[Federal Register Volume 62, Number 140 (Tuesday, July 22, 1997)]
[Notices]
[Pages 39295-39296]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 97-19233]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration


Proposed Advisory Circular (AC) 91-56A, Continuing Structural 
Integrity Program for Large Transport Category Airplanes

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Notice of proposed advisory circular.

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SUMMARY: This notice invites public comment on the proposed revision of 
Advisory Circular (AC) 91-56 which provides guidance material to 
manufacturers and operators of transport category airplanes for use in 
developing a continuing structural integrity program to ensure safe 
operation of older airplanes throughout their operational life.

DATES: Comments must be received on or before October 20, 1997.

ADDRESSES: Send all comments on the proposed AC to: Dorenda Baker, 
Manager, Aging Aircraft Program, ANM-109, FAA Transport Airplane 
Directorate, Aircraft Certification Service, 1601 Lind Ave., SW., 
Renton, WA 98055-4056. Comments may be examined at the above address 
between 7:30 a.m. and 4:00 p.m. weekdays, except Federal holidays.

FOR FURTHER INFORMATION CONTACT:
Pat Siegrist, Regulations Branch, ANM-114, FAA Transport Airplane 
Directorate, Aircraft Certificate Service, 1601 Lind Avenue, SW., 
Renton, WA 98055-4056; telephone (425) 227-2126, facsimile (425) 227-
1320.

[[Page 39296]]

SUPPLEMENTARY INFORMATION: 

Comments Invited

    A copy of the subject AC may be obtained by contacting the person 
named above under FOR FURTHER INFORMATION CONTACT. Interested persons 
are invited to comment on the proposed AC by submitting such written 
data, views, or arguments as they may desire. Commenters must identify 
the title of the AC and submit comments in duplicate to the address 
specified above. All comments received on or before the closing date 
for comments will be considered by the Transport Airplane Directorate 
before issuing the final AC.

Discussion

    The FAA proposes to revise AC 91-56, ``Supplemental Structural 
Inspection Program for Large Transport Category Airplanes,'' to add an 
appendix which provides guidance as to an acceptable means of 
accomplishing a structural evaluation for widespread fatigue damage. It 
revises the original AC to incorporate editorial changes and to reserve 
sections for the Aging Aircraft Modification Program, Corrosion 
Prevention and Control Program, and Repair Evaluation Program. The 
proposed changes would expand the scope of AC 91-56 to cover all 
programs necessary for the continued structural integrity of aging 
aircraft; therefore, the subject of the AC would be changed to 
``Continuing Structural Integrity Program for Large Transport 
Airplanes.''
    The following is a summary of the contents of the appendix on 
widespread fatigue damage.

General

    The likelihood of fatigue damage in an airplane's structure 
increases with the number of damaging repeated load cycles the airplane 
experiences. The manufacturer designs the airplane to keep the 
probability of cracking to a minimum up to the design service goal. It 
is expected that any cracking that occurs during this period will occur 
in isolation, originating from a single source, such as a random 
manufacturing flaw, but uniformly loaded structure may develop cracks 
in adjacent fasteners or in adjacent similar structural details. This 
cracking, known as Widespread Fatigue Damage (WFD) may interact to 
reduce the damage tolerance of the structure. Methods used to date to 
develop structural inspection programs have generally considered only 
localized interactions between fatigue cracks. Since a few cracks of a 
size that may not be reliably detected can cause an unacceptable 
reduction in the structural strength of the aircraft, the manufacturers 
should conduct an evaluation to determine when this damage may occur 
and provide instructions for the verification and removal of WFD in 
airplane structure.

Structural Evaluation for Widespread Fatigue Damage

    The evaluation has three objectives: (1) Identify primary structure 
susceptible to WFD, (2) Predict when it is likely to occur, (3) 
Establish additional maintenance actions, as necessary, to ensure the 
continued safe operation of the airplane. Structure that is susceptible 
to WFD typically has characteristics of similar details operating at 
similar stresses where structural capability could be affected by 
interaction of similar cracking. The proposed AC provides examples of 
generic types of susceptible structure. The evaluation for the onset of 
WFD should include a complete review of service history of the 
susceptible areas, relevant full-scale and component fatigue test data, 
teardown inspections, and any fractographic analysis available. For all 
areas that are identified as susceptible to WFD, the current 
maintenance program should be evaluated to determine if adequate 
structural maintenance and inspection programs exist to safeguard the 
structure against cracking and other structural degradation. The 
initial evaluation validity of the complete airframe should cover a 
significant forward projection of the airplane usage beyond the design 
service goal, typically an assessment through at least an additional 
twenty-five percent of the design service goal would provide a 
realistic forecast.

Documentation

    The manufacturer may revise the Supplemental Structural Inspection 
Program or issue other service information for the inspections and 
procedures and or modification of parts or components necessary to 
preclude WFD.

Responsibility

    It is expected that the evaluation will be conducted in a 
cooperative effort between the operators and the manufacturers with 
participation by airworthiness authorities.

    Issued in Renton, Washington, on July 15, 1997.
Neil D. Schalekamp,
Acting Manager, Transport Standards Staff, Transport Airplane 
Directorate, ANM-110.
[FR Doc. 97-19233 Filed 7-21-97; 8:45 am]
BILLING CODE 4910-13-M