Incident
On 19 September 1999 at 1545 UTC, a Sikorsky S76A (modified), registration G-BJFL, was undergoing a ground run at Norwich Airport. The helicopter had recently had a fuel control unit filter replaced on the No. 1 engine and required leakage and vibration checks. The pilot, occupying the right-hand seat, started the No. 1 engine after receiving clearance. During start-up, the rotor blades began moving as expected with the rotor brake off. The pilot visually confirmed the brake switch was in the upright 'off' position and attempted to engage the brake, but it had no effect. He then advanced the speed select lever to achieve a safe rotor speed. At approximately 48% N1, a DC generator check was performed. At 70% N1, a rapid engine acceleration occurred, requiring the pilot to decelerate. After a five-minute run at 100% rotor RPM, the engine was shut down. As the rotors slowed through 60%, the pilot prepared to select the rotor brake switch to 'on' but observed it was already in that position. Ground personnel then indicated a fire, and the pilot notified air traffic control. The airport fire service extinguished the fire, aided by a cabin extinguisher and a CO2 unit.
Damage
Severe fire damage was found in the area between the main rotor gearbox and engine air intakes. The Kevlar composite components aft of the main rotor gearbox, including engine intake doors, main upper fairing, and internal panels around the tail rotor shaft and rotor brake, were damaged. The rotor brake unit and much of its hydraulic system were destroyed. Heat damage affected wiring, and extinguishant and debris entered the engine compressors. No defects were found in the brake components apart from fire effects, and the only heat source considered capable of initiating the fire was friction between the brake pucks and disk.
Aircraft Systems
The S76A version involved was equipped with a hydraulic rotor brake using an electrically driven pump controlled by a three-position switch. The 'on' position applies the brake until a pressure threshold is reached; the spring-loaded 'release' position opens a dump valve; and 'off' prevents pump operation. A pressure sensor illuminates a ROTOR BRAKE caption on the Central Warning Panel when pressure exceeds 15 psi. However, during single-engine operation, multiple captions are present, making the brake caption less conspicuous. Normal brake application occurs at 50–60% rotor RPM with engines at idle or shut down.
Pilot Background and Actions
The pilot held an Airline Transport Pilot's Licence (Helicopters) with an instrument rating, aged 38, with 4,500 total hours (1,000 on type). Previously, he had flown a later S76 version with a manual rotor brake. Although qualified as a captain on the S76, he was employed as a co-pilot and primarily flew from the left seat. For this ground run, he occupied the right seat. The pilot inadvertently started the engine with the rotor brake off and omitted to reselect the switch to 'off' after attempting to engage it. The brake's failure to stop the rotor likely resulted from being applied too late when engine torque exceeded brake capability. The heat damage indicated sustained pressure between pucks and disk. The pilot's unfamiliar seat position and concern for nearby personnel may have distracted him from correct brake operation.