A smooth run-down could be fuel leak, fuel control, fuel starvation or possibly ice ingestion, flying in very heavy precipitation, prolonged negative 'g'. None of which could be considered 'severe damage'. Whether one should attempt a relight in such a situation is a moot point. Personally I don't like the idea of relighting engines because they are not supposed to run down, but if a worse emergency could develop from not attempting a relight, why not have a go? Or if you did something silly like run a tank dry, of course you would relight and hope no-one noticed.
Severe damage suggests something broken, blades, bearings, near total loss of oil or perhaps an over-temperature due to mis-handling or fuel control problem. Bird strike will do it, and FOD is another possible culprit if on the ground - though the PT-6 and similar designs are fairly resistant to birds and FOD. Lightning strikes have been known to cause considerable damage to PT-6 engines in particular.
Indications of severe damage could include vibration, high TGT/ITT/TIT/EGT, possibly very high oil temperature, possibly the smell of oil or smoke through the air conditioning, a fire warning indication. Slow rotation of the power turbine accompanied by high temperatures is very bad news and of course no rotation of any section that is supposed to be rotating means it is seized and very, very dead. Or you are flying too slowly and about to become very, very dead.
Don't fall into the trap of mis-diagnosing low oil pressure as a failure in itself when it is often simply a result of the engine having already run down. Turbines will run for some time on hardly any oil pressure, provided that some oil is still in the system.
Obviously a relight must NOT be attempted if any of these indications are present.
Last edited by Mach E Avelli; 23rd Oct 2013 at 10:51.