The Impact of Composites to Aircraft Structural Integrity Management

Aircraft structural integrity management programs are aimed at ensuring that aircraft can operate safety and economically throughout its life. As typified by MIL-STD-1530, this is achieved by:a. Establishing and validating the structural integrity of aircraft structures;b. Using operational data to update the status of the structural integrity;c. Providing quantitative data to support decisions related to conduct of inspections and priority for modification; andd. Providing lesson-learnt to be applied to the next generation of aircraft design/modifications.The current aircraft structural integrity management programs are primarily related to metallic structures. Four aircraft incidents are considered milestones in aircraft structure integrity, all of which involve metallic structures. The incidents and the associated impact on aircraft structural integrity were:a. 1954 - De Havilland Comet, loss of two aircraft due to fuselage failure. Impact: awareness that finite fatigue life is important.b. 1969 - General Dynamics F-111 wing failure due to undetected material flaw, which initiated the development of the damage tolerance conceptc. 1977 - Dan Air Boeing 707 loss of tailplane due to fatigue failure of spar, resulting in the realisation that old aircraft become more fatigue critical.d. 1988 - Aloha Airlines Boeing 737 loss of large part of fuselage due to multiple fatigue cracks, which highlighted the issue of Multiple Site fatigue Damage (MSD) in aging aircraft.The use of composites in aircraft construction has increased steadily since being introduced in the 1960s and 70s. Modern combat aircraft, such as the F-22 and F-35 have approximately thirty percent by weight of aircraft A method of simulating Structural Integrity, the Structural Integrity Failure Causation Model (SIFCM) was developed. Using the SIFCM, it was identified that a combination of fail-safe and damage tolerant structures should be used for composite structures anticipated to experience a severe impact environment. This should be combined with:a. High probability of detection for impact damage; b. ASI standards such as MIL-STD-1530C, should be modified to include more specific guidance regarding composite unique degradation modes, specifically impact, andc. Development of impact damage criteria and definition of impact environment to support design and certification activities