The service life of an aircraft is generally limited by metal fatigue caused by takeoff/landing and pressurization/depressurization cycles. For the average jetliner, that can translate to 25-30 years in operation. But metal fatigue is not the only factor - corrosion plays a role too. In some cases, corrosion and fatigue can act together to accelerate damage to important structural components. The fatigue life is governed by the original design and by environmental stresses that cannot be avoided unless the plane isn't used at all. Corrosion, on the other hand, can be at least minimized and controlled through the implementation of a good corrosion prevention and control plan.
This article will review the elements of a corrosion control plan that can maximize an aircraft owner's return on investment while minimizing the risk of corrosion-induced failures.
A good corrosion prevention and control plan starts with a good design. Without a corrosion-conscious design, the jobs of both the aircraft maintenance technician and the corrosion inspector are far more difficult. Good maintenance is an ongoing and critical process. Any plan to extend an aircraft's lifespan must include corrosion control maintenance. At regular intervals, aircraft must be inspected for corrosive damage and a decision made about mitigation techniques, repair efforts, or in extreme cases, aircraft decommissioning.
A good plan will specify the inspection procedures and equipment to be used as well as the documentation that must be kept for each plane. It involves selecting and following the appropriate existing standards and successfully obtaining all necessary certifications. Each of these elements of a good corrosion prevention and control plan is described below.
The major causes of aircraft corrosion are exposure to moisture, salt spray or salty air, spilling corrosive materials like battery acid, and the corrosion/oxidation effects of exhaust gases (particularly those which contain sulfur).
A good design starts with materials that have "enough" corrosion resistance. Corrosion resistance is only one of many competing factors in materials selection, so corrosion-resistant surface treatments and coatings may be needed to help minimize the rate of deterioration. Such coatings must be continuous and well adhered to; otherwise, they may actually accelerate the rate of local corrosion. Good design practice involves careful selection of compatible materials, including fasteners and weld filler metals, to avoid galvanic corrosion.
Avoiding crevices where moisture and debris can gather or provide drainage in low points that may accumulate water are standard procedures where possible. If such features can't be avoided, gaskets, seals, and sealants can help to exclude water.
Designers must also make all parts of the aircraft's primary structural components accessible for inspection. Hidden corrosion can be catastrophic because it remains undetected until it is too late.
Aircraft designs are finalized long before they reach the companies that will actually use them. But aircraft operators can still control their own fate, to a large degree, by religiously following the maintenance procedures laid out in the aircraft maintenance manual and in any other applicable regulations. These will include:
The Federal Aviation Administration (FAA) is the controlling authority over civil aviation in the USA. The FAA issued Airworthiness Directive 8300.12, Corrosion Prevention and Control Programs, in 1993. This document, except as superseded by updates, is the controlling authority over corrosion prevention and control programs in the USA. The most recent major update is Advisory Circular 43-4B, Corrosion Control for Aircraft, published in 2018. This advisory reviews available information on detecting and remediating corrosion in aircraft structural and powerplant components. It clarifies that corrosion prevention and control plans are the responsibility of the aircraft operator. For military equipment, 10 U.S.C. 2228 requires the Department of Defense (DoD) to develop and implement a long-term strategy to address the corrosion of its equipment and infrastructure. A key element of the DoD CPC strategy is programmatic and technical guidance is provided in the Corrosion Prevention and Control Planning Guidebook for Military Systems and Equipment.
Operators must use the original equipment manufacturer's recommendations as their corrosion control program if the manufacturer has published one which, they can expand on if needed. If there is no plan available from the manufacturer, then the operator is free to implement their own corrosion maintenance program and document it in accordance with AC 43-4B. That circular is comprehensive in its review of:
Safe, reliable air travel requires that aircraft operators develop and follow good corrosion prevention and control plans. One of these elements is in place before the airplane is even delivered to its owner: corrosion-conscious design. Another long-term corrosion prevention technique is the application of durable, non-hazardous polyurethane gaskets, seam sealants, and tapes to keep moisture and debris out of sensitive spots. Av-DEC has the proven experience that can help customers select the right sealant product and increase their aircraft's lifespan. Contact an Av-DEC representative today for assistance with your most challenging aircraft corrosion issues.
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