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Corrosion is a major problem no matter where one encounters it. In its simplest definition, corrosion is the weakening and destroying of metal over time. On your car, home appliances, jewelry, some musical instruments such as the trumpet or tuba, a basketball hoop, mailboxes, or swing sets corrosion can and eventually will set in on such metallic structures if not properly treated and prevented. However, there is not a more dangerous place for corrosion to occur than on an aircraft, for if enough corrosion goes undetected or untreated for any length of time that part of the aircraft will deteriorate and potentially break apart. This can very likely cause an aircraft to crash and for this reason great lengths are taken to ensure that aircraft mechanics are trained extensively on how to detect the multiple types of corrosion and how to prevent or treat such occurrences.
There are multiple types of corrosion that can occur on today’s aircrafts. Uniform surface corrosion is perhaps the most common type of corrosion an aircraft mechanic will encounter. As its name suggests, this type of corrosion is characterized by an evenly distributed eating away of a large portion of a metal’s surface. General thinning of the metal occurs until it is too weak to withstand any pressure or work forced upon it (nace.org). This occurs from the metal’s exposure to oxygen, which is why a professional paint application is crucial to the aircraft’s structural integrity. Aliphatic paint is most often used as it provides a protective coating against oxygen, moisture, and other harmful elements.
Uniform Surface Corrosion
Another type of corrosion aircraft mechanics must inspect for is stress corrosion. As its name also suggests, stress corrosion occurs where frequent high stress is applied to the area. Tensile stress on an object can be both direct or indirectly applied stress causing the object or surface to begin to splinter and crack (corrosion-doctors.org). Landing gears and crankshafts are two common parts that endure heavy amounts of stress and are subject to stress corrosion (aopa.org).
Intergranular corrosion is another common metal decay for which an aircraft mechanic must inspect. A lack of uniformity within the metal will cause corrosion around the metal’s grain boundaries (dviaviation.com). The grain boundary becomes anodic compared to the unaffected part of the surface (corrosion-doctors.org). Heating and cooling of the metal during manufacturing can cause these types of imperfections within the metal (dviaviation.com). Narrow lines of corrosion are indicators on or within the metal of intergranular corrosion. This kind of corrosion is particularly dangerous because it can occur within the metal and undetectable to a visual inspection. Left undetected, intergranular corrosion can cause exfoliation whereby the metal’s surface begins to flake apart (dviaviation.com).
Fretting corrosion is caused by two or more metal components aligned closely to each other but designed to be able to slide past one another. The key here is that the metals are made of different materials. Heavy repeated vibration of the two surfaces against each other causes fretting corrosion after the protective film on the metals’ surfaces have been rubbed off (nace.org). Aircraft mechanics will discover pits and oxide debris on the surface as indicators of fretting and must be dealt with immediately. Fretting is often referred to as ‘false brinelling’, since the corrosion resembles brinell markings on the surface (dviaviation.com).
These and other forms of corrosion left untreated can cause serious aircraft damage and result in the loss of human lives. On June 24, 2013, Flight TCX314, an Airbus A330 was attempting to take off from Manchester airport. Upon takeoff, the number two engine failed while increasing its speed to 105 knots resulting in a high power engine surge, a flash of fire, and smoke from the exhaust. The pilot was able to abort takeoff in time and call in the emergency. Upon inspection, one of the turbine blades detached due to undiscovered and thus untreated corrosion pitting (aviation-safety.net).
On April 28, 1988, another flight was affected by corrosion but this time it resulted in the loss of human lives. Aloha Airlines Flight 243, a Boeing 737 departed for Honolulu from Hilo International Airport with five crew members and 89 passengers onboard. Once the aircraft leveled out, a large portion of the fuselage ripped off and threw Chief Flight Attendant Clarabelle Ho Lansing out of the plane. Her body was never recovered. Although the pilot was able to land the aircraft without any further casualties, many passengers suffered injuries from flying debris. Upon inspection, the fuselage suffered from fatigue cracking over the years around certain rivets. An aircraft mechanic never saw the corrosion to make the necessary precautions (aerotime.aero).
These are just two of the many examples of how corrosion eats away at the aircraft components until a section of the aircraft breaks either in part or completely off of the aircraft. As expected, this usually happens during heavy friction of the aircraft during flight and results in lost aircraft utilization, human injuries, and worst of all fatal casualties. If corrosion is found soon enough then there are multiple corrective measures that can be taken to ensure these type of accidents do not occur.
There are many types of nondestructive inspection methods that an aircraft mechanic can use to examine for potential corrosion either on the aircraft’s surface or internal metallic components. The most common are a basic visual inspection, dye penetrant, magnetic particle, eddy current, ultrasonic, x-ray radiography, and isotrope radiography (AC 43.13-1B pg 5-7). All of these inspection methods are performed depending on the type of “critical nature of the component, the material, size, shape, and weight of the part, the type of defect sought, maximum acceptable defect limits in size and distribution, possible locations and orientations of defects, part accessibility or portability, and the number of parts to be inspected”(AC 43.13-1B pgs 5-5 to 5-6).
Once corrosion is found, an aircraft mechanic must take three necessary steps to fix the problem: “remove as much of the corrosion as possible, neutralize any residual material, and restore the protective surface film” (Jeppesen, pg 12-28). For example, if any amount of pitting or etching on aluminum is discovered, the mechanic must remove the corrosion with a specific cleaner named MIL-C-25789, strip the surface with a solvent or MIL-R-25134, remove the corrosion with Brightner or a mechanical method, and finally apply a chromate conversion coating” (Jeppesen, pg. 12-29). A certified aircraft mechanic will be knowledgeable of all inspection methods and be able to refer to AC 43.13-1B on how to treat that exact corrosion type when he/she encounters it.
Aircraft mechanics are tasked with keeping their fleet of aircraft working and airworthy, or ‘safe for flight’. A major part of accomplishing this task is to prevent the buildup of corrosion on their aircraft(s). While it can occur on any metallic surface, it’s development on aircrafts is perhaps the most dangerous place for corrosion to occur. The responsibility of keeping corrosion from harming aircraft and it’s crew and passengers falls on the aircraft mechanic’s shoulders. It is an incredibly lucrative profession, that of the aircraft mechanic, and to end this on a nerdy note, as Spiderman’s uncle once said: “with great power comes great responsibility”.
- General Technician Textbook. Jeppesen Sanderson. 2016
- Aircraft Inspection, Repair and Alterations, Acceptable Methods, Techniques, and Practices. Aviation Supplies and Academics, Inc. 1998.
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