and corrosion control programs will continue to play a major role in the control of corrosion as airplanes age.

Some examples of design improvements to reduce corrosion on the Boeing 777 (Marceau, 1994) include:

• enhanced drainage, especially in the keel of the aircraft;

• the sealing of faying surfaces in corrosion-prone areas;

• the application of improved finish systems;

• liberal use of corrosion-preventive compounds;

• implementation of a good corrosion control maintenance program; and

• improved access for inspection of corrosion-prone areas.

Major airline fleets include aircraft ranging in age from new to 25 years old. Consequently, the degree of corrosion protection incorporated into the airplane varies from limited protection for older aircraft to fairly extensive protection for newer aircraft. Corrosion control programs are tailored to individual fleets, depending on age, prior experience, flight environment and degrees of corrosion protection incorporated prior to the delivery of the aircraft (DeRosa, 1995). All protective finishes are maintained and corrosion prevention compounds are applied during periodic maintenance. Critical areas that are prone to excessive corrosion include areas below the galleys, doorways, lavatories, cargo compartment subfloors, inside external fairings, and the bilges which are all treated at four-year intervals. Landing gear wheel wells and wing spars are treated yearly. Longer intervals of time are allowed between reapplications of corrosion prevention compounds in the case of less-severe environments.

Aging aircraft repairs have typically involved upper-skin lap fastener replacement, nonbonded skin panel replacement, skin lap doubler repairs, frame reinforcement, entryway door and scuff-plate doublers, replacement bushings and clevis joints, bulkhead forging replacement, and selected landing gear component replacement. Based on service experience, the airlines have expectations that manufacturers of new aircraft will (DeRosa, 1995):

• include stress corrosion prevention in all design reviews with airline customers,

• assemble all structure below the aircraft floor with sealant on all faying surfaces,

• coat all detail parts with corrosion-inhibiting primer or polyurethane topcoat before assembly,

• not use adhesive-bonded fuselage skin panels below the floor line or in areas subject to severe corrosion environments such as galleys and lavatories,

• treat all basic fuselage structure with corrosion-preventive compounds, and

• perform a complete (100 percent) inspection for delamination of bonded skin panels prior to the aircraft delivery in order to establish a baseline for subsequent inspection.

The objective of aging aircraft programs is to ensure the continued airworthiness of large transport aircraft as long as they remain in commercial service (Curtis and Lewis, 1992). Because new materials and fabrication processes may yield different degradation and damage mechanisms, a preproduction review should ensure that the new aircraft design includes lessons learned from the existing aging fleet.

Many of the steps needed to improve aging performance are detailed below. Most of these steps have now been incorporated into recent aircraft designs. The susceptibility of aircraft to corrosion and MSD fatigue can be reduced by the following steps:

• eliminating cold-bond lap-joint design details;

• providing adequate drainage to eliminate corrosion in areas where moisture accumulates;

• using the most corrosion-resistant materials and tempers available;

• evaluating galvanic couples with typical coating damage;

• testing dissimilar materials design details for size effects in areas with joints;

• controlling design stress levels, improving design details, and utilizing improved manufacturing and maintenance procedures to preclude the onset of multiple-site damage within the operational lifetime of the airplane;

• developing predictive and monitoring techniques for the onset of MSD (simple and cost-effective techniques should be integrated into the aircraft maintenance plan); and

• providing a complete corrosion prevention and control program within the aircraft maintenance program upon delivery of an aircraft.

The present focus on aging aircraft will lead to better corrosion-resistant treatments for next-generation aircraft. Materials selection in wet areas, the design drainage schemes, the use of insulation standoffs, and sealing and finishing systems have all been improved. The benefits of these improvements should be evident during in-service performance of the Boeing 777 and future aircraft. Liberal use of corrosion-preventive compounds applied in the aircraft assembly process and periodically in service, using a good corrosion control maintenance program, should minimize future corrosion concerns.

As discussed in chapter 4, prior to the latest generation of aircraft, which includes the Airbus A320 and the Boeing 777, structural composites have been used on aircraft flight control surfaces such as elevators, spoilers, ailerons, and rudders, as

Front Matter (R1-R14)

Executive Summary (1-4)

1 Requirements and Drivers (5-10)

2 Structural Concepts (11-25)

3 Metallic Materials and Processes (26-35)

4 Polymetric Composite Materials and Processes (36-44)

5 Environmentally Compliant Materials and Processes (45-48)

6 Methodologies for Assessment of Structural Performance (49-56)

7 Aircraft Maintenance and Repair (57-66)

8 Nondestructive Evaluation (67-70)

9 Committee Findings (71-76)

References (77-82)

Appendix: Biographical Sketches of Committee Members (83-84)