Page 10 - June Volume 10 Volume 6

The Overspeed Governor
and the Test Solenoid
Just before takeoff, or at least before the first flight of the day, your checklist includes testing the overspeed governors. You know that switch on the sub-panel? When you hold it up, the test solenoid opens a port that dumps some oil back into the engine case, and this holds the prop rpm to 150 below takeoff rpm. If the solenoid sticks in the open position after you release the switch, the prop stays at the reduced rpm level. It gets your attention when you’re about to take off.
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 at 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.
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 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 person to recommend a cavalier approach in the cockpit. But if you know the capabilities and limitations of your aircraft well, you can gently bend a rule here or there to find a safe way out of a less-than-optimal situation. That’s a big “if” and I trust you readers are getting my intention here.
Skipping the Test
When the B200 got to my shop, the solenoid was still stuck open. I cured it with a whack of a mallet and
8 • KING AIR MAGAZINE
A torque gauge on a King Air E90 showing the limits in red.
suggested he skip the overspeed governor test from now on. All it does is test the solenoid. It doesn’t, in my opinion, test the overspeed governor itself. Some may disagree with me on this, but there are many seasoned King Air pilots with thousands of King Air hours who agree wholeheartedly with omitting the OSG test before takeoff.
The same thing happened to the pilot of an E90 down in Alabama. He was picking up the aircraft following a Phase Inspection. The shop had just finished checking all the systems (pressurization, auto-feather, auto-ignition, overspeed governors, etc.), so the pilot wasn’t expecting anything to be amiss. But as he was about to take off, he noticed one prop rpm lagging below the other.
He had my number in his cell phone because I’d done the pre-buy for him two years before, so he gave me a call right then and there. He described the problem and I knew right away that the OSG solenoid was stuck open.
by Dean Benedict
JUNE 2016
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