[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
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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
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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
