Accident Details
On September 4, 2022, at 0757 Pacific daylight time, a Bell 407 helicopter, registration N687AM, was substantially damaged during an accident near Mount Baldy, California. The pilot sustained serious injuries. The flight was conducted under Title 14 Code of Federal Regulations Part 91. The helicopter was used to shuttle people, water, and equipment in support of a local charity event between a lower landing zone (LZ) at about 4,300 ft mean sea level (msl) and the summit of Mount Baldy at 10,064 ft msl, about 4 miles away. The pilot estimated five flights were needed.
Pilot Actions
After dropping off the second load at the summit, the pilot began a return to base camp. He intended to fly down-canyon to lose altitude and set up for a steep approach to the lower LZ. While slowing from about 60 to 40 knots on final approach, the low rotor rpm warning light and horn activated. The pilot noted rotor rpm at about 95%, lowered the collective, and rotor rpm increased to 100%. He did not recall checking the throttle. He aborted the approach, turned down a canyon, and made a mayday call, reporting a partial engine failure. He attempted to raise the collective again, but the low rotor rpm warning re-illuminated. He did not switch the Full Authority Digital Engine Controller (FADEC) to Manual mode. While searching for a landing spot, he identified a clearing but overshot it, landing in trees. The helicopter rolled onto its right side, substantially damaging the main rotor blades. The pilot stated he did not autorotate and used available power.
Engine and Flight Data
The engine control unit (ECU) records snapshot data during exceedances. The incident recorder (IR) logged data for nearly 62 seconds. Primary exceedance messages included Torque Rate, followed by Primary Hard Fault and Reversionary Hard Fault, consistent with main rotor impact triggering the IR and engine shutdown. No other anomalies were found. The IR data showed power lever angle (PLA) values between 48°-50° during the 12 seconds before activation, while normal PLA in the FLY position is 70°. The FADEC provides full governing down to 62° PLA. The engine performed normally at the lower PLA setting.
Postaccident Examination
A postaccident engine run and test showed the engine and ECU operated satisfactorily at all test points. Control continuity from the twist grip throttle to the hydromechanical unit (HMU) was verified. The twist grip was manipulated through various positions (Off, Idle, FLY, Max Ng, 48 PLA); the HMU pointer positions and PLA 1 sensor readings indicated correct rigging. The twist grip uses a ball plunger to assist in maintaining the FLY throttle position by engaging a detent. During examination, the detent could be felt when engaged. Force required to move the ball plunger from IDLE to FLY detent was 4 lb, out of FLY detent to IDLE was 4 lb, and out of FLY detent to MAX Ng was 5 lb. Wear was evident where the ball plunger engaged the detent bracket. No other preimpact anomalies were found.
Maintenance
The Bell 407 maintenance manual specifies recurrent inspections of the twist grip throttle within the progressive inspection program (Event No. 3) or airframe periodic inspections. The required force to move the ball plunger through the detent groove is 14–15 + 0.5 lb. Neither the friction check nor the friction adjustment procedure specifies checking for wear at the FLY detent position. According to maintenance records, the last Event No. 3 inspection occurred on May 10, 2022, at a helicopter total time of 6,873.8 hours. The operator’s checklist for that inspection included the throttle friction check and adjustment. The employee who performed the inspection was no longer with the operator and unavailable for verification.