ERA22LA157 · Aeronca 11AC

Before departing on an instructional flight with a new student, the flight instructor added 10 gallons of automotive premium 93 octane fuel that contained ethanol to the airplane, which had a supplemental type certificate (STC) to be able to use automotive fuel. The instructor conducted the takeoff and initial climb to 2,500 ft mean sea level (msl). Once established, the instructor let the student handle the flight controls and instructed the student to maintain their heading but to continue to climb to 3,000 ft msl. After practicing some basic flight maneuvers, the instructor had the student fly straight and level about 2,500 ft msl over a lake. While they were roughly over the center of the lake, the engine sustained a “severe loss of power.” The instructor immediately took the controls and established best glide airspeed. He observed the oil temperature and pressure gauges to be normal. The fuel quantity gauge was still indicating full. He worked the throttle in and out but without effect. He then pulled the carburetor heat to on, but the propeller continued to windmill under no power. The lake was surrounded by high, dense trees, so he elected to conduct a forced landing to the water. During landing the airplane nosed over, resulting in substantial damage to the airplane. The instructor and student were uninjured and were able to exit the airplane and walk to shore. The postaccident examination of the airplane and engine did not reveal any evidence of preimpact failures or malfunctions that would have precluded normal operation. A review of a carburetor icing probability chart indicated that meteorological conditions at the time of the accident were conducive to carburetor icing at glide and cruise power. Guidance published by the automotive fuel-use STC holder advised that automotive fuel containing ethanol should not be used in the airplane as, among other things, ethanol can absorb significant amounts of water in flight, and has an affinity for water and can pull moisture from inlet air on humid days to such extent that the engine may malfunction. Furthermore, carburetor icing is likely to occur at higher ambient temperatures, and at lower humidity with automotive fuel than with aviation fuel. The flight instructor did not know that he should not have been using automobile gasoline that contained ethanol. Thus, the evidence in this case indicates that in addition to not using fuel containing ethanol, the flight instructor should have responded to the carburetor icing by applying full carburetor heat immediately, instead of applying it after the engine had stopped producing power, as the carburetor heat system needed hot air from the exhaust manifold while the engine was running to function properly. He also could have recognized the potential for the formation of carburetor ice and preemptively activated it to prevent the formation of carburetor ice. Therefore, it is likely the engine sustained a complete loss of engine power due to the formation of carburetor ice.

Verbatim NTSB analysis and probable cause from the NTSB dataset