What happened
On November 16, 2002, an ultralight Rans S-7 L was performing a test flight at Viitasaari airfield when it crashed, resulting in injuries to the pilot. The pilot began the takeoff roll on runway 34 using a technique intended to maximize ground acceleration by maintaining a clean aircraft configuration. As the aircraft approached the rotation speed of 55 MPH, the pilot intended to deploy the flaps to a 10-degree setting; however, the flaps were found to be in the full 40-degree position at the time of impact.
As the aircraft reached the end of the runway, the engine suddenly failed. This caused the airspeed to drop rapidly toward the stall speed. Due to a combination of the low speed, the aircraft's configuration, and a center of gravity that was significantly forward of the allowable limit, the nose pitched down violently. The aircraft entered an uncontrollable, steep dive, striking the ground at an angle of approximately 50 to 60 degrees while banking 30 degrees to the left in a spiraling motion.
The investigation
Investigators examined the engine, which had been modified from an automobile engine for aircraft use. They discovered that a significant change had been made to the fuel system. Specifically, the fuel return pipe, designed to prevent vapor lock by cooling the pump, had been incorrectly installed. The return line was connected to the fuel suction line approximately 10 cm from the pump, creating a circulation loop between the return line, suction line, and the pump itself.
Furthermore, the investigation looked into the oversight of the engine modification. It was established that the supervision of the construction work had failed, as the appointed supervisor was incorrectly identified due to an error by the Finnish Aeronautical Association. Additionally, the aircraft's center of gravity was found to be outside of the permitted forward limit, which prevented the pilot from maintaining pitch control during the low-speed engine failure.
Findings
- The primary cause of the accident was the engine failure during takeoff, which led to a total loss of control.
- The engine failure was caused by a vapor lock in the fuel system, which halted fuel delivery.
- The vapor lock resulted from the improper installation of the fuel return pipe, which caused fuel temperatures to rise and increased vaporization.
- A contributing factor to the severity of the crash was the aircraft's excessive forward center of gravity, which rendered the pitch unmanageable at low speeds.
- The failure of the modification oversight process allowed the incorrect fuel system installation to go undetected.