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Ask the Expert
War Stories: Part Four
by Tom Clements
Iam continuing more of my personal King Air “war stories” for this month’s article.
Automatic Unfeathering
We all know how worthwhile the wonderful King Air autofeather system is, right? What you may not realize, however, is that numerous King Airs also have an automatic unfeathering system.
This first came to light for me when we began training on King Airs that had been upgraded with the first examples of the Raisbeck Engineering Inc.’s Quiet Turbofan four-blade propellers. These first appeared on 200s in the early 1980s. A customer of mine had recently replaced his standard three-blade props with the new four-blade ones. During his recurrent flight training session, we shut down one engine to practice single-engine approach and go-around procedures at a
safe altitude, more than 5,000 feet AGL. “That’s odd,” I thought, as I noticed that the feathered propeller never totally stopped rotation, even with the Condition Lever in Fuel Cutoff. I estimated that it was turning about 10-20 RPM.
Later, back on the ground, I phoned my friend and founder of Raisbeck Engineering, James Raisbeck, and asked him about what I had observed. “That’s normal,” was James’ response. “Our propeller blades have such a pronounced twist in them that to actually make them stop turning requires that the outer portion of the blade move past the position that has it exactly facing into the relative wind. In this position, more drag is created than if we simply allow minor rotation.”
“OK. That makes sense,” I thought. As I continued to come across these props that rotated in flight with a shutdown engine I discovered how little rotation is
required to create sufficient prop oil pressure to bring the propeller blades back out of the feathered position. Realize that the oil that operates the propeller is the same oil that lubricates the engine but is not at the same pressure as engine oil. The engine’s oil pressure pump must merely move the oil from the oil tank to the inlet to the propeller governor. There, the pump inside the prop governor picks up the oil and boosts it to a much higher pressure, approaching 400 psi.
A common misconception is that the engine oil pressure pump will not be rotating fast enough, with a shutdown engine in flight, to supply the oil up to the prop governor. Depending on altitude and airspeed, N1 or Ng will still turn between three and 10 percent speed in a shutdown engine with a feathered propeller. It will only have zero speed if the compressor has locked up for some weird reason. That three to 10 percent rotation speed is enough to supply engine oil to and from the governor.
Here is an important step to take when dealing with an engine failure
14 • KING AIR MAGAZINE
NOVEMBER 2018