Caring For Your Engine By Jim and Reese Leach
We like to think of maximizing engine longevity, reducing costs and improving reliability and performance as the engine “Hat Trick”! To some, these may be mutually exclusive, but if a holistic view is taken, we can maximize performance AND minimize operating costs.
Let’s start simply. The engine needs the right combination of compressed air, fuel and spark to create the combustion event in each cylinder. Many factors play a part in ensuring these three elements arrive in each cylinder at the correct time. Sources for obtaining this balance include engine manufacturers’ data, overhaulers’ experience, and personal experiences as an aircraft owner. In addition, our industry has 50-60 years of operating information and best practices for maintaining and operating horizontally opposed, air-cooled piston engines in general aviation aircraft. Windward Aviation services approximately 300 of these engines annually and add this experience to our knowledge base.
We looked at several year’s worth of maintenance records to understand where our customers spend their engine maintenance dollar. The goal was to see if there were areas where our customers have or could save on their maintenance. Most of the data base includes Teledyne Continental Motors(TCM) engines. Many of the findings can be useful to Lycoming operators as well. We found that the work was divided into seven categories: cylinder, exhaust, engine systems(fuel pump and distribution valve), ignition(magnetos, harness, spark plugs), aircraft systems(vacuum pump and alternator), baffling and routine. Naturally, these categories are interrelated. We will provide a summary of common problems in each of these areas and recommendations to either eliminate or at least reduce costs for them.
CYLINDERS: Burnt exhaust valves is by far the most common problem for cylinders. We call it “Public Enemy #1”! Burnt valves are generally caused by operating the engine too close to peak EGT(lean or rich) at higher power settings. With more power comes more heat. As the valve material heats into its plastic range, the metal will creep and deform, no longer sealing the combustion chamber.
Another cause for exhaust valves not fully closing is carbon or lead deposits on the valve seat. Increased clearances between the valve stem and valve guide can also result in poor valve and seat alignment. The remedy is almost always cylinder removal for overhaul or repair. Valve sealing problems will result in loss of power. Non-seating valves can be found during a compression leak check and subsequent borescope inspection.
We have several customers that routinely go to TBO with the original cylinders. “What are they doing that I should be doing?” you may ask. To extend the life of cylinders, our customers have demonstrated we must ensure the correct fuel flow to the engine, operate at 65% power or less in cruise, and maintain the cooling baffles.
TCM publishes many Service Bulletins(SB), Service Information Letters(SIL) and Service Information Directives(SID) on engine operation. These are available directly from TCM or through your maintenance provider. SID 97-3 outlines the procedures for measuring and adjusting fuel pressures at max and idle RPM using calibrated fuel pressure gages. The aircraft fuel gage and tachometer are often inaccurate and should not be used solely for engine set up. If the gages are significantly in error, have them overhauled. Your decisions as pilot are only as good as the information shown on the gages!
TCM Service Bulletin M89-18 discusses leaning recommendations. In short, TCM recommends conservative leaning at 75% power. Their recommendations differ significantly from the performance tables found in most aircraft flight manuals. In fact, many operating manuals have tables for operating at 78% power 20 degrees Celsius rich or lean of peak.. That’s too hot! Our customers have shown that running those power settings will ensure premature cylinder removal. You will have more margin for error in leaning when the power setting is under 65%.
If you are considering operation lean of peak, we recommend installing a multiprobe engine monitor and taking a course on the subject. Too often we find partial or misleading information published in the aviation press leading to a mediocre understanding of correct operating procedures and ultimately burnt exhaust valves. In a nutshell, lean conservatively. When compared to the cost of cylinder replacement, even at $5 a gallon, fuel is still a cheap coolant!
Other causes for premature cylinder removal include carbon and/or lead deposits on the exhaust valve seat. Again proper leaning procedures can help. Leaning for ground operations can also keep sparkplugs from fouling. There is not much the owner can do about valve stem to valve guide clearance. This dimension is set by the manufacturer or overhauler. The only thing within your control is the selection of the cylinder source if repair or replacement is required. Total time for overhauled/exchange cylinders is unknown, so go for new or know the pedigree.
Leading into exhaust repairs, we must consider the exhaust port flange on the cylinders. Poorly sealing exhaust gaskets can cause flange and stud erosion and cylinder replacement. Add this visual check to your pre-flight inspection. Early detection can save the cylinder.
EXHAUST: Cracks and bulges can be seen during the pre-flight inspection. Exhaust failures progress quickly from a small bulge to open hole. Hot exhaust gasses in the nacelle are dangerous as well as costly. Considering most exhaust gasses exceed 1200 degrees Fahrenheit and Aluminum melts at 1100 degrees Fahrenheit, it is best to keep the exhaust contained in the steel manifolds and tailpipes.
ENGINE SYSTEMS: The starter, starter adapter and fuel pump normally last the life of the engine. Factors such as extended periods of being idle can age rubber seal components. Most Beech operators have hours of running per start, but if you have many starts per hour of flight, the starter components will age prematurely. We also recommend the hot start procedure provided by BPPP to reduce wear and tear on the starter and starter drive.
Intake manifolds and balance tubes have rubber connector tubes. Calendar time is more indicative of age than running hours. We recommend replacing most rubber parts at 8-10 years. Intake manifold leaks can artificially produce a lean fuel/air mixture resulting in burnt exhaust valves.
Our aircraft engines were meant to be flown. If we follow TCM’s advice(Lycoming’s, too), we should be flying the aircraft every 10-14 days. If not, harmful corrosion can develop on internal engine parts such as valve lifters and camshaft lobes(see TCM SID 05-1). These are very costly repairs and completely preventable. TCM provides flyable and temporary storage guidelines in Service Information Letter SIL 99-1.
Normally, the fuel distribution valve(spider) will last the time between overhaul for the engine. Again, calendar age is more important than usage for aging the internal diaphragm and seals. The pre-flight inspection can reveal fuel stains around the valve or from the drain fitting. Usually, the engine will also run on at shutdown. If you see stains, have the valve overhauled. Raw fuel spraying on a hot engine is not a good idea.
IGNITION: The heart of the ignition system are the magnetos. They provide the high voltage current that arcs across the sparkplug gap and ignites the compressed fuel/air mixture in the cylinder. Although magnetos have been around for 70+ years and may not be considered high tech, they continue to provide the redundancy and battery-independent electrical ignition source we have come to expect. Like all mechanical and electrical systems, they do require periodic maintenance. We recommend overhaul every 500 hours for Bendix and 500-800 hours for Slick magnetos. Those models with impulse couplings are particularly susceptible to wear and tear.
Of course, good magnetos are only helpful if the ignition harness is in good shape. Ignition leads usually last the life of the engine unless they are not routed correctly and chafing occurs.
Spark plugs come in two varieties: massive electrode and fine wire. Properly maintained sparkplugs will generally last 200-400 hours. Fine wire plugs usually last longer, so in the long run are less expensive. Poorly maintained spark plugs will reduce power output and can put a strain on the exhaust components due to delayed fuel burn in the exhaust manifold. Leaning during ground operations can eliminate fouling.
AIRCRAFT SYSTEMS: By far the most prevalent aircraft component to require attention is the vacuum pump. For most dry pumps, we recommend replacement at 500 hours. Airborne makes this interval mandatory. One of the ways to increase pump life is to replace the pump filters at each Annual Inspection. If your instrument air system is a vacuum system, you might consider a wet pump. Pressure systems do not have this option. The wet pumps generally last the TBO of the engine.
The alternator is a component that doesn‘t give much warning of impending problems. It is for that reason we recommend changing or overhauling the alternator at 800-900 hours. Belt driven models require pulley alignment and tensioning. Direct drive models often require the drive coupling to be changed when the alternator is overhauled. Keeping the belt in good shape and terminals free of corrosion can prolong system life.
Engine shock mounts tend to be another neglected item. Like other rubber parts, they age by calendar rather than operating hours. Some mounts on older Bonanzas require rotation at each Annual Inspection.
BAFFLING: The baffles around the engine direct air into the nacelle and around the cooling fins of each cylinder as well as the oil cooler and fuel pump. Their design is a series of compromises to provide adequate cooling both on the ground at relatively low forward speed as well as flying with relatively high airflow. Air flows from high pressure to low pressure and will take the path of least resistance.
The baffling only works well when cooling air is directed properly. Leaks around the edges or holes in the aft baffle can rob precious cooling air. We have noted the sensitivity of the cylinder cooling. Consider leakage the area of a nickel equal to 10-20 degrees F CHT. It’s that important!
Loose baffle supports not only allow air leakage, but also contribute to chafing damage to the nacelle or the cylinders. Proper routing of ignition harnesses and instrumentation wiring can prolong the life of the baffling components.
Consider that the Bonanza nacelle was initially designed for a 165-185 HP engine. An A36 today has 300 HP. Increased horsepower means increased cooling requirements. The current production baffles as well as aftermarket versions address the increased cooling needs, but cost significantly more than older configurations.
Included in baffling is the air intake system. The goal is to deliver clean air to the engine with minimal losses. Cracked intake tubes or airboxes can result in unfiltered air entering the engine causing dirt or sand erosion on moving parts. Volumetric air flow helps determine power output. Restrict intake air and the result is diminished power output.
ROUTINE: Replacement of air and oil filters are considered routine. The air filter ensures clean air is entering the engine. The oil filter keeps the moving parts of your engine lubricated with clean oil.
Changing the oil regularly removes impurities from the products of combustion. Harmful acids and dirt is collected by the oil. After 30-35 hours, most oil filters are in bypass, meaning unfiltered oil circulates in the engine. We recommend changing the oil at 25-35 hour intervals. Oil is a good filter and coolant. It needs to be changed regularly to achieve optimal results.
Although the oil change is an owner-allowed maintenance item, there is a lot more to it than “watching the oil drain into the bucket”. In general, the tools and procedures, not to mention the lack of workroom are different from automotive engines. The oil filter is cut open for debris analysis. An entire article could be written regarding oil analysis, but suffice it to say, knowing where various metals are used in the engine can prevent problems from occurring in the air.
In general, we look for “what’s wrong with this picture” items such as ignition harnesses, oil and fuel hoses and baffling while the oil is draining, oil filter is changed and oil refilled. The oil change is a good time to service the hydraulic fluid reservoir, tires and battery too.
EGT and CHT probes and wiring must be routinely checked and maintained. Periodic probe replacement and gage calibration ensures the information you use to monitor the engine performance is accurate, i.e. GIGO.
Correcting oil leaks can be classified as routine maintenance. Since “cracks and leaks never get better”, our advise is to correct them early.
Thousands of dollars can be saved by paying attention to operating and leaning techniques. Following engine and component manufacturers’ instructions for operation and maintenance can lead to extended life of your engine. Having a trained and experienced maintenance staff keeping your engine tuned up will give you safety , efficiency and reliability - the engine “HAT TRICK”!