Flow of particulate material from a topdressing aircraft : a thesis presented in partial fulfilment of the requirements of the degree of Doctor of Philosophy in Agricultural Engineering at Massey University, New Zealand
Fixed wing agricultural aircraft apply approximately 40% of the fertiliser used in New
Zealand, the majority of which is applied in hill country. The amount varies from
approximately 600,000 tonnes to 1.2 million tonnes per annum.
About 100 fixed wing aircraft of various types are engaged in agricultural operations
throughout the country and the safety record has been of considerable concern; the
Civil Aviation Authority (CAA) of New Zealand report that there are 12 serious
accidents per 100,000 flying hours which result in 4 deaths, almost 2 annually.
Agricultural aviation stakeholders, including, the Department of Labour, Civil
Aviation Authority, New Zealand Agricultural Aviation Association (NZAAA)
and Federated Farmers are trying to reduce the number of incidents in the sector by
establishing guidelines for airstrips, fertiliser storage facilities, their use and application
A large proportion of incidents have, as contributing factors, poor flowing product
which cannot be jettisoned in time to avert an accident, collisions with obstructions
near the airstrip (20% of all accidents are aircraft hitting fences in proximity to the
airstrip) and damage to aircraft due to an inappropriate surface, such as rutting.
The New Zealand topdressing industry handles many products of which only a few
are homogenous e.g. Urea and fresh Di-ammonium phosphate. The majority of
spreading being undertaken involves products with large variations in particle size and moisture content producing particles from fine dust to concretions. These
characteristics make it very difficult to achieve continuous flow and even spreading
from an aircraft. There have been a disproportionate number of accidents and near
misses in the New Zealand topdressing industry that have occurred whilst spreading
agricultural limestone (lime). Lime has been identified as being particularly
problematic and is being used as a focus for this study. Superphosphate, which is used
as a flow standard in New Zealand Civil Aviation Authority rules, is used as a
comparison in powder flow engineering experiments.
This thesis is a prescribed project concerned with solving specific problems for
industry mainly funded by the Fertiliser Manufacturers’ Research Association.
Specific objectives/aims of the project:
1. Quantify the flow characteristic of products being spread and identify risks
within the system, identifying risk materials and risk situations.
2. Develop a better understanding of material variability in terms of
characterising the different deposits used around New Zealand and relating
these differences to flow properties.
3. Develop a better understanding of the mechanisms creating the variability in
flow properties that relate to production processes, transport and storage and
finally loading and spreading with topdressing aircraft of the different limes
used in New Zealand.
4. Quantify system performance in terms of economic and environmental
impacts.5. Identify suitable test methodologies that can be used within the industry to
determine whether a product is fit for spreading and its flow characteristics.
These would be dispatch tests at the lime quarry or fertiliser plant and a
flowabiltiy test as the material is loaded onto the aircraft.
6. Identify design criteria that determine the performance of aircraft in relation to
safety, flow control and spreading performance. Work with the interested
parties to improve work quality and safety associated with agricultural
Flow properties have been quantified using a shear testing regime and engineering
design parameters established for mass flow have been calculated from interpreting
the powder flow functions. However, as the material from each quarry has variations
in particle size distributions caused by factors such as the moisture of the parent
limestone, age of the crushing hammers and time being crushed; the results are only
an instantaneous solution. All commonly used products except lime are free flowing
and shear testing was undertaken on superphosphate samples as a comparison. All the
limes tend to be on the cohesive – easy flow boundary.
Limes from throughout New Zealand have been classified by mineralogy, have been
analysed by thermal decomposition and have had impurities identified through X-ray
diffraction. Although there were differences in the particle size distributions and loose
and tapped bulk densities between the limes, helium pycnometry testing showed the
limes to have similar particle densities.
In order to achieve free flow conditions with these products they require modification.
The simplest modification that proved effective was the removal of fine particles.This had the effect of reducing the particle size distribution which is important in
reducing the packing density and cohesive strength. This was also achieved by only
having particles within a narrow particle size range, by removing the fine particles the
cohesive strength was reduced and the materials were free flowing.
Although this can be done there is clearly a cost involved, the industry is already
struggling with reduced demand and any increase in cost is likely to be unwelcome
even though it could help to save pilots’ lives and improve the quality of spread
This thesis considers three aspects of topdressing costs in order to estimate the actual
costs of spreading fertiliser and lime. The questions posed are; what are the actual
costs of operating the two main models of aircraft flown in New Zealand? What size
of aircraft fleet is required to fulfil the spreading requirements? What are the on-farm
infrastructure costs that also need to be considered in order to calculate the true costs
of servicing the application of fertiliser to our hill country sector?
Topdressing services mainly the sheep and beef sectors which contribute 22.5% of
New Zealand’s agricultural output. Farm income in this sector is nearly $4 billion.
Application of fertiliser is important to sector productivity and the possible collapse of
the topdressing industry would have far reaching consequences for these farming
sectors and New Zealand’s export earnings.
The model finds that there is no financial return on capital invested in the industry.
Therefore, the best returns are found by applying fertiliser from old aircraft with aged support vehicles all with little capital value. This is clearly unsustainable as even old
aircraft require large injections of capital periodically to maintain airworthiness.
As fertiliser prices have increased, application rates have fallen, which increases
application cost per tonne applied. The agreed fixed price charging model is
traditionally based on an application charge per tonne. It is likely that farmers
perceive increased application charges per tonne as a price increase, whereas it is only
compensating the applicator for the additional time of sowing at a lower rate.
It is clear that although farmers buy fertiliser on a cost per tonne basis this is not the
activity based cost driver for the aerial applicator. Converting the cost per hour
aircraft cost driver, to a cost per tonne for charging farmers; is confusing as
application charges alter by rate and product. The industry needs to alter its charging
mechanism to a cost per aircraft flying hour activity based charging regime.