By James Careless
Corrosion is one of the greatest enemies of airframes, and the culprit in some of aviation’s most serious accidents. One of the best-known occurred on April 28, 1988, when 18 feet of skin ripped off an Aloha Airlines B737-200 in flight. The skin loss resulted in the death of stewardess Clarabelle Lansing during explosive decompression and left the terrified passengers — eight of them seriously injured — exposed to the sky. According to the NTSB investigation, corrosion combined with metal fatigue and stress fractures caused “significant disbonding and fatigue damage” to the aircraft’s skin, which resulted in “lap joint failure and separation.”
Corrosion is also believed to have resulted in engines 3 and 4 falling off the wings of two B747-200 freighters; a China Airlines 747 in 1991 and an EL AL 747 in 1992. In both instances, the aircraft crashed. In the EL AL incident, which occurred in Amsterdam, all four crew members were killed along with 50 people on the ground.
Clearly, corrosion is something that demands detection, treatment, and, where possible, prevention. So how can aircraft technicians do this most effectively, and what tools do they have at their disposal? That’s what this article is all about.
What is corrosion?
There are many forms of aircraft corrosion. The most commonly known occurs when aluminum interacts with water, creating aluminum oxide. It is not just the exterior skin that is vulnerable; as the airframe expands and contracts in response to sunlight, heat, and cold, water can penetrate into rivet holes and thus deeper into the aircraft. As well, water spilled onto galley floors, or leaking around old toilets and sinks, can find its way into the airframe.
“Since all aircraft are primarily made of aluminum, they are particularly vulnerable to this form of corrosion,” says Dr. Jean-Pierre Roberge, an expert on corrosion who teaches at Canada’s Royal Military College in Kingston, Ontario. “Not surprisingly, aluminum oxide has none of the strength and structural properties of uncorroded aluminum. This is why early detection and, where necessary, replacement of overly oxidized aluminum parts is a must.”
A less obvious but equally deadly form of decay is galvanic corrosion. This occurs on the atomic level, when ions flow between two adjacent, chemically different materials. Galvanic corrosion doesn’t just occur between metals; a composite/metal pairing can also result in ions being transferred.
Generally, corrosion is detected visually. Technicians go over every accessible inch of an airframe during maintenance, or when the aircraft has been stripped down to the bare metal for repainting.
“The best inspections look everywhere that water, salt, and acids can seep into,” says Peter Burgher. He owns GSH Corporation, manufacturer of the galvanic corrosion control product NO COR. “This means the galley floors, the toilets, the aircraft belly, wing pockets, and within the ailerons, flaps, and elevators.”
Unfortunately, sometimes the most conscientious of visual inspections isn’t good enough. That was the experience of Florida-based Cessna owner Scott Nelson, who discovered horrific corrosion hidden inside a Cessna 182 that he had lovingly maintained for years. The damage only came to light after the hangar fell down on this aircraft during Hurricane Wilma. After being taken apart for restoration, “What we thought was the safest, most well-maintained 182 out there, actually possessed some of the scariest corrosion I had ever seen,” Nelson writes in the April 2007 edition of Cessna Owner magazine.
The damage included “significant corrosion” on the 182’s hard-to-inspect horizontal stabilizer spar, which was sandwiched inside three riveted together pieces of metal. It was so extensive that “the plane should not have been airworthy,” Nelson writes. Serious corrosion was also found on the forward face of the 182’s vertical fin spar, which is also hard to reach.
To his credit, Scott Nelson decided to purchase a flexible borescope, so that he could look into the 182’s tightest areas. A flexible borescope consists of an eyepiece that is connected by fiberoptic cable to a lens at the other end, which is usually fitted with lights. Like a plumbing snake, the borescope can be bent and guided around corners and angles, making it ideal for inspecting an aircraft’s many nooks and crannies. Since acquiring it, Nelson has found similar corrosion in many other aircraft that have come into his hangar.
There are other visual techniques for detecting corrosion, says Dr. Roberge. “Look carefully at the skin of the aircraft; can you detect bulging?” he tells AMT. “If you can, then chances are that corrosion has taken place within and needs to be attended to.” Other more sophisticated visual inspection methods include penetrant flaw detection, magnetic particles, eddy current flaw detection, ultrasonic flaw detection, mechanical impedance analysis, and X-rays.
Once corrosion has been detected, treatment is a must. In some cases, the damage is superficial enough to be removed, and then for the repaired area to be protected with anti-corrosion products. In others, corroded components may have to be removed and replaced.
Traditionally, technicians have employed wax, oil, and paint in such areas; treatments that are both messy and have to be removed before future inspections can be done. In contrast, newer products such as NO COR are clean, transparent, and do not have to be removed for further inspections. Other anti-corrosion products of note include Corrosion Technologies’ CorrosionX Aviation spray penetrant, which can be sprayed over existing corrosion to displace moisture and prevent the further migration of electrons; and Granitize Aviation’s Xzilon 3AECI Aircraft Exterior Corrosion Inhibitor, which prevents oxidation on exterior paint and bright work.
Vigilance: the best defense
“Airframe corrosion doesn’t happen overnight,” says Peter Burgher. “It can take a year or more for corrosion to seriously weaken an airframe, which is why early detection is so important.”
This being the case, the best defense against corrosion is thorough and regular airframe inspection, using a range of available techniques and tools. The regularity of such inspections is typically governed by an aircraft’s regular maintenance schedule; however, those flying in corrosion-prone areas (such as seaside coastal regions) would be wise to look for corrosion more often.
One piece of advice: Go out and buy a flexible borescope, and conduct nose-to-tail inspections of all your customers’ aircraft. Yes, it will take time, and could even cost you money for those customers who hesitate to pay. However, the hidden corrosion your shop will likely find, and the resulting repairs that you will be paid to perform, will likely more than compensate for the time and effort spent. Most importantly, these efforts may well save some customers’ lives and aircraft, which is what professional aircraft maintenance is really all about.From: http://www.amtonline.com