ERA23FA188 · Miller Glenn F RV-12

Two commercial pilots departed for a local flight in the experimental amateur-built airplane that was undergoing Phase 1 flight testing. The purpose of the flight was to perform aerodynamic stall testing. Earlier in the day, the right-seat pilot had completed a solo flight. Upon returning to the airport, the left-seat pilot boarded the airplane and they taxied for takeoff. During the climb, about 400 ft above ground level, the airplane sustained an abrupt partial loss of engine power. The airplane immediately stopped climbing and the pilot entered a left turn, then turned left again to fly over a highway that paralleled the departure runway. The pilots attempted to troubleshoot the partial loss of engine power by adjusting the throttle and mixture, and by ensuring that both fuel pumps were on, but were unable to restore engine power. The airplane continued toward an overpass, under which a semi-truck was parked, and near which powerlines spanned across the highway. The pilots attempted to fly under the powerlines and over the overpass, but entered an extreme bank angle and impacted the overpass before coming to rest inverted below it. A post-crash fire ignited immediately. The right-seat pilot was pulled from the wreckage by motorists; the left-seat pilot was fatally injured. Examination of the engine found that the Nos. 1 and 2 cylinder spark plug electrode tips were obliterated. The Nos. 3 and 4 cylinder spark plug electrode tips remained intact, but were found blackened. There was no evidence that any of the spark plugs had sustained impact-related damage. A bench test of the spark plugs found that the Nos. 1 and 2 plugs would produce a spark at low compression, but would extinguish under the higher compression levels produced during normal engine operation. The Nos. 3 and 4 cylinder spark plugs produced normal spark at the bench test’s maximum compression. It is likely that the partial loss of engine power was due to the damaged Nos. 1 and 2 spark plug electrode tips. The owner/builder of the airplane stated that he used low-grade 87 octane automotive fuel with the engine in its first 9 hours of operation about two years before the accident. The engine manufacturer required that the engine be operated with at least 89 octane automotive fuel or higher grade. The manual and engine manufacturer further reported that using low grades of automotive fuel could result in engine detonation and/or catastrophic failure. The operations manual further stated that, when stored for 3 months or longer, the airplane should be stored with 100 low lead aviation fuel. The airplane owner reported that the airplane sat idle for nearly two years, during which occasional engine run-ups were performed, and that the fuel onboard during this time was automotive fuel with a fuel preservative/additive. Despite the surviving pilot reporting that he fueled the airplane with 93 octane automotive fuel for his recent flight activity and the accident flight, it is likely that the estimated first 9 hours of engine operation with the lower grade of fuel, and old automotive fuel, likely contributed to the degradation and ultimate failure of the Nos. 1 and 2 spark plug electrodes during the accident flight. Although the engine lacked other signatures of detonation, it is likely that the spark plug damage was due to detonation occurring at some point in the airplane’s 20 total hours of engine operation. There was no evidence that the spark plugs had been inspected or replaced during the required annual condition inspection seven months before the accident. According to the pilot’s operating handbook, the landing distance was 525 ft. During the pilots’ engine troubleshooting, they overflew more than 4,000 ft of a multilane highway with a wide grass median. Had either pilot decided to make an immediate precautionary off-airport landing either on the grass median or the highway, rather than continuing the engine troubleshooting at low altitude, the conflict with powerlines and the collision with the overpass likely could have been prevented. It is also likely that the partial loss of engine power, rather than a total loss of power, exacerbated the confusion and indecision by both pilots on whether an immediate precautionary landing should be made. The engine examination findings and testing of the spark plugs supported a scenario in which the engine likely would continue to run, but could not produce sufficient power to climb. Toxicology testing for the fatally injured left-seat pilot detected Carboxyhemoglobin at 15%, consistent with smoke inhalation after the accident, glucose, and acetaminophen. The testing was negative for ethanol.

Verbatim NTSB analysis and probable cause from the NTSB dataset