On January 16, 2024, at 1130 Pacific standard time, a Sonex experimental amateur-built airplane, N920MB, was substantially damaged when it was involved in an accident near Auburn, Washington. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot reported that during the initial climb, about 100 ft above ground level (agl), the engine exhibited a partial loss of power and the airplane began to descend. The pilot planned to land the airplane on a road straight ahead, but the engine regained power shortly thereafter, enabling the airplane to climb. After the airplane had climbed to about 200 ft agl, the engine surged, then exhibited a total loss of power.

The pilot performed a forced landing onto a road while trying to return to the airport, during which the airplane struck a signpost. The airplane sustained substantial damage to both wings and the fuselage.

The airplane 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 this typical configuration, a paper element filter connected directly to the AeroInjector’s air inlet performs air filtration.

The accident airplane was modified by replacement of the AeroInjector system with a Rotec throttle body injector (TBI). The TBI was mounted underneath and to the rear of the engine and connected to the inlet manifold on top of the engine through a Yshaped inlet pipe. A paper element (similar to the typical configuration), mounted to the TBI air inlet with a honeycomb material spacer, performed air filtration.

The TBI incorporated a separate “on demand metering” fuel regulator, which included a fuel priming feature. The regulator used air pressure tapped from the throttle body bore for reference. This reference was connected to the air side of a diaphragm within the regulator by a flexible, clear plastic line.

Postaccident examination of the TBI revealed that the air pressure line, along with the air side of the diaphragm, was full of water.

The rest of the engine, along with the airframe fuel delivery system, was examined and no additional anomalies were noted that would have precluded normal operation. The airplane’s belly was coated in a stream of oil; however, the engine did not exhibit any 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 at impact.