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.
When I spent a year teaching at a crop dusting school in Oklahoma, we instilled into the minds of all student duster pilots, the importance of keeping their aircraft fastidiously clean. After every flight, each student would wash their Ag aircraft. At this time, they would be inspecting for any corrosion that might be starting to infect their machine. In addition to religiously washing their aircraft they would polish the inside of their engine cowlings as well. This additional action made it easier to spot any oil or fuel leaks that might be developing in the engine compartment so that they could be repaired before catastrophe could strike. In reality, on average, crop dusters wash on a weekly basis but, there is a wide variety of philosophy on aircraft washing.
The dangers of working around crop duster chemicals
The working life of an aircraft technician is not inherently dangerous overall. Spinning propellers, liquid oxygen, high pressure hydraulic systems, high voltage ignitors, sharp aluminum panels, heavy starter generators, high platforms, bad pilots, etc. are all some of the dangers that a technician is exposed to.
However, if you are faced with performing maintenance and repairs to agricultural aircraft, your occupational risk is increased.
As if exposure to methyl ethyl ketone, zinc chromate, jet fuel and many other chemicals are not enough, now you are dealing with agricultural chemicals as well — some of which can be deadly to you, if you do not know how to handle yourself around them. Usually agricultural aircraft carry and dispense four different groups of chemicals. These are insecticides, herbicides, fungicides, and fertilizers.
By the way, fertilizers are the worst source of corrosion. If you know that the aircraft is in contact with a lot of fertilizers (especially powder-type fertilizers that become airborne), you should make a note of it and conduct a much more detailed inspection on the aircraft.
How can you best protect yourself against these harmful chemicals? First of all, it's best not to deal with them at all. If the aircraft looks dirty, don't deal with it.
But if you insist, you've got to do a thorough cleaning of the aircraft before working on it while also complying with all EPA and OSHA regulations. The operator is certified to work with these chemicals, so it's really best to let him do that.
Again, regular inspection of the aircraft is the very best defense against any major problems developing in the future. When an Ag operation is run correctly, a phenomenal amount of wastewater is accumulated. Safe disposal of this water is very important. Usually the aircraft washing area is designed to catch and collect the waste, which can then be pumped into waste disposal containers or recycled as make-up water for a new chemical batch.
If you handle the AG chemical waste in the same way as you do with normal aircraft paint stripping waste, you normally can't go wrong. However, always make sure that any clothes that you have worn to work on an Ag aircraft are washed separately from the rest of your laundry, so as to guard against any contamination of other clothing.
It is always recommended to rinse any items that have come into contact with any Ag chemicals, at least three times before re-use. Please observe this triple-rinse rule at all times. The EPA uses this "triple-rinse" as a standard for de-contamination of containers. When a container has been "triple-rinsed," it can then be recycled as a "de-contaminated" container.
When dealing with a tubular steel framed structure, annual inspection of the tubes with a fabric tester is prudent. Any time the interior of the steel tube is corroded, the fabric tested will indent the tube and you will hear the rust become dislodged. If the tubular structure has small access holes, (sometimes filled with a small rivet), you should coat the inside of the tubes with warm linseed oil or a suitable corrosion protection compound. Drain off the excess and then reinstall the plug rivet. The linseed oil provides a suitable barrier against corrosion occurring internally within the tube.
With aluminum monocoque structure, a regular coating of corrosion inhibitors is also good practice. But, in this instance, be very careful to observe if the corrosion protective solution, which turns to a very thin waxy film, does not become contaminated with Ag chemicals during flight operations. Fresh water, a good acid free detergent soap, a brush, dry towels, and a good polish are the best tools in an Ag technician's armory against corrosion.
Air Tractor Spar Inspections
Although air tractor spars are engineered to withstand some incredible loads and are quite forgiving in terms of corrosion removal, the spars can be continuously subjected to harsh chemicals which exfoliate the aluminum. Typically, the spar is quite beefy, and up to a 1/4 inch of material can be removed from the spar cap depending on the location. But despite this extra margin of safety built into these spars, remember that this aircraft carries over three hundred gallons of agricultural chemicals and pulls G forces that are quite unlike a typical aircraft.
For this reason, Air Tractor provides detailed inspection and corrosion removal instructions for its products. As an example, its service letter No. 90 addresses "Extreme corrosion on wing spar caps."
The following is a portion of this letter, which applies to models AT-300, -301, -302, some -400, and -400A aircraft:
An AT-301 owner in south Texas noticed a bump on the lower wing skin of his aircraft. The bump was immediately below the lower spar cap in the fuel tank area. Inspection plates were removed and the wing lower spar cap was found to have extremely deep corrosion in several places.
The most alarming aspect is that this type of corrosion mushrooms out from the spar cap with the appearance of exfoliation. To prevent the possibility of wing failure, it is strongly suggested that an immediate inspection be made of your aircraft wing spar caps. Inspection plates on the lower side of the wing leading edge skin should be removed, and the spar caps visually inspected with a flashlight and mirror for signs of corrosion.
The inboard portion of the main wing spar can best be inspected with the hopper removed. Hopper removal greatly facilitates inspection of the wing attach angles, tubing, and other highly stressed parts around the wing connection. In some cases, it may be possible to remove the corrosion without replacing the spar cap.
All evidence of corrosion must be removed before refinishing is begun. Do not use steel wool, steel wire brush, or emery cloth to remove the corrosion, because particles may break off and become embedded in the spar cap and cause further corrosion. Sanding with aluminum oxide paper or grinding with an aluminum oxide impregnated wheel is acceptable.
Sanding strokes should always be along the spar in an inboard-outboard direction so that any small scratches that may remain will not be across the spar cap. Finish work should be done with a very fine grit paper or polished with household abrasive powder. The edges of the damaged area must be blended or faired out to the undamaged area.
Before proceeding with finish operations, examine closely, preferably with a 5 to 10 power glass, to be sure that all traces of corrosion have been removed. Any corrosion that remains will soon break out again.