Beacons/anticollision lights and engines

Emily writes:

Next thing I know, he'll probably be arguing that the reference I posted is
incorrect.

Not incorrect (I haven't verified it, but a copy and paste is easy
enough to do), but it says nothing about engines. My question was
whether regulations required anticollision lights or beacons to be lit
whenever engines are operating. I know it seems to be a standard
policy, but I don't know if it is mandatory, especially for GA
aircraft.

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