Rotax 503 TLC- How to Prevent Sudden Engine Stoppage
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The Rotax manual states “WARNING: This engine, by its design, is subject to sudden stoppage. Engine stoppage can result in crash landings, forced landings, or no power landings. Such crash landings can lead to serious bodily injury or death.”(Rotax Operators Manual Page 4.2) No further explanation regarding “sudden engine stoppage” is given at that point in the manual. Certainly an explanation is warranted as to why this is so. Some indicators are given throughout the manual but they are scattered about in the instructions for the various components of the engine. A nice concise summary of the reasons for this critical event is in order so that an operator can take the careful steps to avoid it. The reasons for “sudden engine stoppage” were, however, well explained during the Rotax Level 1 training course for 2 Stroke Air Cooled Engines (read 503) put on by Aero Propulsion Technologies of Saint-Lazare, Quebec. It is highly recommended that any Challenger pilot attend this basic engine course at their local Rotax service center. The Level 1 introductory engine course gave instructions regarding key maintenance issues including disassembling and adjusting the carbs, cleaning the air filter, use of 2 stroke oil, cold weather operation, engine performance limitations, and fuel considerations. Many of these topics were related to the issue of sudden engine stoppage in flight and all attending the course, including some very experienced Challenger pilots and builders, learned how to prevent this from happening. This is a brief summary of what was learned. This summary should in no way be considered as an effective low cost alternative to attending the course. Many of the points are best learned directly from the engine experts in the classroom setting with an engine stripped down as a visual reference. Also, the course provided hands on training and even computer animation of what is happening in the engine. It should also be noted that this is in no way a criticism of the Rotax 503 or the other 2 stroke engines. These units have an excellent reputation and record of service. My local motor cycle shop owner fondly refers to the 503 as a “bullet proof” engine. Like any mechanical device, however, they have to be operated properly and certain situations must be avoided. With that in mind, here is some of what was learned at the basic engine course regarding the subject of “sudden engine stoppage”. The 6 Key Reasons for Rotax 503 Sudden Engine Stoppage:
Let’s look at these 6 main causes of sudden engine stoppage one at a time in greater detail. Fuel Filter Clogged The standard fuel filter sent with the Challenger kit has a paper filter element. This works fine in most operating conditions. However, in cold weather climates, such as Canada and the northern United States, this fuel filter is not suitable for winter operations. A higher quality Bosch model is recommended because it has a metal element. The paper element in the standard fuel filter apparently has a tendency to freeze up if there is any water in the fuel. The metal element is much less likely to do this. Therefore, to prevent sudden engine stoppage in cold weather, use only the Bosch fuel filter with the metal element.(part# 4077)(PA BOSCHN4077) For the cost of only $7.50, the benefits are reliability and peace of mind. Fuel filters should be replaced every 50 hours of service or sooner if it appears that alot of sediment is collecting in the bowl. During the 50 hour inspection is a good time for a fuel filter change. Alcohol in Gasoline One of the great advantages of the Rotax engines is that they run on Mogas (gas for cars). This saves us a lot of money on fuel. Unfortunately, Mogas often has alcohol in the form of ethanol added to it. Alcohol is bad for aircraft engines especially 2 stroke Rotax engines. There have been several well written articles regarding alcohol in fuel in recent aviation magazines including COPA Flight and Kitplanes (April 2008). These all highlighted the dangers presented by alcohol. Alcohol is corrosive and may be incompatible with the rubber seals and other materials, including even some metals, that are used in an aircraft fuel system. If these materials break down because of contact with alcohol, they will cause leakage. Any rubber material may be carried with the fuel and clog the fuel filter or carburators. The alcohol may cause some seals to swell. This swelling may cause a component to stick and malfunction. Alcohol, if in a high enough concentration, may even cause the breakdown of the fibreglass material of a fibreglass gas tank. Some resins may be drawn out of fibreglass material and carried away with the fuel mixture. Before the tank completely dissolves, however, some of this jellied material will clog the fuel system causing an engine stoppage. Ethanol is hygroscopic. This means it has an attraction to water. This causes entrained water to separate from the fuel (this is called phase separation) . Again, in the right conditions, this can cause a blockage in the fuel lines or fuel filter in cold weather or at high altitude. Even in warm weather, this may cause fuel starvation or loss of power. Alcohol separates oil from gasoline. Since a 2 stroke engine is lubricated by the oil mixed into the gasoline, any separation of the two components prior to entering the cylinders is going to cause excessive wear or, in severe cases, even a sudden engine stoppage. The factory states that ethanol has “the effect of reducing the oil’s lubricating properties”. Ethanol also bonds with MTBE (methyl tertiary-butyl ether). MTBE is a fuel additive that oxygenates gasoline. MTBE has been used in gasoline at low levels since the 1970’s to replace lead as an octane enhancer. Unfortunately, alcohol bonds with MTBE to form a gel. This gel will clog a fuel filter and gum up the carburators. Again, the result is a sudden engine stoppage. The solution is to be careful when purchasing fuel. The Rotax factory website ( Rotax Service) says “we recommend using a “premium” type automotive fuel with an octane rating of 91, a minimum of impurities and little or no alcohols (maximum 5%).” From the problems listed above, it would seem that the best policy is to ensure that the fuel does not have any ethanol content. Do a fuel check before every flight. Drain a bit of fuel from the bottom drain and look for water that has separated and collected in the bottom of the tank. Another precaution is to test the fuel for alcohol content before using it. See the Challengers 101 site for the alcohol test method. Finally, be sure to run your engine for 8-10 minutes before flight. If a problem is going to occur, hopefully a full run up will make sure it happens on the ground. Old Stale Fuel It was highlighted during the engine course that gas ages rapidly, especially after it has been mixed with 2 stroke oil. In fac One solution to this problem is simply to use up your fuel and fly alot. That may not always be practical due to bad weather, health, other commitments and so on. Therefore, only add fuel when you are ready to fly. Only mix your 2 stroke fuel when you are ready to fly. Do not feel that you have to leave your gas tanks topped up as is practised in aircraft with metal tanks. If the plane has a plastic or fibreglass fuel tank, it does not need to be left full of fuel. Plastic and fibreglass tanks do not sweat like metal tanks thus creating water from condensation. Buy and mix your fuel when you go flying to keep the gas fresh. It was also recommended that fuel be purchased only from a busy gas station. This would ensure that the fuel purchased was fresh and had not sat in the tanks at the station for weeks thus becoming stale there. A second solution is to add octane boosting fuel preservatives. These are readily available at small engine shops and motorcycle shops. One brand is Klotz octane booster which comes in a 16 fluid ounce can for $15.00 . A couple of ounces per gallon is all that is required. A small cost for safe fuel. Inadvertent Mixture Leaning in Flight In larger GA aircraft, the fuel-air mixture can be easily adjusted by the pilot from the cockpit in flight. The small simple carburators used on the Rotax 503 do not allow for such an easy fuel-air mixture adjustment. To adjust the carburators on the Roatx 503 requires removing the cap off the top of the carburator and changing the position of the snap ring on the needles. This is done with the aircraft on the ground with the engine off. For the first few attempts, it often involves profanity until the skill is perfected. This is usually done seasonally to adjust the mixture to accomodate ambient temperature changes from the warm summer to the cold winter. As the temperature rises so does the position of the snap ring on the needle. This lessens the amount of gas admitted to the cylinder in the thinner summer air. In the winter, the snap ring is moved down a notch on the carburator needle which allows more gasoline to mix with the denser colder air. In really cold winter climates, the bottom notch is used. The fuel-air mixture affects the engine temperatures. A rich mixture will give low cylinder head temperatures (CHT’s) and low exhaust gas temperatures (EGT’s). A lean mixture produces high CHT’s and EGT’s. If the mixture gets too lean, the maximum allowable temperatures for the engine can be exceeded and this can cause the engine to seize and suddenly stop. A full suite of CHT and EGT gauges is recommended on the instrument panel to monitor the engine temperatures and adjust the carburator settings accordingly. How then can the fuel-air mixture be leaned inadvertently? The answer is that under certain flight conditions and engine settings a lean fuel-air mixture can be introduced to the engine. These conditions often occur in close proximity to the ground which is the worst possible time for a sudden engine stoppage. If the aircraft is put into a relatively steep dive and at the same time the throttle is pulled back to idle to prevent excessive speed, the mixture will be leaned. How so? In a high speed dive the propellor becomes unloaded and will spin at a higher rpm than normal. The propellor rpm is, of course, directly proporsional to the engine rpm. The engine with thus be turning faster than it normally would for the given throttle setting. The pistons will thus be sucking in a larger than normal amount of air. However, if the throttle is retarded to idle during this high rpm dive, the amount of gasoline being drawn into the cylinders will be much less than normal for that engine speed. High air volume combined with a small fuel volume is a lean mixture. Therefore in a high speed dive with the engine at idle, an inadvertent lean mixture is introduced to the engine causing engine temperatures to soar. An sudden engine stoppage is likely to occur. Whereas most conscientious Challenger pilots will not be flying their precious aircraft like a dive bombing Stuka pilot, this dangerous combination of flight condition and engine setting may be introduced by an unwary pilot on final approach. Imagine a final approach that suddenly becomes too high. A pilot may react by pushing the nose down and retarding the throttle thus introducing the lean conditions described above. An engine stoppage during a botched approach is a bad situation. The proper response to a high approach is a side slip or a missed approach and go around. Of course, the other situation where the inadvertent lean fuel mixture can occur is a deliberate high speed dive at idle. Avoid this. Leave that stuff to Stuka pilots. Engine Warm Up and Shock Cooling The Rotax 503 engine is manufactured using a lot of aluminum including in the pistons. The cylinder sleeves, however, are manufactured from steel. Whereas the designers no doubt had very good reasons for these material choices, including weight saving, the different metals used in the hot section of the engine can cause sudden engine stoppage and premature wear if misused. The engine course stated that 90% of the wear on these engines occurs during the first minute after start up. The engine should not be over revved above 2500 rpm during this time. Time should be allowed for engine warm up, usually 5 to 8 minutes, for the CHT’s to come up to 300 degrees F before aircraft take off. The reason is that difference in the metals used in the engine. Aluminum and steel have very different properties when reacting the temperature change. Steel and aluminum expand when heated at different rates. They also cool down at different rates. The aluminum pistons cool down and heat up faster than the steel cylinder sleeves. Thus, The aluminum pistons will expand due to heat faster than the steel cylinder sleeves in which they are moving. This can cause excessive wear, piston scoring, and sudden engine stoppage if a large demand for power is required of the engine before it has warmed up sufficiently. The engine is also very small and light weight. This gives it the advantage of a high power to weight ratio which is ideal for ultralight aircraft. It also means it has less mass to absorb heat when their are rapid temperature swings. Also, in an air cooled engine, the temperature does not spread uniformly throughout the engine. The solution is to avoid exposing the engine to rapid temperature changes while it is running. You can avoid rapid temperature changes by doing the following:
Air Filter Clogged The air filter on the Rotax 503 is different from those used on cars or lawn mowers. The conditions they experience are different too. They are also cleaned using a different procedure. The air filter used on a Rotax 503 has a very fine element that is soaked with oil and entrains harmful particles before they enter the engine. Over time, as more and more particles are absorbed, the filter becomes clogged. If allowed to remain untreated, the filter will eventually restrict the amount of air entering the engine. This will change the fuel air mixture to an excessively rich condition. This excessive rich mixture will cause the engine to run rough, lose power and even suddenly stop. The filter should therefore be cleaned after 50 hours of engine operation using the K&N Recharger cleaning kit (part number 99-5050) and following the instructions provided. Wash the filter according to the instructions and with the cleaning fluid provided in the kit. Rinse it in low pressure water from the inside out. Then lubricate it with the special oil from the kit. Do not lubricate the filter element with any other oils such as 2 stroke oil or motor oil as these attract water and will block the filter material. The air filter should not be blasted with compressed air or soaked and rinsed with gasoline as is often done with car filters and lawn mower filters. These K&N filters are a different breed and have different cleaning procedures. The Rotax service centres now offer a special prefilter that is mounted onto and covers the K&N filter. Basically the prefilter protects the K&N filter by removing the coarser particles before they clog the finer filter downstream. Thus the expensive K&N filter is given a longer life and better performance. The K&N filter should be covered when the aircraft is stored whether outdoors or in a hangar. The cover will protect it from contamination and falling debris including rain, dust, blowing dirt, bird droppings, small insects, and so on. This cover can be in the form of an old T shirt. Some after market suppliers, such as Turbulence Aviation, offer a nice vinyl filter cover that offers excellent protection. the K&N filter should be replaced if it tears or appears very dirty and resists cleaning. At any rate, Rotax recommends that the filter should be replaced with a new unit after no more than 300 hours of engine operation. Other Tips for Caring for the Rotax 503
Again, it should be emphasized that the best thing a Rotax owner can do is to take the factory’s basic introductory engine course to learn first hand the proper care and use of the 503 engine. Following its lessons will help you to avoid sudden engine stoppage. |