[Federal Register Volume 63, Number 61 (Tuesday, March 31, 1998)]
[Rules and Regulations]
[Pages 15708-15715]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 98-8379]



[[Page 15707]]

_______________________________________________________________________

Part VII





Department of Transportation





_______________________________________________________________________



Federal Aviation Administration



_______________________________________________________________________



14 CFR Part 25



Fatigue Evaluation of Structure; Final Rule

  Federal Register / Vol. 63, No. 61 / Tuesday, March 31, 1998 / Rules 
and Regulations  

[[Page 15708]]



DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 25

[Docket No. 27358; Amdt. No. 25-96]
RIN 2120-AD42


Fatigue Evaluation of Structure

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Final rule.

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SUMMARY: This action amends the fatigue requirements for damage-
tolerant structure on transport category airplanes to require a 
demonstration using sufficient full-scale fatigue test evidence that 
widespread multiple-site damage will not occur within the design 
service goal of the airplane; and inspection thresholds for certain 
types of structure based on crack growth from likely initial defects. 
This change is needed to ensure the continued airworthiness of 
structures designed to the current damage tolerance requirements, and 
to ensure that should serious fatigue damage occur within the design 
service goal of the airplane, the remaining structure can withstand 
loads that are likely to occur, without failure, until the damage is 
detected and repaired.

EFFECTIVE DATE: April 30, 1998.

FOR FURTHER INFORMATION CONTACT:
Richard Yarges, FAA, Airframe and Airworthiness Branch (ANM-115), 
Transport Airplane Directorate, Aircraft Certification Service, 1601 
Lind Avenue SW., Renton Washington 98055-4056; telephone (425) 227-
2143, facsimile (425) 227-1320.

SUPPLEMENTARY INFORMATION: 

Availability of Final Rules

    This document may be downloaded from the FAA regulations section of 
the FedWorld electronic bulletin board service (telephone: 703-321-
3339), the Federal Register's electronic bulletin board service 
(telephone: 202-512-1661), or the FAA's Aviation rulemaking Advisory 
Committee Bulletin board service (telephone: 800-322-2722 or 202-267-
5948).
    Internet users may access the FAA web page at http://www.faa.gov or 
the Federal Register's web page at http://www.access.gop.gov/su___docs 
to download recently published rulemaking documents.
    Any person may obtain a copy of this final rule by submitting a 
request to the Federal Aviation Administration, Office of Rulemaking, 
ARM-1, 800 Independence Avenue, SW., Washington, DC 20591 or by calling 
202-267-9680. Communications must reference the amendment or docket 
number of this final rule.
    Persons interested in being placed on the mailing list for future 
Notices of Proposed Rulemaking and Final Rules should request a copy of 
Advisory Circular (AC) No. 11-2A, Notice of Proposed Rulemaking 
Distribution System, which describes the application procedure.

Small Entity Inquiries

    The Small Business Regulatory Enforcement Fairness Act of 1996 
(SBREFA) requires the FAA to report inquiries from small entities 
concerning information on, and advice about, compliance with statutes 
and regulations within the FAA's jurisdiction, including interpretation 
and application of the law to specific sets of facts supplied by a 
small entity.
    If you are a small entity and have a question, contact your local 
FAA official. If you do not know how to contact your local FAA 
official, you may contact Charlene Brown, Program Analyst Staff, Office 
of rulemaking, ARM-27, Federal Aviation Administration, 800 
Independence Avenue, SW, Washington, DC 20591, 1-888-551-1594. Internet 
users can find additional information on SBREFA in the ``Quick Jump'' 
section of the FAA's web page at http://www.faa.gov and may send 
electronic inquiries to the following Internet address: 9-AWA-
[email protected].

Background

    This amendment is based on Notice of Proposed Rulemaking (NPRM) 93-
9, which was published in the Federal Register on July 19, 1993 (58 FR 
38642). The notice was issued because of the need: (1) To ensure that 
widespread, multiple site fatigue cracking will not occur during the 
period of service for which the airplane is designed to operate; and 
(2) to prescribe criteria for establishing the thresholds for damage-
tolerance based inspections.
    In addition three minor changes requested by both U.S. and European 
manufacturers of transport category airplanes, aimed at harmonizing the 
U.S. and European certification requirements, were also proposed in 
this notice.
    Section 25.571 of 14 CFR part 25 requires that applicants for an 
airplane type certificate address the technical issue of structural 
fatigue (other than sonic fatigue) in one of two ways: (1) A damage-
tolerance evaluation of the structure; or (2) a safe-life fatigue 
evaluation of the structure.
    Of the two methods of evaluation, the first is preferred and the 
second may only be used if the applicant establishes that it is 
impractical to use a damage-tolerance approach. Even so, several in-
service incidents and accidents resulting from structural fatigue 
failures have demonstrated the need to improve the damage-tolerance 
evaluation requirements of part 25.
    A damage-tolerance evaluation consists of engineering calculations 
and tests aimed at establishing what kind of inspections are needed, 
and how often they need to be repeated on an airplane's structure while 
in service. The inspection frequency is set to assure that, should 
serious fatigue damage begin to develop before the design service goal 
of the airplane is reached, it will be found and repaired before it 
grows to proportions that represent a hazard to the airplane.
    This methodology has proven to be successful in many applications 
and, in fact, is part of the reason for the excellent overall safety 
record that has been achieved in the U.S. Nevertheless, there are two 
issues that have been debated within the technical community that are 
not clearly dealt with by the damage-tolerance methodology:
    1. When in an airplane's life should the first inspection in the 
inspection cycle be conducted (the threshold inspection)?
    2. When in an airplane's life can safety no longer be effectively 
maintained by the damage tolerance inspection program prescribed at the 
time of certification of the airplane type (the onset of widespread 
cracking)?
    These are complex issues that are discussed at some length in 
Notice 93-9. This rulemaking attempts to incorporate into part 25 some 
technical judgments on these issues that offer a high degree of safety 
to the flying public, without overburdening the air transportation 
system with unnecessary inspections or tests. To this end, the FAA 
proposed that Sec. 25.571 of the FAR, ``Damage-tolerance and fatigue 
evaluation of structure,'' be revised:
    1. To require sufficient full-scale fatigue testing to ensure that 
widespread, multiple-site fatigue damage does not occur within the 
design service goal of the airplane; and
    2. To require that thresholds for inspections be based on anaylses 
and tests considering the damage-tolerance concept, manufacturing 
quality, and susceptibility to in-service damage. (The idea of basing 
the time of the threshold inspection on the time it takes a crack to 
grow from a manufacturing defect that is likely to escape manufacturing

[[Page 15709]]

quality control inspection to the time the crack represents a hazard to 
the airplane is known as the ``rogue flaw'' concept for establishing 
inspection thresholds.)
    A revision to companion draft Advisory Circular (AC) 25.571-1A was 
prepared for the proposed rulemaking, to provide guidance on means that 
the FAA would accept as showing compliance with the regulation. As with 
all advisory circulars, this draft was intended only to provide 
guidance on acceptable means of compliance, without eliminating the 
flexibility for future applicants to identify other means of compliance 
with the proposed rule. That draft revision (AC 25.571-1X) was not 
available at the time that Notice 93-9 was issued and was subsequently 
made available to the public for comment on October 19, 1993 (58 FR 
53987). As a result, the FAA has received two sets of comments from the 
public, one in response to the draft AC and one in response to the 
proposed rule. Some of the earlier comments were made without the 
benefit of the commenter knowing the contents of the draft AC. Because 
of this, the FAA has considered both sets of comments in preparing the 
final rule contained herein, and in revising the AC. The announcement 
of the FAA's issuance of the revised AC will be published in the 
Federal Register once it is available to the public.
    Interested persons have been given an opportunity to participate in 
this rulemaking, and due consideration has been given to all matters 
presented. Comments received in response to Notice 93-9 are discussed 
below.

Discussion of Comments

    The FAA received many comments in response to Notice 93-9, most of 
which state support for the added requirement for full-scale fatigue 
testing of new airplane types. Commenters included airplane 
manufacturers, the National Transportation Safety Board, the Airline 
Pilots Association, the Aerospace Industries Association, the General 
Aviation Manufacturers Association, airplane operators, and others. 
Only a few commenters state that full-scale fatigue testing should not 
be required.
    One commenter states that full-scale fatigue tests should not be 
mandated because these tests do not adequately account for actual 
conditions experienced in service and therefore cannot accurately 
predict in-service problems. The commenter further states that such 
tests have never predicted widespread fatigue damage that later became 
a problem in the fleet. The FAA does not concur with this comment. It 
is widely recognized in the aviation engineering community that 
``scatter factors'' need to be applied to fatigue test results, because 
such tests cannot account for the individual construction variations 
and the individual service experience of each airplane. Nevertheless, 
important results have been, and will continue to be, obtained from 
such tests, including the prediction of widespread fatigue damage. The 
FAA, airplane manufacturers, and others have come to recognize that 
full-scale fatigue testing provides an indispensable, although 
admittedly incomplete, source of information about what to expect in 
service from airframe structures. As was pointed out by another 
commenter who favors the new requirement, full-scale fatigue test 
evidence must be coupled with prudent exploratory fleet inspections to 
ensure continued airworthiness.
    The FAA received several comments about the full-scale fatigue 
testing of derivative or modified type designs. These commenters point 
out that full-scale fatigue test data generated during the original 
certification of an airplane type, and other data, can sometimes be 
used to determine when widespread multiple-site fatigue damage will, or 
will not, occur on the modified designs. These commenters state that 
additional full-scale fatigue testing would not be necessary in all 
cases. The FAA concurs with these comments. The working of 
Sec. 25.571(b) in the final rule has been changed along the lines of 
one comment that had been jointly developed by the Aerospace Industries 
Association, the Association Europeenne des Constructeurs de Materiel 
Aerospatial, and the FAA's Technical Oversight Group for Aging 
Airplanes. This change uses the words ``sufficient full-scale fatigue 
test evidence'' in place of ``sufficient full-scale testing.''
    The same commenters also state that guidance should be provided in 
the form of an advisory circular (AC) on the subject of when and how 
much fatigue testing would be necessary for modification and derivative 
certification programs. The FAA concurs. In fact, draft AC 25.571-1X 
does contain some guidance. Based on comments provided to the docket 
for this rulemaking and in response to the draft AC, the FAA has 
revised and expanded the guidance regarding the relevant factors in 
determining whether, and to what extent, fatigue testing may be 
necessary for derivatives and modifications of type designs. Generally, 
these factors relate to the applicability and reliability of previously 
developed test evidence for determining that the airplane will remain 
free of widespread fatigue damage until its design service goal is 
reached.
    Another commenter points out that two lifetimes of fatigue testing 
cannot ``ensure'' that widespread multiple-site damage will not occur 
within the design lifetime of an airplane (since no fatigue test can 
duplicate the exact configuration and operating history of each 
airplane). The commenter states that the requirement of the rule should 
be to ensure hat widespread multiple-site damage will not ``normally'' 
occur. The FAA agrees that two lifetimes of fatigue testing cannot 
ensure that widespread fatigue damage will not occur within the design 
lifetime of an airplane; however, guidance on this statistical fact is 
best addressed in the AC. Therefore, as a result of this comment, the 
FAA has revised the AC in this regard.
    The FAA also received comments that full-scale fatigue testing 
represents a prohibitive expense for small entities that perform 
modifications of type designs produced by others and would put them out 
of business. These commenters note that the FAA has certificated 
airplane modifications for damage tolerance in the past, relying on 
analytical methods that are based upon test data and using conservative 
assumptions, but without full-scale fatigue testing. They state that 
they are small entities that the FAA did not consider.
    As discussed previously, the objective of this rulemaking is to 
ensure that transport category airplanes will remain free of widespread 
fatigue damage within their design service goals. For reasons discussed 
below, the FAA considers that most modifications can be found to meet 
this objective without additional full-scale testing. However, it is 
true that in some cases involving extensive structural modification 
(such as a cargo conversion project) it may be necessary for the FAA to 
require a modifier to conduct full-scale fatigue testing to demonstrate 
freedom from widespread fatigue damage within the design service goal 
of an airplane type. The FAA acknowledges that such testing may be 
expensive. In these cases, the FAA has determined that the safety 
interests of the flying public must take precedence over the economic 
interests of airplane modifiers. This final rule does not preclude 
modifiers from conducting such projects, but, if they cannot otherwise 
meet the objectives of this rule, they will need to consider the costs 
of full-scale fatigue testing along with the other compliance costs 
when they evaluate the economic viability of a particular modification 
project.

[[Page 15710]]

    The FAA does not, however, concur that the overall economic impact 
of this final rule on these small entities is significant. First, as 
discussed in the preceding paragraphs, the full-scale fatigue testing 
requirement of the proposed rule has been revised such that it is not 
always necessary to conduct one for a modification project, and most 
modifications would not necessitate one. The companion AC to the rule 
has been expanded to provide guidance on acceptable means of showing 
compliance for modifications. This guidance discusses how small, simple 
design changes, using a design comparable to the original structure, 
could be analytically determined to be equivalent to the original 
structure in their propensity for widespread fatigue damage (e.g., 
modification of the fuselage structure for mounting an antenna using a 
design that is similar to the original airplane in that area). In 
addition, the amendment will not impose any additional costs on these 
small entities on projects for which they have already applied for 
supplemental type certificates; nor will it impose any additional costs 
on projects for which they would apply for supplemental type 
certificates in the near future, since the designs that would be 
affected by this amendment would probably not enter service until at 
least 5 to 10 years after its adoption. This is because, in general, in 
accordance with Sec. 21.101 of 14 CFR part 21, modifiers of type 
designs need only comply with the regulations that were used to 
certificate the original model.
    One other commenter states that the rule could be interpreted to 
require full-scale fatigue testing of modifications specified in 
service bulletins, which would actually impede safety by delaying the 
issuance of needed service bulletins. The FAA does not concur with this 
comment. Service bulletin modifications are in the same general 
category as other modifications, and most would not necessitate full-
scale fatigue testing. Further, if circumstances necessitate 
airworthiness directive (AD) action to mandate a modification specified 
in a manufacturer's service bulletin before fatigue testing of the 
modification is complete, there is nothing in the rule that prevents 
the FAA from doing so.
    One commenter also suggested replacing the sentence in current 
Sec. 25.571(b) that states, ``Damage at multiple sites due to prior 
fatigue exposure must also be included where the design is such that 
this type of damage is expected to occur,'' with the following 
sentence: ``Special consideration for WFD must be included where the 
design is such that this type of damage could occur.'' Although the 
commenter provided no explanation of this suggestion, the FAA considers 
that it has merit. The FAA concurs that requiring ``special 
consideration for WFD'' emphasizes that, in addition to demonstrating 
that WFD will not occur within the design service goal, the applicant 
for type certificate must also consider ways to prevent or control the 
effects of WFD that may occur beyond the design service goal. This is 
necessary to fulfill the objective of Sec. 25.571(a) to avoid 
catastrophic failure due to fatigue throughout the operational life of 
the airplane.
    Many commenters object to basing all inspection thresholds on the 
so-called ``rogue flaw'' concept, as would be required by the proposed 
amendment to Sec. 25.571(a)(3). These commenters state that 
indiscriminately applying this approach to all airplane structures 
would result in an exorbitant increase in airplane inspection costs, 
because it would necessitate detailed inspections earlier in an 
airplane's life and would not significantly enhance safety. Although 
most of these commenters aknowledge the necessity of using the ``rogue 
flaw'' concept to establish inspection thresholds for certain types of 
airframe design details, it was argued that the current industry 
practice for establishing the inspection thresholds (consisting of 
predicting the onset of cracking from fatigue testing and service 
experience) is adequate for most commonly used airframe designs. Some 
commenters endorsed a proposal that had previously been jointly 
submitted by the Aerospace Industries Association, the Association 
Europeene des Constructeurs De Material Aerospatial (AECMA), and the 
Technical Oversight Group for Aging Airplanes (hereinafter referred to 
as the AIA/AECMA/TOGAA comment). This group proposed that rogue flaw 
based inspection thresholds be limited to single load path structure, 
or other structure where it cannot be demonstrated that load path 
failure, partial failure, or crack arrest will be detected and repaired 
prior to failure of the remaining structure. The FAA concurs with these 
comments. These criteria have been incorporated into the final rule, 
and will ensure that the rogue flaw method of establishing inspection 
thresholds is not applied indiscriminately, but will be applied where 
necessary.
    Following close of the comment period, and after the FAA had 
reviewed these comments and decided to incorporate the language 
proposed by AIA/AECMA/TOGAA into the final rule, Boeing, which had 
participated in the development of the AIA/AECMA/TOGAA comment, became 
aware of the FAA's decision. (This resulted from a series of 
communications between Boeing and the FAA regarding an ongoing program 
to determine the appropriate criteria for establishing fatigue 
inspection thresholds for the Model 757 and 767 airplanes; the 
communications were otherwise unrelated to this rulemaking.) At 
Boeing's request, AIA filed an additional comment, objecting to 
inclusion of this language in the final rule, and recommending instead 
that it be incorporated into AC 25.571-1X. AIA stated that the FAA's 
decision was in conflict with the AIA/AECMA/TOGAA comments, which had 
been based on the commenters' conclusion that the general requirement 
of Sec. 25.571(a)(3) that inspections be established ``as necessary to 
prevent catastrophic failure'' was sufficient to ensure that rogue 
flaws would be considered appropriately, as described in their proposed 
revision to the AC.
    Although the FAA concurs with the commenter's position that rogue 
flaws in certain types of structure must be considered, the FAA does 
not concur that revising the AC alone, and relying on the general 
language of Sec. 25.571(a)(3), is sufficient to ensure adequate 
consideration. Advisory circulars are not mandatory and explicitly 
describe ``one means, but not the only means,'' of complying with the 
relevant regulations. Therefore, because the FAA considers it essential 
that rogue flaws be considered, the final rule has been amended, as 
described previously.
    One commenter states that the sentence added to Sec. 25.571(a)(3) 
should be revised to state that thresholds for inspection should also 
be based on service experience and fatigue testing, followed by a 
``tear-down'' examination of the test article. Although the FAA agrees 
that there may be important factors, it is more appropriate to discuss 
them as acceptable means of compliance in the companion advisory 
circular, and not in the rule itself. This will provide maximum 
flexibility for future applicants to identify means of fulfilling the 
rule's objectives.
    One manufacturer asks for confirmation that its particular method 
of establishing thresholds for inspection be allowed under the current 
rulemaking. The FAA considers it inappropriate, in the context of this 
rulemaking, to evaluate any one manufacturer's particular methodology. 
Such an evaluation would normally be

[[Page 15711]]

accomplished during the certification process for an airplane type.
    One commenter states that the proposed rule implies that simulated 
manufacturing defects must be inflicted on the full-scale fatigue test 
from the start. The FAA disagrees. As proposed, the purpose of the 
full-scale fatigue test requirement is to establish that the structure 
will be substantially free from widespread fatigue damage at least 
until its design service goal is reached. In contrast, the purpose for 
the consideration of manufacturing defects is to establish thresholds 
for inspection (or other procedures) for certain types of structure. 
Although the latter could involve full-scale fatigue testing in which 
the test article is inflicted with simulated manufacturing defects, 
and, in fact, the FAA's certification evaluation of a model type design 
may reveal that this is the necessary way of establishing a threshold 
in exceptional cases, it is not the FAA's intent to require this in 
general.
    One commenter states that it is not normally possible to complete a 
full-scale fatigue test prior to issuance of a type certificate. The 
commenter recommends that AC 25.571-1A be revised to allow completion 
of the full-scale fatigue test after type certification. The FAA agrees 
with this comment. As noted by the commenter, taken literally, the 
proposed rule would have required that the testing be completed prior 
to issuance of a type certificate. However, as reflected in the 
preamble of the NPRM, the FAA recognized that this may not be realistic 
and would have allowed completion of testing after issuance of the type 
certificate. In light of the comment, the FAA has reconsidered this 
issue and determined that the rule must be revised to address this 
potential conflict. As revised, the rule allows testing to be completed 
after issuance of the type certificate, provided:
    1. Before issuance of the type certificate the Administrator has 
approved a plan for completing the required tests, and
    2. The Type Certificate contains an airworthiness limitation in the 
airworthiness limitations section of the instructions for continued 
airworthiness required by Sec. 25.1529 that no airplane may be operated 
beyond a number of cycles equal to one-half the number of cycles 
accumulated on the fatigue test article, until such testing is 
completed.
    The FAA considers that the first condition is necessary to ensure 
that, at the time of type certification, the TC holder has at least 
identified an acceptable method of complying with this rule's 
requirements. The FAA considers that the second condition is necessary 
to ensure that, following type certification, the testing proceeds so 
that the affected airplanes receive the safety benefits that this rule 
is intended to provide. Although these conditions were not specified in 
the NPRM, the final rule actually provides relief from the literal 
requirement of the NPRM to complete testing prior to issuance of a TC. 
It is also a logical outgrowth of the proposal in that it resolves the 
conflict between the proposed rule language and the preamble discussion 
in a way that ensures that the rule's objectives are fulfilled.
    Several commenters recommend that the words ``within the design 
lifetime of the airplane,'' used in the sentence added to 
Sec. 25.571(b), be changed to ``within the design service goal of the 
airplane.'' It was pointed out that it is difficult for manufacturers 
to know at the time an airplane is first certificated exactly how long 
it will be used. The expected service period is set as a goal for 
fatigue design at that time; therefore, the words ``design service 
goal'' are more appropriate. Furthermore, it was pointed out that the 
term ``lifetime'' implies a fixed service period for an airplane, after 
which it would be retired. These commenters state that this does not 
represent the intent of the proposed rule. The FAA concurs with this 
comment, and the words ``design service goal'' have been substituted 
for ``design lifetime.''
    Several commenters state that the proposed requirements for 
operating past the original design service goal are not clear. They 
note that an industry team, the Structural Audit Evaluation Task Group 
(SAETG) of the Airworthiness Assurance Working Group (AAWG), conducted 
an extensive activity to determine possible actions for airplane models 
that reach that point. (The SAETG and AAWG are sub-groups of the 
Aviation Rulemaking Advisory Committee (ARAC), which submits rulemaking 
recommendations to the FAA). The commenters state that the 
recommendations of the SAETG should be addressed concurrently with the 
present change to Sec. 25.571. The FAA does not agree with this 
comment. Although the FAA is addressing the recommendations of the 
SAETG at this time, that action covers only 11 specific models of 
airplanes whose fleet leaders have already exceeded their design 
service goal. These recommendations consist of suggested actions on how 
to implement the guidance material they generated. Although the FAA 
agrees that additional guidance may be appropriate for airplanes 
affected by the present rulemaking on the subject considered by the 
SAETG, the urgency of that action is not great because the design 
service goal of these airplanes would not be reached for at least 
another 20 years. Furthermore, one of the SAETG recommendations is that 
their guidance should not be extended beyond the 11 specific models it 
covers until it has actually been tried. To attempt to establish 
guidance for airplanes affected by the present rulemaking based on the 
SAETG recommendations at this time would only serve to delay the 
issuance of the present rulemaking. Therefore, the most expeditious 
manner of obtaining the benefits of the proposed refinement for the 
damage-tolerance evaluations is to adopt the present rule change and to 
continue discussions with the ARAC and others on how best to address 
the SAETG recommendations.
    One commenter states support for the new requirement for full-scale 
testing, provided the companion Advisory Circular (AC 25.571-1X) 
follows the Certification Maintenance Requirements (CMR) guidelines (AC 
25-19 dated 11/28/94). The CMR guidelines referred to by this commenter 
are guidelines on how inspection programs for airplane systems should 
be established at the time of certification. The FAA does not agree 
with this comment. There are presently fundamental differences in 
methodology between the way inspection programs are established for 
airplane systems and airplane structures. Attempts at resolving those 
differences have not been fruitful in the past, and there is no 
guarantee that they will be any more fruitful in the future. Therefore, 
evaluation of the appropriateness of using the CMR guidelines to 
establish structural inspection programs as part of the present 
rulemaking would result in a delay that the FAA considers unacceptable.
    Several commenters state that the rule should specify the size of 
the initial manufacturing flaw or fatigue scatter factor criteria, 
either in the rule itself or in the accompanying AC. Although the FAA 
does not concur that an absolute size should be specified for the 
initial manufacturing flaw, guidance on acceptable means of compliance 
has been provided in the revised version of the AC on both subjects.

Regulatory Evaluation

    Proposed changes to Federal regulations must undergo several 
economic analyses. First, Executive Order 12866 directs that each 
Federal agency shall propose or adopt a regulation only upon a reasoned 
determination that the benefits of the

[[Page 15712]]

intended regulation justify its costs. Second, the Regulatory 
Flexibility Act of 1980 requires agencies to analyze the economic 
effect of regulatory changes on small entities. Third, the Office of 
Management and Budget directs agencies to assess the effects of 
regulatory changes on international trade. In conducting these 
analyses, the FAA has determined that this rule: (1) will generate 
benefits that justify its costs as defined in the Executive Order; (2) 
is significant as defined in DOT's Regulatory Policies and Procedures; 
(3) will not have a significant impact on a substantial number of small 
entities; and (4) could affect international trade. These analyses, 
available in the docket, are summarized below.

Estimated Costs and Benefits

    Based on the opinions of industry and agency experts, the FAA 
estimates that development and certification costs associated with the 
requirement for an inspection threshold based on initial manufacturing 
defects will be negligible. However, this provision could affect 
operating costs, depending on the degree to which it impinges on the 
timing of initial inspections. This evaluation conservatively estimates 
that an additional 500,000 work hours will be required to inspect a 
fleet of 1,000 airplanes as result of the requirement to base 
inspection thresholds on assumed manufacturing defects. Assuming a 
fully burdened compensation rate of $65 per hour, this provision will 
increase operating costs by approximately $32.5 million over the life 
of a 1,000 airplane fleet.
    The cost of a full-scale fatigue test for a representative 
transport category airplane design is statistically estimated using a 
sample of four different airplane models, ranging from a 45-seat 
airplane to a large widebody transport. In its comments on NPRM 93-9, 
the Aerospace Industries Association (AIA) notes that certification 
requirements could double the number of work hours for such testing. To 
account for this, full-scale fatigue test costs for each airplane model 
were inflated by multiplying the labor cost component by a factor of 
two. The relationship between these adjusted fatigue test costs and 
airplane size--measured by the number of seats--was then estimated 
using ordinary least squares. This yields a cost estimate of $540,000 
for each seat in a proposed model. The cost of a full-scale fatigue 
test for a 162-seat airplane design, for example, would be 
approximately 162 times $540,000 or $87.5 million. for a 1,000 airplane 
fleet, this would equal $87,500 per airplane.
    Total costs are estimated for a representative type certification 
using the following assumptions: (1) The hypothetical airplane model is 
assumed to have 162 seats; (2) 50 percent of testing costs are incurred 
in the year 2000, one-third of the remaining costs are incurred in each 
of the years 2001, 2002, and 2003; (3) production commences in the year 
2002; (4) 100 airplanes are produced per year for 10 years; (5) the 
first airplanes enter service in 2002; (6) for each airplane, 
inspection costs related to the ``rogue flaw'' requirement are 
uniformly distributed in the interval bounded by one-fourth and one-
half the design service goal (i.e., between the 5th and 9th years of 
operation); (7) total burdened cost per work hour is $65; (8) the 
discount rate is 7 percent; and (9) each airplane is retired at the end 
of its 20-year design service goal.
    Under these assumptions, undiscounted fleet certification and 
operating costs--including the costs of a full-scale fatigue test and 
the inspection threshold provision--equal $120.0 million or $120,000 
per airplane. On a discounted (1997 dollar) basis, fleet costs equal 
$78.6 million or $78,600 per airplane.
    The benefits of the rule depend on the inherent variability of 
structural fatigue analysis and on the efficacy of actions taken in 
response to the results of such analysis. For example, the ``rogue-
flaw'' inspection threshold requirement will prevent an accident only 
if: (1) The threshold occurs before an accident would otherwise occur; 
and (2) the resulting inspection identifies the damaged structure. 
Nevertheless, based on the accident history and the likelihood of 
ancillary benefits, the FAA finds that the benefits of the rule justify 
its costs.
    An examination of the service history identified 39 domestic 
accidents or incidents involving structural fatigue during the period 
1974-1990. The National Transportation Safety Board (NTSB) identified 
improper maintenance and/or corrosion as important contributing factors 
in 17 of the events. Of the remaining 22 events, 12 involved the 
landing gear and 10 involved the wing, fuselage, or other structure.
    Although only two of these 10 events resulted in fatalities, 
several other events had catastrophic potential (in one case a wing 
spar failed, and in five other cases passenger cabin decompression 
occurred). In at least one case, the NTSB concluded that the accident 
was the probable result of a manufacturing defect.
    During the same period, air carriers accumulated approximately 148 
million flight hours. Thus, between 1974 and 1990, the overall event 
rate was (10/148)=0.0676 per million flight hours. The historical fatal 
accident rate was (2/148)=0.014.
    Assuming that the average air carrier airplane has 162 seats, 69 
percent of which are occupied; the airplane replacement cost is $30 
million; and the value of an averted fatality is $2.7 million; then the 
economic value of one accident in which an airplane is destroyed and 
there are no survivors is approximately $345.9 million. If the rule 
prevents one such accident, its undiscounted benefits will exceed its 
undiscounted costs by a ratio of $345.9 million/$120.0 million or 2.88. 
Assuming that the probability of an avoided accident is proportional to 
the size of the complying fleet in any given year, then the expected 
discounted benefits of such an avoided accident will exceed discounted 
costs by a ratio of approximately 1.34.
    Prevented accidents, however, do not exhaust the benefits of this 
rule. Full-scale fatigue testing is already industry practice. This 
reflects, in part, benefits such as timely correction of deficiencies 
to prevent early cracking, and verification of inspection and 
maintenance procedures. In addition to obvious safety implications, 
early identification of premature cracking will allow repairs to be 
accomplished during scheduled maintenance visits, thus lessening the 
economic impact of withdrawing an airplane from revenue service. While 
it is difficult to account for these ancillary benefits, the accident 
history gives some indication of their potential.
    A review of records on accidents that occurred between 1974 and 
1989 shows that at least five fleetwide inspections involving 
approximately 900 airplanes were ordered as a result of accidents 
involving failure of airplane structure. During these inspections, at 
least 72 airplanes were found to have fatigue cracks. Cost information 
specifically related to these inspections is unavailable. However, a 
review of some recent Airworthiness Directives (AD) and Service 
Bulletin data indicates that a minimum of 20 work hours (10 hours 
elapsed time) are necessary to carry out the required inspections. 
Minimum out-of-service time is 15-20 hours--approximately one day. If 
the cracking is predicted by full-scale fatigue testing and planned for 
in normal maintenance, unscheduled downtime may be averted. The number 
of required work hours (and downtime) would be much greater of the 
examination reveals extensive

[[Page 15713]]

cracking since this finding would necessitate additional inspections 
and structural repair. If cracking is predicted from a full-scale 
fatigue test, it can be detected at an earlier stage of development in 
the operating airplanes, resulting in less costly repairs, requiring 
less downtime to accomplish.
    The cost of the unscheduled downtime for a fleetwide inspection, in 
which each airplane is withdrawn from revenue service for one day, can 
be estimated using the same production, operating history, and discount 
rate assumptions listed above. Assuming that the probability of an 
unscheduled inspection is uniformly distributed over each airplane's 
service life and that the revenue lost per airplane per day out of 
service is $40,000, the FAA estimates that the expected discounted 
savings from averting an average of one unscheduled inspection/repair 
day per airplane (over the service life of the fleet) is approximately 
$12.1 million. Thus, regardless of the number of accidents avoided, if 
the rule averts an average of 6.5 days of downtime per airplane over 
the life of the fleet, the expected discounted benefits of the rule 
will equal the discounted costs.

Regulatory Flexibility Determination

    The Regulatory Flexibility Act of 1980 (RFA) was enacted by 
Congress to ensure that small entities are not unnecessarily and 
disproportionately burdened by government regulations. The RFA requires 
agencies to review rules which may have a ``significant economic impact 
on a substantial number of small entities.'' Entities potentially 
affected by the rule include manufacturers of transport category 
airplanes and aircraft modification firms.
    While manufacturers of transport category airplanes generally 
support full scale fatigue testing, some aircraft modifiers--including 
some small firms--object to this requirement on the grounds that it 
constitutes and excessive burden. As noted previously, however, the 
final rule may require full-scale fatigue testing--covering, when 
applicable, modifications to future transport airplane designs--for 
four reasons.
    First, the rule will not affect existing airplane types. The 
amendment will not impose additional costs on existing applications for 
supplemental type certification, nor will it affect applications made 
in the near future. The airplanes that would be affected by this 
amendment would not enter service for at least 5 to 10 years after its 
adoption.
    Second, in the case of future type designs, it is difficult to 
predict whether anyone would seek approval for subsequent 
modifications, and, if so, how extensive the modifications would be and 
whether full-scale testing would be necessary for them (based on 
experience, the FAA concludes that most modifications of future designs 
will not require full-scale fatigue testing). Thus, it is impossible to 
conclude that there will be a significant effect on a substantial 
number of small entities.
    Third, even assuming that small entities would propose such 
modifications, the FAA has determined that the safety interests of the 
flying public take precedence over the economic interests of airplane 
modifiers. The FAA finds, that, under the circumstances where existing 
test evidence is insufficient to meet the objectives of this rule, 
there are no alternatives to full-scale testing that would enable small 
entities to meet these objectives.
    Fourth, the FAA remains open to considering technical innovations 
that provide alternatives to full scale testing. Such innovations could 
form the basis for finding that sufficient full-scale test evidence 
exists based on testing performed during initial type certification.

International Trade Impact Assessment

    The Office of Management and Budget directs Federal Agencies to 
assess whether or not a rule or regulation will affect any trade-
sensitive activity. The FAA has assessed the potential for this rule to 
affect domestic transport category airplane manufacturers, aircraft 
modification firms, and air carriers.
    The FAA determines that the rule will have little or no effect on 
trade for either U.S. firms marketing transport category airplanes in 
foreign markets or foreign firms marketing transport category airplanes 
in the U.S. This follows since full scale fatigue testing for such 
airplanes is already industry practice, both domestically and abroad. 
Also, domestic and foreign manufactured airplanes would both be subject 
to the inspection threshold provision of the rule if they are 
certificated in the U.S.
    Similarly, the FAA determines that the rule will have little or no 
effect on foreign firms competing for U.S. aircraft modification work, 
or U.S. firms competing for foreign aircraft modification work.
    The FAA recognizes that the rule could affect the competition for 
international air travel by imposing more conservative inspection 
requirements on U.S. carriers. However, it is unlikely that, in 
validating the FAA's certification of a future airplane design, another 
civil aviation authority would escalate the inspection threshold 
required by this rule. Nevertheless, if a foreign civil aviation 
authority determines that the inspection threshold is too conservative 
and, thus, chooses not to impose this requirement, U.S. carriers 
operating future airplane models subject to this rule could incur 
larger inspection costs relative to foreign carriers operating foreign 
registered airplanes of the same models. The FAA estimates that the 
discounted 20-year cost of the inspection threshold provision is 
approximately $12,000 per airplane. Under the average passenger 
capacity and load factor assumptions described above, and assuming an 
average of 1,600 departures per airplane per year, this equals 
approximately $0.003 per enplaned passenger.

Federalism Implications

    The regulations adopted herein will not have substantial direct 
effects on the States, on the relationship between the national 
government and the States, or on the distribution of power and 
responsibilities among the various levels of government. Therefore, in 
accordance with Executive Order 12612, it is determined that this rule 
will not have sufficient federalism implications to warrant the 
preparation of a Federalism Assessment.

International Civil Aviation Organization (ICAO) and Joint Aviation 
Regulations

    In keeping with U.S. obligations under the Convention on 
International Civil Aviation, it is FAA policy to comply with ICAO 
Standards and Recommended Practices to the maximum extent practicable. 
The FAA has determined that this rule does not conflict with any 
international agreement of the United States.

Paperwork Reduction Act

    In accordance with the Paperwork Reduction Act of 1995 (44 U.S.C. 
3507(d)), there are no reporting or recordkeeping requirements 
associated with this rule.

Regulations Affecting Intrastate Aviation in Alaska

    Section 1205 of the FAA Reauthorization Act of 1996 (110 Stat. 
3213) requires the Administrator, when modifying regulations in Title 
14 of the CFR in a manner affecting intrastate aviation in Alaska, to 
consider the extent to which Alaska is not served by transportation 
modes other than aviation, and to establish such

[[Page 15714]]

regulatory distinctions as he or she considers appropriate. Because 
this final rule applies to the certification of future designs of 
transport category airplanes and their subsequent operation, it could 
affect intrastate aviation in Alaska. The Administrator has considered 
the extent to which Alaska is not served by transportation modes other 
than aviation, and how the final rule could have been applied 
differently to intrastate operations in Alaska. However, the 
Administrator has determined that airplanes operated solely in Alaska 
would present the same safety concerns as all other affected airplanes; 
therefore, it would be inappropriate to establish a regulatory 
distinction for the intrastate operation of affected airplanes in 
Alaska.

Unfunded Mandates Reform Act

    Title II of the Unfunded Mandates Reform Act of 1995 (the Act), 
enacted as Public Law 104-4 on March 22, 1995, requires each Federal 
agency, to the extent permitted by law, to prepare a written assessment 
of the effects of any Federal mandate in a proposed or final rule that 
may result in the expenditure by State, local, and tribal governments, 
in the aggregate, or by the private sector, of $100 million or more 
(adjusted annually for inflation) in any one year. Section 204(a) of 
the Act, 2 U.S.C. 1534(a), requires the Federal agency to develop an 
effective process to permit timely input by elected officers (or their 
designees) of State, local, and tribal governments on a proposed 
``significant intergovernmental mandate.'' A ``significant 
intergovernmental mandate'' under the Act is any provision in a Federal 
agency regulation that would impose an enforceable duty upon State, 
local, and tribal governments, in the aggregate, of $100 million 
(adjusted annually for inflation) in any one year. Section 203 of the 
Act, 2 U.S.C. 1533, which supplements section 204(a), provides that 
before establishing any regulatory requirements that might 
significantly or uniquely affect small governments, the agency shall 
have developed a plan that, among other things, provides for notice to 
potentially affected small governments, if any, and for a meaningful 
and timely opportunity to provide input in the development of 
regulatory proposals.
    This rule does not contain a Federal intergovernmental or private 
sector mandate meeting that criterion, therefore the requirements of 
the Act do not apply.

List of Subjects in 14 CFR Part 25

    Aircraft, Aviation safety, Reporting and recordkeeping 
requirements.

Adoption of the Amendment

    In consideration of the foregoing, the Federal Aviation 
Administration (FAA) amends 14 CFR part 25 of the Federal Aviation 
Regulations (FAR) as follows:

PART 25--AIRWORTHINESS STANDARDS: TRANSPORT CATEGORY AIRPLANES

    1. The authority citation for part 25 continues to read as follows:

    Authority: 49 U.S.C. 106(g), 40113, 44701-44702, 44704.

    2. Section 25.571 is amended by revising the introductory text of 
paragraph (a), and paragraph (a)(3), the introductory text of paragraph 
(b), and paragraphs (b)(1), (b)(5)(ii), and (e)(1) to read as follows:


Sec. 25.571  Damage-tolerance and fatigue evaluation of structure.

    (a) General. An evaluation of the strength, detail design, and 
fabrication must show that catastrophic failure due to fatigue, 
corrosion, manufacturing defects, or accidental damage, will be avoided 
throughout the operational life of the airplane. This evaluation must 
be conducted in accordance with the provisions of paragraphs (b) and 
(e) of this section, except as specified in paragraph (c) of this 
section, for each part of the structure that could contribute to a 
catastrophic failure (such as wing, empennage, control surfaces and 
their systems, the fuselage, engine mounting, landing gear, and their 
related primary attachments). For turbojet powered airplanes, those 
parts that could contribute to a catastrophic failure must also be 
evaluated under paragraph (d) of this section. In addition, the 
following apply:
* * * * *
    (3) Based on the evaluations required by this section, inspections 
or other procedures must be established, as necessary, to prevent 
catastrophic failure, and must be included in the Airworthiness 
Limitations Section of the Instructions for Continued Airworthiness 
required by Sec. 25.1529. Inspection thresholds for the following types 
of structure must be established based on crack growth analyses and/or 
tests, assuming the structure contains an initial flaw of the maximum 
probable size that could exist as a result of manufacturing or service-
induced damage:
    (i) Single load path structure, and
    (ii) Multiple load path ``fail-safe'' structure and crack arrest 
``fail-safe'' structure, where it cannot be demonstrated that load path 
failure, partial failure, or crack arrest will be detected and repaired 
during normal maintenance, inspection, or operation of an airplane 
prior to failure of the remaining structure.
    (b) Damage-tolerance evaluation. The evaluation must include a 
determination of the probable locations and modes of damage due to 
fatigue, corrosion, or accidental damage. Repeated load and static 
analyses supported by test evidence and (if available) service 
experience must also be incorporated in the evaluation. Special 
consideration for widespread fatigue damage must be included where the 
design is such that this type of damage could occur. It must be 
demonstrated with sufficient full-scale fatigue test evidence that 
widespread fatigue damage will not occur within the design service goal 
of the airplane. The type certificate may be issued prior to completion 
of full-scale fatigue testing, provided the Administrator has approved 
a plan for competing the required tests, and the airworthiness 
limitations section of the instructions for continued airworthiness 
required by Sec. 25.1529 of this part specifies that no airplane may be 
operated beyond a number of cycles equal to \1/2\ the number of cycles 
accumulated on the fatigue test article, until such testing is 
completed. The extent of damage for residual strength evaluation at any 
time within the operational life of the airplane must be consistent 
with the initial detectability and subsequent growth under repeated 
loads. The residual strength evaluation must show that the remaining 
structure is able to withstand loads (considered as static ultimate 
loads) corresponding to the following conditions:
    (1) The limit symmetrical maneuvering conditions specified in 
Sec. 25.337 at all speeds up to Vc and in Sec. 25.345.
* * * * *
    (5) * * *
    (ii) The maximum value of normal operating differential pressure 
(including the expected external aerodynamic pressures during 1 g level 
flight) multiplied by a factor of 1.15, omitting other loads.
* * * * *
    (e) * * *
    (1) Impact with a 4-pound bird when the velocity of the airplane 
relative to the bird along the airplane's flight path is equal to 
Vc at sea level or 0.85Vc at 8,000 feet, 
whichever is more critical;
* * * * *

[[Page 15715]]

    Issued in Washington, D.C. on March 26, 1998.
Jane F. Garvey,
Administrator.
[FR Doc. 98-8379 Filed 3-30-98; 8:45 am]
BILLING CODE 4910-13-M