14 DAY TRIAL // Composite carbon fiber materials are traditionally used to manufacture strong, lightweight components, which can be used at aircraft fabrication. But the tests which are made to detect the so-called material fatigue might not be so conclusive as previously thought. Studies regarding the 'tap test', which should reveal any imperfections in the material, suggest that this technique cannot detect small flaws that could pose serious security issues for the airplane industry.
The tests used to detect material fatigue have been imposed as routine checks that must be made at certain time intervals to ensure that the components are in perfect condition and do not pose any danger to the aircrafts. The basic test involves gently taping the surface of a component, such as a tailplane or a rudder and listen for any change in the pitch of the reflected sound. It is supposed to reveal any gaps in the layers of the composite material, which might come apart, but according to the Canadian air-safety investigation board, TSB, the small gaps which could later grow, remain mostly undetected.
The study comes as a result of an investigation of an aircraft incident which took place almost two years ago, when an Airbus A310 lost in middle of the flight its rudder made out of a carbon composite material, during a routine flight from Cuba to Canada. Eventually the aircraft landed safely, due to the pilot's skill, who was able to stabilize the airplane and fly it using the remaining functional controls.
The simulation of the incident showed that the airplane executed a so-called Dutch roll, due to the fact that the 8.2 meter rudder started vibrating uncontrollably, as a result of a serious damage. However, because the weather conditions were ideal, the investigators studying the incident started to suspect that the tail composite component might have presented certain flaws that hadn't been detected during the last service checks.
Experiments of already damaged A310 rudders and other components, which were placed in airless chambers revealed that the minor damages that were already present, almost doubled their surface area, while experiencing violent pressure changes. The experiment replicated the changes in pressure, similar to those an airplane would be exposed to while flying at certain altitudes.
The test conducted by the tap method concluded that the component is completely safe, however after repeated cycles of pressure changes, the rudders eventually blew up, losing most of their strength and stiffness, fluttering and breaking up.
The results of this research come as a shock for the aircraft companies which have invested large amounts of money in the development of aircrafts, with fuselages made completely out of composite materials.
Both major passenger aircraft manufacturers, Boeing and Airbus, plan to launch composite aircrafts such as the Boeing designed 787 Dreamliner which is expected to be operational by 2008 and Airbus' A350 in 2012. None of them uses other tests, aside from the tap test, to verify the composite-made components.