Questions and Answers – Engine Overspeeds

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August 2012

Q: Dear Steve,

I have been flying my 1974 Cessna 182 a lot lately as I work toward getting my instrument rating. My other flying buddies have questioned the wisdom of using my airplane for my training—they tell me that I would save money if I rented a less complex airplane like the flight school’s 172—but I want to train in the airplane I’ll be flying in actual IFR conditions. So far, I think it’s been the right decision.

However (and there’s always a “however”) during one of my instrument approaches yesterday, my instructor suddenly told me to go around. In my haste to comply (I was under the hood), I quickly applied power. After we landed and reviewed the lesson, my instructor told me that I had jammed the throttle forward so fast that the engine RPM had gone past the tachometer redline.

His point was that I should never rush my actions when flying IFR, but I’m worried about my engine. Do you think I damaged it?

—Redline Buster

 

 A: Dear Buster,

First off, I want to commend you for pursuing your instrument rating. I believe the instrument rating is the graduate school of flying.

Now, back to your engine. Engine overspeeds are more common that most pilots realize. The most important thing pilots can do to eliminate overspeeds is to commit to making smooth power changes. Never cram the throttle forward, or jerk it back to the idle stop; idle to full throttle should take a minimum of three seconds. Count ‘em out…“one, one thousand; two, one thousand; three, one thousand.” Here’s why.

The RPM of the O-470R engine on your 182—an engine equipped with a constant speed propeller—is controlled by the prop governor that is working to balance the interplay between opposing forces.

Forces in the propeller that drive the blade angle to a flat pitch (high RPM) position in the hub during engine operation are (1) a low pitch spring in the propeller hub; and (2) the centrifugal twisting moment (CTM), a force generated during propeller rotation. The spring and CTM apply pressures that push a piston in the prop hub forward.

The forces that drive the blades to coarse pitch (low RPM) are (1) aerodynamic twisting force (think of lift generated by an airfoil); and (2) oil pressure from a small high-pressure oil pump inside the propeller governor. When governor oil pressure is directed to the forward side of the piston in the prop hub, the force is sufficient to drive the piston aft in the hub.

Your prop governor is mounted on a mounting pad located at the forward end of the right engine case half and is driven at crankshaft RPM (1.0:1 gear ratio).

The governor senses engine speed and responds to RPM changes by either directing oil under pressure to the hydraulic cylinder, or by releasing oil pressure from the cylinder. Changes in volume of the cylinder (by movement of the piston) changes blade angles. The amount of oil and direction of oil flow are controlled by the movement of a pilot valve in the governor.

The pilot valve is also positioned by opposing forces. A spring—called the speeder spring—pushes down on the pilot valve. Each time you adjust the prop control you are adjusting speeder spring tension. A set of flyweights in the governor rotate at crankshaft RPM to provide the forces that oppose the speeder spring down push. The position of the flyweights responds to changes in engine RPM. During stable cruise flight the opposing forces are balanced; therefore, the pilot valve moves only very slightly as it sends oil to, or vents oil from, the piston.

When the throttle is crammed forward to increase power, or pulled rapidly aft to decrease power, the pilot valve, the flyweights that control the pilot valve, the oil flow to the piston, the piston movement, and the blade angle changes aren’t able to change rapidly enough to strictly govern; therefore, an overspeed takes place.

Often pilots don’t really realize an overspeed has occurred because analog (moving needle) tachometers aren’t very accurate. These gauges were once accurate, but over the years the increase of friction in the needle movement and a lessening of the magnetic connection between the needle and the drive plate reduce accuracy.

Digital tachometers are very accurate. Horizon Instruments’ stand-alone P-1000 unit has an overspeed warning light. Full-featured engine monitors by JP Instruments, Electronics International, Flightline Systems and Insight Avionics all track and record engine RPMs, and can be programmed for RPM overspeed alerts. Engine monitoring sections of modern flat panel systems can also warn of and track the extent and duration of engine overspeeds.

HOW MUCH IS TOO MUCH?

The Teledyne Continental Motors Service Bulletin (SB) that spells out overspeed limitations is SB05-2. This SB was last updated in January 2005 and superseded M89-15.

The redline RPM for your O-470R is 2,600. The SB breaks overspeeds down into three categories. Each category (I, II and III) spells out the overspeed limits and the maintenance action required in each category.

Category I and Category II are further broken down by duration of overspeed. For instance, no maintenance is required in Category I if the time exceeding 2,900 RPM is less than 10 seconds. If over 10 seconds, maintenance requirements include draining the oil, removing and inspecting all oil screens and filters, and removing all valve covers and inspecting all “top end” components in accordance with the latest TCM service publication. In addition, an inspection for damage in the engine’s accessory drive section is conducted by checking for excessive backlash in the drives.

If no damage is found during this inspection, the bulletin requires the oil, screens and filters part of the inspection to be done again after five flight hours.

If an overspeed on an O-470R exceeds 3,200 RPM, Category II kicks in unless the overspeed is less than 10 seconds. If the overspeed exceeds 10 seconds, the likelihood of internal damage increases and all the cylinders need to be removed to check for damage.

It’s very unlikely that the momentary overspeed caused by your abrupt and rapid throttle “jamming” caused enough of an overspeed to require any maintenance action. But if you want to be extra safe, drain and screen the oil for contaminants, and remove and inspect the screen or filter. Based on your description of the event, you should be okay to continue flying.


Happy flying,

Steve

 

Know your FAR/AIM and check with your mechanic before starting any work.

 


Steve Ells has been an A&P/IA for 38 years and is a commercial pilot with Instrument and Multi-Engine ratings. Ells also loves utility and bush-style airplanes and operations. He’s a former tech rep and editor for Cessna Pilots Association and served as Associate Editor for AOPA Pilot until 2008. Ells is the owner of Ells Aviation (EllsAviation.com) and lives in Paso Robles, Calif. with his wife Audrey. Send questions and comments to .

 

 

Resources

Electronics International

buy-ei.com

 

Flightline Systems

ultra-fei.com

 

Horizon Instruments

horizoninstruments.com

 

Insight Instrument Corp.

insightavionics.com

 

JP Instruments

jpinstruments.com

 

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