Accident Details
On August 28, 2015, at approximately 1100 mountain daylight time, a Robinson R44 II helicopter, registration N440SA, was substantially damaged during a hard landing near Cheyenne Regional Airport (CYS), Cheyenne, Wyoming. The pilot and two passengers sustained minor injuries. The helicopter was registered to Submissive Air, Inc., doing business as Eagle Sky Patrol, based in Deadwood, South Dakota, and was operated under 14 CFR Part 91 as a business flight. Day visual meteorological conditions prevailed, and no flight plan was filed.
Flight Purpose and Events
The flight was intended for pipeline patrol near the Colorado/Wyoming border. After picking up two passengers, the helicopter proceeded to CYS for fuel. The pilot reported adding about 25 gallons of fuel, for a total of about 36 gallons on board. During initial takeoff, a low rotor rpm warning was received, but the pilot was able to reposition the helicopter and make a normal takeoff. During the patrol flight, one passenger needed to replace a laptop power supply, prompting a return to CYS.
Landing Sequence
The pilot stated that during the approach for landing, the rotor rpm began to decay when the helicopter was about 40 feet above the ground. He lowered the collective and attempted to fly into "clean air," but the helicopter was too low and continued descending until ground contact. Both passengers reported hearing the low rotor rpm horn and seeing an instrument panel light illuminate during the first landing at CYS. The front seat passenger also heard the horn during the subsequent takeoff and noted that the pilot seemed to have difficulty getting the aircraft to fly. During the second landing, both passengers again heard the horn and saw the light before the helicopter lost lift and landed hard.
Impact and Damage
The helicopter impacted the ground from an altitude of about 15 feet and slid 8 to 10 feet. Both skids were spread apart and broken, and the helicopter came to rest on its fuselage. Both tail rotor blades separated, and the vertical stabilizer was damaged. Fuselage buckling was observed. The tail boom and main rotor blades appeared undamaged, and no discernible tail strike occurred.
Environmental Conditions
Cheyenne Regional Airport has an elevation of 6,160 feet mean sea level. At the time of the accident, the temperature was 23°C, dew point 7°C, and altimeter setting 30.30 inches of mercury. Wind was variable from 6 to 15 knots. Density altitude was calculated to be 8,181 feet. The helicopter was operating about 50 pounds below its maximum gross weight given the occupants and fuel onboard.
Manufacturer's Performance Information
According to the manufacturer, the aircraft would have been able to hover in ground effect (IGE) at 7,700 feet, while the out-of-ground-effect (OGE) hover ceiling would have been a little over 4,000 feet. The pilot would have been in a situation where he could not hover OGE but could hover IGE. If he slowed the aircraft too much while OGE, he could run out of power, leading to settling or low rotor rpm.
The FAA Helicopter Flying Handbook (FAA-H-8083-21A) explains that helicopter performance revolves around hover capability; more power is required during hover than any other regime. The IGE hover ceiling is usually higher than OGE due to ground effect. As density altitude increases, more power is required, and at some point power required equals power available, establishing the hovering ceiling. Under conditions of high weight, temperature, or density altitude, a pilot may encounter low rotor rpm; despite maximum throttle, rotor rpm may be low, and lifting power diminished because blade angle of attack creates drag exceeding engine power.