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.
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