NTSB Factual Report

On July 28, 2017, at 1532 central daylight time, a Helicopteres Guimbal Cabri G2 helicopter, N722TX, was substantially damaged when it impacted terrain while maneuvering at the Beaumont Municipal Airport (BMT), Beaumont, Texas. The flight instructor and student pilot sustained minor injuries. The helicopter was registered to American Helicopter Leasing Corp and operated by Texas Rotorwing LLC as a Title 14 Code of Federal Regulations Part 91 instructional flight. Day visual meteorological conditions prevailed. The flight was not operated on a flight plan. The local flight originated from BMT about 1500.

The flight instructor reported that he and the student had completed three normal approaches and then practiced basic hover skills at the north ramp area of the airport. A high engine oil temperature condition occurred during the hovering maneuver due to the ambient air temperature and the reduced airflow available for engine cooling. To increase engine airflow and reduce the engine temperature, he performed a low altitude circuit at 60 knots, about 100 ft above ground level (agl), around the perimeter of the airport. About halfway around the perimeter, the oil temperature had returned to the normal operating range and all other engine indications were normal at that time. However, during the turn at the southwest corner of the airport, the helicopter began to lose altitude and would not respond to his control inputs. His efforts to regain control were not successful and the helicopter impacted the open grass area on the airport southwest of the runway.

The instructor noted that the helicopter was loaded within the gross weight and center-of-gravity limitations. He added that the turns were never more than 45 degrees angle of bank. He reported that there were no anomalies with regard to the helicopter, and that the high temperatures, low wind, and high density altitude likely resulted in the accident.

A Federal Aviation Administration inspector responded to the accident site and conducted an examination of the helicopter. The flight instructor informed him that he was completing a turn, during which the helicopter was unable to maintain altitude. The helicopter came to rest on its right side. The main rotor blades were fragmented, and the tail boom was partially separated. Impact marks suggested that the main rotor had struck the ground followed by a landing skid. His examination of the flight control system did not reveal any pre-impact anomalies.

Weather conditions reported by the BMT Automated Weather Observing System (AWOS) did not include any wind information. The most recent observation recorded a temperature and dew point of 35 degrees Celsius and 21 degrees Celsius, respectively. The calculated density altitude was about 2,650 feet.

Wind conditions reported by the Jack Brooks Regional Airport (BPT) Automated Surface Observing System (ASOS), located about 12 miles southeast of BMT, were variable at 4 knots at the 1453 observation and variable at 3 knots at the 1553 observation.

The maximum engine power was derated as installed in the Cabri G2 helicopter to 145 hp for both takeoff and continuous operation. However, under the accident conditions, the engine was capable of producing approximately 160 hp. Application of full throttle by the pilot would be expected to provide 160 hp under the accident conditions. Engine performance was referenced to the 145 hp continuous limit, with 100% power corresponding to 145 hp and 111% power corresponding to 160 hp. In addition, the helicopter geometry implied that an angle of bank in excess of 40° would result in the main rotors impacting the ground before the landing skids.

The helicopter manufacturer stated that at an operating weight of 1,440 lbs. (650 kg) about 90% engine power would be required to maintain an in-ground-effect (IGE) hover at 32 ft agl and 32°C. Application of full throttle (111% power) would have provided a 21% power margin for an IGE hover under the accident conditions. However, maintaining altitude in a 45° bank turn would require a 1.4 load factor which exceeded the available 21% power margin.

NTSB Probable Cause

The flight instructor's low-altitude maneuvering, which exceeded the helicopter's performance capability in the high-density altitude conditions and resulted in a loss of control.