What happened
On August 12, 2006, during a 50th-anniversary fly-in at the Keiheuvel airfield, a Fokker S-11 performing aerobatic maneuvers crashed at a very low altitude. After completing a series of maneuvers including loops and rolls, the pilot attempted a low-speed pass at approximately 30 to 40 meters above the ground to greet the spectators. During this pass, the pilot had the canopy open and was waving to the crowd. As the pilot attempted to increase engine power, the engine choked. This loss of power, combined with the aircraft's low speed and left-hand slip, caused the aircraft to stall and enter an incipient spin, impacting the ground nose-first.
The investigation
The AAIU(Be) examined the wreckage of PH-HTC and the circumstances of the flight. Investigators noted that the pilot had a valid ATPL license but had limited recent experience on this specific aircraft type. The investigation focused on the engine's behavior and the cockpit controls. While the engine's power loss could not be definitively confirmed, investigators analyzed the position of the mixture control lever, which was found in the forward (lean) position. The investigation also looked into the aircraft's design, noting that the mixture control lever's movement is contrary to modern standard practices, where forward movement typically increases power rather than leaning the mixture.
Findings
- The aircraft was flying at an extremely low altitude (30-40m) during a maneuver that could be classified as aerobatic.
- The pilot was performing a low-speed pass with the aircraft in a slip with the rudder fully deflected left.
- The engine choked when the pilot attempted to increase power, leading to the loss of airspeed.
- The aircraft entered a stall and a left-hand spin due to the combination of low speed and the banking turn.
- The mixture control lever on this specific model has an uncommon reverse-motion characteristic that may lead to inadvertent engine choking if moved forward.
Safety action
- The investigation recommended that the Belgian Civil Aviation Authority (BCAA) launch a sensitization campaign to warn pilots about the extreme dangers of stall and spin accidents at low altitudes.
- A recommendation was made to the Dutch authority (IVW) to evaluate modifications for vintage aircraft with non-standard controls, such as introducing locking mechanisms or warning placards.
- The BCAA was advised to ensure aeroclubs notify authorities of fly-in events involving aerobatic maneuvers to ensure proper safety oversight.