What happened
On April 26, 2017, a Bell 206 B3 helicopter, registration VT-DDA, was performing a charter flight to transport paramilitary personnel in Chhattisgarh, India. After completing several flight sectors, the aircraft was approaching the Chintagufa helipad for landing. The pilot had performed a reconnaissance flight at approximately 600 feet above ground level and received a smoke signal indicating the landing area was clear.
Due to high ambient temperatures of approximately 40°C, the pilot executed a shallow approach to minimize power requirements, maintaining 100% torque. As the helicopter leveled off roughly 3 feet above the ground, it encountered sudden thermal convection and crosswinds blowing from the left. During this hover in ground effect, the aircraft experienced a sudden loss of directional stability, characterized by an unexpected yaw to the left. The pilot attempted to correct this by applying right pedal, but the aft portion of the left skid struck the ground. The momentum caused the aircraft to bounce and strike the right skid, leading to a dynamic rollover where the main rotor blades struck the ground, causing extensive damage to the airframe.
The investigation
AAIB India examined the wreckage and flight data to reconstruct the sequence of events. The investigation focused on the aircraft's performance under high-temperature conditions and the impact of environmental factors on flight control effectiveness. Investigators analyzed the damage to the main rotor blades, mast, and tail rotor drive shaft, concluding that all damage was consequential to the impact and that the engine was still running at the time of the accident. The investigation also reviewed the pilot's credentials, finding the pilot was fully qualified with a valid CPL (H) and FATA, and that the aircraft's maintenance and weight/center of gravity were within regulatory limits.
Findings
- The aircraft was operating at 100% torque during a shallow approach to manage performance in high heat.
- High outside temperatures and the aircraft's position in ground effect led to reduced tail rotor control effectiveness.
- Thermal convection and left-to-right crosswinds contributed to an unexpected yaw to the left.
- The application of right pedal to correct the yaw, combined with the initial contact of the left skid, initiated the rollover sequence.
- The aircraft's center of gravity moved beyond the pivot point of the right skid, resulting in the main rotor blades striking the ground.