Sudden Engine Failure; Where Are the Regulators?

Engine reliability has a lot to do with staying airborne and choosing a suitable place to land. When the engine sputters and shuts down, the options for landing are severely limited. Given that general aviation pilots generally have less experience than transport category airline pilots, one would think that assured engine reliability would be a matter of high priority, especially with regard to certification by the Federal Aviation Administration (FAA). It appears, however, that light sports aircraft engines (and other components) are failure prone, showing a weakness in the regulations and in certification of engines for this category of aircraft.

Case in point: the Rotax 503 TLC piston engine, which has found favor in ultralight and experimental aircraft. Recently, two pilots from Winter Haven, Florida, were forced to land their Challenger experimental plane on a highway when the engine failed (more than once) in flight. The co-pilot was the owner of an audio visual production company, and he outfitted the airplane with two cameras for the plane ride.


While the cockpit camera was not specifically oriented to capture the flying pilot’s control inputs or instrument readings, the video does show why the National Transportation Safety Board (NTSB) wants image recorders in cockpits of on-demand and charter aircraft not now equipped with cockpit voice or flight data recorders.

The pilot of this particular aircraft exhibited good airmanship skills, and he was also lucky that truck or auto traffic was absent from the side of the highway on which he chose to land. The engine failure occurred shortly after takeoff, but the runway was just on the edge of the airplane’s glide range.

According to Lake Aviation, which produces the Challenger kit for assembly, sudden engine stoppage is not unusual for the two-cylinder two-cycle engine. Herewith, quotations from Lake Aviation, abbreviated from their website:

“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 … Some indicators are given throughout the manual but they are scattered about in the instructions … A nice concise summary for this critical event is in order so that an operator can take the careful steps to avoid it …

“The 6 key reasons for Rotax 503 sudden engine stoppage:

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 [filter] is recommended because it has a metal element ….

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 …

“Alcohol is corrosive and may be incompatible with the rubber seals and other materials. 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 filter or carburetors. The alcohol may cause some seals to swell. This swelling may cause a component to stick or malfunction …

“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 ….

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 fact, the Rotax factory states ‘when fuel is premixed with 2-stroke oil, the octane rating is reduced by about 2 points. An 87 octane fuel would therefore become 85 octane.’ (The minimum recommended octane for the engine is 87.) Over the course of a couple of weeks, the octane rating of gas drops off even more. Over a couple of months maybe 10 points. Thus, old mixed fuel in the tank may well have an octane rating well below the minimum required for the engine to continue running ….

Inadvertent Mixture Leaning in Flight

“In a high speed dive the propeller becomes unloaded and will spin at a higher rpm than normal. The propeller rpm is, of course, directly proportional to the engine rpm. The engine will 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. A sudden engine stoppage is likely to occur ….

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 …

Air Filter Clogged

“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 ….”

It seems that the Rotax engine may be okay for snowmobiles but a poor safety bet for a light sports aircraft’s primary source of power.

When one looks at light sport aircraft, there appear to be numerous examples of regulatory slackitude, as it were. The designs don’t pass muster, and/or the aircraft are not adequately inspected by FAA officials during and following their construction. Here is one example:

February 2008, 3 injured, none killed: The NTSB determined that the cause of this amateur-built experimental airplane crash was “a total loss of engine power while on approach due to the electrical overload of a fuse caused by the inadequate design and installation by the builder of the electrical ignition switch.”