Maintenance Tip: The Overspeed Governor and the Test Solenoid

Maintenance Tip: The Overspeed Governor and the Test Solenoid

A torque gauge on a King Air E90 showing the limits in red.
A torque gauge on a King Air E90 showing the limits in red.

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 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. I told him to taxi back to the shop, tell them he had spoken to me about the problem, and have them smack the OSG solenoid with a mallet. That did the trick.

I gave that pilot the same advice – quit testing the overspeed governor because you’re just asking for a stuck solenoid.

The Power Lever Double-Check

Back in the ‘70s and ‘80s, we had a chronic problem with solenoids sticking open. We would do the final ground runs on a King Air and everything would be fine, but when the owner went to leave, one prop wouldn’t come up to takeoff rpm. It happened a lot and it was maddening.

In response, I formed the habit of releasing the test switch and running the power levers back up through the test zone. This way, if the solenoid stuck open after the test, I could catch it on my ground run and fix it before the owner picked up his aircraft. To this day, if I touch that OSG test switch, I do a double-check with the power levers afterwards.

In the old days, those solenoids failed so frequently that we kept them in stock. I don’t know if it was a vendor problem or a change in design, but the tendency for stuck solenoids on the overspeed governors seemed to lessen over time. It still happens, however, and always at an inopportune time. Being able to identify and fix the problem is very handy.

Malletization Beats
Beaucoup Bucks

So why didn’t I replace the solenoid in that B200, or tell the E90 pilot to have his replaced?

Once you look up the price you’ll see why. They have become absurdly expensive. Currently, the price at Beech (Textron) is a hair below $7,000.

Malletization, or hitting something with a soft-blow hammer, is a
time-honored solution to lots of problems. Customers don’t want to hear that their expensive aircraft was whacked with a hammer, hence the term “malletization” was born. What was once an inside joke among mechanics, is now listed in the “Urban Dictionary” on the internet. Solenoids, valves, certain switches, etc., respond beautifully to malletization, provided you know what you’re doing, and it beats the heck out of replacement pricing (pun intended).

A pen pointing to the test solenoid of a King Air 200, with the overspeed governor directly to its right.
A pen pointing to the test solenoid of a King Air 200, with the overspeed governor directly to its right.

The E90 Conundrum

You E90 drivers have two torque limits on your torque gauges – one for 2,200 rpm (TQ value of 1315) and one for 1900 rpm (TQ value 1520). I ran into a puzzling situation with the owner of an E90 with Raisbeck four-blade props who complained that his OSGs had never tested in the time he owned the airplane. Of course, the first place I went was the solenoids. I verified that both had power, I removed them for bench check and they both passed with flying colors … hmmm.

I was beginning to suspect bad OSGs when I had an “Aha!” moment. The takeoff rpm on an E90 with three-blade props is 2200, so the normal setting for the OSG is about 150 rpm below that (1950-2050 rpm is typical). But on an E90 such as this one, with four-blade props, the takeoff rpm is 1900, so the corresponding OSG setting would need to be around 1750 rpm. I found the prop governors on this E90 were correctly set for takeoff rpm at 1900, but the OSGs were still set at 2050 to align with a takeoff rpm of 2200. Bingo! The OSGs will never test at that setting. Once both OSGs were properly adjusted to 1750 rpm, everything worked as advertised and the OSGs tested every time.

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. And in the 40-plus years I’ve been working on King Airs, I have yet to hear of one with a prop governor failure. I’m not saying they have never failed on a King Air; I’m just saying I’ve never experienced it.

Therefore, based on my experience, the OSG test is an unreliable and/or misleading test of a secondary system which is backing up a very reliable primary system.

The other point I want to make is this: You could perform the overspeed governor 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 the overspeed governor removed, you pay $3,500 for an exchange unit with a $15,000 core deposit (Beech/Textron current pricing), and then 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 is more about the test solenoid than the OSG itself. This is exactly why many seasoned King Air pilots skip the OSG test altogether.

Yes, the OSG test does check the function of the OSG, but it also tests the test solenoid. Unfortunately, that pesky solenoid is far more likely to malfunction than the OSG, leading you to think you have an OSG problem when you don’t.

If you want to test your OSGs, by all means do so. But consider doing a double-check with your power levers immediately following; and maybe stash a mallet somewhere … just in case.

About the Author

Leave a Reply