How essential is nondestructive testing (NDT) to airplanes?
In the aerospace industry, as with other transportation industries, NDT can make the difference between life and death. Aircraft components are inspected before they are assembled into the aircraft and then they are periodically inspected throughout their useful life. Aircraft parts are designed to be as light as possible while still performing their intended function. This generally means that components carry very high loads relative to their material strength and small flaws can cause a component to fail. Since aircraft are cycled (loaded and unloaded) as they fly, land, taxi, and pressurize the cabin, many components are prone to fatigue cracking after some length of time. If you are unfamiliar with the term "fatigue cracking" think about what happens when you bend a paper clip or piece of wire back and forth ...eventually it will break. Even parts that are loaded well below the level that causes them to deform can develop fatigue cracks after being cycled for a long time. This is what happens in aircraft. After they are used for a while, fatigue cracks start growing in some of their parts. Cracking can also occur due to other things like a lightning strike. Aircraft have some protection against lightning strikes but occasionally they occur and can results in cracks forming at the strike location like the one shown in the picture.
Another problem that aircraft have is that they are under the constant attack of corrosion. When an aircraft lands and the door is opened, the inside of the plane often fills with warm moist air. When the plane takes flight, and reaches altitude, the skin of the aircraft becomes very cold due to the temperature of the outside air. This cause the moisture held by the air inside the cabin to condense on the inside of the aircraft skin. The water will collect at low areas and serve as the electrolyte needed for corrosion to occur.
The good news is that aircraft are designed to withstand a certain amount of damage from cracking and corrosion without cause for concern, and NDT inspectors are trained to find the damage before it becomes a major problem. The rigorous process used to design aircraft either allow for a certain amount of damage to occur before a part fails, or in many cases, a part can fail completely and performance of the aircraft will not be affected. The job of the NDT inspector is to find the damage while it is within acceptable limits.
What kind of NDT techniques are used to ensure the safety of the airplane?
Over 80 percent of the inspections done to an aircraft are visual inspections. At regular intervals inspectors look at various component of the aircraft for signs of damage. During heavy maintenance work, much of the interior of the aircraft is stripped out so inspectors can look for damage on the inside surface of the fuselage. However, not all areas of the aircraft can be accessed for visual inspection and not all damage can be detected by visual means. This is were NDT plays a critical role in thoroughly inspecting airplanes.
NDT methods allow inspectors to inspect areas of the plane that would otherwise be uninspectable without disassembling structure to gain access to the internal areas. NDT methods also allow inspectors to detect damage that is too small to be detected by visual means. Eddy current and ultrasonic inspection methods are used extensively to locate tiny cracks that would otherwise be undetectable. These techniques are also used to measure the thickness of the aircraft skin from the outside and detect metal thinning from corrosion on the inside surface of the skin. X-ray techniques are used to find defects buried deep within the structure and to locate areas were water has penetrated into certain structure. Obviously, this task requires trained professionals who are capable of performing a variety of different NDT techniques to get a complete and accurate status of the airplane.
There is no question that the success of the airplane industry is dependent on NDT. Without NDT, the cost of maintaining and flying in airplanes would increase dramatically, while the safety of flying would decrease. When people step into an airplane they trust that it will get them to their destination with as little turbulence as possible. NDT plays a vital role in keeping air travel one of the safest modes of transportation.