Page 27 - Volume 13 Number 6

Ask the Expert
Why Feather at Shutdown?
by Tom Clements
It’s going to do it anyway, right? Since the feathering springs and blade counterweights are always trying to move the propeller blades to high pitch – and the extreme of high pitch is the feathered position – and propeller oil pressure is what prevents the springs and counterweights from succeeding in their job, then as the engine stops turning and hence oil pressure is lost, the blades must feather. Right? So why does the checklist tell us to feather them ourselves?
There is a one­word answer to this question: Safety. Let me explain.
The engine’s oil system serves four functions. First, as in all engines, it provides vital lubrication and cooling for all of the engine’s moving parts. Second, it is used in measuring the torque being supplied to the output (propeller) shaft so that the amount of torque may be displayed in the cockpit. Third, it is used to heat the fuel, to decrease the likelihood of liquid water that may be suspended in the fuel from turning into ice crystals that could clog filters and passages. Lastly, it is used to make propeller blade angle changes. The oil that is used for all four of these functions is the same oil, most commonly the “2380” turbo oil variety that has had many different names over the years: Esso 2380, Exxon 2380, BP (British Petroleum) 2380, and now Eastman BP 2380. As the oil is pumped from and scavenged back to the integral oil tank, a molecule that was in the oil­to­ fuel heat exchanger a moment ago may now be in the propeller dome and one that was in the propeller may now be spraying onto a bearing.
However, just because this is all the same oil does not mean that it is at the same pressure. The single oil pump that sends the oil to everything except the propeller has a discharge pressure – depending on the exact PT6 model – of between 60 and 135 psig (pounds per square inch gauge). The oil inside the propeller dome or hub is at a much higher pressure to be capable of overcoming the springs and counterweights. The engine’s oil pump supplies oil from the tank to the Primary Propeller Governor (PPG). This device is mounted on a pad at the front of the engine, on top at the 12 o’clock position. In addition to the speeder spring, flyweights and oil passages it also contains a pump that takes the incoming oil and increases its pressure up to about 400 psig. The position of the governor’s pilot valve determines the exact pressure inside of the dome.
JUNE 2019
Since the PPG is mounted on a drive pad at the front of the engine, it is rather obvious that it is geared into the propeller shaft (whose speed is represented by the symbol Np) instead of into the compressor or gas generator shaft, N1 or Ng.
You’ve heard that a PT6 may be started while the propeller is restrained from turning, right? It’s true. If a rope – or a gutsy person – is preventing propeller rotation, then the pump inside the PPG is also not turning so the propeller dome receives no high­pressure oil. The blades remain feathered. Only when the propeller is allowed to start turning is the oil pressure created that permits the blades to flatten their angle or “bite.” Watch carefully when a PT6 is started normally. This is especially obvious when you stand looking parallel to the propeller disk. You will notice that rotation begins while the blade angle is still in the highest pitch, feathered position. Then its
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