What aircraft was involved and where did it happen?

The accident on 2001-10-28 involved a SPORTAVIA-PÜTZER GMBH & CO.KG RF 4 D, registration HB-2006, at Spiringen, UR, CH.

What was the probable cause of the accident?

The accident was caused by the pilot losing control of the aircraft during a low-altitude maneuver, leading to a collision with an obstacle.

Fatal Crash of Touring Motor Glider in Schächental

A pilot lost control of a Fournier RF4D motor glider at low altitude, resulting in a fatal collision with a tree near Spiringen, Switzerland.

What happened

On October 28, 2001, a Fournier RF4D motor glider, registration HB-2006, departed from Birrfeld airfield for a visual flight rules (VFR) excursion through the central Alps. The pilot had planned a route spanning from Titlis to Jungfrau, with an expected flight duration of approximately 2 hours and 45 minutes.

During the flight, the aircraft was observed flying at low altitudes through the Schächental valley. Witnesses noted the aircraft flying toward the Klausen Pass before performing a turn to return toward Spiringen. Shortly thereafter, observers witnessed the aircraft enter a sharp left-hand turn and suddenly pitch forward into a steep descent. The aircraft struck a large, 8-meter-tall maple tree at a high rate of sink and low horizontal speed. The impact caused the aircraft to disintegrate, and the pilot sustained fatal injuries.

The investigation

The Swiss Transportation Safety Investigation Board (SUST) examined the wreckage and interviewed several witnesses. Technical inspections of the HB-2006 revealed that the engine and propeller were functional at the time of the accident, and all flight control cables and mechanisms were in working order, with no evidence of pre-existing mechanical failure or material fatigue.

Investigators also reviewed the aircraft's weight and balance. The investigation established that the aircraft was overloaded by 34 kg at the time of the crash, though the center of gravity remained within permissible limits. Medical examinations of the pilot showed no evidence of sudden incapacitation, nor were any traces of alcohol, drugs, or medication found. The investigation also ruled out bird strikes or collisions with other aircraft.

Findings

The flight was conducted at an unusually low altitude relative to the terrain, which is particularly hazardous in the mountainous Schächental region.
The aircraft was operating in an overloaded state, exceeding the maximum takeoff mass by 34 kg.
The aircraft entered a steep, knife-edge right bank and a rolling motion during the final moments before hitting the tree.
The pilot was flying in a tight left-hand curve at low altitude when control was lost.
The seatbelts failed to withstand the forces of the impact.