N861CCBell OH-58A · 2013

On August 9, 2023, about 1230 Pacific daylight time, a Bell OH-58A helicopter, N861CC, sustained substantial damage when it was involved in an accident near Guernsey, California. The pilot sustained minor injuries. The helicopter was operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight.

The pilot reported that before conducting his third aerial application flight of the day, he had reloaded the hopper and refueled the helicopter. He departed the staging area, then conducted “a complete scan of all temperatures and pressures of vital helicopter systems, all of which were within normal operating limits.” After completing his first pass from north to south about 65 mph and 25 ft above ground level (agl), he initiated a 180° climbing left turn. Halfway through the turn, when the helicopter was on an easterly heading, about 100 ft agl and with an airspeed about 35 kts, the helicopter’s low rotor rpm warning light illuminated and the pilot heard the low rotor audio tone in his headset.

The helicopter was banked about 30° left when the pilot lowered the collective to regain main rotor rpm within normal operating limits; he immediately noticed a “stiffness and lag” in the flight controls, which he associated with a loss of hydraulic pressure. He continued the descending left turn and was able to return the helicopter to a level attitude and touch down upright on both skids; however, the right skid contacted a drainage culvert and the helicopter rolled over onto its right side.

The field elevation at the accident site was 194 ft, and the pilot estimated the temperature was 90°F, with variable winds at 2–7 kts and 5-kt gusts. Using these estimates, the density altitude was about 2,387 ft. Automated weather reporting, located at Hanford Municipal Airport (HJO), Hanford, California, about 12 nautical miles northeast of the accident site at an elevation of 249 ft mean sea level, reported the temperature was 88°F, with wind from 190° at 4 kts.

Postaccident examination of the helicopter revealed impact damage to the right side of the fuselage, and the upper-right fuselage airframe was fracture separated at the front door post and windscreen. The tailboom revealed a concave depression about mid-span on the left side and downward flexing of the tail rotor driveshaft. The tailboom was completely separated just aft of the horizontal stabilizers, severing the tail rotor and gear box. Witness marks from the main rotor blades were observed on the tail rotor driveshaft cover, aft of the horizontal stabilizers. The tail rotor assembly remained attached to the gearbox and the vertical stabilizers sustained minor impact damage. The main rotor hub was separated from the mast. Both main rotor blade leading edges revealed yellow paint transfer on the outboard third of each blade. The outboard third of one of the main rotor blades was completely severed.

Examination of the transmission revealed no deformations to the gear teeth on the drive or vertical shaft. The rotor tachometer generator was intact. The oil pump and variable delivery hydraulic pump were removed from the transmission and damage to the rotational shaft was observed. The hydraulic fluid reservoir contained ample hydraulic fluid, with the suction and return lines tight to touch. The servo actuators and valve circuits were secure. The hydraulic boost solenoid circuit breaker and force trim circuit breakers were not popped.

Postaccident examination of the engine revealed no preaccident mechanical malfunctions or failures that would have precluded normal operation. The engine performed within manufacturer specifications during a test run at various power settings.

The manufacturer’s technical manual stated that “hydraulic power failure will be evident when the force required for control movement increases; a moderate feedback in the cyclic and collective controls is felt and the HYD PRESS caution light illuminates. Control movements will result in normal aircraft response in every respect.” According to the FAA Helicopter Flying Handbook (FAA-H-8083-21B), an impending hydraulic failure can be recognized by a “grinding or howling noise from the pump or actuators, increased control forces and feedback, and limited control movement.” The pilot reported that he did not know if the hydraulic pressure caution light illuminated because he was tending to the low rotor rpm.

The pilot estimated that the weight of the helicopter at the time of the accident was 3,179 lbs, about 21 lbs less than its maximum gross weight. The manufacturer’s technical manual stated that rotor rpm limitations were 93% minimum and 110% maximum. Furthermore, the manual stated that the low rotor warning system is activated when rotor rpm drops below 95 ± 1.4%; the rotor rpm is governed by the engine rpm during powered flight.

According to the performance section of the manufacturer’s technical manual, when operating in the environmental conditions estimated by the pilot, the torque required for the helicopter to conduct an out-of-ground effect hover was a minimum of 86%. The 5minute continuous power limitation was 85–100% torque. Additionally, the manual cautioned that low-altitude maneuvering below 35 kts is not recommended in conditions where the power required to hover out of ground effect exceeds maximum continuous power. The maximum continuous power in the environmental conditions estimated by the pilot was about 92% torque.

The pilot reported that the helicopter was in a 30° left bank during the onset of the lower rotor rpm condition. The FAA Helicopter Flying Handbook states, “When you bank a helicopter while maintaining a constant altitude, the ‘G’ load or load factor increases…To overcome this additional load factor, the helicopter must be able to produce more lift.” The Handbook further states that at 30° of bank or pitch, the load factor, and thus the lift required to maintain altitude, is increased by 16% of the helicopter’s gross weight.

According to the FAA Helicopter Flying Handbook, good practices to follow during maneuvering flight include understanding the following flight characteristics:

• Left turns, torque increases (more antitorque).

• Application of forward cyclic (especially when immediately following aft cyclic application), torque increases and rotor speed decreases.

• Know where the winds are.

• In steep turns, the nose drops. In most cases, energy (airspeed) must be traded to maintain altitude as the required excess engine power may not be available (to maintain airspeed in a 2G/60° turn, rotor thrust/engine power must increase by 100%). Failure to anticipate this at low altitude endangers the crew and passengers. The rate of pitch change is proportional to gross weight and density altitude.

The FAA Helicopter Flying Handbook also states that low rotor rpm can lead to a power-on rotor stall:

Known as “overpitching,” this can easily occur at higher density altitudes where the engine is already producing its maximum horsepower and the pilot raises the collective. The corresponding increased angle of attack of the blades requires more engine horsepower to maintain the speed of the blades; however, the engine cannot produce any additional horsepower, so the speed of the blades decreases.

According to the manufacturer’s technical manual, “Droop is defined as the speed change in N2 rpm as power is increased from a no-load condition.” Additionally, the manual stated, “If N2 power is allowed to droop, other than momentarily, the reduction in rotor speed could become critical. If N2 droop occurs, but low rpm warning is not activated and N2 recovers to 100% within 5 seconds, and further droop is not experienced, this is considered a normal flight characteristic.”