[Federal Register Volume 62, Number 152 (Thursday, August 7, 1997)]
[Proposed Rules]
[Pages 42433-42439]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 97-20732]
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DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
14 CFR Part 39
[Docket No. 96-NM-264-AD]
RIN 2120-AA64
Airworthiness Directives; Boeing Model 737-100 and -200 Series
Airplanes
AGENCY: Federal Aviation Administration, DOT.
ACTION: Notice of proposed rulemaking (NPRM).
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SUMMARY: This document proposes the supersedure of an existing
airworthiness directive (AD), applicable to certain Boeing Model 737-
100 and -200 series airplanes, that currently requires that the FAA-
approved maintenance inspection program be revised to include
inspections that will give no less than the required damage tolerance
rating for each Structural Significant Item, and repair of cracked
structure. That AD was prompted by a structural re-evaluation by the
manufacturer which identified additional structural elements where, if
damage were to occur, supplemental inspections may be required for
timely detection. This action would require additional and expanded
inspections, and repair of cracked structure. This action also would
expand the applicability of the existing AD to include additional
airplanes. The actions specified by the proposed AD are intended to
ensure the continued structural integrity of the Boeing Model 737-100
and -200 fleet.
DATES: Comments must be received by September 16, 1997.
ADDRESSES: Submit comments in triplicate to the Federal Aviation
Administration (FAA), Transport Airplane Directorate, ANM-103,
Attention: Rules Docket No. 96-NM-264-AD, 1601 Lind Avenue, SW.,
Renton, Washington 98055-4056. Comments may be inspected at this
location between 9:00 a.m. and 3:00 p.m., Monday through Friday, except
Federal holidays.
The service information referenced in the proposed rule may be
obtained from Boeing Commercial Airplane Group, P.O. Box 3707, Seattle,
Washington 98124-2207. This information may be examined at the FAA,
Transport Airplane Directorate, 1601 Lind Avenue, SW., Renton,
Washington.
FOR FURTHER INFORMATION CONTACT: Greg Schneider, Aerospace Engineer,
Airframe Branch, ANM-120S, FAA, Seattle Aircraft Certification Office,
1601 Lind Avenue, SW., Washington; telephone (425) 227-2028; fax (425)
227-1181.
SUPPLEMENTARY INFORMATION:
Comments Invited
Interested persons are invited to participate in the making of the
proposed rule by submitting such written data, views, or arguments as
they may desire. Communications shall identify the rules docket number
and be submitted in triplicate to the address specified above. All
communications received on or before the closing date for comments,
specified above, will be considered before taking action on the
proposed rule. The proposals contained in this notice may be changed in
light of the comments received.
Comments are specifically invited on the overall regulatory,
economic, environmental, and energy aspects of the proposed rule. All
comments submitted will be available, both before and after the closing
date for comments,
[[Page 42434]]
in the rules docket for examination by interested persons. A report
summarizing each FAA-public contact concerned with the substance of
this proposal will be filed in the rules docket.
Commenters wishing the FAA to acknowledge receipt of their comments
submitted in response to this notice must submit a self-addressed,
stamped postcard on which the following statement is made: ``Comments
to Docket Number 96-NM-264-AD.'' The postcard will be date stamped and
returned to the commenter.
Availability of NPRMs
Any person may obtain a copy of this NPRM by submitting a request
to the FAA, Transport Airplane Directorate, ANM-103, Attention: Rules
Docket No. 96-NM-264-AD, 1601 Lind Avenue, SW., Renton, Washington
98055-4056.
Determination To Develop the Supplemental Structural Inspection
Program
As part of its continuing work to maintain the structural integrity
of older transport category airplanes, in the early 1980's, the FAA
concluded that the incidence of fatigue cracking may increase as these
airplanes reach or exceed their design service objective (DSO). A
significant number of these airplanes were approaching or had exceeded
the DSO on which the initial type certification approval was
predicated. In light of this, and as a result of increased utilization,
longer operational lives, and the high levels of safety expected of the
currently operated transport category airplanes, the FAA determined
that a supplemental structural inspection program (SSIP) was necessary
to ensure a high level of structural integrity for all airplanes in the
transport fleet.
Issuance of Advisory Circular
As a follow-on from that determination, the FAA issued Advisory
Circular (AC No. 91-56), ``Supplemental Structural Inspection Program
for Large Transport Category Airplanes,'' dated May 6, 1981. The AC
provides guidance material to manufacturers and operators for use in
developing a continuing structural integrity program to ensure safe
operation of older airplanes throughout their operational lives. This
guidance material applies to large transport airplanes that were
certified under the fail-safe requirements of Civil Air Regulations 4b
or damage tolerance structural requirements of 14 CFR part 25, and that
have a maximum gross weight greater than 75,000 pounds. The procedures
set forth in this AC are applicable to the large transport category
airplanes operated under subpart F of 14 CFR part 91 and parts 121,
123, 125, and 135. The objective of the SSIP was to establish
inspection programs to ensure timely detection of fatigue cracking.
Development of the Supplemental Structural Inspection Program
In order to evaluate the effect of increased fatigue cracking with
respect to maintaining fail-safe design and damage tolerance of the
structure of Boeing Model 737-100 and -200 series airplanes, Boeing
conducted a structural reassessment of those airplanes, using modern
damage tolerance evaluation techniques. Boeing accomplished this
reassessment using the criteria contained in AC No. 91-56, as well as
14 CFR 25.571; Amdt. 25-45. During the reassessment, members of the
airline industry participated with Boeing in working group sessions and
developed the SSIP for Model 737-100 and -200 series airplanes.
Engineers and maintenance specialists from the FAA also attended these
sessions to observe these developments. Subsequently, based on the
working group's recommendations, Boeing developed the Supplemental
Structural Inspection Document (SSID) for Model 737-100 and -200 series
airplanes.
Issuance of AD 91-14-20, Amendment 39-7061
On August 9, 1991, the FAA issued AD 91-14-20, amendment 39-7061
(56 FR 30680, July 5, 1991), which is applicable to certain Boeing
Model 737 series airplanes. That AD currently requires that the FAA-
approved maintenance inspection program be revised to include
inspections that will give no less than the required damage tolerance
rating (DTR) for each Structural Significant Item (SSI), and repair of
cracked structure. The AD references Boeing Document No. D6-37089,
``Supplemental Structural Inspection Document'' (SSID), Revision B,
dated February 18, 1987, and Revision C, dated January 1990, as the
appropriate source of service information. That action was prompted by
a structural re-evaluation that identified additional structural
components where fatigue cracking is likely to occur. The requirements
of that AD are intended to ensure the continued structural integrity of
the entire Boeing Model 737 fleet.
Actions Since Issuance of Previous AD
Since issuance of AD 91-14-20, the FAA has reconsidered the
following four aspects of the existing SSID:
1. Classification of Fuselage Skin as ``Damage Obvious'' or
``Malfunction Evident''
AC No. 91-56, Change 2, dated April 15, 1983, recommends that the
SSID should contain inspections of all critical parts or components for
each airplane to ensure the continued safe operation of the existing
fleet. The fuselage skin is an example of a critical component.
Cracking in any critical part or component, if not detected and
corrected in a timely manner, could result in reduced structural
integrity of the airplane.
Revisions B and C of the SSID excluded the fuselage skin from
directed inspections, since it was classified as ``damage obvious'' or
``malfunction evident.'' At the time of this classification, Revisions
B and C of the SSID relied on venting or flapping to indicate cracks in
the fuselage skin.
Venting is a gradual loss of cabin pressure as a result of cracking
in the pressurized area of the fuselage skin. Based on the design
philosophy of flapping, these cracks in the fuselage skin would grow
only to a specific length and then turn direction because of certain
structural components. Because venting and flapping were considered to
be readily apparent, Boeing considered that it was unnecessary to
provide for additional inspections of the fuselage skin. Reliance also
was placed on venting or flapping to allow for the safe operation of an
airplane with such cracks. This technique worked well in ground tests
and in some in-service incidents, but proved to be unreliable in other
cases.
In one such case, a large portion of Section 43 of the fuselage
structure separated from a Boeing Model 737 series airplane. Results of
a National Transportation Safety Board (NTSB) investigation revealed
that this incident occurred as a result of the catastrophic failure of
the fuselage skin at a lap joint. The results also revealed that,
contrary to the design philosophy, controlled decompression of the
structure (i.e., flapping or venting) did not occur due to the presence
of widespread fatigue damage. As a result of this failure, the NTSB
recommended that the SSID be revised to discontinue classification of
the fuselage skin as ``damage obvious'' or ``malfunction evident.''
The FAA concurs with the NTSB's recommendation. Therefore, the FAA
has determined that additional inspections are necessary to ensure
timely detection of cracks in the fuselage skin structure.
[[Page 42435]]
2. Deletions of Modified, Altered, or Repaired Structure from the SSIP
Paragraph 1.4 of Appendix 1, ``Guidelines for Development of
Supplemental Inspection Document,'' of AC No. 91-56, Change 2, dated
April 15, 1983, states, ``the effect of repairs and modifications
approved by the manufacturer should also be taken into account. In
addition, it may be necessary to consider the effect of repairs and
operator-approved modifications on individual airplanes. The operator
has the responsibility for ensuring notification and consideration of
any such aspects.''
In addition, the FAA's current policy is that operators of
transport category airplanes that are subject to AD's that mandate SSID
programs should follow the guidelines of AC No. 91-56 and should
continue to inspect any SSI that is modified, altered, or repaired in
any way. Any modification that affects the loading spectrum, stress
levels, or damage tolerance characteristics of the structure must be
reassessed to determine its impact on the inspection program. Such a
reassessment may require the development of additional inspection
requirements for that modification.
The FAA's policy also states that, ``* * * the [SSID] programs are
based on type design crack growth data generated from analysis or
structural tests using a realistic and conservative loading spectrum,
material properties, part geometry, etc. For this reason, structural
modifications that may increase stress levels in load carrying
structures, including maximum weight limit increases, cargo door
installations, and repairs to load carrying structures, must be
reassessed for its impact on the structural inspection program.''
(Reference: Transport Airplane Directorate's Policy Letter,
Information: Policy Regarding Impact of Modifications and Repairs on
the Damage Tolerance Characteristics of Transport Category Airplanes,
dated October 27, 1989. This letter will be retained in Rules Docket
No. 96-NM-264-AD.)
Section 5.0 of Revisions B and C of the SSID contains provisions
that allow for the deletion of modified, altered, or repaired areas
from the SSIP because Boeing considers these areas not to be
``representative of the fleet.'' The FAA is aware that there have been
a significant number of such deletions. As a result, contrary to the
FAA's policy discussed above, operators are not following the
guidelines of AC No. 91-56 and not continuing to inspect any SSI that
is modified, altered, or repaired in any way.
In addition, for Boeing Model 737-100 and -200 series airplanes
that have been converted from a passenger configuration to an all-cargo
configuration by the Supplemental Type Certification (STC) process, the
FAA finds that Revisions B and C of the SSID do not include procedures
for inspection of new SSI's created by this conversion, or unmodified
SSI's affected by this conversion. (There are approximately 100 of
these airplanes in the worldwide fleet of which several are listed in
the effectivity listing of Revisions B and C of the SSID.) These
conversions have the effect of removing SSI's from the SSIP and
creating a large number of new SSI's that have not been assessed.
Consequently, airplanes that have been converted to an all-cargo
configuration do not have a SSID that specifies an inspection method
and compliance time for each new SSI. Additionally, an unmodified SSI
also could require a new inspection method and compliance time because
the modification may increase the loads or change the load
distributions in that SSI. These conditions would necessitate that the
inspection interval for that affected, unmodified SSI be shorter than
required in the Boeing SSID. Hence, the FAA finds that the objectives
of the SSIP are not being met for these modified airplanes.
Likewise, a design change (such as an increase in the maximum
certified weight or in the center of gravity limits) also may cause an
increase in the loads or change the load distributions in the affected,
unmodified SSI's. The effect of this increase or change would be
similar to the effect that a cargo conversion would have on an
unmodified SSI. As a result, the inspection interval for an affected,
unmodified SSI may need to be lower than required in the Boeing SSID.
Thus, the DTR specified in the SSID for any SSI affected by a design
change may no longer be applicable. Therefore, the FAA finds that the
objectives of the SSIP are not being met for airplanes with such design
changes.
Furthermore, in consideration of AC No. 91-56 and current FAA
policy, the FAA has determined that new inspection methods and
compliance times are necessary for areas that have been modified,
altered, or repaired to ensure timely detection of cracking in those
areas. The FAA also has determined that new inspection methods and
compliance times are necessary for those areas that were deleted from
the SSIP by previously approved alternative methods of compliance,
which includes those areas deleted in accordance with the requirements
of Section 5.0 of the SSID. Furthermore, the new inspection methods and
compliance times should meet the requirements of 14 CFR 25.1529, Amdt.
25-45; 14 CFR 25.571, Amdt. 25-45; 14 CFR 25.571, Amdt. 25-54; 14 CFR
25.571, Amdt. 25-72; or the guidelines of AC 91-56.
3. Candidate Fleet vs. Inspection Threshold Approach
Paragraph 4.4 of AC No. 91-56, Change 2, dated April 15, 1983,
states, ``Inspection thresholds for supplemental inspections should be
established. These inspections would be supplemental to the normal
inspection including the detailed internal inspections.'' Moreover,
paragraph 4.4.2 of AC No. 91-56 states, ``* * * this threshold should
be such as to include sufficient [high-cycle] airplanes in the
inspection to develop added confidence in the integrity of the
structure * * *.''
A properly established inspection threshold ensures that: (1) The
SSI inspections are accomplished; (2) fatigue cracks in SSI's are
detected in a timely manner; (3) airplanes are automatically added to
the SSIP; and (4) the SSIP includes a statistically valid number of
airplanes.
Among other things, Revisions B and C of the SSID define a
candidate fleet approach to ensure that fatigue cracks in SSI's are
detected in a timely manner in the entire fleet. The initial Boeing
Model 737 candidate fleet consisted of a number of airplanes that had
exceeded 37,500 flight cycles by April 30, 1983. In other words, Boeing
considered 37,500 flight cycles to be the threshold for the airplanes
in the candidate fleet. These airplanes were the most likely in the
fleet to experience initial fatigue damage since they had the highest
number of flight cycles. Boeing produced this SSID with the assumption
that the airplanes in the candidate fleet would continue to represent
the entire fleet and would have the highest number of flight cycles in
the fleet.
Under the existing SSIP, Boeing intended to periodically review the
airplanes in the candidate fleet for significant changes in fleet
distribution, composition, or utilization, and update the candidate
fleet, if any significant change was detected. It was intended that the
FAA would then mandate any change to the SSID through the rulemaking
process.
The FAA finds that the candidate fleet approach is deviating from
Boeing's original philosophy in that the candidate fleet has not been
updated to reflect changes (such as cargo conversions) in the fleet.
This situation
[[Page 42436]]
could result in a statistically invalid number of airplanes in the SSIP
and undetected fatigue cracks in SSI's. The candidate fleet approach
also does not automatically account for non-candidate airplanes that
eventually accumulate more flight cycles than that of certain candidate
airplanes. High-cycle airplanes are more likely to experience initial
fatigue damage in the fleet. The confidence in the structural integrity
of the fleet of airplanes could be reduced if high-cycle airplanes are
excluded from the SSIP.
The FAA has reconsidered the candidate fleet approach described in
Revisions B and C of the SSID, since it does not meet the guidelines of
AC No. 91-56. The FAA has determined that the Boeing Model 737 SSIP
must contain inspection thresholds for all Boeing Model 737-100 and -
200 series airplanes to ensure the timely detection of fatigue cracks
in the SSI's. (The FAA is currently considering a separate rulemaking
action to address the problems associated with fatigue cracking on all
Boeing Model 737-300, -400, and -500 series airplanes.)
The FAA has reviewed the thresholds derived from Boeing's
reliability analysis. The analysis is based on a certain probability
that cracks will be detected in the inspected fleet before they
initiate on other airplanes that have not been inspected. The FAA has
determined that the thresholds recommended in the analysis of past
service experience of the Boeing Model 737 fleet are acceptable.
Therefore, for Model 737-200C series airplanes, the FAA has determined
that a threshold of 46,000 total flight cycles is necessary in order to
produce a statistically valid assessment of the service history for
these airplanes. For other Model 737-100 and -200 series airplanes, the
FAA has determined that a threshold of 66,000 total flight cycles is
necessary to produce a valid assessment. The threshold for Model 737-
200C series airplanes is lower than that of other Model 737 series
airplanes since Model 737-200C series airplanes have a lower
utilization rate and fewer airplanes in the fleet. Since the
utilization rate is lower for Model 737-200C series airplanes, these
airplanes have accumulated fewer flight cycles and have fewer airplanes
with higher flight cycles than that of the remaining fleet.
It should be noted that, although the proposed AD requires a
threshold, the FAA may approve requests for adjustments to the
compliance time [i.e., under paragraph (h)(1) of this proposed AD]
provided that no cracking is detected in the airplane's SSI's. The
request should include a new inspection threshold and must include data
to substantiate that such an adjustment would provide an acceptable
level of safety.
Operators should note that the alternative inspection threshold may
be based solely on the analysis of the data of the existing fleet.
However, the FAA has determined that the analysis that derives the new
inspection threshold must include: (1) Data relevant to a sufficient
number of high-cycle airplanes, and (2) data that shows accomplishment
of the inspections of the SSI's. An adequate statistical sampling size
will provide confidence in the structural integrity of the fleet of
airplanes. Therefore, additional airplanes may need to be added to the
inspected fleet until a sufficient number of airplanes have been
inspected with no crack findings.
4. Transferability of Airplanes
Since issuance of the SSID and AD 91-14-20, the FAA has issued
several AD's that implement Corrosion Prevention and Control Programs
(CPCP) for aging airplanes. While developing the AD's that mandated the
CPCP, the FAA recognized that an operator of an airplane that has been
transferred from another operator could revise its maintenance program
to restart the compliance times for the required corrosion tasks. This
situation could lead to corrosion not being detected and corrected in a
timely manner, which could reduce the structural integrity of the
airplane.
As a result, the CPCP AD's require that operators establish a
program for accomplishment of the subject corrosion tasks before any
airplane can be added to an air carrier's operations specification.
Establishment of this program will ensure that airplanes transferred
from operator to operator are inspected and that corrosion is detected
in a timely manner.
The FAA's intent in AD 91-14-20 was that operators of candidate
fleet airplanes that have been previously operated under an FAA-
approved maintenance program accomplish the SSID inspections within the
compliance time established by the previous operator. The FAA assumed
that, under the existing SSID, these airplanes would be inspected in a
manner similar to CPCP requirements. However, the SSID and AD 91-14-20
do not address the transfer of airplanes in the candidate fleet from
one operator to another.
AD 91-14-20 currently requires that the revision to the maintenance
program be included and be implemented in accordance with the
procedures specified in Sections 5.0 and 6.0 of the SSID. However, the
FAA finds that these sections do not provide explicit instructions to
repetitively inspect airplanes that have been transferred from one
operator to another. It also does not specify that new operators must
continue the SSID inspections at the same frequency established by the
previous operator.
In addition, as AD 91-14-20 is currently worded, the FAA finds that
operators that acquire candidate fleet airplanes that have been
previously operated under a maintenance inspection program could revise
their programs to restart the compliance times. This situation is
contrary to standard AD requirements. An AD typically mandates an
initial compliance time and a repetitive interval that remains
unchanged for all operators of the affected airplanes.
As a result of these omissions, the SSID inspections of a candidate
fleet airplane could be deferred until it is required by the
maintenance inspection program of the new operator. For airplanes that
are transferred frequently, this situation could continue for the life
of the airplane. As a result, fewer Boeing Model 737 candidate fleet
airplanes are being inspected; thus, the size of the candidate fleet is
in effect reduced. Even if airplanes are ultimately inspected under
these circumstances, inspections would not be performed frequently
enough to maintain the applicable DTR. The FAA has determined that such
a reduction does not ensure the continued structural integrity of the
entire Boeing Model 737 fleet.
Implementation of procedures in the SSID that are similar to the
CPCP will ensure that: (1) Airplanes transferred from operator to
operator are inspected; (2) the SSIP includes a statistically valid
number of airplanes; and (3) fatigue cracks are detected in a timely
manner.
Therefore, the FAA finds that, to ensure the continued structural
integrity of the entire Model 737 fleet, the AD 91-14-20 must be
revised to include provisions that address the transfer of airplanes.
The FAA also finds that a program must be established to accomplish the
inspections before any airplane that is subject to this proposal can be
added to an air carrier's operations specifications.
FAA's Conclusions
In light of all the factors discussed above, the FAA has determined
that AD 91-14-20 does not adequately ensure timely detection of fatigue
cracking in SSI's. Fatigue cracking in those items, if not detected and
corrected in a timely manner, could result in reduced structural
integrity of the airplane.
[[Page 42437]]
Explanation of New Relevant Service Information
The FAA has reviewed and approved Boeing Document No. D6-37089,
``Supplemental Structural Inspection Document'' (SSID), Revision D,
dated June 1995, which describes procedures for revising the FAA-
approved maintenance inspection program for all Boeing Model 737-100
and -200 series airplanes. This revision of the Model 737 SSID
incorporates additional and expanded inspections from those that were
contained in the previous version and mandated by AD 91-14-20. The
fuselage skin structure that was the subject of an NTSB recommendation
is included in these inspections. The FAA finds that accomplishment of
these inspections will ensure the continuing structural integrity of
the Boeing Model 737-100 and -200 fleet.
Explanation of Requirements of Proposed Rule
Since an unsafe condition has been identified that is likely to
exist or develop on other products of this same type design, the
proposed AD would supersede AD 91-14-20.
Paragraph (a) of the proposed AD restates the requirements of AD
91-14-20.
Paragraph (b) of the proposed AD would require incorporation of a
revision into the FAA-approved maintenance inspection program that
provides no less than the required DTR for each SSI listed in Revision
D of the SSID.
Paragraph (c) of the proposed AD would establish specific
compliance times for performing the initial inspection of the structure
identified in Revision D of the SSID. Once the initial inspection has
been performed, operators would be required to perform repetitive
inspections at the intervals specified in the Document in order to
remain in compliance with their maintenance inspection programs, which
would have been revised in accordance with paragraph (b) of this
proposed AD.
Paragraph (d) of the proposed AD would require, for airplanes on
which any design change or repair has been accomplished prior to the
effective date of this proposed AD, a revision to the FAA-approved
maintenance inspection program to include an inspection method for any
new or affected SSI, and to include the compliance times for this
inspection. This paragraph also would require that any new inspection
method and the compliance times be approved by the FAA.
Paragraph (e) of the proposed AD would require that the repair of
any cracked structure is to be accomplished in accordance with an FAA-
approved method.
Paragraph (f) of the proposed AD would require, for airplanes on
which any design change or repair has been accomplished after the
effective date of this proposed AD, a revision to the FAA-approved
maintenance inspection program to include a new inspection method for
any new or affected SSI, and to include the compliance times for this
inspection. This paragraph also would require that any new inspection
method and the compliance times be approved by the FAA.
Before any airplane that is subject to this proposed AD can be
added to an air carrier's operations specifications, a program for the
accomplishment of the inspections required by this proposed AD must be
established. Paragraph (g) of the proposed AD would require
accomplishment of the following:
1. For airplanes that have been inspected in accordance with this
proposed AD, the inspection of each SSI must be accomplished by the new
operator in accordance with the previous operator's schedule and
inspection method, or the new operator's schedule and inspection
method, whichever would result in the earlier accomplishment date for
that SSI inspection. The compliance time for accomplishment of this
inspection must be measured from the last inspection accomplished by
the previous operator. After each inspection has been performed once,
each subsequent inspection must be performed in accordance with the new
operator's schedule and inspection method.
2. For airplanes that have not been inspected in accordance with
this proposed AD, the inspection of each SSI must be accomplished
either prior to adding the airplane to the air carrier's operations
specification, or in accordance with a schedule and an inspection
method approved by the FAA. After each inspection has been performed
once, each subsequent inspection must be performed in accordance with
the new operator's schedule.
Accomplishment of these inspections will ensure that: (1)
Operators' newly acquired airplanes comply with its SSIP before being
operated; and (2) frequently transferred aircraft are not permitted to
operate without accomplishment of the inspections defined in the SSID.
Differences Between SSID and Proposed AD
Operators should note the following differences between the
procedures specified in Revision D of the SSID and the proposed
requirements of this AD:
1. Paragraphs 5.1.17 and 5.1.18 of the General Instructions of
Revision D of the SSID permit deletions of modified, altered, or
repaired structure from the SSIP. As described previously in Item 2 of
the ``Actions Since Issuance of Previous AD'' section of this preamble,
the FAA has determined that such deletions are unacceptable. Therefore,
for airplanes on which the areas specified in the SSID have been
modified, altered, or repaired, the proposed AD would require a
revision to the operator's existing SSIP to include procedures for
accomplishing a new FAA-approved inspection method that provides a new
DTR for that SSI.
2. Revision D of the SSID bases the supplemental inspections on
specific high-cycle airplanes (i.e., candidate fleet airplanes) and
does not include an inspection threshold for those airplanes. It also
does not automatically add airplanes to the candidate fleet. Based on
the discussion described previously in Item 3 of the ``Actions Since
Issuance of Previous AD'' section of this preamble, the FAA has
determined that the proposed AD would expand the applicability of this
AD action to include all Model 737-100 and -200 series airplanes. In
addition, for Model 737-200C series airplanes, the proposed inspection
of all SSI's would be required to be accomplished prior to the
accumulation of 46,000 total flight cycles, or within 18 months,
whichever occurs later. For other Model 737-100 and -200 series
airplanes, the proposed inspection of all SSI's would be required to be
accomplished prior to the accumulation of 66,000 total flight cycles,
or within 18 months, whichever occurs later.
Cost Impact
There are approximately 1,021 Boeing Model 737-100 and -200 series
airplanes of the affected design in the worldwide fleet. The FAA
estimates that 404 airplanes of U.S. registry would be affected by this
proposed AD.
The actions that are proposed in this AD action would take
approximately 1,200 work hours per airplane to accomplish, at an
average labor rate of $60 per work hour. Based on these figures, the
cost impact of the proposed requirements of this AD on U.S. operators
is estimated to be $29,088,000, or $72,000 per airplane, per inspection
cycle.
The cost impact figure discussed above is based on assumptions that
no operator has yet accomplished any of the current or proposed
requirements of this AD action, and that no operator
[[Page 42438]]
would accomplish those actions in the future if this AD were not
adopted.
The number of required work hours, as indicated above, is presented
as if the accomplishment of the actions proposed in this AD were to be
conducted as ``stand alone'' actions. However, in actual practice,
these actions for the most part would be accomplished coincidentally or
in combination with normally scheduled airplane inspections and other
maintenance program tasks. Therefore, the actual number of necessary
additional work hours would be minimal in many instances. Additionally,
any costs associated with special airplane scheduling would be minimal.
Regulatory Impact
The regulations proposed herein would 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
proposal would not have sufficient federalism implications to warrant
the preparation of a Federalism Assessment.
For the reasons discussed above, I certify that this proposed
regulation (1) is not a ``significant regulatory action'' under
Executive Order 12866; (2) is not a ``significant rule'' under the DOT
Regulatory Policies and Procedures (44 FR 11034, February 26, 1979);
and (3) if promulgated, will not have a significant economic impact,
positive or negative, on a substantial number of small entities under
the criteria of the Regulatory Flexibility Act. A copy of the draft
regulatory evaluation prepared for this action is contained in the
rules docket. A copy of it may be obtained by contacting the Rules
Docket at the location provided under the caption ADDRESSES.
List of Subjects in 14 CFR Part 39
Air transportation, Aircraft, Aviation safety, Safety.
The Proposed Amendment
Accordingly, pursuant to the authority delegated to me by the
Administrator, the Federal Aviation Administration proposes to amend
part 39 of the Federal Aviation Regulations (14 CFR part 39) as
follows:
PART 39--AIRWORTHINESS DIRECTIVES
1. The authority citation for part 39 continues to read as follows:
Authority: 49 U.S.C. 106(g), 40113, 44701.
Sec. 39.13 [Amended]
2. Section 39.13 is amended by removing amendment 39-7061 (56 FR
30680, July 5, 1991), and by adding a new airworthiness directive (AD),
to read as follows:
Boeing: Docket 96-NM-264-AD. Supersedes AD 91-14-20, Amendment 39-
7061.
Applicability: All Model 737-100 and -200 series airplanes,
certificated in any category.
Compliance: Required as indicated, unless accomplished
previously.
To ensure the continued structural integrity of the Boeing Model
737-100 and -200 fleet, accomplish the following:
Note 1: Where there are differences between the AD and the
Supplemental Structural Inspection Document, the AD prevails.
(a) For airplanes listed in Section 3.0 of Boeing Document No. D6-
37089, ``Supplemental Structural Inspection Document'' (SSID), Revision
B, dated February 18, 1987, and Revision C, dated January 1990: Within
12 months after August 9, 1991 (the effective date of AD 91-14-20,
amendment 39-7061), incorporate a revision into the FAA-approved
maintenance inspection program which provides no less than the required
damage tolerance rating (DTR) for each Structural Significant Item
(SSI) listed in that document. (The required DTR value for each SSI is
listed in the document.) The revision to the maintenance program shall
include and shall be implemented in accordance with the procedures in
Sections 5.0 and 6.0 of the SSID. This revision shall be deleted
following accomplishment of the requirements of paragraph (b) of this
AD.
Note 2: For the purposes of this AD, an SSI is defined as a
principal structural element that could fail and consequently reduce
the structural integrity of the airplane.
(b) Within 12 months after the effective date of this AD,
incorporate a revision into the FAA-approved maintenance inspection
program that provides no less than the required DTR for each SSI listed
in Boeing Document No. D6-37089, ``Supplemental Structural Inspection
Document'' (SSID), Revision D, dated June 1995 (hereinafter referred to
as ``Revision D''). (The required DTR value for each SSI is listed in
the document.) The revision to the maintenance program shall include
and shall be implemented in accordance with the procedures in Section
5.0, ``Damage Tolerance Rating (DTR) System Application'' and Section
6.0, ``SSI Discrepancy Reporting'' of Revision H. Upon incorporation of
the revision required by this paragraph, the revision required by
paragraph (a) of this AD may be deleted.
(c) Except as provided in paragraph (d) or (f) of this AD, as
applicable, perform an inspection to detect cracks in all structure
identified in Revision D at the time specified in paragraph (c)(1) or
(c)(2) of this AD, as applicable.
(1) For Model 737-200C series airplanes: Inspect prior to the
accumulation of 46,000 total flight cycles, or within 18 months
after the effective date of this AD, whichever occurs later.
(2) For Model 737-100 and -200 series airplanes, except for
those airplanes identified in paragraph (c)(1) of this AD: Inspect
prior to the accumulation of 66,000 total flight cycles, or within
18 months after the effective date of this AD, whichever occurs
later.
Note 3: Once the initial inspection has been performed,
operators are required to perform repetitive inspections at the
intervals specified in Revision D in order to remain in compliance
with their maintenance inspection programs, as revised in accordance
with paragraph (b) of this AD.
(d) For airplanes on which the structure identified in Revision
D is affected by any design change or repair that was accomplished
prior to the effective date of this AD: Within 18 months after the
effective date of this AD, revise the FAA-approved maintenance
inspection program to include an inspection method for any new or
affected SSI, and to include the compliance times for initial and
repetitive accomplishment of this inspection. For purposes of this
section, an SSI is ``affected'' if it has been altered or repaired,
or if the loads acting on the SSI have been increased or
redistributed. Following accomplishment of the revision and within
the compliance times established, perform an inspection to detect
cracks in the structure affected by any design change or repair, in
accordance with the new inspection method. The new inspection method
and the compliance times shall be approved by the Manager, Seattle
Aircraft Certification Office (ACO), FAA, Transport Airplane
Directorate, 1601 Lind Avenue, SW., Renton, Washington 98055-4056,
fax (425) 227-1181.
Note 4: Notwithstanding the provisions of paragraphs 5.1.17 and
5.1.18 of the General Instructions of Revision D, which would permit
deletions of modified, altered, or repaired structure from the
Supplemental Structural Inspection Program (SSIP), the inspection of
SSI's that are modified, altered, or repaired shall be done in
accordance with a method approved by the Manager, Seattle ACO.
Note 5: For the purposes of this AD, a design change is defined
as any modification, alteration, or change to operating limitations.
(e) Cracked structure found during any inspection required by
this AD shall be repaired, prior to further flight, in accordance
with an FAA-approved method.
(f) For airplanes on which the structure identified in Revision
D is affected by any design change or repair that is accomplished
after the effective date of this AD: Within 12 months after that
modification, alteration, or repair for any new or affected SSI,
revise the FAA-approved maintenance inspection
[[Page 42439]]
program to include an inspection method for any new or affected SSI,
and to include the compliance times for initial and repetitive
accomplishment of this inspection. For purposes of this section, an
SSI is ``affected'' if it has been altered or repaired, or if the
loads acting on the SSI have been increased or redistributed.
Following accomplishment of the revision and within the compliance
times established, perform an inspection to detect cracks in the
structure affected by any design change or repair, in accordance
with the new inspection method. The new inspection method and the
compliance times shall be approved by the Manager, Seattle ACO.
Note 6: Notwithstanding the provisions of paragraphs 5.1.17 and
5.1.18 of the General Instructions of Revision D, which would permit
deletions of modified, altered, or repaired structure from the SIP,
the inspection of SSI's that are modified, altered, or repaired
shall be done in accordance with a method approved by the Manager,
Seattle ACO.
(g) Before any airplane that is subject to this AD and that has
exceeded the applicable compliance times specified in paragraph (c)
of this AD can be added to an air carrier's operations
specifications, a program for the accomplishment of the inspections
required by this AD must be established in accordance with paragraph
(g)(1) or (g)(2) of this AD, as applicable.
(1) For airplanes that have been inspected in accordance with
this AD, the inspection of each SSI must be accomplished by the new
operator in accordance with the previous operator's schedule and
inspection method, or the new operator's schedule and inspection
method, whichever would result in the earlier accomplishment date
for that SSI inspection. The compliance time for accomplishment of
this inspection must be measured from the last inspection
accomplished by the previous operator. After each inspection has
been performed once, each subsequent inspection must be performed in
accordance with the new operator's schedule and inspection method.
(2) For airplanes that have not been inspected in accordance
with this AD, the inspection of each SSI required by this AD must be
accomplished either prior to adding the airplane to the air
carrier's operations specification, or in accordance with a schedule
and an inspection method approved by the Manager, Seattle ACO. After
each inspection has been performed once, each subsequent inspection
must be performed in accordance with the new operator's schedule.
(h)(1) An alternative method of compliance or adjustment of the
compliance time that provides an acceptable level of safety may be
used if approved by the Manager, Seattle ACO. Operators shall submit
their requests through an appropriate FAA Principal Maintenance
Inspector, who may add comments and then send it to the Manager,
Seattle ACO.
Note 7: Information concerning the existence of approved
alternative methods of compliance with this AD, if any, may be
obtained from the Seattle ACO.
(2) Alternative methods of compliance, approved previously in
accordance with AD 91-14-20, amendment 39-7061, are not considered
to be approved as alternative methods of compliance with this AD.
(i) Special flight permits may be issued in accordance with
sections 21.197 and 21.199 of the Federal Aviation Regulations (14
CFR 21.197 and 21.199) to operate the airplane to a location where
the requirements of this AD can be accomplished.
Issued in Renton, Washington, on July 31, 1997.
Darrell M. Pederson,
Acting Manager, Transport Airplane Directorate, Aircraft Certification
Service.
[FR Doc. 97-20732 Filed 8-6-97; 8:45 am]
BILLING CODE 4910-13-O
