Accident Overview
On October 1, 2020, at about 1500 Pacific daylight time, an experimental, amateur-built Zodiac 601XL (registration N601SA) was substantially damaged when it impacted terrain during a test flight in Burlington, Washington. The private pilot, who was the sole occupant, was not injured. The flight was conducted under Title 14 Code of Federal Regulations Part 91 as a test flight.
Pilot Report and Sequence of Events
The pilot reported that before takeoff, he added fuel containing a lead scavenging additive. After departure, he remained in the airport traffic pattern. While turning from base to final at about 300 ft above ground level (agl), the engine lost total power. He immediately retracted the flaps and engaged the starter in an unsuccessful attempt to restart the engine. The airplane impacted the ground about 1,500 ft short of the runway. As ground speed decreased, the airplane transitioned from a dirt surface to an asphalt road running perpendicular to its direction of travel, coming to rest upright.
Post-Accident Examination
Federal Aviation Administration (FAA) inspectors interviewed the pilot at the scene. The pilot initially stated that the fuel selector was on the left wing tank at the time of power loss, but later provided a written statement indicating it was on the right wing tank. Inspectors observed minimal fuel in the left wing tank (about 1/2 inch from the pickup) and considerably more fuel in the right tank. Three weeks after the accident, during an FAA examination of the fuel system, 5.5 gallons were drained from the right tank and about two gallons from the left tank. The pilot later stated he had drained fuel from the wing tanks before that examination. The exact amount of fuel onboard at the time of the accident could not be determined.
The fuel tanks were intact, and fuel quantity indicators were accurate. Fuel line continuity from both wing tanks to the three-way fuel selector was confirmed. The fuel selector valve was operational, with positions Off, Left, and Right. The gascolator exterior was undamaged but exhibited a green film on the line from the gascolator to the engine-driven fuel pump. The line from the gascolator to the carburetor was disconnected and purged. With the selector in Right, no fuel could be extracted; with the selector in Left, about one pint of a lime green substance was purged. The throttle body also contained the green substance.
Laboratory Analysis
The NTSB Materials Laboratory Senior Chemist examined the lime green substance, which revealed an artificially high octane level, but the cause of this out-of-specification finding could not be determined. D86 distillation results were within specified ranges for Avgas 100LL. Fourier Transform Infrared (FTIR) spectrum and distillation results indicated the liquid was aviation gasoline capable of supporting combustion in an internal combustion engine.
According to an aviation gasoline manufacturer, Avgas 100LL turning green may result from a contaminant broadly yellow to red in color, such as fine ferrous or copper particulate. Another possibility is fuel degradation from standing in the selector for a long period, particularly if the selector has fuel-wetted surfaces made of catalytic metal like copper. The manufacturer did not recommend the use of additional lead scavenging additives, as the fuel is already formulated with such additives.
Additional material testing could not be performed with the available fuel sample, so the exact cause of the color change was undetermined.
Ignition System Examination
The airplane was released to the pilot, who examined the ignition system. He identified an instrument panel with two ignition switches: Ignition A (placarded 'Start') and Ignition B (placarded 'Run'). Ignition A was used to start the engine; Ignition B for continuous operation. With the engine running at about 1,000 rpm, testing showed that when both switches were On, operation was normal. However, with Ignition B On and Ignition A turned Off, the engine lost total power. The pilot cited the coil selector (Mallory 29000) as the cause. The coil selector is a directional dual-inlet, single-outlet device for high-performance dual ignition systems.
To isolate the failure, the pilot interchanged coil wires for Ignition A and B. After modifications, a successful start was achieved with Ignition A On and engine ran with Ignition B On, but again, turning Ignition A Off caused total power loss. The pilot then disconnected and relocated wires 'R' and 'Y' to Coil A, and wire 'G' to Coil B. Circuit breaker toggle switches for the ignition remained closed during the test. The pilot's test did not examine the distributor or ignition control box.