> It's a minor difference though as there would be no way to detect thata single grounding screw had failed and then the failure of the second screw would cause the same problem.
Actually, having two grounding screws would have avoided this problem - the flap switch would never have been connected to the ground via the feathering mechanism (unless both screws got loose and somehow entangled). In that case, either the flap mechanism would have failed, which would have been obvious, or the second terminal of the feathering switch would cease to be grounded, which could lead to sudden feathering with sufficient EMI. While still not great, a dual ground-screw setup would have been much better.
Two circuits sharing a common ground path causes electronic noobs no end of confusing grief. Because the two circuits are coupled via the ground inductance and resistance.
And a circuit that forms with it's ground a large loop is another source of confusing grief. A big loop is an inductor. And will pickup magnetic energy passing through it.
The diagram in the article described the alternate design. Both wires connected to the same terminal block. It was redundant ground screws for the terminal block that was still shared between the two systems.
Actually, having two grounding screws would have avoided this problem - the flap switch would never have been connected to the ground via the feathering mechanism (unless both screws got loose and somehow entangled). In that case, either the flap mechanism would have failed, which would have been obvious, or the second terminal of the feathering switch would cease to be grounded, which could lead to sudden feathering with sufficient EMI. While still not great, a dual ground-screw setup would have been much better.