https://data.ntsb.gov/carol-repgen/api/ ... 107039/pdf
Probable Cause and Findings
The National Transportation Safety Board determines the probable cause(s) of this accident to be:
The airplane owner/builder’s inappropriate use of a lower grade fuel than that required by the airplane and engine operations manual, which resulted in engine detonation and the degradation and eventual obliteration of the Nos. 1- and 2-cylinder spark plug electrode tips. Contributing to the outcome was the pilots’ decision to continue flight at low altitude following a partial loss of engine power instead of performing an immediate precautionary landing, which resulted in collision with an overpass and terrain.
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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.
