Resolving the Agricultural Crushed Limestone Flow Problem from Fixed-Wing Aircraft

Abstract

There are on average 13 aerial topdressing accidents and about four deaths per 100,000 flying hours in New Zealand; currently, one pilot is killed every 215 days. The number of fixed-wing flying hours undertaken each year varies between 30,000 and 55,000. Crushed agricultural limestone is the most cohesive material commonly applied as a bulk solid from fixed-wing agricultural aircraft in New Zealand, and limestone is present in a disproportionate number of these accidents. The failure of limestone to flow out of the aircraft when desired is one probable cause. In addition, unless products are free flowing, controlling the spread is impossible, and they cannot be applied at the desired application rate from an aircraft. Shear tests on ten commercial Fertmark-registered crushed limestones showed them all to be either cohesive or easy-flow materials as rated by the Jenike flow index. Loose bulk densities and tapped densities were measured for each of the limestones. These were much more variable than the particle densities, which were measured using helium pycnometry. Variations in bulk density are caused by the way the particles pack, which largely reflects differences in the particle size distributions. Removing the fines smaller than 325 µm in a sample of Whiterock limestone, thus reducing the span of the particle size distribution, produced a free-flowing bulk solid whose flow rate was consistent with that calculated using an established equation and parameters. In contrast, the as-received material was a borderline cohesive/easy-flow bulk solid. The cohesion problem, therefore, is apparently caused by the packing of fine particles into the void spaces, which occurs as a result of the wide particle size distributions in as-received commercial products.