Background
The aircraft, a Slingsby T67C Firefly registered G-FORS, was manufactured in 1990 and powered by a single Lycoming O-320-D2A piston engine. On 25 May 2005, it was being used for a training flight with an instructor and a student aboard. The instructor held a JAA Airline Transport Pilot's Licence with instructor ratings and had 6,000 flying hours, including at least 25 on type. The student had no previous flying experience and had flown with the instructor on 12 prior occasions, most in G-FORS. No training records were found for those flights, but exercises were noted in the student's logbook.
Flight History
The pilots arrived at Turweston Aerodrome around 1530 hrs. No formal briefing was recorded, so the intended content of the flight was unknown. The aircraft took off from Runway 27 at 1545 hrs with a surface wind of 200°/13 kt. After takeoff, it turned north. No radio calls were made on the Turweston frequency, and no emergency calls were heard. Radar data showed primary returns from approximately 1551 hrs, indicating the aircraft manoeuvred north of Turweston, including a left turn of at least 360°. The final radar returns were near the accident site at about 1605 hrs.
Witness Observations
Multiple witnesses observed the aircraft. One saw it flying slowly in a straight line at about 500 ft, then saw the right wing drop and the aircraft enter a steep dive, appearing to turn from side to side before spinning clockwise. The witness ran to call emergency services. His wife also saw the spin and heard no engine noise. Another witness saw the aircraft at tree-top level in a high rate of descent. A further witness reported the aircraft spiralling out of control, estimating two to four spins before it disappeared behind trees. No other aircraft were reported in the area.
Wreckage and Impact
The aircraft struck the ground at a nose-down angle of 35° on a heading of 020°, with little forward motion. The left wing hit first, then the fuselage rotated anti-clockwise. The tail section came to rest on a heading of 326°. Two wreckage trails were found: one forward of the engine containing canopy fragments, and another on a heading of 155° containing cockpit items. The propeller blades showed evidence of impact, with one blade bent and scored. The engine had detached from its mount. No fuel was found in the tanks, but clean fuel was present in the line between the selector valve and filter. Flight controls were examined: ailerons and elevator operated normally; rudder cables were detached from pedals but still connected to the rudder; elevator trim cable was pulled out, so trim position could not be determined; flaps were in the up position. The engine could not be tested due to damage, but no mechanical failure was identified. Carburettor icing was possible given weather conditions.
Investigation Findings
The investigation concluded that the aircraft probably entered an unintentional spin during an exercise involving oscillatory stalling. This exercise is not part of the UK Private Pilot's Licence syllabus and was considered inappropriate for ab initio training. A recommendation was made to the CAA to ensure instructors do not include oscillatory stalling during early flight training. It was also possible that the engine stopped during the spin, but this could not be confirmed. There was no evidence of a mechanical problem.