Cheyenne, WY, USA
N440SA
ROBINSON R44 II
The commercial helicopter pilot and two passengers were conducting a pipeline inspection flight. After picking up the passengers and prior to conducting the inspection, the pilot landed the helicopter at an airport to obtain fuel. During approach for landing, the passengers heard the low rotor rpm warning horn activate, but the helicopter landed normally. The pilot fueled the helicopter, and during the subsequent departure, the low rotor rpm warning horn activated. The pilot was able to "reposition" the helicopter and depart without incident. After completing a portion of the flight, the helicopter returned to a pipeline terminal near the airport for landing. The pilot stated that, during the off-airport landing, the main rotor rpm began to decay, so he lowered the collective and tried to fly "into clean air;" however, the helicopter was too low, and he realized it was going to hit the ground. The pilot increased collective and leveled the helicopter to cushion the impact. The helicopter impacted the ground from an altitude of about 15 ft and slid about 10 ft before coming to rest. The helicopter was operating about 50 lbs under its maximum gross weight, and the density altitude around the time of the accident was about 8,000 ft. Given these conditions, the helicopter's out-of-ground-effect hover ceiling was about 4,400 ft; its in-ground-effect hover ceiling was about 7,700 ft. Therefore, the helicopter would not have been able to hover out of ground effect, but could hover in ground effect. Operating under such conditions could lead to a settling-with-power or low rotor rpm situation if the helicopter became too slow while out of ground effect. It could not be determined whether the pilot conducted preflight performance planning; however, the repeated activation of the low rotor rpm warning suggests that he did not adequately adjust the helicopter's takeoff and landing profiles to account for the flight conditions present at the time of the accident.
***This report was modified on November 14, 2016. Please see the docket for this accident to view the original report. *** On August 28, 2015, about 1100 mountain daylight time, a Robinson R44 II, N440SA, was substantially damaged during a hard landing near Cheyenne Regional Airport (CYS), Cheyenne, Wyoming. The pilot and two passengers received minor injuries. The helicopter was registered to Submissive Air, Inc., dba Eagle Sky Patrol, Deadwood, South Dakota, and was operated under the provisions of 14 Code of Federal Regulations Part 91 as a business flight. Day visual meteorological conditions prevailed at the time of the accident, and no flight plan had been filed. The purpose of the flight was to conduct pipeline patrol near the Colorado/Wyoming border. After picking up two passengers, the flight proceeded to CYS to obtain fuel. The pilot reported to a Federal Aviation Administration (FAA) inspector that he added about 25 gallons of fuel to the helicopter, for a total of about 36 gallons on board. During takeoff, he received a low rotor rpm warning, but was able to "reposition" the helicopter and "make a normal takeoff." During the patrol flight, one of the passengers needed to replace his laptop computer power supply, and the flight returned to CYS. The pilot stated that during approach for landing, the rotor rpm began to decay about 40 ft above the ground, and he lowered the collective and tried to fly "into clean air." The helicopter was too low, however; and continued to descend to ground contact. Both the front and rear seat passengers stated that they heard the low rotor rpm horn and saw a light on the instrument panel illuminate during the first landing at CYS. The front seat passenger stated that, during the subsequent takeoff, he again heard the low rotor rpm horn. He also stated that it seemed like the pilot was "having a little trouble getting the aircraft to fly." During the second landing at CYS, both passengers again heard the low rotor rpm horn and saw the light illuminate before the helicopter lost lift and landed hard. The helicopter impacted the ground from an altitude of about 15 feet and slid about 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. There was also buckling of the fuselage. The tail boom and main rotor blades appeared to be undamaged, and there was no discernible tail strike. The elevation of CYS is 6,160 feet mean sea level (msl). About the time of the accident, the temperature was 23 degrees C, the dew point was 7 degrees C, and the altimeter setting was 30.30 inches of mercury. The wind was variable from 6 to 15 knots. Density altitude was calculated to be 8,181 feet. Given the weight of the occupants and the fuel onboard at the time of the accident, the helicopter was operating about 50 lbs below its maximum gross weight. According to the manufacturer, "the aircraft would have been able to hover in ground effect (IGE) at 7700 feet, and the out of ground effect hover ceiling would have been a little over 4,000 feet. Therefore the pilot would have been in a situation where he could not hover out of ground effect, but could hover in ground effect. In this situation, if he slowed the aircraft too much while out of ground effect, he could run out of power, leading to settling or low rotor rpm." The FAA Helicopter Flying Handbook, FAA-H-8083-21A , stated, "Helicopter performance revolves around whether or not the helicopter can be hovered. More power is required during the hover than any other flight regime. Obstructions aside, if a hover can be maintained, a takeoff can be made, especially with the additional benefit of translational lift. Hover charts are provided for in ground effect (IGE) hover and out of ground effect (OGE) hover under various conditions of gross weight, altitude, temperature, and power. The IGE hover ceiling is usually higher than the OGE hover ceiling because of the added lift benefit produced by ground effect…As density altitude increases, more power is required to hover. At some point, the power required is equal to the power available. This establishes the hovering ceiling under the existing conditions." The publication also stated, "Under certain conditions of high weight, high temperature, or high density altitude, a pilot may get into a low rotor rpm situation. Although the pilot is using maximum throttle, the rotor rpm is low and the lifting power of the main rotor blades is greatly diminished. In this situation, the main rotor blades have an [angle of attack] that has created so much drag that engine power is not sufficient to maintain or attain normal operating rpm."
The pilot's improper preflight performance planning, which resulted in a hard landing due to low rotor rpm while operating near maximum gross weight in high density altitude conditions.
Source: NTSB Aviation Accident Database
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