to the rotor. Fixed wing aircraft can have - fixed wing aircraft fly by means of structure which can be made inherently fail-safe. Helicopters fly by means of a mechanism which is a mechanical unit and is thereby potentially more prone to failure and that really is the issue between fixed wing and rotary wing aircraft, my Lord. One flies by means of a mechanism; the other flies by means of its structure and that gives you different levels of achievability in terms of safety and redundancy."
(my underline, gotta love the statement: helicopters fly by means of a mechanism. (anyway helicopters use wings in tension whereas aeroplanes use wings in sheer.))
So:
Section 6(1)(d) - The defects, if any, in any system of working which contributed to the deaths or the accident resulting in the deaths
On the evidence, no such defects exist.
... but then they didn't consider that where a highly complex gearbox is required there is an increased inherent risk.
That's how many gearbox fatals in twins now? (Less engine fatals more 'other systems' fatals instead - obvious no?)
Blackadder: The proportion of mechanical failures is too high!
Baldrick: Let's increase the pilot-error rate then !