Page 23 - February 2022

In Conclusion
If skipping that OSG test altogether sounds a tad radical to you, let me tell you where I’m coming from: The overspeed governor is a backup to the prop governor; in the 45-plus years I’ve been working with King Airs I have yet to hear of a prop governor failure. I’m not saying it has never happened, I’m just saying I’ve never run across it or heard it from anyone else. Prop governors on King Airs have proven to be extremely reliable.
In the unlikely event that one should fail, you have the overspeed governor to keep the propeller in check. It does this via a complex series of counterweights that open up the oil channel going back into the engine case. Actual OSG function cannot be effectively tested on the ground. Therefore the OSG test solenoid simulates oil reduction to the prop by opening a valve that allows oil to drain into the channel. When you perform the test, and see the prop RPM drop by 150 RPM, the OSG is responding to oil reduction triggered by the solenoid.
In summary, the OSG check is testing a secondary system that backs up an extremely reliable primary system. The test solenoid involved, although greatly improved from 25 years ago, is still subject to failure. When it stays open, you can’t make full power for takeoff. Worse yet, it leads you to believe you have a bad OSG. Sometimes a stuck solenoid can be coaxed to close with a few deft strokes of a soft-blow hammer.
There’s one more point I want to make: Let’s say you perform an OSG test and get a failure, i.e., you pull the switch but the prop RPM doesn’t stop and goes all the way to takeoff RPM. So you have that OSG removed, you pay $3,500 for an exchange unit (with a $15,000 core deposit). After your core unit is evaluated, you get a $7,000 bill-back on your core for a bad solenoid. Your OSG was fine but the solenoid failed to actuate! So, again, this OSG test speaks more about the test solenoid than the OSG itself. For this reason, many seasoned King Air pilots skip the OSG test altogether.
I realize that some pilots are not comfortable skipping the OSG test on the Before Takeoff checklist. If you want to test your OSGs, by all means do so. You are the pilot. Just consider completing a double-check with your power levers after you release the test solenoid switch ... and maybe stash a mallet somewhere ... just in case! KA
Dean Benedict is a certified A&P, AI with nearly 45 years’ experience in King Air maintenance. He’s the founder and former owner of Honest Air Inc., a “King Air maintenance boutique” (with some Dukes and Barons on the side). Now, with BeechMedic LLC, Dean consults with King Air owners and operators on all things King Air related: maintenance, troubleshooting, pre-buys, etc. He can be reached at dr.dean@ beechmedic.com or 702-773-1800.
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FEBRUARY 2022
KING AIR MAGAZINE • 21