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Engine Care - when you do not fly
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We have received a lot of requests from our members regarding the need to keep our engines running, on the ground, during the period when we cannot fly. The real question remains whether the practice of ground engine run is a good idea or not. What happens when you run an engine on the ground without flying?
Note: the author was Global Marketing Director of Continental Aerospace Technologies, formerly Continental Motors, for seven years and had the privilege to work alongside the engineers who design, prototype, certify and industrialize the production of piston engines for aircraft.
Today, there are only 4 engine manufacturers, worldwide, who share the world of certified piston engines. And again, one of them only produces diesel engines and another recommends the use of auto gasoline instead of AVGAS.
Everyone agrees that an aircraft engine that does not run is an engine that will experience an early demise. In this period of confinement, we are right to ask ourselves questions about the health of our engines, since we cannot fly and operate the engines properly.
An exception to the strict containment rule has appeared in recent days in France, allowing flying clubs to delegate a pilot to defy the containment rules, travel to the hangar to run the engines of the club's aircraft. It involves taxiing planes on the ground or performing a ground engine run for a maximum of twenty minutes. The underlying idea is that it will do the engine good, wash it from the inside and leave a layer of protective oil on all metal surfaces.
It’s a misconception. The practice of the ground engine run is strongly discouraged by the engine manufacturers. They explain clearly that it is an aberration, only promoting an exponential and rapid growth of internal corrosion.
Why limit as much as possible the ground engine runs on the ground or taxiing on the ground?As you all know, piston engines burn a mixture of air and gasoline. This operation causes the explosion which allows the pistons to come to life.
But we also get combustion residues. Naturally, these combustion residues end up in the oil pan, where they are covered by the oil. In fact, in the depths of the engine, there is oil, residue, acid and water. " Some water? No way! " Will cry out some voices. To be able to convince oneself of the presence of water in our oil sumps, it suffices to remember a few principles of elementary physics. Warm humid air experiencing a sudden drop in temperature will condense into fine droplets. A hot engine, when the mixture lever is pulled to turn the engine off, contains hot humid air (just like ambient air, but heated up by the temperature of the engine). The engine cools quickly, ambiant temperature air enters the still warm engine, heat ups and the humidity condensates, producing water droplets. At each engine stop, a certain amount of water is "created" which cannot escape from the crankcase. The water naturally goes directly to the bottom of the crankcase. Those who still doubt may pour a little cooking oil into a glass and then a few drops of water. The result is obvious. The water stays beneath the oil and is isolated from the ambient air that would have allowed, after a lengthy period of time, the water to evaporate. Being isolated, the water will remain in its current state: liquid!
Water, oil and what else?Unfortunately, when the engine is running and burning AVGAS in the cylinders, combustion is never complete (Diesel engine run much cleaner because of the nature of the Diesel cycle and always run lean). Depending on the richness of the mixture, the state of the engine, the setting of the magnetos and many other parameters, the detonation takes place at the appropriate moment or is slightly offset from the ideal moment. This is one of the major shortcomings of our ignition systems (magnetos associated with a mechanical injection system or a carburetor) compared to a fully electronic system controlled by a microprocessor or FADEC.
This incomplete combustion causes the appearance of combustion residues. In AVGAS, there is lead, but also carbon, various more or less carbonized chemicals, but also acid residues. Part of this waste leaves the engine through the exhaust valves, but a part remains trapped inside and finds its way to the oil pan.
Depending on the mixture settings and operating temperatures, the amount of waste varies enormously. Cool CHT's (350°F and below), the greater the amount of waste. The shorter the operating time, the less deposits are burned. The axiom reads as follows: "an engine which runs for a short time at low temperature fouls much faster than an engine which runs for a long time at its ideal speed of rotation and at nominal temperature". (Remember how you "clean up" your car on the highway after three months of heavy city use.
This image of a car engine clearly shows how the oil, gradually broken down by the combined actions of water, acids and combustion residues, turns into a sticky material, incapable of lubricating anything. To avoid this, with engines devoid of any electronic injection and ignition system, only one solution: drain the oil regularly, change the filter at the same time and run the engine in flight and at nominal temperatures. If all this turns out to be impossible, there remains the storage solution.
Temperature and healthIt might sound ironic and in bad taste to talk about health in the present situation ... But it is essential for the well-being of our engines. A new belief has spread in recent years in the world of general aviation. This belief says the following: "the lower the temperature CHT (temperature of the cylinder heads), the better it is for the health of the engine". It’s one more aberration! An engine is designed to run within well-defined parameter limits. The cylinder head temperature is ultimately an imprecise indication. It gives an average temperature of the metal, but in no way an precise indication of the temperature variations observed inside the cylinder. A cold mixture is injected into the combustion chamber, whereas a few milliseconds earlier an explosion has just occurred. Can you imagine the temperature differentials inside the cylinder when this cycle occurs several times per second? We have already admitted that there are combustion residues which are produced on a cycle. We also accepted the fact that some of it vaporized through the exhaust valve. But the rest, what becomes of it? Ideally, it is burned by the rise in temperature during the next explosion cycle. But if this does not reach a sufficient temperature, because the stoichiometric ratio of 15 parts of air for one part of fuel is not respected, or because the power displayed is low, with a low outside temperature and an excessively rich mixture (or any combination of these parameters), the temperature generated will not allow the residues to burn throughly. Just think of your kitchen oven. If you put a wonderful chicken in it, the heat will be enough to melt the fat. On hot metal, that fat will burn, leaving a foul smelling greasy and coaly deposit. You’ll be left with your own version of Dante’s hell-while trying to clean the mess with a sponge and soap. Or you are going to use the pyrolysis cycle of the oven. After the pyrolysis cycle, deposits that are so difficult to clean will have disappeared, leaving only a few ashes, whose volume is ridiculous compared to the initial fat mass. A simple sponge will be enough to remove the ashes leaving a clean oven. Heat burns residues, if you allow the engine to reach the temperature at which this natural phenomenon will occur. Run your engine too cool and this will not happen.
Another very important factor is the temperature of the oil. Our synthetic, semi-synthetic or mineral oils are designed to achieve their lubricating properties within a given temperature range. Too cold, they are not effective enough, too hot, they lose some of their faculties. Engine oil therefore needs to reach a certain temperature to be fully effective. The oil most often contains chemical additives intended to promote "washing" or thoroughly burn combustion residues, transforming them into ashes, like in your oven ("Ashless Dispersant Oil"). These additives reach their full effectiveness within a given temperature range. A bit like in the case of pyrolysis of an oven (but the temperatures are not the same). Give the oil a chance to play its role to its full extent!
It's time to mix oil and waterActually, its exactly what you don’t want to happen! The fact remains that we have water in the bottom of the crankcase. What causes corrosion in an engine is the mixing of acid and water on metal surfaces that werent treated against corrosion. In other words, the cylinders, the pistons, the connecting rods, the camshaft, the crankshaft are the best suited to see corrosion appear. And this, despite the nitriding of certain parts. When you perform a ground engine run of twenty minutes, the rise in oil temperature is not sufficient to cause all the water in the mixture to evaporate. This is understandable: for water to come out of the engine for good, it must be in the vapor state. The most efficient way to do this is to bring the temperature above 100 ° C at sea level. We will remember that the boiling point of water depends on atmospheric pressure, therefore on altitude ... Altitude and water boiling pointWhat is the boiling temperature of water at an altitude of 700 m? Let's break it down: 700 = 500 + 200 brings us to the intersection of the 500 m line and the 200 m column: boiling point of 97.7 ° C.
This phenomenon is well known to mountaineers who want to make a soup or a tea. But it is unknown to the pilots. That's too bad. However, it explains ipso facto why it is much faster to fly an airplane at altitude to get rid of the water in the engine in the most expeditious way. Going up to 9000 ft, it is enough that the oil reaches 89 ° instead of 100 ° for the water to be transformed into vapor. This example is valid at 1013.25 HPA. If the air pressure drops, the boiling point will drop faster. This is the principle of the pressure cooker.
The ideal scenarioWhat Lycoming and Continental recommend is the same: At least once a month, fly the aircraft for a period sufficient for:
Returning from the flight, the water has completely or almost disappeared from the oil pan, the combustion residues generated by the flight have been pyrolyzed, part of the "grime" contained in the oil has done the same. The bottom of the crankcase is cleaner, most of the acids have disappeared. A relatively clean protective film of oil has settled on the inside of the engine. A little condensation will form, but, the whole circuit being cleaner, and the amount of water negligible, the engine will stay healthy for much longer and mostly corrosion free, internaly.
How to corrode your engine?By making repeated ground engine runs on the ground and leaving the engine off for a significant period after that. By running an engine on the ground, the engine is subjected to several attacks: The engine, designed to be air and oil cooled, is not cooled normally. Air is missing due to the missing relative speed of the aircraft in flight. It’s a huge shortage. The engine parts and the oil will not heat up homogeneously. The engine will naturally run “full rich” on the ground, for a relatively long period of time. So, with a too rich mixture. We will foul the spark plugs and create more combustion residues. While performing the ground engine run, several things are happening:
If a little corrosion exists in the barrels of the cylinders, the scraper segment will eliminate it. As long as it’s just surface corrosion, it’s not a big deal. The detached corrosion will end up in the bottom of the engine, which is why it is recommended to perform an oil change immediatly after an engine has not been running for a period ranging from 1 to three months. (Again, the case of airplanes exposed to sea air or hot and humid climate is another story). We should fly for an hour or two, and then a complete oil change. An oil filter and 8 liters of oil are cheaper than a cylinder or a camshaft. Never keep the oil that was in the engine if the plane has not been running for two to three months, even if the oil has been changed shortly before. Manufacturers recommend storing an engine if it is to be idle for more than three months. You can imagine that an American manufacturer (Lycoming and Continental give the same storage instructions) takes very serious safety margins for all these recommendations. Imagine the number of lawsuits that these manufacturers would have experienced, if after two months of inactivity, the engines were corroded to the point of requiring a top overhaul...
What to do if the situation persists?Much better than running an engine on the ground, the engine should be stored. It should be filled with a special oil, following the recommended manufacturer's procedure. Repeat the operation at the intervals recommended by the manufacturer (every six months or a year depending on the procedure chosen). To resume flights, have your mechanic apply the procedure once again, making sure that everything is ok.. From experience, this is by far the best solution if you do not want to leave an engine idle, regardless of the downtime. If you know that an aircraft will be immobilized for two months or more, storage is the best solution. An exception.Aircraft engines that are based in a marine climate or a hot and humid climate. In this case, storage of the engine should occur asap.
The problem in general aviation is that a lot of information comes from bar of the aeroclub. We put more trust in the man who has seen the man who thinks he might have seen the bear, than in the engineers and specialists who have knowledge, experience, statistical data and manufacturing and certification data.
Please, for the health of your engine do not make ground engine runs repeatedly! Follow the manufacturers' recommendations: an example:
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