Following a fatal crash in early August, Swiss civil aviation officials have temporarily grounded JU-Air's two remaining aircraft over concerns about structural corrosion. The small Swiss-based airline has offered chartered flights aboard their vintage tri-motored planes, the Junkers Ju 52s, for over 30 years without incident. Investigators are still unsure about the exact cause of the crash but ordered the airline to produce certificates of airworthiness before normal operations can resume. According to a report by Flight Global, the initial inspection of crash debris by the Swiss Transportation Safety Investigation Board (STSB) found "significant corrosion" in several key areas of the aircraft. Among the affected areas, the spars, hinges and wing components displayed the most obvious signs of long-term corrosion. As part of the investigation, aviation officials also inspected the aircraft's maintenance documentation and found discrepancies relating to part modification and management, though no serious charges have been brought against the airline.
One of the most troubling details about this incident was that the Junker Ju 52s had been examined by the Swiss civil aviation office just four months prior, raising broader concerns about the need for more rigorous aircraft inspection guidelines around the world.
All aircraft contend with corrosion damage
Vintage planes aren't the only class of aircraft that require rigorous maintenance and inspection. In the most literal sense, corrosion is an inevitable hazard for all metal structures, especially those that spend a lot of time outdoors. When determining the risk potential of any given airplane, safety officials consider a range of factors such as age, operational environment and how frequently it's cleaned. According to the Aircraft Owners & Pilots Association, the first sign of corrosion is the formation of rust on the outermost metal components, recognizable by its distinct reddish hue. But not all types of corrosion come with such obvious markers, so it's important for owners and pilots to closely examine their aircraft on a regular basis – the only way to ensure it's airworthy is by keeping up with maintenance and looking out for these common forms of corrosion:
- Surface corrosion: This type of corrosion is considered the most common for aircraft as it naturally forms when a plane's metal exterior comes in contact with the oxygen and moisture in the atmosphere. Over time, a plane's protective coating of paint wears and fades from operational weathering, exposing the surface of the metal to the elements and creating rust as a byproduct of the reaction. Frequent travel in regions with high humidity can accelerate the spread of surface corrosion, but since it's easy to spot there is little risk of it going unnoticed for long.
- Crevice corrosion: While similar to surface corrosion in principle, this type can pose a serious risk to an aircraft's overall structural integrity. As a plane travels through the air, moisture builds on its surface and works its way into the crevices between adjoining metal components, such as wing joints and panel rivets. This slowly weakens the aircraft's joints, increasing the likelihood of a serious accident during a bumpy takeoff and/or landing.
- Stress corrosion: This form of corrosion typically affects specific metals (like aluminum, copper and some stainless steels) when they are subjected to immense weight and/or pressure. Aircraft components that are routinely under high operational tension, like landing gears and engine crankshafts, are at the most risk for developing stress corrosion. Continued use without treatment or replacement can cause the metal to fracture, which will expand over time as moisture seeps into the opening.
- Filiform corrosion: Of all the corrosion types to look out for, filiform corrosion is perhaps the most dangerous for aircraft because of how difficult it can be to detect. Even upon close visual inspection, filiform corrosion can be easily overlooked because it spreads beneath the surface coating and does not leave any noticeable deposits. More often than not, this type is caused by mistakes in the chemical treatment process of surface panels, leading to small blisters and tubes under the paint's surface.
The common factor in almost every type of aircraft corrosion is moisture, which makes operating in coastal regions more dangerous than in other climates. This is one reason airline companies have invested so much energy in corrosion testing and protection solutions, as a majority of commercial aircraft will spend at least some time in high-moisture environments. Without a solid corrosion testing process, aircrafts like the Junkers Ju 52s can become a major liability in just a few years time.