Legal or NOT?

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beg2differ

SEL, IFR with PAX is do-able in pistons and there are a handful of perators in the US doing so. It does however require equipment to be installed in the aircraft that is not normally accomplished. A list can be found in FAR 135.163(f). Additionally, an oil engine analysis (every 100/50 hours depending on Manufacturers recommendation) constitutes a trend monitoring program for the SINGLE PISTON engine.

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[Jet_Dreamer]

I have the understanding that if you fly pax in a single engine piston you must be able to descend under VFR conditions in the event of an engine failure. i am pretty sure this written somewhere in the 135 section of your FAR's.