August 1, 2004
Steven W. Ells
Airframe and powerplant mechanics do their work on the ground — pilots can help by learning to spot problems in the air.
Tracking compression readings, inspecting oil filters and screens, and, to some degree, interpreting oil analysis samples are prosaic parts in the troubleshooting puzzle that helps mechanics determine if an airplane engine is healthy enough to be approved for return to service for another year.
One of a mechanic's most valuable tools has nothing to do with compression gauges or with the art of interpreting an oil analysis. It has to do with his person on the spot — the pilot.
Why is the pilot's feedback so valuable? Because a pilot acts as the mechanic's eyes and ears during every moment of engine operation. An involved pilot who understands what the panel instruments are revealing can recognize problems early on — before they get expensive or compromise safety.
Even the tachometer can reveal performance information for the savvy pilot.
Yet many pilots pay scant attention to these gauges — because no one has ever explained what to look for. One mistake that pilots with fixed-pitch propellers make is to think that the tach is just a rough method of keeping track of power. In reality, engine rpm varies widely because of changes in atmospheric conditions.
An engine is certified to produce its rated power under one atmospheric condition — a standard temperature of 59 degrees Fahrenheit, an atmospheric pressure of 29.92 inches of mercury, a sea-level elevation, and zero wind. Any change in these conditions causes a change in engine power output. The effect of changes in atmospheric conditions is reflected in a certification document called the airplane type certificate data sheet (TCDS). This document lists the models of propellers that the airplane manufacturer has approved for the airplane. For instance, one of the propellers listed in the TCDS for a Cessna 172M is a McCauley 1C160/CTM 7553. The numbers following CTM specify the overall propeller length in inches (75) and the blade pitch (53). Just below the propeller specifications the TCDS further lists the engine static (airplane not moving) rpm. This engine propeller combination has full throttle rpm limits of no greater than 2,370 nor less than 2,270. This 100-rpm window shows how much atmospheric conditions affect engine power output.
A well-matched propeller-engine combination will result in a static rpm of 80 to 85 percent of full-rated rpm on a standard day. True static rpm is the average of four tests with the airplane turned 90 degrees between tests to negate the effect of wind.
Pilots can look up the rpm limits for their airplane online ( www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/MainFrame?OpenFrameSet). Click on Type Certificate Data Sheets, go to Current Models By Make, select the airplane model, and then scroll down the TCDS — propeller specifications are immediately below the engine listings.
These rpm limits provide an easy-to-use guideline for determining engine health.
There's a pretakeoff checklist item calling for an idle rpm check. There are a couple of good reasons to perform this check. First, almost every general aviation engine with a correctly adjusted carburetor or fuel-injection system will idle smoothly at 600 rpm. If the idle fuel-air mixture is set correctly, there won't be any hesitation as the throttle is pushed forward, nor will the engine die as the throttle is pulled all the way to the aft stop.
An idle mixture check is done by setting the engine rpm at a fixed rpm (800 to 1,000 rpm) and watching the tachometer for an rpm rise as the mixture control is pulled smoothly aft to shut down the engine. A properly adjusted idle mixture will cause the engine to increase speed slightly just before it quits — 25 to 50 rpm is perfect. If there is no rise, the idle mixture is set too lean — if the rpm rises more than 50 rpm the idle mixture is set too rich. Because of seasonal temperature changes, the idle mixture should be reset at least twice a year.
Single-probe exhaust gas temperature (EGT) gauges, though helpful for coarse mixture leaning, don't provide enough information for comprehensive engine troubleshooting. But they're not useless. During a full-rich takeoff (at sea level) EGTs should be at least 200 to 300 degrees below the peak EGTs that are indicated at cruise power settings. This gap indicates that the carburetor or fuel-injection system is correctly adjusted, the cylinders are not leaking during the compression stroke, and the induction system is leak free.
Engine monitors — systems with an exhaust gas temperature probe and a cylinder head temperature (CHT) probe for each cylinder — provide chapters of real-time engine troubleshooting data. Many monitors now have data-logging provisions that save every piece of engine operating data (all EGTs, all CHTs, oil temp, oil pressure, outside air temperature, manifold pressure, and rpm) for future reference. Downloading this data and storing it is like an insurance policy, especially if proof of proper engine operating techniques is needed during a warranty claim. In engine troubleshooting, deviation from an established norm is engine-speak for "pay attention." The more sophisticated monitors make deviation spotting easy because of built-in pilot-selectable alarms that sound alerts when engine parameters such as temperatures or pressures deviate from the norm.
Electronics International ( www.buy-ei.com), an established manufacturer of engine monitors and other engine gauges, has rewritten its manual titled Diagnosing Engine Problems. It can be downloaded from the EI Web site. The manual reviews basic engine science and is full of well-documented engine operating tips and temperature guidelines. EI says that carbureted engines have EGT splits of around 150 degrees, while a fuel-injected engine (with nonprecision injection nozzles) will have spreads of approximately 80 degrees except for the IO-520 and IO-470, which have spreads of around 110 degrees. The same manual says that average CHT spreads are around 65 degrees F.
Ron Roberts, author of the EI manual, has clarified the use of EGT for troubleshooting by simmering engine energy principles down to an easy-to-digest sauce. In Roberts' concept EGT equals the total temperature potential in the fuel-air charge less the energy (expressed in temperature or heat) that's used to produce engine power. The total temperature of the fuel-air charge potential energy equals approximately 4,000 degrees F.
A cylinder that is operating correctly with the proper valve lift and timing, a leak-free and properly adjusted induction and fuel metering system, and an energetic and properly timed ignition system will use potential energy at a predictable rate — and the result is an EGT of approximately 1,300 degrees F. Roberts says that anything that reduces the effective cylinder compression ratio, such as leaks around the valves or rings, will reduce the efficiency of the cylinder. The result is that less of the potential energy will be converted to power. And since less energy is converted to power, more will go out the exhaust pipe, resulting in higher EGTs. Temperatures and pressures in airplane engines don't just happen — everything is related to cause and effect.
The EI manual also contains six tests that, when used in combination with an engine monitor, help establish engine-operating baselines and provide guidance for troubleshooting.
Twenty-first-century engine monitors such as EI's UBG-16, the Vision Microsystems ( www.visionmicrosystems.com) VM 1000, and the J.P. Instruments ( www.jpinstruments.com) EDM-700 and -800 analyzers are state-of-the-art systems that record and analyze engine data. One of the advantages of this advanced technology is alerting systems that let the pilot know when temperatures or pressures stray out of normal ranges.
Again, the main purpose of the monitor is, as in all engine health equations, to detect any deviation from the established norms. A spark plug that has become lead fouled, a fuel injector that has become partially clogged because of contamination, or a baffle seal that was folded the wrong way during cowling reinstallation will be picked up very quickly with a modern engine monitor.
Pilots can increase their awareness of engine cause and effect by studying the concepts in the EI manual, or by visiting the JPI Web site and studying their product operating manuals.
Compression tests (see " Airframe & Powerplant: The Healthy Engine," April Pilot) are a time-proven tool that determines how well the rings and valves seal the compression chamber. With the engine at operating temperature, and using a special pressure regulator and set of gauges, compressed air is piped into the compression chamber of the cylinder. The test is performed with the piston at top dead center of the compression stroke — in this position both the intake and exhaust valves are closed.
Since the test pressure (typically 80 psi) is so much less than internal combustion pressures (800 to 1,200 psi) that occur during cruise power settings, the effectiveness of this test is limited to finding gross leaks past the intake and exhaust valves. These leaks are identified by listening for the sound of air squirting through leaks between the valve and valve seat or past the compression rings into the engine case. The sound is not hard to hear — just listen at the exhaust pipe, induction air filter, or oil filler tube. Excessive leakage past an exhaust valve is the most common type of failure seen during a compression test.
Teledyne Continental Motors recently revised its 19-year-old compression test service bulletin. The revision is SB 03-3, which can be viewed at the TCM Web site ( www.tcmlink.com) and AOPA Online. The major change is TCM's recommendation that a bore scope inspection of each cylinder be conducted as part of every compression test — TCM believes that this visual internal cylinder inspection improves the chances of identifying potential problems such as worn cylinder walls, broken rings, or burned valves.
Engines are predictable. Although some owners might argue the point, an engine does not have a personality nor is it subject to the vagaries of emotional upsets — given identical operating conditions a healthy engine will not use more oil or fuel per hour one day than it will the next. Unless something in the engine changes, that is. Therefore, one of the simplest engine health monitoring tools is a fuel and oil usage log. Keeping track of how many gallons of fuel per hour an engine uses, and how many hours pass before a quart of oil is added, is the first chapter in engine health 101.
Any change in the oil consumption numbers is an alert. The cause of the increase (there's rarely a decrease) may be something minor such as a dried-up crankshaft oil seal that's allowing ram air to push past the seal, pressurizing the engine case and pushing oily vapors out the engine breather tube at an accelerated rate. Or it could be excessive valve guide wear. Any rapid change in oil consumption is a big red flag. Marked changes in fuel consumption that can't be accounted for by changes in operation profiles — training flights with multiple touch and goes will always use more fuel and oil per hour than cross-country flights — should also prompt action to determine the cause of the change.
Oil filters and oil screens separate out all the visible and most of the microscopic contaminants that get swept up and circulated in the engine oil system. Since oil plays so many roles in an airplane engine — lubricating, cooling, cleaning, and sealing — cutting open and examining oil filters is a critical step in diagnosing engine health.
Inside each filter are 40 or 50 inches of pleated paper designed to capture the circulating contaminants before they damage bearings, pistons, and other internal engine parts. During this inspection the filtering paper is cut off the center stem of the filter and opened up. It's not uncommon to find little clumps of carbon, but any appreciable amount of ferrous or nonferrous metals calls for further tests. If metal is found, the amount of metal is critical — a few slivers isn't a big deal, especially if the engine has just been rebuilt or a cylinder has been recently changed. But any amount over and above a few slivers is another one of those red flags. When excess metal is found every bit of the filter paper and all the contaminants should be put in a clean container — a plastic bag works fine — and shipped to either the engine manufacturer or other qualified lab to determine the makeup of the metal.
Oil analysis is not a panacea for identifying engine problems, nor should an oil analysis anomaly be given too much weight — it's just one small piece of the engine health puzzle. Oil analysis information is always used to support or reinforce conclusions about anomalies discovered using other tests. And like the other tools referred to here, oil analysis only works when a norm, or baseline of data, has been established over time.
A well-educated pilot armed with his engine's temperature and pressure norms flying a well-instrumented airplane and looking for deviations from those norms is the first identifier of engine health.
E-mail the author at firstname.lastname@example.org.
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