T
he 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.
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 person to recommend a cavalier approach in the cockpit. However, 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 my customer brought his B200 over to my shop, that 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, directly test the function of 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, autofeather, 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 cellphone and 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 the E90 pilot the same advice to 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 those solenoids sticking open. We would execute 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 frequently and it was maddening.
In response, I formed the habit of releasing the OSG 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 afterward.
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. Five years ago the price at Beech (Textron) was close to $7,000.
Malletization, or hitting something with a soft-blow hammer, is a time-honored solution to lots of mechanical hangups. Years ago, the term “malletization” was an inside joke among savvy mechanics. It was a sophisticated way to say they fixed it with a couple whacks of a mallet. I was amused to find “malletization” 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 sure beats (pun intended) the heck out of the exorbitant cost of replacement.
The E90 Conundrum
Years ago, I ran into a puzzling situation with the owner of an E90 who complained that his OSGs had never been tested in the time he owned the airplane. It’s important to know that E90s are the only King Air model with two torque limits marked on the torque gauges: 2200 RPM (TQ value of 1315) and 1900 (TQ value 1520).
As soon as this E90 showed up at my shop, I went straight to those pesky OSG test solenoids. I verified they both had power, I removed them for bench check and they both passed with flying colors. That wasn’t what I expected. My lead mechanic was convinced the OSGs were bad, but I was doubtful. It was highly unlikely that both would fail. Then I had my “Aha” moment – this E90 had 4-blade Raisbeck props!
The takeoff RPM on an E90 with 3-blade props is 2200, so the OSG setting should be 150 RPM below that (1950-2050 RPM is typical). But the takeoff RPM for 4-blade props is 1900, and the corresponding OSG setting should be around 1750 RPM. I checked the prop governors first and found them set correctly for takeoff RPM at 1900. But the overspeed governors had not been adjusted. Bingo! They were still set at 2050 to align with the 3-blade takeoff RPM of 2200. Those OSGs would never test at that setting. We adjusted them properly to 1750 RPM, aligning with a takeoff RPM of 1900. Finally, the OSGs tested properly and everything worked as advertised.
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!