[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
