Composite materials, such as fiberglass and carbon fiber, are easily molded into complex and aerodynamic shapes. They can create very strong structures tolerant to wide variations in temperature and requiring little maintenance. Today, the use of composites has replaced that of aluminum in many areas and is found in all areas of aviation, from airliners to Light Sport Aircraft.
The use of composites, though, introduces different (not necessarily more complex) structural analysis and testing methods that are often not well understood.
There are two primary purposes for testing aircraft structures: the first is to validate computer models and confirm that the aircraft meets structural strength objectives (discussed above); the second, more sophisticated objective is to simulate the effects of long-term use on the airplane structure and develop a “useful life.”
Depending on the materials used this takes one of two directions: aluminum airplanes emphasize fatigue testing (metal structures typically develop cracks after extended use); the composite counterpart is damage-tolerance testing (composites don’t develop fatigue cracks but can have hidden damage).
In both cases major aircraft assemblies are cycled through simulated takeoffs, cruise, turbulence, landings (some good, some not so good!) and then taken to destruction and shown to be strong enough to be “certified” even after this simulated lifetime. Premature failures can be dealt with by regular structural inspections (if the parts involved can be replaced).
All this structural testing, both for strength and damage tolerance, on a Cirrus is performed with major components built from sub-standard parts. This may not be intuitive but the idea is to test the structure with all parts being at the minimum end of the acceptable range in manufacturing – and then damaged further to allow for field incidents not visible during a pre-flight inspection. You can be confident that an average Cirrus airplane comes from the factory with much better components than the items we use for our testing.
DAMAGE TOLERANCE TEST OF CIRRUS WING
Useful Life of Older Airplanes
Testing and publication of useful life for a general aviation airplane is a relatively new development. Prior to the mid-1990s general aviation aircraft were not required to have either fatigue or damage-tolerance testing.
Obviously this is not the same as having “infinite life,” as is sometimes argued. It simply means that no testing was done and the useful life is unknown.
These airplanes rely on routine annual inspections to identify fatigue or other damage. This is then corrected on a case by case basis or by publication of Service Bulletins and Airworthiness Directives. These can lead to unanticipated expense and inconvenience.