What happened
On 28 November 1998, a Robinson R22 Beta, registration G-DLDL, was performing a private training flight when the crew experienced mechanical issues during their approach to Cambridge Airfield. While descending through 1,000 feet agl at approximately 75 kt, the crew heard a brief scraping sound. Shortly after, the transmission clutch warning light illuminated. Following established emergency procedures, the pilot pulled the clutch circuit breaker.
As the helicopter approached a hover, the main rotor RPM began to decay immediately after the collective lever was raised. This led to a rapid descent, though the pilot managed to perform a successful run-on landing on the taxiway. Once the aircraft stopped, a thud was heard from the engine compartment, followed by the appearance of smoke and flames. The crew shut down the engine, issued a 'Mayday' call, and evacuated the aircraft. The fire was subsequently extinguished using the onboard fire extinguisher. There were no injuries to the two occupants.
The investigation
Investigators examined the engine compartment and discovered that the oil cooler matrix and the supply and return lines to the oil cooler had been fractured. This damage was caused by a flailing drive belt striking the components, which resulted in oil spraying onto the exhaust and igniting a fire. The investigation also revealed charring on electrical cables and smoke damage.
Upon inspecting the drive train, a small nick was found on the rim of one of the upper pulleys. It was determined that a foreign object likely caused this damage, which subsequently led to the failure of one of the transmission belts. The aircraft had flown approximately 500 hours since new, with only 10 hours elapsed since the previous 100-hour inspection. While the belts are maintained on condition rather than a fixed service life, the investigation focused on the sequence of the belt's departure from the pulley assembly.
Findings
- The initial scraping noise heard by the crew was likely the moment the first transmission belt failed and departed the aircraft.
- The loss of the first belt reduced the tension loads, causing the actuator to extend to compensate.
- The remaining belt was able to maintain torque requirements until the pilot raised the collective lever during the approach to the hover.
- The failure of the second belt caused it to strike the oil cooler lines and matrix, leading to the engine compartment fire.