What happened
On 19 September 1999, a Sikelsky S76A (Modified), registration G-BJFL, was undergoing ground testing at Norwich Airport. The aircraft was undergoing an engine run following the replacement of a fuel control unit filter on the number one engine. During the start-up of the first engine, the pilot and a supervising engineer noticed the rotor blades moving, indicating the rotor brake was not engaged.
Although the pilot visually confirmed the brake switch was in the 'off' position, an attempt to engage the brake had no effect. Prioritising the safety of ground personnel standing near the rotor disc, the pilot proceeded to increase the rotor speed to a stable level. After the engine run was completed and the engine shut down, the pilot attempted to apply the brake as the rotors slowed. However, the pilot discovered the switch was already in the 'on' position. As the rotors came to a halt, ground personnel alerted the pilot that the aircraft was on fire. The fire, which caused severe fire damage to the area between the engine air intakes and the main rotor gearbox, was eventually extinguished by the airport fire service and onboard extinguishers.
The investigation
Investigators examined the damage to the aircraft, which included the destruction of the rotor brake unit and significant damage to the hydraulic system, wiring, and Kevlar composite components. No mechanical defects were found in the brake unit itself; the damage was entirely consistent with extreme heat. The investigation focused on the sequence of events leading to the friction-induced heat.
It was noted that the pilot was operating from the right-hand seat, which was not his usual position, as most of his experience on the type was in the left-hand seat. Furthermore, the Central Warning Panel was displaying multiple captions related to the non-operating engine, which likely obscured the specific warning for the brake system pressure.
Findings
- The fire was caused by intense friction between the brake pucks and the disc.
- The pilot inadvertently started the engine with the rotor brake in the 'off' position.
- After an initial failed attempt to stop the rotating blades, the pilot failed to return the switch to the 'off' position.
- The brake failed to stop the rotors because the engine torque exceeded the brake's capacity at that specific stage of the run.
- The pilot's attention was likely diverted by the presence of ground personnel and the need to manage the rotor disc speed.