The number of commercial airframes exceeding twenty years of service continues to grow. In addition, Service Life Extension Programs are attempting to extend the “economic” service life of commercial airframes to thirty years. The use of bonded composites may offer the airframe manufacturers and aircraft maintenance facilities a cost effective method to extend the lives of their aircraft. The Federal Aviation Administration has sponsored a project at its Airworthiness Assurance NDI Validation Center (AANC) to validate the use of bonded composite doublers on commercial aircraft.
A specific application was chosen - reinforcement of an L-1011 door frame - in order to provide the proof-of-concept driving force behind this test and analysis project. However, the data stemming from this study serves as a comprehensive evaluation of bonded composite doublers for general use. The associated documentation package provides guidance regarding the design, analysis, installation, damage tolerance, and nondestructive inspection of these doublers. An industry team consisting of an Original Equipment Manufacturer (Lockheed-Martin), an airline (Delta Air Lines), and the AANC was formed. FAA oversight was provided through the Atlanta Aircraft Certification Office and the FAA's William J. Hughes Technical Center. Textron Systems Division provided their expertise in doubler installation and conducted a training class for Delta personnel.
This paper provides an overview of the FAA project and details the design, analysis, and test activities which were conducted in order to gain FAA approval for composite doubler use on commercial aircraft. Structural tests evaluated the damage tolerance and fatigue performance of composite doublers while finite element models were generated to study doubler design issues. Nondestructive inspection procedures were developed and validated using full-scale test articles. Finally, installation dry-runs demonstrated the viability of applying composite doublers in hangar environments. The first use of the project's documentation package was to support the installation of an FAA-approved Boron-Epoxy composite repair on a Lockheed L-1011 aircraft. This represents the first (non-decal) use of a bonded composite doubler on a U.S. commercial aircraft. A second important product of the results cited above is a Lockheed Service Bulletin which allows the door corner composite doubler to be installed on all L-1011 aircraft.