but I have seen a C30 hand out free samples of expensive engine bits and pieces flung far and wide
A good reason for not doing ground runs to find where the problem is when the trend check has taken a massive nose dive.
the S76 tail rotor drive is geared between the two engine inputs
Sir Korsky, most of the schematics I've seen would have you interpret that as being the case, because the orientation of the graphic is so poor. See below for a clearer view. You can see the tail rotor is only driven from the #1 engine side. As Sikorsky says
Each engine has a separate power train all the way up to the bull gear through a single spur and a single bevel mesh. The tail takeoff is from the left engine power train. In case of a left engine failure the tail takeoff still drives through the right engine to the bull gear back through the bevel set to the tail take off.
So its theoretically possible for a #1 engine drive failure into the MGB where #2 is what supplies main rotor power and you you have to keep #1 running to drive the tail rotor ie shutting down #1 will deprive you of tail rotor drive. A fact I've never seen pointed out in training or simulator.
Sikorsky Archives | S-76