For Immediate Release
October 1, 2004
Contact: Les Dorr/Paul Takemoto
- Composites have been used in airplanes since the 1950s.
- Critical applications of composites started in the early 1980s and the related service history has been excellent.
- Composite materials have some fundamentally different characteristics from metals.
- Composite certification testing is conservative in comparison to similar parts manufactured from metal.
- Detailed quality inspection and testing assure that parts entering service have no unacceptable internal defects.
- Allowable production defects do not grow and do not adversely affect strength as confirmed by extensive "real world" testing.
- Maintenance programs are specifically tailored to address composites and differ from programs for metal structures
What's a "composite?"
- In industry, a composite is defined as two or more materials
that retain their identities in the combination while yielding properties superior to either.
- Common composite types include fibrous, laminate and particulate.
- They can employ glass, aramid (Kevlar or nylon) or carbon fibers.
- Various resins are used as the "matrix" bonding individual materials together into the desired form.
Composites in Aviation
- Composites were introduced in commercial aviation design in the late 1950s.
- Fiberglass was the predominant composite reinforcement in use through the early 1980s.
- Advanced composites (e.g., carbon reinforcement) were introduced in the early 1980s.
- In-service experience with composite structures that carry high loads has been excellent.
Benefits of Composites vs. Metals
- Weight reduction
- Better resistance to crack spreading (fatigue)
- Corrosion prevention
Composite Material Design Practices
- Composite primary aircraft structures are designed to withstand the highest loads ever expected in service, multiplied by a safety factor, given the worst case environment, plus a lifetime worth of operating loads and damage that can't be found by the maintenance program.
- Manufacturing defects and non-detectable service damage are deliberately inserted into test articles to validate that, with these conditions present, the composite structure will withstand all expected loads and conditions experienced in service.
Philosophy for Composites In-service Inspection Program
- Per the design criteria, if damage is not visible, structural integrity is not affected.
- Fatigue life and load carrying capability are sustained.
- Damage will not propagate to a significant extent during aircraft life.
- If damage is visible, a more detailed nondestructive inspection (NDI) must be performed to determine the extent of the damage and establish repair requirements.
Different Maintenance for Metals and Composites
- Aircraft maintenance programs are tailored to detect and fix primary damage concerns.
- Primary damage concerns for metals are fatigue, corrosion and accidents.
- Maintenance programs are established to find and repair cracks, corrosion and accidental damage before they can reduce strength to critical levels.
- Primary concern for composites is accidental damage (e.g., impact).
- Maintenance programs are established to find and repair accidental damage that can reduce strength below acceptable levels.
Manufacturing Quality Control
- Nondestructive inspections (NDI) are a key part of a composite quality control system
- NDIs are performed at the manufacturing plant as part of a company's overall FAA-approved quality system.
- Good quality control systems prevent defects beyond allowable limits in composite structures about to enter service.
Composites in Aircraft Tail Structures
- In-service experience from numerous transport aircraft with composite components that carry most of the tail loads has been excellent.
- No reports of fatigue or corrosion problems
- Main sources of significant damage are accidental impacts, lightning strikes or burn damage
- Composite tail structures have proven to be "damage tolerant" to large flaws.
- Composites are unaffected by the worst of hail storms