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Aging Airplane Systems

As the average age of commercial airplanes currently in service increases, concerns continue to be raised about existing maintenance programs and how effectively they can help ensure the continued airworthiness of older airplanes. Although no specific deficiencies have been identified, the U.S. Federal Aviation Administration (FAA) and industry representatives are working together on a program to address these concerns. The purpose of the program is to determine what action must be taken to mitigate the effects of age on systems in older airplanes.

Approximately 63 percent of the 10,500 Boeing commercial airplanes in service were built according to type designs that are more than 20 years old. These airplanes may not be more than 20 years old, but all Boeing DC-8, DC-9, DC-10, 707, 727, 737-100/-200, and 747-100/-200/-300 airplanes were designed before 1979 and have accumulated 67 percent of the 403 million total hours flown. A plan that responds to concerns about the condition of aging nonstructural systems on these airplanes was recently released. The plan resulted from a program similar to one established in 1988 to evaluate aging airplane structures.

The U.S. White House Commission on Aviation Safety and Security recommended that the FAA work with operators and original equipment manufacturers (OEM) to expand the aging aircraft program to include nonstructural components. The plan released by the FAA last year outlines seven initiatives to address aging airplane systems:

  1. Establish an aging transport systems oversight committee to coordinate the various aging systems initiatives within the FAA.
  2. Conduct an in-depth review of the aging transport fleet and make model-specific safety recommendations related to airplane systems.
  3. Enhance airplane maintenance to better address aging airplane systems.
  4. Add aging systems tasks to the aging airplane research program.
  5. Improve reporting of accident, incident, and maintenance actions involving wiring system components.
  6. Evaluate the need for additional maintenance of transport airplane fuel system wiring and address any potentially unsafe conditions.
  7. Improve wiring installation drawings and instructions for continuing airworthiness.

Earlier this year, the FAA formed a committee to propose revisions to applicable Federal Aviation Regulations and associated guidance material. The goal of the committee is to revise the materials as necessary to ensure the continued airworthiness of nonstructural systems on aging transport airplanes.

Known as the Aging Transport Systems Rulemaking Advisory Committee (RAC), the group includes representatives of the Aerospace Industries Association of America, Air Transport Association, Air Line Pilots Association, Airbus Industrie, America West Airlines, Boeing Commercial Airplanes Group, European Association of Aerospace Industries, FAA, General Aviation Manufacturers Association, Joint Aviation Authorities, National Electrical Manufacturers Association, Transport Canada, U.S. Department of Defense, and U.S. National Aeronautics and Space Administration.

The RAC has developed five specific tasks to address one or more elements of the FAA plan:

  1. Evaluate fleet condition.
  2. Review fleet service history.
  3. Improve maintenance criteria.
  4. Review and update standard practices for wiring.
  5. Review inspection and repair training programs.

Other White House Commission recommendations, such as evaluating the need for additional maintenance of commercial airplane fuel-system wiring, are being addressed in separate industry programs.

1. EVALUATE FLEET CONDITION
The first task will be to conduct a sampling inspection of the fleet, to include establishing the airplane models to be evaluated, determining the evaluation criteria, developing a plan, and evaluating the fleet. Airplanes to be surveyed include those undergoing heavy maintenance, those sitting out of service, and those representing newer airplanes of each affected model.

Airplanes undergoing heavy maintenance.
The applicable OEM and airline representatives for each airplane model have identified significant electrical systems that are susceptible to the effects of aging. They plan to survey those systems on older airplanes for which previous inspection data does not exist or is incomplete. Surveys of wiring and related electrical systems on DC-8, DC-9, DC-10, 727, 737, 747, and L-1011 airplanes were scheduled to be completed in May 1999, when the RAC was slated to review the results. Surveys of the Airbus A300, which was recently added to the roster of aging airplanes, are expected to be complete in December 1999.

Airplanes sitting out of service.
The condition of systems on recently retired airplanes that are no longer economically feasible for commercial service operation will also be evaluated. Intrusive, detailed examination of these airplanes should identify conditions that could potentially affect the proper operation of similar systems on airplanes still in service.

Newer airplanes representing affected models.
Information from surveys of both in-service airplanes and recently retired airplanes will be used to survey newer airplanes that represent the affected models. This aspect determines at which point general conditions related to aging begin to appear. The results will be reflected in recommendations for proper maintenance intervals.

Because of the complexity and difficulty of reporting incidents on electrical systems and because of the difficulty in obtaining historical data specific to airplane wiring, the short-term focus will be on electrical systems. A decision whether surveys of other airplane systems are required will be made at the end of 1999.

2. REVIEW FLEET SERVICE HISTORY
As airplanes age, changes in operational, maintenance, and design practices prompt production changes to the airplane that cause the design to continuously evolve. Many changes benefit operators and are offered for retrofit on earlier airplanes. Operator and regulatory consideration of these postdelivery changes for airplane fleet installation is based on economic and safety factors. If a change is not immediately mandated by regulatory agencies, economic factors determine whether any postdelivery changes are installed on commercial airplanes.

The service history of each airplane model will also be evaluated to determine whether to recommend that the FAA mandate certain production modifications on in-service airplanes. Depending on the outcome of this evaluation, a similar program may be instituted on airplanes with type designs less than 20 years old.

3. IMPROVE MAINTENANCE CRITERIA
Boeing recommends a general plan for maintenance of each airplane model, but many operators customize these plans based on their unique requirements and experiences. These maintenance plans are reviewed and approved by the applicable regulatory agencies but may not completely address the effects of aging. The FAA has recommended that general maintenance procedures be evaluated and that a model-specific maintenance program be developed.

Specific areas under review are developing an improved electrical system inspection criteria; defining practices to eliminate wire bundle contamination or damage; determining an acceptable criteria for corrosion of components; establishing a process to link line, base, and shop maintenance actions; and reviewing the maintenance steering group-3 (MSG-3) process. The effects of aging noted during the surveys of both in-service and out-of-service airplanes will be considered when updating the criteria. The MSG-3 process establishes the initial minimum maintenance and inspection requirements for operators to use when developing an approved continuous airworthiness maintenance program for airframe, engines, systems, and components.

4. REVIEW AND UPDATE STANDARD PRACTICES FOR WIRING
The FAA has recommended that operators and the OEM develop a customized standard wiring practices manual to clarify and simplify electrical system installation and repair practices. This manual provides procedures for all types of electrical parts, some of which may not be used by a particular maintenance or repair facility. Removing unnecessary information and tailoring the procedures to specific repair facilities are expected to streamline the repair of electrical components.

In addition, the FAA recommended that a process that trains maintenance personnel to recognize aging effects on electrical systems and alerts them to proper repair procedures be defined.

5. REVIEW INSPECTION AND REPAIR TRAINING PROGRAMS
As with maintenance programs, operators also customize their inspection and repair training programs based on their own requirements. The FAA has recommended collecting the best practices from each training program and establishing a model inspection and repair training program that reflects industry best practices.

The Air Transport Association has already compiled some best practices from its member airlines, including operator-amended recommendations from Boeing on proven maintenance, training, and procedural practices. This information will serve as the basis for a maintenance best-practices videotape and any future inspection and repair training programs.

SUMMARY

After continuously monitoring its in-service airplanes and conducting an historical analysis of airplane accidents, Boeing has not identified aging aircraft systems as a major problem. These results are based on data from thousands of airplanes that have accumulated millions of flight hours over the last 25 years. As the result of scheduled and unscheduled line and base maintenance, many nonstructural system components and the wires, tubes, and hoses that link these components are inspected, repaired, or replaced. However, Boeing is working with the aviation industry on a program established by the FAA to assess the effects of age on systems in older airplanes. When completed, the program will provide all members of the industry with the necessary information to ensure the continued safety of nonstructural systems on these airplanes.


ON THE WEB

AGING AIRPLANE SYSTEMS
Additional information regarding the White House Commission on Aviation Safety and Security recommendations can be found at

http://www.dot.gov/affairs/whcsec1.htm

The FAA response to the White House Commission on Aviation Safety and Security is located at

http://www.faa.gov/apa/publicat/fatnspcov.htm

DON ANDERSEN
MANAGER
SERVICE ENGINEERING
BOEING COMMERCIAL AIRPLANES GROUP


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