N83702Bell 47G-3B-1 · 1966

On August 6, 2013, about 1030 central daylight time, a Bell 47G-3B-1 helicopter, N83702, made a forced landing after experiencing a decrease in engine performance near Vinton, Iowa. The commercial pilot, who was the sole occupant, was not injured. The helicopter sustained substantial damage. The helicopter was registered to Benedict Fluke and operated by Speas Aviation Inc., under the provisions of 14 Code of Federal Regulations Part 137 as an aerial application flight. Visual meteorological conditions prevailed for the flight which operated without a flight plan. The local flight was departing at the time of the accident.

In a statement provided by the pilot, the helicopter was loaded with 60 gallons of spray product. When the helicopter lifted off of the truck bed, the pilot noted that the rotor rpm decreased so he landed. The pilot then decided to lift off from the ground and did not detect any anomalies with the helicopter. He flew the helicopter forward at 20 feet AGL and 20-25 miles per hour groundspeed over a bean field. The rotor rpm decreased again so the pilot elected to land in the field. The helicopter struck the ground with the tail low, which resulted in damage to the tail rotor and tail boom.

The pilot, age 46, held a commercial pilot certificate for a helicopter and an instrument helicopter rating. He also held a flight instructor certificate for helicopters. The pilot reported having accumulated 1,352 total flight hours, of which 1,315 hours were in a helicopter, and 210 hours were in the make and model of the accident helicopter. He had accumulated 41 hours in the accident helicopter in the preceding 30 days.

The Bell 47G-3B-1 four-seat, single engine, agricultural equipped helicopter was manufactured in 1966. A review of the maintenance logbooks revealed that an annual inspection was completed on July 17, 2013. According to the pilot, the helicopter accumulated 77 hours of total flight time and 41 hours since the last annual inspection.

The helicopter had a maximum gross weight of 2,850 pounds.

At 1035, an automated weather report at the Vinton Veterens Memorial Airpark, which was about 10 miles northeast of the accident, reported wind from 240 degrees at 3 knots, 10 miles visibility, clear skies, temperature 27 degrees Celsius (C), dew point 22 degrees C, and a barometric pressure of 29.89 inches of mercury.

The pilot reported that the wind was variable and favoring 180 degrees at 3 to 5 knots.

The Bell 47G-3B-1 four-seat, single engine, agricultural equipped helicopter was manufactured in 1966. A review of the maintenance logbooks revealed that an annual inspection was completed on July 17, 2013. According to the pilot, the helicopter accumulated 77 hours of total flight time and 41 hours since the last annual inspection.

The helicopter had a maximum gross weight of 2,850 pounds.

The helicopter came to rest upright in a bean field. The main rotor blades and tail rotor blades impacted the ground and sustained damage. The tail boom was separated from the helicopter. A postaccident examination was conducted by the FAA inspector. The examination did not reveal any mechanical malfunctions or failures with the airframe or engine that would have precluded normal operation.

Video Footage

The flight was being recorded by a hand held recording device. A review of the video footage revealed that the pilot flew the helicopter off of the truck bed, flew about 50 yards while maintaining an altitude of about 10 feet above the ground (AGL), and then landed in tall grass. Shortly after, the helicopter lifted off again and flew over a bean field for about 100 yards. The helicopter descended and the pilot executed a forced landing straight ahead in the bean field. The helicopter struck the ground with the tail low, which resulted in damage to the tail boom and tail rotor.

The video footage was reviewed by a representative of Scott's Helicopter Services, the helicopter's type-certificate holder and a party to the investigation. He noted that when the helicopter first settled in the grass, the pilot should have assessed the weight and balance and possibly off-loaded applicant. The flight path of the second attempt, the pilot did not fly along the road, but chose a flight path over standing crops. The engine sounds were normal during the takeoff but then sounded as though the engine was "lugged down" as the takeoff continued.

Weight and Balance and Density Altitude Calculations

According the responding Federal Aviation Administration (FAA) inspector, the weight and balance calculations revealed that the helicopter was loaded 16 pounds under the maximum gross weight. The density altitude at the time of the accident was calculated about 2,800 feet.

Helicopter Flying Handbook – FAA-H-8083-21A

Chapter 2 "Aerodynamics" defines "In Ground Effect (IGE)" as the increased efficiency of the rotor system caused by interference of the airflow when near the ground. The air pressure or density is increased, which acts to decrease the downward velocity of air. Ground effect permits relative wind to be more horizontal, lift vector to be more vertical, and induced drag to be reduced. These conditions allow the rotor system to be more efficient. Maximum ground effect is achieved when hovering over smooth hard surfaces. When hovering over surfaces as tall grass, trees, bushes, rough terrain, and water, maximum ground effect is reduced. Rotor efficiency is increased by ground effect to a height of about one rotor diameter (measured from the ground to the rotor disk) for most helicopters.

An excerpt from Chapter 7 "Helicopter Performance" states:

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. Any adjustment to the gross weight by varying fuel, payload, or both, affects the hovering ceiling. The heavier the gross weight, the lower the hovering ceiling.