Page 20 - February 2022

MAINTENANCE TIP
The Overspeed
Governor and the
Test Solenoid 2.0
by Dean Benedict
The Before Takeoff checklist includes a test of the overspeed governors. You know that switch on the sub- panel? When you hold it up, it sends a current to a test solenoid which opens a valve and dumps some of the oil going to the prop into a channel going to the engine case. The reduction of oil to the propellor holds the prop RPM down 150 RPM below takeoff RPM. You release the switch, the valve closes, the oil flow returns to normal and your prop RPM should go back up to takeoff level.
Many pilots, once they see the drop in RPM, release the switch and pull the power lever back and move on with the checklist. BUT, if the solenoid sticks in the open position after the switch is released, that prop won’t reach takeoff RPM. That gets your attention when you’re rolling out!
A customer of mine with a B200 was heading home at the end of a weekend trip. He started to roll on takeoff but noticed the prop RPM on one side was hanging up about 150 RPM shy of takeoff requirements. The aircraft started to yaw because the torque on that side was now disproportionately high. He aborted the takeoff and gave me a call.
18 • KING AIR MAGAZINE
As soon as he outlined the scenario, I immediately suspected the solenoid on the overspeed governor (OSG). No maintenance personnel were available, so I had him flick the test switch several times to see if it would release the solenoid, but it wouldn’t budge. We decided that if he pulled the other prop back to match the lower prop RPM, he would be able to take off safely and get home so I could have a look at it. His location that day was not much above sea level so I knew he would get enough horsepower for takeoff. Good thing he wasn’t in Telluride.
Of course, I have to insert a caveat for safety here, because I’m a “by the book” kind of guy. I’m the last
An overspeed governor on a King Air B200, with the solenoid pointed out (at left).
FEBRUARY 2022