Accident Overview
On December 23, 2023, about 2230 Pacific standard time, a Bell 206B helicopter, registration N111PM, was substantially damaged during an accident near El Centro, California. The pilot was not injured. The helicopter was operating under Title 14 Code of Federal Regulations Part 137 as an aerial application flight.
Pilot Account
According to the pilot, the helicopter departed from a flatbed truck with approximately 73 gallons of pesticide and 200 pounds of fuel on board. It then flew toward the intended application site at about 100 feet above ground level. Abruptly, the helicopter began producing a loud noise and vibrating badly. Initially unsure if the engine was losing rotor rpm, the pilot lowered the collective and initiated a run-on landing to a field.
Postaccident Examination
Examination of the airframe and engine revealed substantial damage to the engine-to-transmission driveshaft, known as a KAflex driveshaft, which had a fractured flex plate and gouging of the isolation mount. No other mechanical malfunction or failure was observed that would have precluded normal operation.
The National Transportation Safety Board Materials Laboratory examined the KAflex driveshaft. The forward flex plate at the forward end of the forward flex coupling was fractured where it had been attached to the forward driveshaft fitting. Fracture surfaces exhibited flat features with curving crack arrest lines consistent with fatigue fracture. Fatigue features emanated across most of the fracture surface on both sides of a hole, consistent with high-cycle fatigue at relatively low nominal stress levels.
The joint between the forward driveshaft fitting and the forward flex plate was loose, with visible gaps when flexed by hand. Bolts attaching the forward flex plate to the adjacent plate were removed. Examination of the aft face of the forward flex plate at the fracture and opposite hole revealed elongated holes with dished surfaces and thinning, consistent with wear from relative motion between the plate and bolts. Bolt shanks at both locations were thinned from contact wear.
Torque paint evidence was found at the joint. Yellowish orange paint remnants on the bolt head and forward flex plate, and green paint on the nut, washer, and fitting lug, were misaligned if the plate was in its original position. On the opposite side, a yellowish orange paint remnant was on the flex plate, with paint absent elsewhere. Most attachment hardware showed evidence of yellowish orange torque paint, but on the aft coupling, torque paint was missing, broken, or incomplete on 3 of 10 bolt heads and all 10 nuts. On the forward coupling, torque paint was missing, broken, or incomplete on half the bolt heads and 6 of 10 nuts, including at the forward flex plate. In four joints with missing paint, the outer flex plate surface showed circumferential rubbing contact.
Maintenance Records
According to Kamatics Service Instruction SIN2348 Rev. K for the KAflex driveshaft, the assembly requires daily preflight inspection for loose or missing hardware and torque strip slippage. If loose or missing hardware is found, the driveshaft must be returned to Kamatics, with a warning not to disturb or tighten fasteners.
Maintenance records showed the most recent routine 100/200/300-hour inspection was completed three weeks before the accident. The KAflex driveshaft had been installed on June 23, 2016, and no evidence of removal after installation was found in the records.