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Q&A: Replacing a worn starter ring gear, and the proper use of carburetor heat

Q&A: Replacing a worn starter ring gear, and the proper use of carburetor heat

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January 2015-

Q: Hi Steve,
I am the proud owner of a 1967 PA-28-235 and I like to do real good preflight inspections, but I sometimes run into something that puzzles me.
At my last preflight I discovered a tooth missing on what looks to me like a flywheel. I think the teeth are engaged by the starter to start the engine. As far as I can tell, the missing tooth hasn't caused any starting problems. It starts just like it always has.
Will the missing tooth cause a problem? What do I have to do to fix it?

—Eagle-Eyed Ed

 

A: Dear Ed,
Taking the time before each flight to perform a detailed preflight is one of the most important risk-reducing tasks entrusted to a pilot, and many owner's manuals and all POHs include a detailed preflight checklist in a walkaround format. Printing a copy and referring to it during the actual walkaround helps.
The broken tooth you found is one of 149 teeth on the part that's listed as a "gear, starter ring 12/14 pitch" in the Lycoming parts manual. (Mechanics just call it a ring gear.) The part number is 72566. The same gear is used on a wide range of Lycoming engines including -290, -320, -360, -435, -540, and -541 series engines.
Lycoming Service Instruction (SI) 1141A dated May 9, 2014 outlines the procedures for removing the old ring gear and installing a new one. (This recent instruction supersedes SI 1141 dated Oct. 20, 1967. —Ed.)
The first step it to inspect the holes in the aluminum support assembly to see if they are worn or out of round. If the support assembly is airworthy, the ring gear is removed by grinding. (Do not grind into the support assembly.) With only a thin ring of steel holding the ring gear, simply insert a chisel into the gap and tap it to break the gear loose.
Heat the new ring gear to approximately 450 degrees in an oven or with a torch. The heated gear will have expanded so much that it will be easy to drop it into position on the support assembly. Make sure that the chamfered edge of the teeth is on the aft side (generator or alternator pulley side) of the support assembly.
Inspect to make sure it's securely seated against the flange of the support assembly—there's no indexing needed. As the ring gear cools, it will shrink to provide a tight fit.

Happy flying.

Q: Hi Steve,
The latest discussion around the home 'drome coffee shop revolves around the use—or misuse—of carburetor heat.
Like most of the gang, when I first started flying I was taught that carb heat is an all-or-nothing affair. Either put it fully on, or leave it fully off. As I've gained experience, and flown with more experienced pilots and instructors, the "all-or-nothing" rule has become muddied.
One of the locals insists that airplanes with Continental engines are very prone to induction (carburetor) icing, therefore pilots in Continental powered airplanes have to be extra careful to watch for loss of rpm when flying an airplane with a fixed pitch prop—or loss of manifold pressure (MP) when flying an airplane with a constant speed prop.
The same guy says that Lycoming engines are rarely subject to carburetor ice since the carburetor is bolted onto the oil sump which heats the carburetor throat, thus preventing icing.
Finally, a new guy has shown up and he says it's okay to fly around with partial carburetor heat. He claims that increasing the temperature of the air flowing through the carburetor causes better atomization of the fuel. This, he claims, results in a more even distribution of the fuel/air mixture to the cylinders.
Would you like to weigh in on this?

—Het-Up Hal
A: Dear Hal,
I'd be happy to. The statement about carburetor icing in Continental versus Lycoming engines is semi-correct, and for exactly the reasons you cited. But all pilots need to memorize this fact: any carburetor-equipped engine—of any make—will, under the right conditions, form carburetor ice.
Factors such as vaporization of fuel and the expansion of air as it passes through the carburetor throat (i.e., the Venturi effect) can cause the inlet air to drop as much as 70 degrees F. This is why carburetor icing occurs when outside air temperature (OAT) is well above freezing.
Since Continental engine carburetors aren't warmed by heat from the engine oil sump, carb ice occurs in Continentals at a higher ambient temperature than it does in Lycoming engines. Warmer air can hold more moisture than cooler air, and the potential for severe carburetor icing is greater in Continental engines.
If carburetor ice is suspected, full carburetor heat should be applied. If there's no ice present, the warm, less dense air ducted into the carburetor air box by the carburetor heat system will cause a slight loss of power. It's normal to hear and feel a change in the engine.
FAA certification rules mandate that carburetor heat systems must be sufficient to produce a temperature change of 90 degrees F. According to one engine expert, full application of carb heat may reduce power by as much as 13 percent.
If ice is present in the carburetor throat, the application of carb heat will result in engine roughness and a loss of rpm or MP before power is restored. It's a little disconcerting to feel the engine getting rougher when the carb heat is applied, but that's normal.
The application of partial carburetor heat during cruise flight is mentioned and approved by Lycoming Service Instruction 1148C, provided the aircraft has a carburetor temperature gauge. The Service Instruction specifies that aircraft with a carburetor temperature gauge may be used to keep the mixture temperature above the freezing point of water (32 degrees F).
A number of companies including Electronics International, JP Instruments, Mid-Continent Instruments and Avionics, UMA Instruments and Westach manufacture carburetor temperature gauges. Many of these products are available through Chief Aircraft and Aircraft Spruce as well as directly from the manufacturers.
Applying partial heat in accordance with the Lycoming SI eliminates carburetor throat ice. It also has two other effects. The first effect of using carb heat in cruise is slightly less power. Secondly, the warmer air causes the fuel metered by the carburetor to atomize more completely. This creates a fuel/air mixture that flows better and is more evenly distributed to the individual cylinders.
You can test the "better distribution with heat" effect if your airplane has a carburetor temperature gauge and EGT probes installed in each cylinder. In cruise (below 75 percent power if Lycoming-powered; below 65 percent power if Continental-powered) and after leaning for cruise, wait five minutes for the engine to stabilize, then slowly start applying carburetor heat.
Add enough heat to get a reading of 50 degrees F on the carburetor temperature gauge. You will need to lean the mixture slightly due to the less dense air (same amount of fuel), but you should see the EGT spread lessen as the fuel/air mixture is more evenly distributed. Theoretically, the engine should run smoother. It's up to you whether you adopt this practice.
Don't forget that whenever carburetor heat is applied, the engine inlet filter is bypassed and any airborne dirt will enter the engine.

Happy flying.

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

Steve Ells has been an A&P/IA for 43 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 the proud owner of a 1960 Piper Comanche. He lives in Paso Robles, Calif. with his wife Audrey. Send questions and comments to .

RESOURCES >>>>>
Ring gear replacement
"Replacement of Worn Starter Ring Gears"
Lycoming Service Instruction 1141A
PiperFlyer.org/Lyc1141A

Carburetor heat
"Use of Carburetor Heat Control"
Lycoming Service Instruction 1148C
PiperFlyer.org/Lyc1148C

Carburetor temperature gauges
Electronics International Inc.
buy-ei.com

JP Instruments
jpinstruments.com

Mid-Continent Instrument Co. Inc.
mcico.com

UMA Inc.
umainstruments.com

Westberg Manufacturing Inc. (Westach)
westach.com

 

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