Summary
On Saturday, 4 August 1962, at 1500 Central European Time, a de Havilland DH-82A Tiger Moth, registration HB-UBF, departed from Herten airfield west of Rheinfelden, Germany, with a pilot and one passenger on a flight to Bohlhof airfield east of Waldshut. While climbing near Möhlin, Switzerland, the aircraft entered a left turn that developed into a spin. It struck the ground at approximately 1507 in a wooded area about 160 meters from the Rhine riverbank, south of the river. The two occupants sustained serious injuries, and the aircraft was destroyed. Third-party damage was minor.
Aircraft and Crew
The aircraft, a single-engine biplane constructed by de Havilland Aircraft Co. Ltd. (works number N-6476, built around 1940), was privately owned. At the time of the accident, ownership had been transferred to three individuals in May 1962 without notification to the Swiss Federal Air Office. The aircraft was equipped with a Gipsy Major I engine (120 hp) and a two-blade wooden propeller. Maximum takeoff weight was 830 kg; the actual weight at the time was approximately 815 kg. The center of gravity was positioned 43 cm behind the reference point, slightly aft of the permissible limit of 42 cm due to 14 kg of tow material stowed in the baggage compartment.
The pilot, born 1917, held valid Swiss and German private pilot licenses with glider-towing endorsements. He had accumulated about 400 flight hours during wartime training and 56 hours of motor flight since 1955, including 6.5 hours on the accident type. The passenger, born 1926, held a German glider pilot license and had about 50 hours of glider experience.
Accident Flight
Before departure, the aircraft was refueled with automobile gasoline (Shell ICA) instead of the required unleaded aviation fuel with a minimum octane rating of 73. The pilot noted that the takeoff roll seemed abnormally long, a phenomenon he had observed on the preceding four flights. During the climb, he reported maintaining 1900 rpm and about 60 knots, achieving an altitude of approximately 300 meters near Möhlin. The aircraft then entered an unintended left turn. The pilot initially thought the passenger was steering toward his home in Öflingen. When he attempted to correct, the aircraft became unresponsive, losing altitude in an increasingly steep and tight turn. He identified the motion as a spin and applied full throttle, opposite rudder, and pulled back on the elevator, but could not prevent the crash. The passenger stated he never touched the controls and observed the aircraft entering a tightening left turn that led to a spin after about two rotations.
Investigation and Findings
Examination of the wreckage revealed that the engine was operating at high rpm at impact. The slats (hand operated) were extended on both sides, and their locking mechanism in the cockpit was released. The airspeed indicator in the front cockpit read 46 knots, approximately 6 knots above the red line. The controls showed no pre-existing defects. Fuel analysis indicated a tetraethyl lead content of 0.4 ml/l and an octane rating of 85.5, confirming the use of automotive gasoline.
The aircraft's stall characteristics were described as benign when slats were operational and within the approved center-of-gravity range, but more abrupt with an aft center of gravity. The accident center of gravity was aft of the limit, which may have degraded handling but was not a direct cause.
Probable Cause
The accident was attributed with high probability to a loss of speed, although the underlying reasons could not be determined. The investigation could not identify a specific technical failure or pilot error that initiated the stall-spin sequence.