Originally Posted by
HeliboyDreamer
Is this only for that type of aircraft or does it apply generally to all types?
Heliboy, this procedure applies to pretty much all single-engine helicopters with so-called "free turbine" engines - in other words, helicopters that don't have a clutch between the engine and transmission.
When the engine turns, it drives the transmission through a freewheeling unit. Like the hub on the back wheel of a bicycle, this device "decouples" the engine (pedals) and allows the transmission (back wheel) to keep turning if the engine stops. So on the ground when the engine is off, you can turn the blades in their normal direction and nothing happens. But if you turn the blades "backward," then the sprag clutch in the freewheeling unit engages and turns the engine backward too. IF you feel resistance when turning the engine, then it's "Houston, we have a problem."
It works this way in the Bell 206, 407, MD/Hughes 500's, and the French Astars/EC120/130, etc. Sometimes, if carbon has formed in the bearings or seals within the engine, it will resist turning - backwards *or* forwards. There is usually a checklist item that specifies that the rotor must be turning by such-and-such engine RPM. If it doesn't, it could mean that the interior bits of the engine are suffering from this "coking" problem we've been discussing.
It's all basic stuff to those of us who've been in the business for a while, but I'm sure it confounds newbies as it did me when I was first learning about these crazy machines, which was back when helicopters were still powered by steam engines - which, by the way is where we get the expression "steam gauges" from to describe mechanical, analog gauges. But you didn't ask about that.