Accident Narrative
On January 16, 2024, at 1130 Pacific standard time, a Sonex experimental amateur-built airplane, registration N920MB, was substantially damaged in an accident near Auburn, Washington. The pilot, who was the sole occupant, reported no injuries. The flight was conducted under Title 14 Code of Federal Regulations Part 91 as a personal operation.
According to the pilot, during the initial climb at about 100 feet above ground level (agl), the engine experienced a partial loss of power, causing the airplane to descend. The pilot intended to land on a straight ahead road, but the engine regained power shortly afterward, allowing the airplane to climb again. When the airplane reached about 200 feet agl, the engine surged and then suffered a total loss of power.
The pilot performed a forced landing onto a road while attempting to return to the airport, during which the airplane struck a signpost. The accident resulted in substantial damage to both wings and the fuselage.
Aircraft and Engine Details
The accident airplane was a Sonex experimental amateur-built design. It was equipped with an AeroConversions Aerovee engine, which typically includes an AeroInjector throttle body fuel delivery system connected to the engine’s inlet manifold. In the standard configuration, a paper element filter is attached directly to the AeroInjector’s air inlet for air filtration.
However, the accident airplane had been modified: the AeroInjector system was replaced with a Rotec throttle body injector (TBI). The TBI was mounted underneath and to the rear of the engine, connected to the inlet manifold on top of the engine via a Y-shaped inlet pipe. A paper element filter, similar to the typical configuration, was mounted to the TBI air inlet with a honeycomb material spacer.
The Rotec TBI included a separate “on demand metering” fuel regulator with a fuel priming feature. The regulator used air pressure tapped from the throttle body bore as a reference, connected to the air side of a diaphragm within the regulator by a flexible, clear plastic line.
Postaccident Examination
Postaccident examination of the TBI revealed that the air pressure line, along with the air side of the diaphragm, was filled with water. The rest of the engine and the airframe fuel delivery system were examined, and no additional anomalies were found that would have precluded normal operation.
The airplane’s belly was coated with a stream of oil; however, the engine showed no evidence of oil starvation, seizure, or catastrophic failure. Additionally, significant quantities of oil were found pooled under the engine at the accident site in an area where one of the rocker covers had detached upon impact.