Preventive Maintenance
Much has been done to improve the corrosion resistance of aircraft, such as improvements in materials, surface treatments, insulation, and modern protective finishes. All of these have been aimed at reducing the overall maintenance effort, as well as improving reliability. In spite of these improvements, corrosion and its control is a very real problem that requires continuous preventive maintenance. During any corrosion control maintenance, consult the Safety Data Sheet (SDS) for information on any chemicals used in the process.
Corrosion preventive maintenance includes the following specific functions:
- Adequate cleaning
- Thorough periodic lubrication
- Detailed inspection for corrosion and failure of protective systems
- Prompt treatment of corrosion and touch up of damaged paint areas
- Accurate record keeping and reporting of material or design deficiencies to the manufacturer and the FAA
- Use of appropriate materials, equipment, technical publications, and adequately-training personnel
- Maintenance of the basic finish systems
- Keeping drain holes free of obstructions
- Daily draining of fuel cell sumps
- Daily wipe down of exposed critical areas
- Sealing of aircraft against water during foul weather and proper ventilation on warm, sunny days
- Replacing deteriorated or damaged gaskets and sealants to avoid water intrusion and/or entrapment
- Maximum use of protective covers on parked aircraft
After any period where regular corrosion preventive maintenance is interrupted, the amount of maintenance required to repair accumulated corrosion damage and bring the aircraft back up to standard is usually quite high.
Inspection
Inspection for corrosion is a continuing problem and must be handled daily. Overemphasizing a particular corrosion problem when it is discovered and then forgetting about corrosion until the next crisis is an unsafe, costly, and troublesome practice. Most scheduled maintenance checklists are complete enough to cover all parts of the aircraft or engine, thus no part of the aircraft goes uninspected. Use these checklists as a general guide when an area is to be inspected for corrosion. Through experience, one learns that most aircraft have trouble areas where, despite routine inspection and maintenance, corrosion still sets in.
All corrosion inspections start with a thorough cleaning of the area to be inspected. A general visual inspection of the area follows using a flashlight, inspection mirror, and a 5– l0X magnifying glass. The general inspection is to look for obvious defects and suspected areas. A detailed inspection of damage or suspected areas found during the general inspection follows.
Visual inspection is the most widely used technique and is an effective method for the detection and evaluation of corrosion. Visual inspection employs the eyes to look directly at an aircraft surface or at a low angle of incidence to detect corrosion. Using the sense of touch is also an effective inspection method for the detection of hidden, well-developed corrosion. Other tools used during the visual inspection are mirrors, optical micrometers, and depth gauges.
Sometimes the inspection areas are obscured by structural members, equipment installations, or for some reason are awkward to check visually. Adequate access for inspection must be obtained by removing access panels and adjacent equipment, cleaning the area as necessary, and removing loose or cracked sealants and paints. Mirrors, borescopes, and fiber optics are useful in providing the means of observing obscure areas.
In addition to visual inspection, there are several NDI methods, such as liquid penetrant, magnetic particle, eddy current, x-ray, ultrasonic, and acoustical emission, that may be of value in the detection of corrosion. These methods have limitations and must be performed only by qualified and certified NDI personnel. Eddy current, x-ray, and ultrasonic inspection methods require properly calibrated (each time used) equipment and a controlling reference standard to obtain reliable results.
In addition to routine maintenance inspections, amphibians or seaplanes must be checked daily and critical areas cleaned or treated, as necessary.