What happened
On April 23, 2008, a Bell 206 B Jet Ranger, registration PK-DAT, was conducting a scheduled tourist flight over Bali, Indonesia. After completing an initial flight from Ngurahrai Airport to Kintamani, the aircraft underwent refueling before commencing a second mission carrying four passengers for a sightseeing tour of the region.
Approximately one hour and 45 minutes into the second flight, while operating near the Sanur area, the pilot noticed the aircraft's nose dropping. A check of the turbine indicator (N2) revealed a sudden loss of engine power. The pilot immediately initiated engine shutdown procedures and executed an emergency landing in a rice field near the Pantai Kramas area in Labih Village, Gianyar. While the aircraft sustained substantial damage, including a broken tail boom and damaged rotor blades, the pilot and all four passengers escaped without injury.
The investigation
The investigation focused on the mechanical failure of the Allison 250-C20 engine. Investigators examined the Fuel Control Unit (FCU) and identified that a plumbing connection, specifically a "B" nut at the Pc port, had detached. The absence of torque paint marks at the connection indicated that the nut had slipped from its position.
Technical analysis revealed that because the Pc port was detached, the compressor discharge pressure could no longer be properly sensed. This allowed outside air pressure to enter the system, causing the fuel metering valve to revert to an idle position. Consequently, despite the pilot's attempt to maintain power via the twist grip, the engine could not produce sufficient RPM, leading to the power loss.
Findings
- The primary cause of the engine power loss was the detachment of the B-nut plumbing connection at the Pc port of the Fuel Control Unit.
- The detachment of this connection prevented the sensing of compressor pressure, which forced the fuel metering valve into an idle position.
- The pilot and passengers were unharmed during the emergency landing.
- The aircraft's tail boom was broken by the main rotor shaft during the impact with the ground.
Safety action
Following the investigation, the NTSC recommended that the Directorate General Civil Aviation ensure all operators utilizing Allison 250 series engines strictly adhere to EASA Airworthiness Directive 2004-0009 R2, which mandates regular inspections of control system plumbing and the verification of B-nut torque and slippage marks.