Discussed briefly in this section are most of the trouble areas common to all aircraft. However, this coverage is not necessarily complete and may be amplified and expanded to cover the special characteristics of the particular aircraft model involved by referring to the applicable maintenance manual.
Exhaust Trail Areas
Both jet and reciprocating engine exhaust deposits are very corrosive and give particular trouble where gaps, seams, hinges, and fairings are located down the exhaust path and where deposits may be trapped and not reached by normal cleaning methods. Special attention should be paid to areas around rivet heads and in skin crevices. Fairings and access plates in the exhaust areas should be removed for inspection. Exhaust deposit buildup in remote areas such as the empennage surfaces should not be overlooked. Buildup in these areas will be slower and sometimes completely absent, but it has become a problem on some currently operating aircraft.
Battery Compartments and Battery Vent Openings
Despite improvements in protective paint finishes and in methods of sealing and venting, battery compartments continue to be corrosion problem areas. Fumes from overheated electrolyte are difficult to contain and will spread to adjacent cavities and cause a rapid, corrosive attack on all unprotected metal surfaces. Battery vent openings on the aircraft skin should be included in the battery compartment inspection and maintenance procedure. Regular cleaning and neutralization of acid deposits will minimize corrosion from this cause.
These are natural sumps for waste hydraulic fluids, water, dirt, and odds and ends of debris. Residual oil quite often masks small quantities of water which settle to the bottom and set up a hidden chemical cell. Seaplane and amphibian aircraft bilge areas are protected by small bags of potassium dichromate inhibitor suspended near the low point in each bilge compartment. These crystals dissolve in any waste water and tend to inhibit the attack on exposed metal surfaces.
Inspection procedures should include replacement of these bags when most of the chemical has been dissolved. Particular attention must be paid to areas located under galleys and lavatories and to human waste disposal openings on the aircraft exteriors. Human waste products and the chemicals used in lavatories are very corrosive to the common aircraft metals. Clean these areas frequently and keep the paint touched up.
Wheel Well and Landing Gear
This area probably receives more punishment due to mud, water, salt, gravel, and other flying debris than any other area on the aircraft.
Because of the many complicated shapes, assemblies, and fittings, complete area paint film coverage is difficult to attain. A partially applied preservative tends to mask corrosion rather than prevent it. Due to heat generated by braking action, preservatives cannot be used on some main landing gear wheels. During inspection of this area, pay particular attention to the following trouble spots:
1. Magnesium wheels, especially around boltheads, lugs, and wheel web areas, particularly for the presence of entrapped water or its effects.
2. Exposed rigid tubing, especially at B-nuts and ferrules, under clamps and tubing identification tapes.
3. Exposed position indicator switches and other electrical equipment.
4. Crevices between stiffeners, ribs, and lower skin surfaces, which are typical water and debris traps.
Water Entrapment Areas
Design specifications require that aircraft have drains installed in all areas where water may collect. Daily inspection of low point drains should be a standard requirement. If this inspection is neglected, the drains may become ineffective because of accumulated debris, grease, or sealants.
Engine Frontal Areas and Cooling Air Vents
These areas are being constantly abraded with airborne dirt and dust, bits of gravel from runways, and rain erosion which tend to remove the protective finish. Inspection of these areas should include all sections in the cooling air path, with special attention to places where salt deposits may be built up during marine operations. It is imperative that incipient corrosion be inhibited and that paint touchup and hard film preservative coatings be maintained intact on seaplane and amphibian engine surfaces at all times.
Wing Flap and Spoiler Recesses
Dirt and water may collect in flap and spoiler recesses and go unnoticed because they are normally retracted. For this reason these recesses are potential corrosion problem areas.
External Skin Areas
External aircraft surfaces are readily visible and accessible for inspection and maintenance. Even here, certain types of configurations or combinations of materials become troublesome under certain operating conditions and require special attention.
Relatively little corrosion trouble is experienced with magnesium skins if the original surface finish and insulation are adequately maintained. Trimming, drilling, and riveting destroy some of the original surface treatment which is never completely restored by touchup procedures. Any inspection for corrosion should include all magnesium skin surfaces with special attention to edges, areas around fasteners, and cracked, chipped, or missing paint.
Piano-type hinges are prime spots for corrosion due to the dissimilar metal contact between the steel pin and aluminum hinge. They are also natural traps for dirt, salt, and moisture. Inspection of hinges should include lubrication and actuation through several cycles to ensure complete lubricant penetration.
Corrosion of metal skin joined by spot welding is the result of the entrance and entrapment of corrosive agents between the layers of metal. This type of corrosion is evidenced by corrosion products appearing at the crevices through which the corrosive agents enter. More advanced corrosive attack causes skin buckling and eventual spot weld fracture. Skin buckling in its early stages may be detected by sighting along spot welded seams or by using a straightedge. The only technique for preventing this condition is to keep potential moisture entry points, including seams and holes created by broken spot welds, filled with a sealant or a suitable preservative compound.
Miscellaneous Trouble Areas
Helicopter rotor heads and gearboxes, in addition to being constantly exposed to the elements, contain bare steel surfaces, many external working parts, and dissimilar metal contacts. These areas should be inspected frequently for evidence of corrosion. The proper maintenance, lubrication, and the use of preservative coatings can prevent corrosion in these areas.
All control cables, whether plain carbon steel or corrosion resistant steel, should be inspected to determine their condition at each inspection period. Cables should be inspected for corrosion by random cleaning of short sections with solvent soaked cloths. If external corrosion is evident, tension should be relieved and the cable checked for internal corrosion. Cables with internal corrosion should be replaced. Light external corrosion should be removed with a steel wire brush. When corrosion products have been removed, recoat the cable with preservative.