What happened
On 10 August 2003, a Rans S7, Courier, registration G-CBNF, was conducting a mandatory five-hour test flight near Bagby Airfield, North Yorkshire, to satisfy requirements for a Permit to Fly. During the flight, while at an altitude of approximately 400 feet, the engine began experiencing intermittent power surges that exceeded the 5,000 RPM limit. The pilot noticed a smell of burning rubber and realized that the engine was failing to transmit full power to the propeller.
Unable to reach the runway due to a high rate of descent, the pilot attempted an emergency landing in a nearby stubble field. The windmilling propeller created significant drag, further increasing the sink rate. As the aircraft approached the landing site, the pilot encountered a wire and post fence, prompting him to raise the nose. This maneuver caused the right wing to drop, leading the aircraft into a spin at roughly 30 feet. The aircraft sustained substantial damage to the landing gear and forward fuselage, though the pilot escaped with no injuries.
The investigation
Investigators examined the engine compartment and found that both engine drive belts had failed; the front belt was shredded into strips, while the rear belt had lost its teeth. The propeller shaft remained free to rotate, and no external cause for the failure was immediately apparent.
Research into the Verner 1400 engine revealed that the drive belt system has been superseded by a gearbox in newer models. While a previous instance of belt failure was attributed to incorrect tension, the engine in G-CBNF had only been operated for a few hours, and the tension had not been altered from the manufacturer's settings. The pilot noted that the engine undergoes significant vibration and violent acceleration during the initial start-up sequence, which may contribute to belt stress.
Findings
- Both engine drive belts failed during the flight, resulting in a loss of power transmission to the propeller.
- The pilot's attempt to clear a fence during the forced landing caused the aircraft to enter a spin.
- The engine's start-up procedure involves high-amplitude vibrations that could potentially impact the integrity of the drive train.