N3761LCessna 172G

On July 22, 2019, about 1300 Pacific daylight time, a Cessna 172G, N3761L, was substantially damaged when it was involved in an accident near Madras Municipal Airport (S33), Madras, Oregon. The flight instructor was not injured. Both the student pilot and passenger received minor injuries. The airplane was operated under Title 14 Code of Federal Regulations (CFR) Part 91 as an instructional flight.

According to the flight instructor, she asked the student pilot to perform a preflight inspection of the airplane while she concluded a debrief with another student. When the instructor returned, she delivered a safety briefing and overview of the upcoming flight to the rear-seat passenger. An uneventful engine start, and subsequent engine run-up were performed before they departed the home airport. As the airplane was about 15 nautical miles (nm) from S33, the instructor and student retrieved weather information from the automated terminal information service, which did not show any hazards.

During the descent they made a radio call over the uncontrolled airport’s common traffic advisory frequency to announce their position and intentions before they entered the extended left base leg of the airport traffic pattern for runway 34. After a successful touch-and-go maneuver, they re-entered the left traffic pattern and heard an advisory call over the radio from a nearby airplane with intentions of entering the pattern. Abeam the runway 34 designation numbers, the student applied carburetor heat, reduced engine power to 1,500 rpm, and configured the airplane to land.

The instructor recounted that the approach progressed normally as she scanned the area for the incoming traffic. After another radio call from the incoming airplane, the instructor observed the traffic and issued instructions to the student to turn left to the final approach leg. Almost immediately after she announced their position, she heard the student say “…it won’t go in, it won’t go in!” While instructor attempted to comprehend the student’s statement, she noticed that the rpm indication on the tachometer showed 1,000 rpm. She immediately retarded and advanced the throttle, advanced the mixture to FULL RICH, then recycled the throttle again but did not observe any response from the engine. From about 300 ft above ground level, the instructor took control of the airplane and instructed both the student and the passenger to brace for impact. The airplane touched down in a field south of runway 34, impacted a barbed wire fence, and traversed an embankment and paved road before it came to rest on its nose. The fuselage was substantially damaged.

The student’s statement provided some additional information about the accident flight but was otherwise consistent with the flight instructor’s recount. Before the accident occurred, the student pulled the carburetor heat lever out to the ON position, retarded the throttle to 1,500 rpm, and deployed wing flaps. While the flight instructor searched for incoming traffic, the student turned to the final approach leg of the airport traffic pattern and observed a sudden reduction in engine power to 1,000 rpm. The student then advanced the throttle slowly until it was in the full open position but did not observe an increase in engine power. She notified the flight instructor who acknowledged the loss of engine power and took control of the airplane. The airplane impacted the ground seconds later. The student recalled that the fuel selector was in the both position for the entire flight.

According to Federal Aviation Administration Special Airworthiness Information Bulletin CE-09-35, the airplane was operating in conditions conducive to carburetor at glide and cruise power (see figure).

Figure: Carburetor Ice Plot

Additionally, an article published by Transport Canada in April 2011 and available in the Transport Canada Aeronautical Information Manual, stated in part,

Due to its higher volatility, MOGAS is more susceptible to the formation of carburetor ice [than 100 low lead aviation grade gasoline]. In severe cases, ice may form at outside air temperatures up to 20° higher than with aviation gasoline.

The airplane was equipped with a Teledyne Continental O-300-D air cooled, 145 hp reciprocating engine. On February 20, 2019, the owner began operating the engine in accordance with Supplemental Type Certificate (STC) SE1943CH, which permitted the use of unleaded automotive gasoline 87 octane minimum. According to the Experimental Aircraft Association, who owned and authored the STC, the use of the STC did not require any upgrades or modifications to the Teledyne Continental O-300-D engine, which was approved for use of the STC. The operator of the airplane reported that he used non-ethanol 92 octane automotive fuel regularly and 100 low lead aviation grade gasoline about every 15 hours.

Review of the airplane’s maintenance records showed that the engine’s last major overhaul was accomplished 3,325.5 flight hours prior to the accident. The manufacturer’s standard practice maintenance manual indicates a recommended engine time between overhaul period (TBO) of 1,800 flight hours or every 12 years. This guidance includes a caveat that TBO can be extended by 200 hours if the engine consistently accumulates 40 or more hours per month.

According to a technical brief released by the engine manufacturer on April 8, 1994,

Automotive gasoline is manufactured to the industry specification ASTM D4814 which does not control or establish limits for octane rating, major anti-knock constituents, or energy density (lower heating value). In addition, critical properties such as vapor pressure and level of contaminants are not tightly controlled as with AVGAS. Vapor characteristics for auto gas are inferior to AVGAS and result in a tendency for auto gas to more readily convert to vapor. In addition, the lower octane rating for auto fuel can lead to detonation and pre-ignition which may damage the engine. Alcohol content of auto fuels may also result in damage to o-rings, seals, and other elastomer components in the fuel system.

An examination was performed at a recovery facility by representatives of the airframe and engine manufacturers with oversight from the National Transportation Safety Board (NTSB) investigator-in-charge. Flight control continuity was established for the rudder, aileron, and elevator from the cockpit to their respective control surfaces. Continuity of the fuel system was verified by transferring air through the supply and vent system. Most of the fuel system was free of blockages with the exception of a small piece of rolled up masking tape that was recovered from the left-wing fuel tank vent line. Continuity of the carburetor heat control system was confirmed.

Several ounces of fuel, which were captured during recovery process, were submitted for laboratory analysis. Results of the analysis showed that the sample was consistent with the American Society for Testing and Materials D4814 specifications for automotive gasoline without any contaminants.

A functional test of the carburetor was performed, and examination of the unit revealed trace amounts of fibrous material in the fuel inlet screen; however, the drain plug did not display any foreign material and the main fuel discharge nozzle was not obstructed. The carburetor exhibited mostly normal function during testing with some indications of weak discharge flow from the accelerator pump, but the unit did not leak.

The engine was tested at the manufacturer’s facility and a total of six engine runs were performed. The engine ran sluggish during the first two tests but ran smooth and continuous during all subsequent runs after the left and right magnetos were timed from 12.5° and 11.5° before top dead center (BTDC), to 20° BTDC in accordance with the manufacturer’s specifications, and the fuel/air mixture was enrichened. There were no signatures on either magneto to indicate that they had been moved during the impact sequence.