We've seen enough problems with this during the power plant failure item on type ratings and/or FAR 61.58 checkrides that we thought it might be advantageous to discuss it here. This would seem to be a better place than during the oral or the flight test. I certainly don't mean to say here that we don't see problem areas during the feathering portion, perish the thought. But this is also a vital bit of propeller knowledge, and maybe we can separate it from the other part long enough here to shed a little bit light on it.
First of all, on the propellers we're discussing, the feather button is a simple push button. When pushed into the depressed (feather) position, it's held there by an electrical holding coil. Some of the later ones, i.e., postwar and later, had reverse and all those goodies. On these, the feather button became a push in - pull out type. That complicated life a bunch and is outside the area of this discussion! Let's just confine this to the 23E50 propellers, commonly used in
the airplanes we are directing this discussion towards.
Now then, to unfeather the prop, all that you should really have to do is push the feather button and hold it down! Obviously, it's going to want to "pop" up all by itself because the oil pressure that built up in the dome to feather it also released the holding coil, it's up to you to hold it down for the unfeathering process to occur. Hold it down until reaching 800 RPM, by that time the prop has reached a blade angle where it'll finish unfeathering by itself. If the tachometer malfunctions (which, by the way, happens fairly often) you'll have to
substitute your own judgment for the 800 RPM. A good way (that we used to teach) to do this is to release it when you can no longer detect the individual propeller blades.
As soon as the propeller has unfeathered, you need to immediately check for engine oil pressure. Also, don’t immediately place the mixture into the RICH position, wait long enough to check that the tachometer rises to and then stabilizes at the same RPM you experienced when checking the propeller for minimum RPM governing during your pre-takeoff run-up. This is usually in the vicinity of 1100-1200 RPM. Also remember that one reason you checked it on run-up is that this is the only solution you'll have available if the feathering pump fails during
flight when you need to feather after an engine failure. After it stabilizes you can move the mixture to RICH and begin the warming-up process. If you disregard this important RPM check you just might be putting a mixture to a potential runaway since the prop governor may not have assumed control.
Another thing here,
One last thing here, how about the situation of your releasing the button - but the contacts weld (stick) or something causes the feather motor to continue running. In this case the propeller blades would be driven by the feather pump to the low pitch stops, another way of saying high RPM or – can you say "an overspeed"? In this case, what can you do to immediately stop the feather motor? You can simply (and immediately) place both generators to OFF and all battery switches to OFF. This
eliminates all electrical power and, voi-la and eureka, since the feathering pump is electrically powered, it stops!. Then, in this case, I believe I'd then utilize one battery switch only to "bump" the feather motor to feather (stop) the propeller, easier to control that way, I believe. But if you ever do this, remember that you cannot subsequently restore any source of electrical power! If you do, the feather motor will again start to operate. You’re stuck with this situation for the duration! Hope you’ve kept adequate track of your position on a map – since you are now without radios.
R. Sohn © 1999
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