Accident Overview
On July 22, 2019, at about 1300 Pacific daylight time, a Cessna 172G, registration N3761L, was substantially damaged following a loss of engine power during approach to Madras Municipal Airport (S33), Madras, Oregon. The aircraft was operated as an instructional flight under Title 14 Code of Federal Regulations Part 91. The flight instructor was not injured; the student pilot and a passenger received minor injuries.
Flight Sequence
According to the flight instructor, she asked the student to perform a preflight inspection while she debriefed another student. Afterward, she provided a safety briefing and overview to the rear-seat passenger. An uneventful engine start and run-up were performed before departure. About 15 nautical miles from S33, the instructor and student retrieved weather information from the automated terminal information service, which showed no hazards.
During descent, they announced their position and intentions over the common traffic advisory frequency and entered the left base leg for runway 34. After a touch-and-go, they re-entered the left traffic pattern. Abeam the runway 34 designation numbers, the student applied carburetor heat, reduced engine power to 1,500 rpm, and configured for landing.
The approach progressed normally while the instructor scanned for incoming traffic. After a radio call from another airplane, the instructor observed the traffic and directed the student to turn left to final. Almost immediately after announcing their position, the student said, “...it won’t go in, it won’t go in!” The instructor noticed the tachometer indicated 1,000 rpm. She retarded and advanced the throttle, advanced the mixture to FULL RICH, and recycled the throttle, but observed no engine response. From about 300 ft above ground level, she took control and instructed both occupants to brace. The airplane touched down in a field south of runway 34, struck a barbed wire fence, traversed an embankment and paved road, and came to rest on its nose. The fuselage sustained substantial damage.
Student Account
The student’s statement was consistent with the instructor’s. The student reported pulling the carburetor heat lever to ON, retarding the throttle to 1,500 rpm, and deploying wing flaps. While the instructor searched for traffic, the student turned to final and observed a sudden rpm drop to 1,000. She advanced the throttle slowly to full open with no increase in power. She notified the instructor, who acknowledged the loss and took control. The student recalled that the fuel selector was in the BOTH position throughout the flight.
Carburetor Icing Conditions
Federal Aviation Administration Special Airworthiness Information Bulletin CE-09-35 indicated that the airplane was operating in conditions conducive to carburetor icing at glide and cruise power. Additionally, a Transport Canada article noted that automotive gasoline (MOGAS) is more susceptible to carburetor ice formation than aviation gasoline, with ice possible at outside temperatures up to 20° higher.
Aircraft and Engine Details
The airplane was equipped with a Teledyne Continental O-300-D, 145 hp reciprocating engine. On February 20, 2019, the owner began operating the engine under Supplemental Type Certificate (STC) SE1943CH, permitting use of unleaded automotive gasoline with a minimum 87 octane. The operator reported using non-ethanol 92 octane automotive fuel regularly and 100 low lead aviation gasoline about every 15 hours.
Maintenance records showed the engine’s last major overhaul was 3,325.5 flight hours prior, exceeding the manufacturer’s recommended time between overhaul of 1,800 hours (or 12 years), with a possible extension of 200 hours if the engine accumulated 40+ hours per month.
A technical brief from the engine manufacturer stated that automotive gasoline does not control octane rating, anti-knock constituents, or energy density as tightly as aviation gasoline, and has inferior vapor characteristics that can lead to detonation and pre-ignition, with alcohol content potentially damaging fuel system components.
Post-Accident Examination
Representatives from the airframe and engine manufacturers, with NTSB oversight, examined the aircraft at a recovery facility. Flight control continuity was established for rudder, aileron, and elevator. Fuel system continuity was verified, and most of the system was free of blockages except a small piece of rolled masking tape in the left-wing fuel tank vent line. Carburetor heat control continuity was confirmed.
A fuel sample from the recovery was consistent with ASTM D4814 specifications for automotive gasoline, with no contaminants. A functional test of the carburetor revealed trace fibrous material in the fuel inlet screen, but the drain plug and main fuel discharge nozzle were unobstructed. The carburetor functioned mostly normally, though with weak accelerator pump discharge flow.
The engine was tested at the manufacturer’s facility over six runs. The first two runs were sluggish, but after timing both magnetos from 12.5° and 11.5° to 20° before top dead center (per manufacturer specs) and enrichening the mixture, the engine ran smoothly. No evidence indicated magneto movement during impact.