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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! KA
King Air expert Tom Clements has been flying and instructing in King Airs for over 50 years and is the author of “The King Air Book” and “The King Air Book II.” He is a Gold Seal CFI and has over 23,000 total hours with more than 15,000 in King Airs. For information on ordering his books, contact Tom direct at twcaz@msn.com. Tom is actively mentoring the instructors at King Air Academy in Phoenix.
If you have a question you’d like Tom to answer, please send it to Editor Kim Blonigen at editor@blonigen.net.
MAY 2024
KING AIR MAGAZINE • 23