The aerial application of dusts, granules, sprays, and other materials for agricultural purposes.
The first patent relating to the use of aircraft in agriculture was granted to Alfred Zimmermann in 1911 by the Imperial Patent Office in Berlin, Germany, for an invention allowing the aerial application of lime water to control moth damage to pine forests. In practice, neither Zimmermann’s invention nor other sporadic efforts succeeded until two Americans, C. R. Neillie and J. S. Houser, controlled Catalpa sphinx caterpillars by dusting them with lead arsenate from a Curtiss JN-6H Jenny biplane near Troy, Ohio, on August 31, 1921.
Agricultural aircraft are designed or equipped to enhance visibility in all directions, to avoid pilot exposure to chemicals with special ventilation, to reduce pilot fatigue, to ensure pilot safety, to protect the aircraft and equipment from corrosive chemical mixtures, and to ensure high performance at slow speeds with heavy loads. The types of aircraft employed in crop dusting include specially designed agricultural monoplanes and biplanes, ex-military and ex-civil aircraft, and helicopters. Specially designed agricultural aircraft, such as the Cessna Ag-Truck, Turbo Thrush, and Ag-Cat and the Skyfarmer T-300A, tend to be more expensive than nonspecialized aircraft that can be adapted to agricultural use. Both ex-military aircraft, such as the Grumman Avenger and the Boeing Stearman, and ex-civil aircraft such as the Douglas DC-6, DHC Beaver, Piper Aztec, Russian Antonov AN-2M, and Pilatus Turbo Porter, have been used as agricultural aircraft.
The use of helicopters in crop dusting has increased worldwide because of helicopters’ many advantages over fixed-wing aircraft, including their superior efficiency as sprayers due to their greater downwash at slower speeds, their ability to land almost anywhere without the need for an airstrip, their greater maneuverability, and their superior visibility. Helicopters’ advantages may be outweighed by their disadvantages, however. Helicopters are far more expensive to purchase, and their operation is much more affected by changes in temperature and humidity than that of other types of aircraft. In addition, helicopters have more moving parts than other aircraft, requiring more maintenance, and they also have smaller centers of gravity, necessitating more careful loading.
During the 1920’s, hand-cranked and horse-drawn ground dusters had been employed with some success against boll weevil infestations devastating the cotton crops of the American South, but application rates were too slow for control. Early crop-dusting airplanes were equipped with sheet metal containers, called hoppers, mounted behind the pilot in the rear seat. An assistant balancing behind the hopper was required to turn the hopper’s feeder crank, causing calcium arsenate to be discharged through a tube in the bottom of the fuselage. Dusting from aircraft proved successful for pest control over large areas.
Experience taught crop dusters that the pesticide dusts had to be constantly agitated and sifted in order for them to be dispersed evenly, leading to hopper designs incorporating rotating paddles or sweeping, windshield-wiper-like blades. Hoppers centrally mounted under the fuselage and equipped with pilot-operated levers eliminated the need for assistants. Tanks suitable for carrying liquid applications, accurate gauges to measure loads, and efficient filters and pumps evolved through trial and error.
Dispersal equipment also evolved, the most popular being the boom and nozzle for spraying liquids and the spreader for dispersing dry materials. The boom, a long, rigid pipe usually attached behind or below the wing of an airplane, supports movable nozzles that can be variably spaced along the boom and rotated to point in any direction to vary the spray pattern. In addition, the size and shape of the orifices on the nozzle can be changed like those on a showerhead to vary droplet size and spray intensity. The most frequently used spreader is suspended beneath the hopper and the fuselage so that the airstream can help blow the hopper contents out through the opening gate of the hopper to be deflected by the spreader. Some spreaders are fan- or wing-shaped, whereas others have divided outlets or rotating disks to disperse materials.
Applications of insecticides, fumigants, herbicides, fungicides, and defoliants are most often associated with crop dusting, but many other materials are commonly applied, including fertilizers, trace elements, and poison bait dropped over wide areas for the control of animal pests such as rabbits and mice. Even seeds for crops such as rice, grass, and vetch are sown from aircraft.
The usual practice in crop dusting is to fly back and forth in straight, parallel lines across the field being treated. If the area is too steep or irregular, the flight lines should follow the contours of the land. Coverage should begin downwind, so that the aircraft can make each swath without passing through an already-sprayed area. Flags are often posted at each end of a field to mark the spacing for each swath.
Turnarounds are the most likely maneuvers to cause accidents and must be made carefully, to prevent both crashes and the accidental dispersal of chemicals over adjoining areas. Maintaining a constant speed and height is important for even distribution. Speed and height of the aircraft are determined by atmospheric conditions and by the material to be dispersed. Controlling drift is necessary to prevent the exposure of humans, livestock, water sources, adjacent crops and pasture, and structures to contamination by hazardous materials. Drift is influenced by many factors, including weather conditions, particle size, specific gravity, evaporation rate, height of release, horizontal and vertical air movement, and aerodynamic effects of the aircraft. These factors lead to concerns about the potential use of crop dusters to disperse hazardous biological or chemical material in a terrorist attack. These planes were grounded temporarily following the hijackings of September 11, 2001.
Anderson, Mabry I. Low and Slow: An Insider’s History of Agricultural Aviation. San Francisco: California Farmer, 1986. Entertaining, informative, and well-illustrated account of the history of agricultural aviation in the United States by a professional crop duster. Quantick, H. R. Aviation in Crop Protection, Pollution, and Insect Control. London: Collins Professional and Technical Books, 1985. Well-written, encyclopedic coverage of all aspects of agricultural aviation. United States Animal and Plant Health Inspection Service. Aerial Application of Agricultural Chemicals. Washington, D.C.: U.S. Department of Agriculture, 1976. A practical manual containing clearly presented technical information and simple, accurate illustrations that enhance comprehension.
Pilots and copilots