“The distribution of the baits from a helicopter is far from certain. While DGPS has allowed more accurate placement of the aircraft there are other factors involved :-

1. The groundspeed of the machine during the delivery run.

Over level terrain this is quite easily controlled but over steep terrain (where the helicopter is most ‘useful’) every act of climbing and descending to match the contour changes the ground speed.
This is difficult to control because the pilot (if he’s smart) will be concentrating on terrain avoidance and personal welfare to the detriment of accurate groundspeed monitoring.
Another negative factor is wind speed and its degree of crosswind (or even tail wind) component. Again, in level terrain the crosswind component is relatively constant and can be allowed for but in broken country wind changes are legendary including lift, lee turbulence, rotor action, and a raft of others which sensible pilots do their best to avoid.
The changing wind speeds and directions rapidly change the aircrafts airspeed indication for which the pilot must compensate to maintain the target groundspeed. It also rapidly changes the aircraft direction of travel and rate of climb which has to be constantly corrected. When you combine terrain avoidance with constant altitude changes and violently changing wind speed and direction it is not hard to see that accurate distribution of baits is not likely.

2. The height of the machine above the ground during the delivery run.

It is assumed that the hopper delivery system is designed to produce a certain pattern on the ground when delivered from a certain height. If the delivery is from a lesser height then the pattern will not have reached its full dimension and a narrow strip of high bait concentration will be laid. If the delivery is from a greater height than the target then the delivery pattern may be wider with reduced bait density. If delivery from the ideal height results in the baits falling vertically downwards then distribution from a greater height is okay except that the baits are longer in the air and are therefore more susceptible to wind currents moving them out of the intended delivery zone. Trying to maintain an accurate height above ground in steep and broken country while trying to maintain groundspeed and avoid terrain creates a scenario ripe for failure.

3. Accurate delivery relies on the baits exiting the hopper and dropping onto the spinning distribution disc at a constant rate.

This is highly unlikely. The greater the head (weight) of baits in the hopper the greater is the pressure available to push the bottom baits out. As the hopper level reduces the pressure reduces and so the flow of baits also reduces. It is possible that there is some compensation for this negative affect provided by the reducing friction losses against the hopper walls as the hopper level falls but given slippery carrots and free flowing cereal baits it is hard to imagine this mitigation being enough. It is a pretty safe bet that the distribution density of baits at the start of a run is greater than at the end.

4. Overlap.

If a circle of material is continuously dropped while in motion then the path along the centre line of the circle will receive twice as much material as the edges. This is because the material which is thrown forward is added to by the material which is thrown backwards over exactly the same path whereas the material thrown to the extreme edges cannot be duplicated. This provides a distribution pattern with the highest concentration along the centreline of the direction of travel gradually reducing to the minimum distribution density along both edges. It is not possible to produce an even distribution density from a moving 360° rotating disc. It is assumed the applicators mitigate against this effect by overlapping each delivery run so the low distribution density strips along the edges are ‘double dosed’ to achieve the target density.
Again it is assumed this need to overlap is programmed into the tracks to be flown under GPS guidance but is another error inherent in the entire delivery system.

Unintended by-kill is closely correlated with bait distribution.

Having a DGPS in the cockpit of the delivery aircraft is certainly a help but many other factors need assessment and controls added to improve the chance of accurate delivery.
ERMA failed to investigate and understand the variables associated with bait delivery and how poor distribution might easily result and instead nominated DGPS as a ‘cure all’ which it cannot be.”

Look closely at the photo above in the header image and you will see baits flying in all directions. This helicopter is low to the bush. Imagine that same helicopter at height and you can start to realize the inaccuracy of a bait drop.