I was told by my previous company's chief engineer that starter/generators for the F1 /F2 should be switched off for engine start because of a possible weakness in the genny driveshafts. There may be a recent mod; I'm not sure as I haven't flown them for a while.
When in starter mode, the drive quill is stressed in one direction (driving engine). When in generator mode, it is stressed in the opposite sense (being driven by engine).
Going immediately from one to the other can overload the genny driveshaft quill in torsion and cause a failure. If the generator is not selected on for start, it gives the drive quill chance to unload when the starter mode drops off line, before being driven under electrical load by the engine. Obviously, the drive stays engaged with the genny off but isn't stressed as highly.
The 355N can be started with gennies on because it has a stronger drive system (Arrius engines). That model has very pilot friendly engines, because of its DECUs. Booster pump on, switch on engine, watch it accelerate the rotor to fly all by itself. Start second engine. No need to move ECLs at all, they stay in FLY position. Much rated by police pilots as it enables a very rapid departure and a good OEI performance to boot! They don't leak oil either.
A possible "gotcha!" with the 'N' is that the collective MUST be locked down for start, for obvious reasons...so that the single engine hover performance isn't explored inadvertently..Yes, it has been done!
[ 10 November 2001: Message edited by: ShyTorque ]