Originally Posted by
Kepi
The NTSB believed that since the propeller had damage from rotation, that the engine must have had power.
The extent of the damage to the prop can often give an idea whether it was under power or not. Rotation not under power generally doesn't have any force behind it, so it will typically be light damage, as far as the rotational aspect is concerned. There are other witness marks though, like inside the propeller hub. If there is a sudden stoppage at full power, the propeller hub can often be used to corroborate the theory, as there are very specific marks made in specific spots if there is sudden stoppage at high power, besides governors, fuel linkages, any engine logging equipment, and all the other things used to determine if it producing power. The NTSB also gets highly involved in computer modeling and simulation, they'll take the flight and re-create it from the beginning, same load, fuel, weather conditions, etc, taking off from the same spot, then they'll cut out the engine and see how far it will go, vs. continuing to the spot under power.
The NTSB uses the parties involved to conduct the investigation, that means the airframe manufacturer, the engine manufacturer, the company, etc. These entities want to know if they are to blame, because it cuts their litigation costs, rather than going to a lengthy trial and then having to pay out a bunch of money on top of everything else. Of course each one hopes they aren't to blame, but these companies always have credible, educated and accredited investigators that they send to the NTSB investigation, for one reason, to help identify the probable cause.
Of course, the NTSB are humans too and could have made an error, but they try to establish probable cause, not "beyond all doubt", because it's unfortunately not possible.