Comparing SET to MEP:
A PT6 may have a MTBF (including prop, fuel, etc.) that is 10 times higher than that of a well maintained piston engine, which would result in 20 times better engine reliability in a SET than in a MEP. It is very difficult to get good data, so this is just a very rough guess looking at accident data.
So for initial climb, which is critical in most cases in a MEP, you are safer in a SET, especially if pilot skills are not excellent and climb out is difficult. However, for long flights over water, mountains or remote areas or in bad weather, especially at night, a MEP will offer much more safety if handled by a professional pilot and by the book. Most turbocharged MEPs will provide sufficient single engine performance after initial climb, at most places.
If MTBF on a piston is 1 per 10,000 hours, the probability to lose the second engine when within 1 hour of an alternate in average is 1:5,000 (OEI) * 1:10,000 (second engine out within one hour). This, of course, requires to save the good engine.
A Seneca III will keep you OEI at 14,000 ft on an ISA day with full tanks on departure and 4 pax. A Cessna 421C with the same loading will maintain 20,000 ft on one engine. At MTOM, which is full tanks and 500 kg of crew + pax on the C421C I fly, the ISA single engine service ceiling is 15,000 ft. In most cases, 70 % MCP will get you a good single engine cruise above all you want to clear.
I would not frequently do flights in a SET in conditions where an emergency landing would probably be lethal for me or my passengers. Evacuating a child or older persons after ditching and getting them in a raft would be a nightmare even if it is the Mediterranean Sea in summer. I feel SET - either new or used - are way overpriced compared to MET (e.g. a new King Air or a used Conquest) or MEP.
Reliability on many piston engines is largely affected by maintenance and pilots. Turbines are seeing professional handling and pilots in most cases. The engines are not guilty if the owner doesn't care and the pilot doesn't either.
Last edited by AndiKunzi; 8th Feb 2017 at 07:33.