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 (PT6) 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 one-word 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 oil-to-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.
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 high-pressure 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 rotation creates the oil pressure that flattens the blades. As the blades flatten, the lesser air bite means less rotational resistance so the propeller speed rises until normal idle conditions are met with the propeller now on its Low Pitch Stop (LPS).
As long as the propeller is rotating then the pump inside the PPG keeps supplying the necessary oil to the dome to prevent the blades from feathering. To demonstrate this, I often have a new King Air pilot not pull the propeller levers into feather after we pull the condition levers into fuel cutoff as we shut down. Usually it is well over one minute before the propeller finally stops turning. At this point we can observe that the blade angle is quite large, in the order of 45 degrees, half-way to feather. As we watch, we can actually see the blade angle slowly becoming larger as the feathering springs force the remaining oil out of the dome and back into the engine’s nose case. Often I will then ask the pilot to pull only one propeller lever all the way back into the feather position. When this is done – opening the passage in the PPG to allow oil to return freely into the nose case – the blade angle moves rapidly the rest of the way and the blades stop moving when they reach the metal-to-metal stop at feather. It takes in the order of two seconds for this to happen. The other side may take another five minutes or more to leak into the fully feathered position.
Try it yourself. Pick a deadhead leg and make sure the ramp is empty of nearby people when you shutdown. Leave the propeller levers alone and watch what happens. It takes a
l-o-n-g time for the propeller to stop, eh? In fact, I have done this facing into a strong Kansas “breeze” and the propeller never stopped rotating! There was sufficient windmill effect to keep the not-yet-feathered propeller turning indefinitely.
Do you see why I stated the reason for feathering is “safety”? The lineperson waiting to install your nose chocks, a curious bystander or the poorly briefed passenger rushing to get to the meeting … there is a lot more chance of someone getting hurt by a rotating propeller than by one that has stopped. When we make the propeller blade angle go to its maximum bite position immediately at shutdown – yielding the maximum amount of rotational resistance – it lessens the dangerous rotating time immensely.
Can you think of a situation in which feathering manually at shutdown is not a good idea? Yes! You are correct: When parked on a very slippery, icy ramp, the thrust that the propellers provide as the blade angle suddenly increases can cause the airplane to slide forward with no control whatsoever. It’s best here to let them coast to a gentle stop on their own.
By the way, do any of you feather first and pull the condition levers second? Believe it or not that was how the checklist procedure was written for many years in the early days of King Airs. Back then, the Environmental Protection Agency (EPA) was not yet in existence and there were no restrictions on turbine airplanes dumping a little fuel out at shutdown. As fuel pressure decreased, a dump valve would return to its spring-loaded open position and allow residual fuel to dump onto the ramp instead of into the hot combustion chamber liner – “burner can” – where it caused smoking and coking problems. (“Coking” refers to leaving deposits of carbon in the fuel nozzles.)
In King Airs, this fuel dumped out of the oil breather tube that terminated just behind the oil cooler … and directly in front of the main tire(s). Prop wash tended to blow this kerosene back onto the tires, leading to their decreased service life. Around 1974 was when King Airs began being manufactured and retrofitted with fuel drain collector systems – usually referred to as “EPA kits” – to prevent fuel from dumping onto the ramp at shutdown. Shortly thereafter checklists were revised to feather after the fuel is cut off.
There is certainly no harm done by doing it the old way. In fact, feathering while taxiing at idle is a great way to keep the airplane quiet and avoid brake usage, especially when on a long, straight taxiway with a strong tail wind. However, there is a very definite momentary increase in thrust as the blades move through the big bite position on their way to feather. You can feel the acceleration for a moment while rolling down the taxiway.
Likewise, if we feather before – or too soon after – we have cut off the fuel at shutdown, there is enough airflow through the engine that again thrust increases. We won’t feel it as acceleration – unless the brakes aren’t set – but the nose strut will do a noticeable compression bounce. I have found that pulling the condition levers and then waiting for the propeller speed to hit 600 RPM before feathering is both very smooth and yet gets the props stopped in a reasonable, safe timeframe.
Before I wrap this up, I want to emphasize the fact that the propeller feathering itself at shutdown is purely a ground, not flight, phenomenon. Remember when I said the propeller never feathered in the strong Kansas wind? Well, imagine the strength of the relative wind when flying. Unless you are doing slow flight while fuel is cut off in flight, the propeller doesn’t even slow down! In fact, do you know why 140 KIAS is specified as the minimum speed for a windmilling airstart? It’s because that is the airspeed at which maximum propeller rotational speed can be achieved while the prop is being driven by windmilling force only, with no fuel, no exhaust gases driving the power turbine.
Some alert pilots have asked me this question: “How does the oil keep getting supplied to the PPG and its pump? If the engine is shut down and the No. 1 shaft, compressor shaft, is not rotating, then the engine’s oil pump is also not turning. So how does oil get to the prop governor?”
Excellent question. The answer is “Because the N1 shaft usually does not stop rotating.” Unless some bearing jammed and indeed caused the compressor and all of its accessories to not be turning, then eventually the oil to the governor would no longer be supplied and feathering would have to occur. But in a more normal situation of a shutdown due to fuel starvation, the ram air through the engine keeps N1 turning. In my experience, the windmilling N1 in flight, with a windmilling propeller, varies between five and 15%, based upon altitude and airspeed. That is plenty for the engine’s oil supply and scavenge pumps to circulate the oil to and from the governor.
Get it? Got it? Good!