I don't fly PT6 but have had some peripheral involvement with them so the following has more of an engineering than pilot slant.
1. Flat rating. Normal engine characteristic is for output to increase as OAT decreases at constant altitude. On many engines the mechanicals become the limiting factor (typically gearboxes, combustor pressures, or whatever) and the machine output has to be restricted at OAT below that at which the mechanical limitation becomes critical. Alternatively, the OEM may chose to limit the engine to a lower than maximum output for a variety of reasons. In this lower OAT region, the engine is said to be flat rated as the power or thrust output is limited to provide the relevant maximum which the mechanicals can tolerate or the OEM choses to adopt.
Operationally this means that the RTOW fall off with increasing OAT for a given runway is modest until the engine is no longer flat rated and then falls off much more rapidly with increasing OAT.
Derated certification. If an engine is capable of a certain output with whatever level of progressive deterioration in service, then it is reasonable to presume that the mechanicals and the hot end will last longer (ie cheaper maintenance) if we hold the output below the maximum. There are two ways to approach this
(a) flexible thrust settings (normally used on jets) where the engine is operated in a manner which uses a lower than maximum thrust output .. but the limitations are based on the certificated maximum thrust capability. For US rules, the maximum reduction is 25 percent.
(b) derate thrust where the engine is limited by the electronics or mechanical settings to be able to produce no more than a specific percentage of the maximum capability. This has significance cost implications and may have some performance advantages in certain circumstances. Many of the modern jet engines are certificated to several thrust rating levels, selectable by the flightcrew via FMS or similar wizardry.
Derate and flex can be used in conjunction. This may well result in considerably lower thrust output than the 25 percent flex limit for the maximum thrust certification.
2. Cranking power probably is the most significant factor. Others include wind (especially if from astern).
3. Cranking loads during turbine start are very high .. typically something in excess of 1000 amps. The generator is not much use for these sorts of current drain figures. The secret is to use ground power for starts to save the ship's battery
4 Failure of autofeather is very serious on a direct turboprop (typically the older engines) as the windmilling prop has to drive a whole bunch of turbomachinery through the gearbox. Much less of a problem on a free turbine but still of interest.
5 SFAR41, now defunct but still relevant to older SFAR41 aircraft, upped the ante of FAR23 a bit towards FAR25 to facilitate the development of early commuter aircraft. There is a useful FAA memo on the subject which you might like to
read.
6 Two significant problems with Avtur is that the fuel's colour often masks water and water may stay in suspension for considerable time. As a result it is usual to employ chemical means to detect the presence of water and there are several processes in common use.
7 I will dig out some charts for a couple of dash variants and come back with some numbers ..
8 Better for rated pilots to address this one for the particular engine.