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on the overspeed governor when the torque drops to 200 lb-ft. It’s oil pressure from engine torque that keeps prop blades in flat pitch. When the oil pressure dumps, the prop feathers. Autofeather function is wired on the same side ... in other words, the left engine controls the feathering of the left prop and vice versa. Only the arming of each autofeather system is wired to the opposite side. There’s a good reason for this and I’ll explain that shortly.
Flickering Lights and Oscillating Blades
I am often asked about autofeather lights blinking during test. When a prop goes into feather, particularly while on the ground, it increases the engine torque. In the example above where you just feathered the left prop, if the torque pushes far enough above 200 lb-ft, the left autofeather light will come back on and the left prop will flatten out. Flat pitch reduces torque. If it falls back down to 200 lb-ft, the left annunciator light will go out again and the left prop will feather again.
This flicker of the annunciator lights and oscillating of the prop blade is not unusual during autofeather testing on the ground. But it’s equally normal for that annunciator light to go out and stay out. There are lots of variables (engine rigging, N1 settings, pressure switch adjustments) that influence whether or not you’ll have
a blinking annunciator during autofeather test. Either way you are good to go. Now let’s bring the left engine back to speed and do the same with the right engine.
Autofeather Test – Last Step
Once you have brought each engine, one at a time, down to 200 lb-ft and feathered each prop, there’s one more check to do. Assuming that you feathered the left prop first and now you have your right prop in feather, pull back the left power lever and make sure the left prop does not go into feather. Also make sure that the righthand prop comes out of feather. This is a crucial test. It ensures all switches and wiring are operating properly. If, in this last test step, the left prop feathered along with the right, you have a problem. Or, if the right prop doesn’t come out of feather you have another problem. Either way, your autofeather system needs attention.
Test Versus Arm
You are now ready for takeoff, put the autofeather switch in the “Arm” position and take the runway. You go through 500, 600, 700 lb-ft of torque but the autofeather lights don’t come on. Why? Because you have not yet gone through 92% N1. When you pass that N1 threshold, your autofeather annunciators should come on. Both sides are armed and ready to go.
Cessna Piston Twins
18 • KING AIR MAGAZINE
APRIL 2023