November 1, 2002
STEVEN W. ELLS
Your airplane has been ground-bound for eight weeks because it's getting an engine-ectomy. Bolting a new, rebuilt, or overhauled engine onto an airplane's engine mounts is just a part of readying that "new" engine for flight. If any of the 101 small details that are part of a complete engine installation are overlooked or neglected, or if the engine is not operated correctly during the first few minutes, running a new engine can quickly go from a "flying time" to a "crying time" experience.
A well-executed engine installation starts the minute the cowling is off, prior to the removal of the old engine. This is the best time to start planning for the comprehensive refurbishment of the engine accessories and airframe systems that make up a complete engine installation. One trick many shops use is to buy a disposable camera for each engine change. Shops have saved hours by spending 20 or 30 minutes capturing the details of the existing engine installation prior to the start of disassembly.
A pre-removal inspection includes checking items such as the cowl flap and hinges, the engine mount (for rust, cracks, and corrosion), cowl supports and cowling attachment hardware, cooling baffles and cooling baffle seals, flexible hoses (for age as well as condition), exhaust system, turbocharger and wastegate, and induction filter and induction ducting sheetmetal.
Teledyne Continental Motors (TCM) and Lycoming factory engines will include a new or rebuilt starter, new or rebuilt magnetos, new ignition harness (wires) and spark plugs, a new or rebuilt carburetor or fuel-injection system (including an engine-driven fuel pump), and exhaust gaskets. These accessories are considered part of the engine. If you buy a field-overhauled engine, these accessories may or may not be included as part of the purchase price (see " Waypoints: Overhaul Options," October Pilot).
Accessories like the propeller, propeller governor (if equipped), alternator (or generator), and exhaust system are rarely included in the overhaul price. The decision to rebuild these accessories is usually determined on a unit-by-unit basis. If a propeller upgrade was purchased new and installed three years ago, then the decision to send the prop in for overhaul will depend on factors such as time in service, condition, and operating environment.
Digging into the records (logbooks) for accessory replacements can help owners make accessory overhaul decisions. It's also a good idea to prepare a spreadsheet so that your mechanic has an easy reference for how much time (calendar and hours) is on each accessory.
Of all the accessories that contribute to an airworthy installation, the exhaust system (see " Airframe and Powerplant: Exhausted, and Often Forgotten," March 2001 Pilot) is the system that's most likely to be pulled off, given a cursory look, and then shoved under the workbench until it's time for reinstallation. The best policy is to send the complete exhaust system to an exhaust system repair shop — just like all the other accessories.
The many pieces of specially shaped and cut aluminum that form the cylinder cooling baffles, and the flexible, rubberized material that nestles against the cowling to create the engine cooling system are vital for good engine health. Most engines come with intercylinder baffles that direct the cooling air to the cylinder cooling fins. These baffles are considered engine parts — all other baffle parts are airframe parts. Every piece, no matter how small, is vital. If the pieces are worn, cracked, or missing, buy new ones or repair any that are marginal yet worth salvaging.
The cooling requirements of common light-airplane engines are specified in cubic feet per minute and by the pressure differential (measured in inches of water) between the air before it's been forced past the cylinders and the air after it's flowed past the cylinders. There's rarely a problem with the volume of air because designers traditionally have opted for huge cooling air inlets in the cowling. The critical number is the pressure differential.
Tests have shown that a hole as small as one square inch in a sheet metal baffle will result in the loss of 20 percent of the differential needed for satisfactory cooling. It can't be said often enough — worn, cracked baffles and old, stiff baffle seals are key contributors to short engine life.
Engines must be pre-oiled prior to the first start. Pre-oiling is not a subject that's open for debate. Teledyne Continental's document on engine break-in ( www.tcmlink.com/carenfeed/brkin.pdf) recommends pumping heated oil through the engine before the first start.
Lycoming Service Instruction 1241 specifies the following method. Leave out the bottom spark plugs, fill the oil tank with non-compounded (mineral) oil, and use the starter to spin the propeller. This bulletin says to limit continuous starter operation to 15 to 20 seconds, then wait for five minutes and give it another 20 seconds. At some point the oil pressure needle on the cockpit gauge will come off the peg and start to climb. When there is oil pressure in the cockpit, the filter is full of oil, indicating that a layer of oil has been pumped into the main bearings. To refine Lycoming's starter-driven version of pre-oiling, heat the oil before it's added to the engine oil sump — the oil will flow more completely throughout the engine.
Almost all engine builders run their engines on a test stand for at least an hour before the engine is shipped; these runs do not break in an engine. This process is termed a run-in or standard acceptance test and it's conducted so that the builder can be assured that the engine is ready to ship.
The first ground run is done with the cowling off. The first start consists of a low rpm operational check. Start the engine and run it at 1,200 rpm until the oil pressure starts to come off the peg — then increase the rpm to 1,800 to relieve camshaft lobe pressures. Limit the first run to three minutes. Don't touch the mixture and don't touch the prop lever. Move the magneto switch to the left and right positions (momentarily in each position). Testing the left and right magnetos should cause a small rpm drop-off; if the rpm drops like a rock when the switch is in either the left or right position, let the engine die, and then fix the discrepancy before the engine is restarted. Turning the magneto back on could damage the engine because of the presence of excess fuel in the cylinders when the spark plugs fire.
Near the end of the three-minute time limit, pull the rpm back to idle and move the magneto switch very quickly to off, then right back on — the engine should sag, then come back on. Be quick on the off-position test. This test ensures that the magnetos are properly grounded. If a thorough visual inspection shows that there are no leaks and all systems operate normally, it's time to take the airplane for a flight.
Try to plan the first flight during the cool part of the day, and try to use the longest runway on the airport. Since cylinder cooling is almost nonexistent when the airplane is moving at less than 40 miles per hour, ground runups should be avoided.
Do not cycle the prop — leave the prop control full forward (high rpm) for at least 10 minutes after takeoff. Cycling the propeller creates a low-rpm, high-combustion-pressure situation that may delay or limit ring seating.
Every one of these procedures is designed to help the pilot manage the heat of combustion during the first few hours of engine operation.
When cylinders are new or refurbished, the cylinder walls are not smooth. They've purposely been manufactured to a rough surface (technicians call this a ring finish) consisting of peaks and valleys; a side view looks like the profile of a hacksaw blade. During the first hour or two of flight, the rings rapidly wear off the tops of the peaks, and a plateau-and-valley profile results. When the peaks are worn away, the ring-to-cylinder wall seal improves dramatically.
Before an engine is fully broken in, the rings and cylinder walls have not worn together to form an effective seal. Since the rings can't seal well, the very hot, high-pressure combustion gases are free to blow past the ring-to-cylinder wall seal and heat up the cylinder walls.
If the pilot is not very careful to take steps to control the heat buildup on the cylinder walls, these gases will overheat the oil in the cylinder wall valleys. If the valleys become filled with scorched, or oxidized oil (this is called glazing), the break-in process stops and the rings simply glide across the smooth, slick surface. The result is an ineffective ring-to-cylinder wall seal.
One characteristic of glazing is high oil consumption. This is caused by the poor ring seating — combustion gases leaking past the rings pressurize the engine crankcase, which blows oil overboard through the engine breather tube. Glazing can occur almost instantaneously, especially during high-power ground runs before the initial break-in flight.
If the engine ground running is excessive, the airplane is not flown in a manner that assures adequate cylinder cooling, or the baffles and baffle seals have been neglected, the rings may never seat. If this occurs, the only remedy is to remove the cylinders, break the glaze, and start over.
As the takeoff roll is begun after turning onto the runway, slowly increase the engine rpm to the normal magneto check speed for your airplane and switch from Both to Left, back to Both, and then to Right. As long as there's no backfiring or a serious rpm drop-off, you're good to go.
Limit the engine rpm to 2,000 until the airspeed needle has gone through 40 mph. After 40, gradually (three or four seconds) push the throttle all the way in. Scan the engine instruments for any abnormalities.
Pilots flying the first flight on a Continental fuel-injected engine will need to check to see where the fuel flow is during full-power operation. According to engine break-in instructions from RAM Aircraft ( www.ramaircraft.com), a company that specializes in power and performance upgrades for Cessna twin-engine aircraft with fuel-injected TCM engines, if the fuel pressure is more than one gallon per hour (6 lb) off target from the values specified in TCM SID 97-3, pull the power back and make the proper adjustments before taking off. Fuel flows that are too low will cause high temperatures.
During climb, select a gradual climb profile to maximize cooling airflow. Leave the cowl flaps open and the mixture rich.
Recommendations vary slightly between the manufacturers about when to reduce power. Both Lycoming and TCM say this should occur during climb. Engine Components Inc. (ECI), an engine parts refurbishment shop and manufacturer that has been in continuous business since the 1960s ( www.eci2fly.com), and RAM say to reduce to climb power by reducing the manifold pressure as soon as practical. Pilots in airplanes with constant-speed propellers can reduce rpm to recommended climb settings after 10 to 20 minutes of flight.
After climbing to a safe altitude (not so high that 75-percent power is unattainable), level off, and pull the power back to 75 percent. The mixture should not be leaned more than 125 degrees rich of peak. RAM recommends that cruise power mixtures can be set by noting the takeoff EGT indication and leaning back to the same indication during level cruise flight.
Opinions vary regarding the time spent cruising at a fixed power setting — TCM and Lycoming say one hour, while RAM says 30 minutes. Depending on the amount of time you choose, the next step is the same. Vary the power settings between 65 and 75 percent. RAM's turbocharged engine schedule calls for 10 minutes at 2,500 rpm and 30 inches, followed by one minute at 2,700 rpm and 30 inches, then 10 minutes at 2,400 rpm and 30 inches.
This pattern goes on with the rpm being increased in turn to 2,600. Then the cycle starts again for an hour. Lycoming recommends that the break-in flight be completed by increasing power to full rated for an additional 30 minutes before returning to land.
During descent, gradually reduce power to achieve a 300 to 500 fpm descent. Do not push the mixture to full rich — gradually richen as necessary to maintain the same EGT that was noted during 75-percent power cruise flight. Unless a high-power go-around is imminent, there's never a need to move the mixture to full rich.
Lycoming and TCM say that break-in is best accomplished if power settings are maintained between 65 and 75 percent for the first 50 hours of flight.
Lycoming recommends inspecting both the oil filter (or pressure screen) and suction screen after the initial flight. ECI suggests that the oil and filter be changed at 10 hours, 35 hours, and 60 hours after the initial start-up. The critical time in the break-in of a new engine starts the moment the engine is uncrated and doesn't end until the second hour of flight.
If heat management methods are understood and thoroughness and patience are practiced, good things happen — if any steps are neglected, you'll pay and pay and pay.
E-mail the author at firstname.lastname@example.org.
There are three basic types of oil in stock at most general aviation maintenance shops today. These are usually referred to as mineral oil, ashless dispersant (AD) single-viscosity oil, and ashless dispersant (AD) multivicosity oil. Actually, they're all wholly or partially mineral oils — the AD oils have ashless additives and dispersant additives, and the mineral oil doesn't.
Lycoming and TCM recommend using "a straight weight non-dispersant mineral oil conforming to SAE J1996" during the initial break-in flight. There are exceptions to this rule. Both TCM and Lycoming note that when the engine has a turbocharger (or turbonormalizer) ashless dispersant oils must be used during break-in. RAM and ECI specify the use of Phillips X/C SAE 20W-50 (type M), a multiviscosity mineral oil, during break-in.
Oil that has dispersant additives holds wear particles and dirt in suspension as the oil circulates. The contaminants are then captured in the engine oil filter. Non-dispersant oil lets the wear particles and dirt fall out into the pan. Both non-dispersant and dispersant oil will work during break-in as long as the proper break-in procedures are used. But it's best to follow the recommendations of your overhaulers so as not to violate your warranty. — SWE
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