What happened
On 6 May 2010, a PZL-104 Wilga 35A, registration G-BWDF, was conducting aerial work at Hinton-in-the-Hedges Airfield in Northamptonshire. The mission involved towing an advertising banner using a grapple hook system. During the operation, the pilot made several passes to engage the tow line held between two poles.
After successfully engaging the line, the pilot noted significant forward pressure on the control column, requiring both hands to maintain level flight. Due to the difficulty in controlling the aircraft, the pilot opted for a shallow, flapless approach to a grass area north of the main runway rather than the recommended steep approach.
As the aircraft descended to approximately 35 ft agl on final approach, the banner made contact with the ground. This contact caused a momentary increase in tension on the tow line, which pulled the control column forward with irresistible force. The aircraft's nose dropped, and despite the pilot's efforts to raise the nose, the aircraft struck the ground with a high rate of descent. The impact caused the engine to separate from its mounts and the landing gear to collapse. The aircraft pivoted and came to rest inverted. The flight resulted in two serious injuries to the crew.
The investigation
The AAIB examined the aircraft and reviewed photographic evidence and ground observer reports. The investigation established that while the pilot believed the grapple hook had deployed correctly, the hook had actually failed to release immediately.
Evidence showed that the tow line had wrapped around the left elevator, specifically between the trailing edge of the tailplane and the hinge of the elevator horn balance. This entanglement created the heavy nose-down elevator input experienced during the flight. The investigation also noted that the operator's manual provided procedures for a tow line snagged on the landing gear, but did not specifically address the fouling of the elevator.