Page 18 - April 23

switch in the autofeather system. When the oil pressure drops, the prop feathers. But that’s not happening here, even with a new pressure switch. My thoughts were: Either the engine is way out of rig, the torque meter is off ... or both.
Sure enough, the torque meter was out of calibration. This prevented the 200 lb-ft switch from activating, so the prop would not feather. This was a brain teaser, I admit. The autofeather system in King Airs is fairly simple and it works great; but to troubleshoot it (or to help your shop troubleshoot it), you really need to know how it works. Accordingly, I’m repeating part of my article, “Autofeather Review” from 2018.
Autofeather Test
On takeoff, the autofeather system arms when the power level reaches 92% N1 or higher. But to test this on the ground in your preflight run up, you’d have to stand on your brakes and hope your fillings don’t pop out while pulling each power lever back one at a time. Fortunately, the “Test” position of your autofeather switch eliminates this problem. In Autofeather Test, the power lever switches in the pedestal (the ones set at approximately 92% N1) are
bypassed, which enables you to test autofeather function at a much lower power value. Note: The Test position of the autofeather switch is spring-loaded so that you can’t mistakenly leave it in that mode.
“The autofeather system in King Airs is fairly simple and it works great; but to troubleshoot it ... you really need to know how it works.”
16 • KING AIR MAGAZINE
APRIL 2023