Crop Duster Corrosion
Written by Jeremy R.C. Cox, A&P IA and ex-duster pilot.
In order to understand corrosion on crop dusters, it is helpful to understand a bit about the chemicals that the aircraft is being exposed to.
Virtually all agricultural chemicals are water-soluble. In fact, most Ag chemicals are purchased in an undiluted form and are then mixed with water prior to pumping into a spray tank for deposition onto a crop. Hopper size varies from aircraft to aircraft depending upon the performance capabilities of each. Generally, hoppers are between 200 and 800 gallons each. With this amount of water/chemical mix being placed on board an aircraft, corrosion is a very serious hazard to the structural integrity of the airframe. Hoppers are generally constructed of reinforced fiberglass. This saves weight and also cuts down on normal tank corrosion.
The liquid pump system is normally made from cast aluminum with plastic components. The distribution and delivery system is normally constructed of stainless steel and plastic. Most hose connectors/couplings are Viton or another type of composite rubber. These hose connectors are normally clamped onto the stainless pipes and fittings with hose clamps.
If the chemical delivery system is functioning correctly without any leaks, corrosion will not occur. In a perfect world, from a maintenance and inspection point of view, it would only be necessary to look for and guard against surface corrosion caused from the delivery of the chemical mix. However, most corrosion found on an agricultural aircraft can usually be tied back to improper chemical loading procedures. All solid chemicals, mainly fertilizers, come in a granulated, powder, or dust form.
These materials are loaded into the hopper at the top of the tank, by a funnel loader that is positioned over the top of the aircraft by a front-end loader. Liquids can be loaded by the same method, but usually to save time a system just like the single point refueling system on the average jet aircraft is incorporated into the Ag aircraft loading system. If the loading personnel are not careful to watch the pressure and speed of delivery into the aircraft, the hopper can either rupture or overflow. Possibly the most common occurrence of aircraft chemical contamination simply due to the environment the aircraft is flying in. Fabric covered, 4130 steel-tube frame construction is not very common, but can still be found in Ag aircraft design as fabric covering was very effective at keeping the chemical mix away from the primary structure. However, aluminum construction aircraft have far surpassed the popularity of fabric covered aircraft because of its durability and reduced the need for routine maintenance.
Obviously, fabric covering is not universal in all Ag aircraft and in a lot of designs, the airframe has aluminum skins. In this instance, a rigid schedule of keeping the paint protective finish in good condition is paramount. But internally, the only thing that can be done to protect against chemical corrosion is to continually inspect the internal structure, clean up any spills, and vigorously attack any corrosion that is starting — immediately.
Most Ag aircraft have large removable access panels that allow unhindered access to the fuselage interior. Any anti-rub tapes or insulation materials that have been contaminated must be replaced, otherwise, they will hold the damaging chemical/water mix and encourage corrosion to attack the structure beneath.
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