Structures Engineering Support for Out-of-Production Airplanes


As an increasing number of commercial airplanes approach or exceed their design service objectives, Boeing continues to take the lead in enhancing the safe operation of these out-of-production or aging airplanes. In addition to offering maintenance recommendations and ongoing structural studies, the company works with manufacturers, operators, and regulatory agencies to ensure proper maintenance of these airplanes and to gather data about them for future safety improvements.

More than 7,200 out-of-production commercial airplanes will be flying by the year 2000, according to recent estimates, with many airplanes operating beyond their model design service objectives (DSO) for a second or third operator. Boeing works consistently to assist airlines in safely operating these airplanes according to globally accepted airworthiness standards.

The number of active Boeing- and Douglas-designed out-of-production airplanes (see chart 1 below) now totals over 4,500. Out-of-production airplanes are those models that are no longer being manufactured, but are still operating. Boeing- and Douglas-designed airplanes in this category include the 707, 727, 737-100/-200, 747-100/-200/-300, DC-8, DC-9, and DC-10.

These airplanes are supported by an extensive network of inspection and maintenance programs. The focus of these programs is monitoring maintenance practices, adjusting maintenance schedules for airplane safety, and accumulating information to improve design of the next generation of airplanes.

A number of these airplanes are now also entering the "aging" category. Aging airplanes are defined as those that are flying beyond their DSO, which traditionally has been 20 years. For newer models that figure has grown to as long as 30 years. Approximately 20 percent of all commercial jet airplanes flying today are considered to be aging airplanes. As that number increases, so does the need for heightened fleet monitoring by airlines and manufacturers and continued intense focus and surveillance of these activities by regulatory agencies.

In order to handle increased maintenance needs, Boeing continues to establish partnerships, train its customers, and extend its facilities in future high-volume areas, such as China. The company also sponsors a structures safety conference in major areas of the world to train operators in the use of Boeing-provided technology for maintenance, corrosion prevention, and all other forms of structural degradation. The conference is one part of the three-pronged industry effort to reach manufacturers, who must design the airplanes with maintenance in mind; regulatory agencies, who are responsible for surveillance; and operators, who must monitor, report, and implement structural maintenance. The conference also provides a platform for receiving first-hand information from operators about Boeing airplanes and informing local regulatory agencies about the latest laws and regulations.

Support for Boeing- and Douglas-designed out-of-production airplanes--and for aging airplanes built by all manufacturers--is provided through two major sources:

1. Industry standards for aging-fleet structure programs.
2. Boeing-specific programs.

1 Industry Standards For Aging-Fleet Structure Programs
These standards were developed by an international working group whose members represent manufacturers, regulatory agencies, and operators. Though not all activities aimed at safe operation of out-of-production and aging airplanes are mandated, the highest level of safety possible depends on two factors: a well-established inspection and maintenance plan for each airplane (see chart 2 below) and the exchange of performance and maintenance information among Boeing, operators, and regulatory agencies. Data from inspections allows new-airplane design to incorporate features for cost-effective upkeep of the structures and systems, which in turn allows extended safe operation in less-than-ideal environments.

Boeing has been conducting thorough teardown and testing of older airplanes since 1970, when the focus was on high-time airplanes, to help understand and analyze areas that could adversely affect the future safety of airplanes. Many of the programs that grew from this effort are now formalized as industry standards.

During the Aging Fleet Conference in 1988, which was sponsored by the U.S. Federal Aviation Administration (FAA), the Air Transport Association of America and the Aerospace Industries Association committed to identifying and implementing procedures to ensure the continuing structural airworthiness of aging transport-category airplanes. From this conference, five aging-fleet structures programs were proposed for development. Of the five, three have been mandated by the FAA for the industry, and they are monitored by government agencies around the world. The Airworthiness Assurance Working Group, which consists of manufacturers, operators, and certifying agencies, oversees the development and implementation of these five programs:

STRUCTURAL MODIFICATION AND INSPECTION.
Continuing airworthiness has traditionally been dependent on repetitive inspections. However, concerns about aging airplanes--as well as a better understanding of the human factors associated with numerous repetitive actions--prompted a reassessment of the viability of indefinite repetitive inspections. The FAA, manufacturers, and operators agreed that continued operational safety of airplanes could be better served by placing less emphasis on repetitive inspections and more emphasis on design improvements and material replacement.

The Structures Task Group (STG), which consists of manufacturers, operators, and certifying agencies, was given the task of selecting service bulletins for each airplane model for incorporation of mandatory structural modification at a given threshold to maintain safety. In the event of known fatigue cracking, corrosion problems, or both, the STG normally introduces a service bulletin to define inspections. If damage is found during the defined inspections, the STG introduces another service bulletin containing instructions for repairs and modifications at a specific threshold.

CORROSION PREVENTION AND CONTROL.
The Boeing corrosion task force reviewed all Boeing sources of information related to known corrosion problems. A program was then developed that, in some cases, may require more frequent maintenance visits to accomplish corrosion inspections. These inspections were designed to control corrosion damage to acceptable minimum levels that will not adversely affect safety. Changes were required to operators' maintenance programs if they could not demonstrate that corrosion was being kept within the acceptable minimums. Design improvements based on data collected from these inspections and from service experience have been incorporated into current-production airplanes.

STRUCTURAL MAINTENANCE PROGRAM GUIDELINES.
Although manufacturers have published maintenance programs for each model, no comprehensive guidelines existed to properly address aging-fleet concerns. A structural maintenance program guidelines document was created to identify and summarize all of the relevant structural maintenance and modification tasks for a particular area of structure. In addition, model-specific documents were created to collect all data for a particular model airplane (such as maintenance planning data, service bulletins, and service letters) in a single document. Use of the document is optional.

SUPPLEMENTAL STRUCTURAL INSPECTIONS.
Supplemental structural inspection (SSI) documents are currently in place for aging 707, 720, 727, 737, 747-100/-200/-300, DC-8, DC-9, and DC-10 airplanes. These documents help ensure the continued safe operation of the aging fleet through timely detection of potential fatigue damage at locations previously not reported as damaged. The documents are updated regularly to reflect service experience and operator inputs. An SSI program for 757 and 767 airplanes has recently been released.

REPAIR ASSESSMENT.
Traditionally, repairs to airplane structures have primarily focused on static strength and fail safety to the structure after the repair. Recent aging-airplane concerns and regulatory changes have increased the emphasis on the effect repairs may have on the damage tolerance of the airplane structure. As a result, a repair assessment document has been developed to ensure the continued airworthiness of repaired structures. Boeing has developed repair assessment guideline documents and sent them to the FAA for approval. The use of these guidelines will be required by the operational rule currently being proposed by the FAA.

WIDESPREAD FATIGUE DAMAGE ASSESSMENT AND INSPECTIONS.
In 1997 widespread fatigue damage evaluations for aging airplanes were added to the tasks of the STG, making the effort the first new task added since the introduction of the initial five aging-airplane programs. Model-specific audits are being performed on airplane models that are approaching or have exceeded their DSO. These audits will include determining areas of potential susceptibility to multisite damage and accessing each suspect area to maintain levels of safety required by existing maintenance programs (e.g., the Service Bulletin Modification and Inspection Program or the Supplemental Structural Inspection Program).

2 Boeing-Specific Programs
Boeing has voluntarily implemented other programs that exceed FAA requirements. These programs are designed to accomplish several goals: to ensure that the company's airworthiness objectives are being met, to develop improved maintenance guidelines, to identify appropriate areas on which to focus preventive maintenance actions, and to provide information for new-airplane development or specialized technology advancements. The following programs are examples of these Boeing-specific efforts:

FLEET SURVEY.
Since 1987, teams of Boeing structural specialists have been performing detailed evaluations of selected high-time airplanes during the airplane's heavy maintenance checks at airline facilities around the world. The evaluations include monitoring ongoing maintenance programs and providing recommendations to the owner. These checks of in-service airplanes provide Boeing with invaluable structural performance data taken under different operating and maintenance environments. Airline maintenance program performance data is also recorded. To date, a total of 187 airplanes of all models have been inspected, with an average of about 20 airplanes a year.

TEST AND TEARDOWN INSPECTION.
Bolt by bolt, Boeing takes apart a retired airplane to inspect it (see chart 3 below) for fatigue, corrosion, and any anomalies that might provide a clue to future structural improvements and possible areas for maintenance scrutiny in working aircraft. Fatigue testing and teardown of in-service structure provides validation of airplane design concepts and preventive maintenance actions for the fleet. Testing generally covers twice the minimum DSO and involves cyclic pressure testing of the fuselage and pressure barrel testing of large fuselage panels. Teardown inspections of high-flight-cycle airframes allow a detailed assessment of in-service structural integrity that cannot be carried out with a working airplane. A full-scale fatigue test of a high-cycle 727 was completed in 1995, followed by a teardown inspection. The data from this test is currently being evaluated. A second full-scale fatigue test of a high-cycle 737 airplane has been proposed, and it would be followed by a teardown inspection when conducted.

Current-production Boeing airplanes are more durable, damage tolerant, and corrosion resistant than previous generations of airplanes, as well as being more cost-effective to maintain. The improvements are due, in large part, to the extensive structural performance data gathered through the aging-fleet survey programs and through the Boeing voluntary programs.

Summary
The number of out-of-production and aging airplanes in operation is predicted to increase sharply by the turn of the century, and Boeing is preparing itself to handle the growth in the number of these airplanes. Through several mandated and voluntary programs, the company is working with regulatory agencies and operators to help maintain the Boeing commitment to quality, durability, damage tolerance, and corrosion prevention.


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Chart 1:

Active Out-of-Production Airplanes
Airplane
model
Number
active
Design goals Number of airplanes
exceeding goals
Average age
(years)
Flights Hours Years Flights Hours Years
707 60 20,000 60,000 20 44 36 60 28.7
727 1,164 60,000 50,000 20 12 635 664 22.6
737-100 271 75,000 51,000 20 2 15 17 28.7
737-200 883 75,000 51,000 20 19 221 260 18.5
747-100

747-200

747-300

131

348

79

20,000

20,000

20,000

60,000

60,000

60,000

20

20

20

52

27

0

92

232

11

101

87

0

23.4

17.5

11.2

DC-8 300 25,000 50,000 20 103 259 300 29.7
DC-9 863 40,000 30,000 20 742 838 751 26.3
DC-10 446 42,000 60,000 20 0 233 211 19.8



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Chart 2:




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Chart 3:

Teardown inspections
707 Wing and center section 1965
707 Wing 1968
707 Wing, center section, and fuselage 1973
707 Empennage 1978
727 Forward fuselage 1978
737 Wing, center section, forward fuselage, and empennage 1987
DC-9 Fuselage, wing, and empennage 1987
737 Rear fuselage 1988
747 Wing and empennage 1989
747 Fuselage 1991
727 Wing and empennage 1994
727 Fuselage 1995

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