[Federal Register Volume 63, Number 96 (Tuesday, May 19, 1998)]
[Rules and Regulations]
[Pages 27465-27474]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 98-13078]
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DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
14 CFR Part 39
[Docket No. 96-NM-264-AD; Amendment 39-10531; AD 98-11-04]
RIN 2120-AA64
Airworthiness Directives; Boeing Model 737-100 and -200 Series
Airplanes
AGENCY: Federal Aviation Administration, DOT.
ACTION: Final rule.
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SUMMARY: This amendment supersedes an existing airworthiness directive
(AD), applicable to all 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
amendment requires additional and expanded inspections, and repair of
cracked structure. This amendment also expands the applicability of the
existing AD to include additional airplanes. The actions specified by
this AD are intended to ensure the continued structural integrity of
the entire Boeing Model 737-100 and -200 fleet.
DATES: Effective June 23, 1998.
The incorporation by reference of certain publications is approved
by the Director of the Federal Register as June 23, 1998.
ADDRESSES: The service information referenced in this AD may be
obtained from Boeing Commercial Airplane Group, P.O. Box 3707, Seattle,
Washington 98124-2207. This information may be examined at the Federal
Aviation Administration (FAA), Transport Airplane Directorate, Rules
Docket, 1601 Lind Avenue, SW., Renton, Washington; or at the Office of
the Federal Register, 800 North Capitol Street, NW., suite 700,
Washington, DC.
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: A proposal to amend part 39 of the Federal
Aviation Regulations (14 CFR part 39) by superseding airworthiness
directive (AD) 91-14-20, amendment 39-7061 (56 FR 30680, July 5, 1991),
which is applicable to all Boeing Model 737-100 and -200 series
airplanes, was published in the Federal Register on August 7, 1997 (62
FR 42433). That action proposed to supersede AD 91-14-20 to continue to
require that the FAA-approved maintenance program be revised to include
inspections that will give no less than the required damage tolerance
rating for each Structural Significant Item (SSI). That action also
proposed to require additional and expanded inspections, and repair of
cracked structure. In addition, that action proposed to expand the
applicability of the existing AD to include additional airplanes. [A
similar proposal applicable to all Boeing Model 727 series airplanes
also was published in the Federal Register on May 29, 1997 (62 FR
29081).]
Comments
Interested persons have been afforded an opportunity to participate
in the making of this amendment. Due consideration has been given to
the comments received.
The FAA has received comments in response to the two NPRM's
discussed previously (i.e., Docket Nos. 96-NM-263-AD and 96-NM-264-AD).
Because in most cases the issues raised by the commenters are generally
relevant to both NPRM's, each final rule includes a discussion of all
comments received.
Two commenters support the proposed rule.
Delete Repairs and Type Certificate Holder Modifications
Several commenters request that, for the reasons stated below, the
FAA delete the requirements that address repairs and Boeing
modifications (i.e., modifications specified in service bulletins or
other technical data issued by Boeing), as specified in paragraphs (d)
and (f) of the proposed AD.
Several commenters contend that the intent of the Boeing
Supplemental Structural Inspection Program (SSIP) was to evaluate the
original structure of candidate fleet airplanes using the latest damage
tolerance methods, not to bring all airplanes up to damage tolerance
design. They note that the Boeing Supplemental Structural Inspection
Document (SSID) explicitly excluded SSI's that had been modified or
repaired, because they were no longer considered to be representative
of the configuration of the fleet. One of these commenters also states
that Boeing should retain the authority to determine whether repaired
SSI's are representative.
The FAA infers that the commenters believe that the purpose of the
SSIP for Boeing airplanes is limited to protecting the original
airplane structure. As discussed in the notice of proposed rulemaking
(NPRM), FAA Advisory Circular (AC) No. 91-56, Change 2, dated April 15,
1983, states that assessments should be accomplished on modified or
repaired structure to determine whether special inspections are needed
to ensure continued airworthiness, regardless of whether the structure
continues to be ``representative'' of the original structure.
Consistent with this policy, the FAA has previously issued other SSIP
AD's that effectively require assessment of repairs and modifications:
For McDonnell Douglas Model DC-8 series airplanes: AD 93-
01-15, amendment 39-8464 (58 FR 5576, January 22, 1993);
For McDonnell Douglas Model DC-9 series airplanes: AD 96-
13-03, amendment 39-9671 (61 FR 31009, June 19, 1996); and
For McDonnell Douglas Model DC-10 series airplanes: AD 95-
23-09, amendment 39-9429 (60 FR 61649, December 1, 1995).
One of the purposes of this AD is to correct this deficiency in the
Boeing SSIP. The commenters have not provided any information to call
this basic policy into question. The FAA finds that repaired or
modified SSI's should be included in the Boeing SSIP
[[Page 27466]]
to ensure timely detection of cracking in those areas. Boeing does
retain the authority to determine whether repaired or modified SSI's
are ``representative,'' but that determination will no longer have the
effect of deleting repaired or modified SSI's from the Boeing SSIP.
Several commenters also state that, in consideration of their
request to delete repaired SSI's or Boeing modifications from the SSIP,
reducing the inspection thresholds specified in the proposed AD would
offset the FAA's concern regarding the reduction in the number of
inspected SSI's. One of these commenters suggests that the FAA reduce
the inspection thresholds specified in the proposed AD by an
incremental amount to increase the inspected fleet by 10 percent. Such
a reduction would compensate for the subject deletions. Another
commenter states that lowering the threshold would require less time
and lower labor costs than that required to develop special inspections
for repairs and modifications. The FAA does not concur. As discussed
previously, the purpose of the SSIP is to ensure the continued
airworthiness of all airplanes, including those that have been repaired
or modified. The commenters' proposal would not achieve this objective.
In contrast to the previous comments, several commenters state that
SSI's affected by standard repairs or Boeing modifications do not need
to be included in the Boeing SSIP, because the original structure is
``representative'' of the durability of repaired or modified structure.
The FAA does not concur. Although repaired or modified structure may be
similar to original structure, operators must accomplish an assessment
to determine if the inspection program specified in the SSID is
effective. It should be noted that, if the assessment indicates that
the applicable inspection specified in the SSID is effective, no change
to the Boeing SSIP is required.
Several commenters state that paragraphs (d) and (f) of the
proposed AD are unnecessary because other airworthiness programs and
documents, such as the proposed repair assessment program (RAP) for
pressurized fuselages, will require operators to assess repairs and
modifications. [The FAA has issued Notice No. 97-16, Docket No. 29104
(63 FR 126, January 2, 1998) that would require operators of certain
transport category airplanes, including the Model 737, to adopt RAP's
into their maintenance or inspection programs.] Two of these commenters
state that the 737 Structures Task Group (STG) (a group consisting of
737 operators and Boeing) has taken the position that only repairs to
the fuselage skins and pressure webs need to be assessed for damage
tolerance, not repairs to other areas of the airplane structure (e.g.,
wing and empennage SSI's).
For two reasons, the FAA does not concur that the proposed RAP is
adequate to address potential fatigue cracking of modified or repaired
SSI's. First, the proposed RAP does not address either the damage
tolerance characteristics of SSI's in supplemental type certificate
(STC) modified structure that has not been repaired, or the effects of
such modifications on original SSI's.
Second, the FAA does not concur with the commenters that only the
pressure boundary should be subject to a damage tolerance assessment.
The STG's conclusion that only repairs to the pressure boundary need to
be assessed is based on a small sampling of existing repairs and on an
assumption that those repairs are representative of all repairs. This
approach would not give any consideration to repairs that are internal
to the fuselage skin, or repairs to the wings or empennage. The FAA is
aware that a significant number of these types of repairs have been
installed on Model 737 airplanes, and that these repairs have not been
assessed, to the extent practicable, in accordance with the principles
of the current damage tolerance standards (14 CFR 25.571, Amdt. 25-45).
For those repairs that affect SSI's, the failure of which could be
catastrophic, reliance on an assumption that these repairs are free of
fatigue cracking is inappropriate.
Therefore, reliance on the proposed RAP is inconsistent with the
policy of AC No. 91-56, which does not draw a distinction between
original structure and modified or repaired structure in describing the
need for damage tolerance assessments of SSI's to ensure the structural
integrity of the airplane. As discussed in the NPRM, the FAA continues
to consider that appropriate damage tolerance based inspections are a
necessary means to ensure long-term structural integrity of all SSI's,
including those that have been modified or repaired. It should be noted
that this AD and the proposed RAP are complementary for the structure
associated with fuselage skins and pressure webs. Compliance with the
SSID may be facilitated by use of the repair assessment guidelines
developed in conjunction with the proposed RAP; and, assuming that the
FAA adopts the proposed RAP, compliance with this AD will facilitate
compliance with the requirements of the proposed RAP.
One commenter states that the existing Corrosion Prevention and
Control Program (CPCP), in concert with the proposed RAP, makes the
inspections specified in the proposed AD unnecessary and redundant. In
addition, this commenter states that the CPCP requires 100 percent
(visual) inspections of all SSI's, including repaired or modified
SSI's.
The FAA does not concur. The relationship of this AD to the
proposed RAP is discussed previously. The CPCP AD's require visual
inspections to detect corrosion of SSI's. In contrast, the SSIP AD's
require various inspection methods (e.g., visual, eddy current,
ultrasonic) to detect fatigue cracks in SSI's. Because the purposes of
the two programs are different, in many cases, the corrosion
inspections would not be adequate to detect fatigue cracking. In
conclusion, the FAA has determined that the Boeing SSIP is necessary to
maintain the airworthiness of the Boeing Model 737 fleet, and that it
is not redundant with the proposed RAP and CPCP.
Extend Compliance Time for Assessing Existing Repairs and Boeing
Modifications
Several commenters request that the FAA revise paragraph (d) of the
proposed AD to extend the compliance time of 18 months for existing
repairs and Boeing modifications. The commenters state that repairs and
Boeing modifications are likely to have fatigue characteristics that
are similar to the original structure and, therefore, are not of
immediate concern. These commenters also state that compliance within
18 months would cause an undue burden on operators because of the size
of the fleet, the number of repairs and modifications on each airplane
that would need to be identified and evaluated, the difficulty of
accessing the affected structure, and the total number of work hours
necessary to comply with the requirement. The commenters state that,
because the purpose of the inspections is to identify potential unsafe
conditions, rather than address known unsafe conditions, the level of
effort necessary to comply within 18 months is unjustified. One
commenter states that there is a shortage of sufficiently trained
personnel to develop necessary non-destructive test (NDT) procedures to
conduct the required inspections within the proposed compliance time.
Another commenter proposes that operators be able to address repairs
during the required SSID inspections.
The FAA concurs that an extension of the compliance time is
appropriate. The FAA agrees that Boeing repairs and modifications are
likely to have fatigue
[[Page 27467]]
characteristics that are similar to the original structure and,
therefore, are not of immediate concern. For other repairs, although
their fatigue characteristics may be different, the FAA recognizes that
the records and data necessary to identify and evaluate these repairs
may not be readily available.
Therefore, the FAA has revised the final rule to include a new
paragraph (e) to specifically address repairs and design changes other
than STC's. Operators are required to identify each repair or design
change to an SSI at the time of the first inspection of each SSI after
the effective date of the AD in accordance with Revision D of the SSID.
Within 12 months after such identification, operators are required to
assess the damage tolerance characteristics of each SSI created or
affected by each repair or design change to determine the effectiveness
of the applicable SSID inspection for each SSI and, if not effective,
revise the FAA-approved maintenance or inspection program to include an
inspection method and compliance times for each new or affected SSI.
This change will enable operators to identify these repairs and
modifications at the time of the required SSID inspection, so that no
additional inspections will be necessary. This change also will allow
for the timely development of NDT procedures. The requirement to revise
the maintenance or inspection program within 12 months after
identification of each repair or design change is consistent with both
the guidance of AC No. 25-1529-1, dated August 1, 1991, and the long-
standing practice under the McDonnell Douglas SSIP's.
Evaluation of Existing STC Design Changes
Several commenters state that paragraph (d) of the proposed AD
should retain the requirement to revise the maintenance or inspection
program to address STC design changes within 18 months after the
effective date of this AD. The commenters state that the durability of
individual airplanes is affected by STC design changes, which affect
existing SSI's and create new SSI's. Thus, the inspection times for
these SSI's might need to be revised to account for changes in
durability. The commenters also state that the STC documentation should
be readily available. This would permit a timely paperwork evaluation
of the effect on the Boeing SSIP without an extensive airplane
inspection. In contrast, another commenter requests an extension of the
18-month compliance time to 5 years for implementing program revisions
for addressing STC's. This commenter notes that STC holders are not
equipped to perform the assessments of affected SSI's.
The FAA concurs partially. Although most of these commenters
support the proposed requirements of paragraph (d) for STC design
changes, the FAA has revised paragraph (d) of the final rule to limit
its applicability to airplanes on which STC's have been incorporated,
and to provide an option that would extend the compliance time for
identifying and evaluating SSI's created or affected by STC's and
revising the maintenance or inspection programs to reflect those
evaluations. The FAA has recently reviewed several STC's regarding the
installation of cargo doors on 727 airplanes and determined that the
substantiating data for many of these STC's do not include internal
loads data. Without the internal loads data for the modified structure,
it would be difficult to perform an adequate damage tolerance
assessment.
In accordance with the guidance provided in AC No. 91-56, external
(flight, pressure, and ground) loads are necessary to complete a
structural damage tolerance assessment and must be obtained from the
type certificate (TC) holder or be developed by another source. Those
external loads must then be applied to the structure and resolved into
an internal distribution within the STC structural components (this
includes original structure that is not modified but could be affected
by the STC design change). All STC structural parts, whose failure
could reduce the structural integrity of the airplane, then must be
identified (as SSI's), and a damage tolerance assessment must be
performed. Subsequently, the inspection methods compliance times (i.e.,
thresholds and repetitive intervals) must be developed for these SSI's
and added to the operator's maintenance or inspection program.
Therefore, the FAA has determined that operators may need more time to
assess STC design changes on their airplanes.
To avail themselves of the option of extending the 18-month
compliance time, operators are required to accomplish the following
three actions:
1. Within 18 months after the effective date of this AD, submit a
plan to ensure that they are developing data, as described above, that
supports their revision to the FAA-approved maintenance or inspection
program (i.e., compliance times and inspection methods for new or
affected SSI's), and to demonstrate that they are able to complete the
required tasks within 48 months after the effective date of this AD.
2. Within 18 months after the effective date of this AD, and
thereafter at intervals not to exceed 18 months, accomplish a detailed
visual inspection of all structure identified in Revision D of the SSID
that has been modified in accordance with an STC (this repetitive
inspection will be terminated by accomplishment of the third action).
The detailed visual inspection and the repair of any crack shall be
accomplished in accordance with a method approved by the Manager of the
Seattle Aircraft Certification Office (ACO).
3. Within 48 months after the effective date of this AD, revise the
maintenance or inspection program to include an inspection method for
each new or affected SSI and to include the compliance times for
initial and repetitive accomplishment of these inspections.
The plan that an operator submits to the FAA for approval should
include a detailed description of the: (1) STC; (2) methodology for
identifying new or affected SSI's; (3) method for developing loads and
validating the analysis; (4) methodology for evaluating and analyzing
the damage tolerance characteristics of each new or affected SSI (see
discussion below); and (5) proposed inspection methods. The plan would
not need to include all of these elements if the operator can otherwise
demonstrate that its plan will result in implementation of an
acceptable program within 48 months after the effective date of this
AD. For this option, the final rule requires that the plan be submitted
to the Manager of the Seattle ACO within 18 months after the effective
date of the AD.
As indicated by the commenters, STC modifications may pose a
greater risk of fatigue cracking than standard repairs or Boeing
modifications. However, STC holders normally do not have access to
Boeing type certification data. Therefore, STC modified structure may
not have the same durability as the original structure or structure
that has been subject to standard repairs or Boeing modifications. In
order to ensure the structural integrity of STC modified structure
during the 48-month compliance time provided for the development of a
revision of the maintenance or inspection program to address STC's, the
FAA considers it necessary to require repetitive detailed visual
inspections of that structure.
These visual inspection methods are required to be approved by the
Manager of the Seattle ACO to ensure that adequate access is provided
and that the inspection area is adequately defined. In addition, the
repair of any crack must be
[[Page 27468]]
approved by the Manager of the Seattle ACO. This contrasts with the
repair provision of paragraph (f) of the final rule, which requires
that cracks be repaired in accordance with any FAA-approved method.
Seattle ACO approval for these repairs is necessary because, as
discussed previously, the durability of these STC's is unknown, and
findings of cracks may indicate the need for additional corrective
action. The FAA has revised paragraph (f) of the final rule to
reference the ACO approval as an exception to the general provisions
allowing repairs in accordance with an FAA-approved method. The FAA
selected an 18-month inspection interval to coincide with most
operators' normal maintenance schedules. It should be noted that these
visual inspections would not be required for operators who adopt a
damage tolerance based revision to the maintenance or inspection
program to address STC modifications within 18 months after the
effective date of this AD, as proposed in the NPRM.
One commenter also requests that the FAA develop guidelines to
assist operators in assessing STC's. The FAA does not consider that
there is a need for further guidance at this time. As discussed
previously, AC No. 91-56 provides extensive guidance on methods for
assessing the airplane structure using damage tolerance principles to
the extent practicable. This guidance is also applicable to STC's.
Revise Compliance Time to Assess Future Repairs and Modifications
Several commenters concur with the requirements of paragraph (f) of
the proposed AD.
Several other commenters request that paragraph (f) be revised to
extend the compliance time for assessment of repairs and modifications
installed after the effective date of this AD. Rather than completing a
damage tolerance assessment within 12 months after installation of the
repair or modification, as proposed in the NPRM, these commenters
suggest that operators should be required to complete an assessment
within 12 months after accomplishment of the next SSID inspection of
the SSI following such an installation.
The FAA does not concur. The FAA has determined that delaying the
assessment until after the next SSID inspection is not appropriate. At
the time of the installation, operators have all the data necessary to
define the repair or modification that would be used in an assessment.
Delaying the assessment until after the subsequent SSID inspection may
result in loss of these data. Requiring an assessment within 12 months
after installation of the repair or modification provides sufficient
time and ensures that the inspection program accurately reflects the
actual airplane structure. As stated previously, the requirement to
revise the maintenance or inspection program within 12 months after
installation is consistent with both the guidance of AC No. 25-1529 and
the long-standing practice under the McDonnell Douglas SSIP's.
Clarify What ``Affected'' Means
One commenter requests clarification of the meaning of the word
``affected'' in paragraphs (d) and (f) of the proposed AD. The
commenter states that the definition provided in the proposed AD is
vague. As an example, the commenter states that it was not clear
whether an operator needs to obtain a new inspection method and
threshold or interval for a corrosion blend-out repair that does not
include a doubler to reinforce the structure.
The FAA concurs that clarification is necessary. As defined in
paragraphs (d) and (f) of the proposed AD, the term ``affected'' means
that an SSI has been changed such that the original structure has been
physically modified or that the loads acting on the SSI have been
increased or redistributed.
For existing altered or repaired SSI's, the FAA has determined that
it is evident when an SSI is ``affected'' because of a physical change
to the structure. For existing changes where the loads acting on the
SSI have been increased or redistributed, the FAA has determined that
it may not be readily evident that an SSI is ``affected'' because there
has not been a physical change to the structure. Because of this, it
may not be possible for operators to identify all ``affected'' SSI's
without performing a damage tolerance assessment. For these reasons,
the FAA has changed paragraph (d) to require identification of
structure that has been ``physically altered,'' rather than
``affected,'' in accordance with an STC; and has added a new paragraph
(e) to require identification of other structure that has been
``physically altered or repaired.''
In the cited example of a corrosion blend-out to an SSI not
requiring reinforcement, the operator would be required to assess
whether the repair reduced the effectiveness of the original SSID
inspection method and repetitive interval. However, a blend-out would
not normally reduce the effectiveness of the original inspection
method, because the structure is essentially unchanged. The repetitive
interval would continue to be appropriate because the blend-out would
not appreciably affect the durability of the structure.
After the effective date of this AD, when SSI's are altered or
repaired or when the loads acting on an SSI are increased or
redistributed, it should be evident to the operator that SSI's are
``affected.'' The FAA has determined that, at the time of the
installation, operators should have all the data necessary to define
the repair or modification that would be used in an assessment. For
this reason, the FAA has determined that the word ``affected'' in
paragraph (g) [proposed paragraph (f)] is appropriate.
If an SSI is determined to be ``affected,'' an operator must
perform an assessment of the damage tolerance characteristics of the
SSI to determine the effectiveness of the applicable SSID inspection
for that SSI. It is only if that inspection is determined not to be
effective that the operator must revise the FAA-approved maintenance or
inspection program to include an inspection method and compliance times
for that SSI. Accordingly, the FAA has revised paragraph (d)(1) of the
final rule [which corresponds to paragraph (d) of the proposed AD as it
applied to STC modified structure] to require the operator to assess
the damage tolerance characteristics of each SSI created or affected by
each repair or design change to determine the effectiveness of the
applicable SSID inspection for each SSI. If it is not effective, the
operator is required to revise the FAA-approved maintenance or
inspection program to include an inspection method and compliance times
for each new or affected SSI. The FAA will monitor operators'
compliance with these provisions to determine whether future revisions
to this AD are necessary to fulfill the intent of AC No. 91-56.
Threshold for STC Modified Airplanes
One commenter questions whether airplanes that have been converted
from a passenger configuration to an all-cargo configuration by the STC
process are subject to the requirements of paragraph (c)(1) or (c)(2)
of the proposed AD. (This comment specifically addresses Model 727
airplanes identified in NPRM Docket No. 96-NM-263-AD; however, the
comment also applies to this AD.) The commenter's concern appears to
result from the fact that, when some passenger airplanes were converted
to cargo airplanes, the modifier revised the airplane records to
reflect a different model number (e.g., a -200 may be reidentified as -
200C). The FAA's intent is that the references to model numbers in the
AD correspond to the model numbers specified on the type
[[Page 27469]]
certificate data sheet (TCDS). Because these converted airplanes are
not identified as Model 737-200C series airplanes on the TCDS,
paragraph (c)(1) does not apply, and paragraph (c)(2) does. As
discussed previously, for SSI's altered by the conversion, operators
also must consider the provisions of paragraph (d) of this AD, which
require a damage tolerance evaluation to determine what structure needs
to be inspected, what inspection methods are needed, and when the
inspections are to occur. The FAA has revised the final rule to include
a new NOTE following paragraph (c)(1) that clarifies this point.
Candidate Fleet Approach
One commenter suggests that the FAA delete the threshold approach
defined in paragraph (c) of the proposed AD and retain the candidate
fleet approach defined in AD 84-24-05 and the SSID. The commenter
proposes that the candidate fleet be updated annually to reflect
changes in the fleet (e.g., when an airplane is modified from a
passenger configuration to a cargo configuration). (This comment
specifically addresses Model 727 airplanes identified in NPRM Docket
No. 96-NM-263-AD; however, the comment also applies to this AD.)
The FAA does not concur. As stated in the NPRM, the policy
established in AC No. 91-56 anticipated that all SSIP's would establish
thresholds. The candidate fleet approach was originally based on an
understanding 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. This would be achieved by periodic updates
to the candidate fleet. In practice, this approach has not fulfilled
the intent of AC No. 91-56. Because of the extensive modifications and
repairs of both candidate fleet airplanes and non-candidate fleet
airplanes, the candidate fleet is no longer representative.
In addition, the FAA finds that the candidate fleet no longer
includes all of the highest time airplanes in the fleet. Even if the
SSID were updated annually to reflect changes to the fleet, this
approach would be impractical for both operators and the FAA. Because
of the frequency of modifications and changes in utilization of the
affected airplanes, even annual updates would quickly be rendered
obsolete. Annual changes in the composition of the candidate fleet
would deprive operators of the predictability needed for long-term
maintenance planning provided by the approach of defining the
thresholds as adopted in this AD. For these reasons, the FAA has
determined that the 737 SSIP must contain inspection thresholds for all
Model 737 series airplanes to ensure the timely detection of fatigue
cracks in the SSI's.
Extend Compliance Time for Revising the Maintenance or Inspection
Program
Several commenters request that the compliance time of 12 months in
paragraph (b) of the proposed AD be extended to provide operators more
time to incorporate Revision D of the SSID into their FAA-approved
maintenance or inspection program. These commenters state that an
operator should not be required to revise its FAA-approved maintenance
or inspection program to incorporate Revision D of the SSID until its
airplanes are at or near the threshold specified in paragraph (c) of
the proposed AD. The commenters state that, as paragraph (b) of the
proposed AD is currently worded, all operators are required to
incorporate the change regardless of the cycle age of an airplane. This
requirement poses an undue burden (cost and time) to those operators
that are not required to inspect until much later. Several other
commenters also state that the safety of the fleet is not increased by
requiring incorporation of Revision D of the SSID into an inspection
program on low-cycle airplanes.
The FAA concurs with the commenters' requests to extend the
compliance time of paragraph (b) to prior to reaching the threshold
specified in paragraph (c) of the AD, or within 12 months after the
effective date of this AD, whichever occurs later. The FAA has revised
the final rule accordingly. However, as discussed previously in this
AD, operators are required to comply with the requirements of
paragraphs (d) and (g) of this AD, which may necessitate action before
reaching the threshold.
Extend Grace Period for Initial Inspections
Several commenters request that the 18-month grace period specified
in paragraph (c) of the proposed AD be extended to provide operators
that are near or over the threshold more time to accomplish the initial
inspection. Many inspections included in the SSID require several work
hours to accomplish. These commenters point out that the proposed AD
allows 12 months to implement Revision D of the SSID, but allows only 6
months thereafter to accomplish inspections (18 months total from the
effective date). The commenters contend that accomplishment of all the
inspections within the 18-month grace period will significantly affect
an operator's planned maintenance schedule and program, especially
operators of large fleets.
Several of these same commenters state that the original SSID AD
91-14-20 permitted the initial compliance time to be the repeat
interval (after incorporation of the revision into a maintenance or
inspection program). Several commenters also state that other AD's that
mandate maintenance type programs, such as the CPCP for aging
airplanes, give operators one repeat interval to come into compliance;
therefore, the initial inspection should be similar in concept to such
maintenance type programs (i.e., the grace period should be 18, 36, 48,
60, and 72-month intervals depending on the inspection).
One commenter states that no service, test, or engineering analysis
could justify the inspection of new SSI's within 18 months. Another
commenter states that the approach used in the proposed AD appeared to
be the same as for a service bulletin with a known fatigue problem.
This commenter also states that this approach was not appropriate for
damage tolerance based inspections contained in the Boeing SSID, which
are exploratory inspections and are not intended to address identified
problems. Another commenter states that the SSID threshold is somewhat
arbitrary, because it is based on a reliability analysis rather than a
true fatigue analysis. The threshold is derived from calculations that
ensure that a statistically accurate representation of the fleet is
being inspected, rather than a true crack growth analysis. One of these
commenters suggests that the grace period be based on flight cycles
instead of calendar time because the SSID addresses structural fatigue.
Several commenters state that a major maintenance check would be a more
appropriate grace period for accomplishing the inspections specified in
the SSID.
The FAA concurs that more time should be provided to accomplish the
initial inspections specified in paragraph (c) of the proposed AD.
However, the FAA does not concur that the grace period should be tied
to the repeat interval established in the Boeing SSID because some of
the repeat inspections have extremely long compliance times. The
existing Boeing SSID is not like the CPCP document which establishes an
initial compliance time (threshold) within the document. As discussed
in Item 3. of the ``Action Since Issuance of Previous AD'' Section of
the NPRM, the FAA has determined that a grace period based on a repeat
[[Page 27470]]
interval does not ensure that the SSI inspections are accomplished, and
that fatigue cracks in SSI's are detected, in a timely manner.
The FAA finds that it would be appropriate to base the grace period
on the number of accumulated flight cycles rather than calendar time,
because the Boeing SSIP is based on fatigue and crack-growth analyses.
In addition, the FAA concurs that the grace period should begin at the
time when operators are required to have revised their maintenance or
inspection programs to incorporate Revision D of the SSID. The FAA has
determined that such a grace period would provide operators with more
time to accomplish the inspection; yet it also would ensure that the
SSI inspections are accomplished, and that fatigue cracks in SSI's are
detected, in a timely manner. As a result, the FAA has revised the
final rule to specify a grace period of 4,000 flight cycles measured
from the date 12 months after the effective date of the AD. The 4,000-
flight cycle grace period corresponds to a typical maintenance interval
for most operators and, therefore, minimizes the need for special
maintenance scheduling.
Modify Criteria for Adjusting the Threshold
Several commenters request that the criteria for adjusting the
thresholds specified in paragraph (c) of the proposed AD (discussed in
Item 3 of the ``Actions Since Issuance of Previous AD'' Section of the
NPRM) should allow for the threshold to be reasonably adjusted. These
commenters suggest that the FAA allow operators to use the rate of risk
methodology to extend the threshold in the future.
The FAA concurs. The rate of risk methodology is a means of
determining the probability that cracks will be detected in the
inspected fleet before they initiate on other airplanes that have not
been inspected. As discussed in the NPRM, in accordance with paragraph
(i)(1) of the final rule, the FAA would approve threshold increases if
it can be shown by sufficient data that the increase in the threshold
does not result in an increased risk that damage will occur in the
uninspected fleet before it is detected in the inspected fleet.
Some of these commenters state that the following statement in the
NPRM is unreasonable: ``* * * the FAA may approve requests for
adjustments to the compliance time * * * provided that no cracking is
detected in the airplane structure.'' Confirmed fatigue cracks should
not restrict the ability to adjust the SSIP threshold. The commenters
state that the present philosophy for addressing an SSI with a
confirmed fatigue crack is to remove that SSI from the SSID and to
issue a service bulletin to correct the problem. The FAA then issues an
AD to mandate the action, if the FAA deems it necessary. Once this SSI
has been removed from the SSID, it should not affect the ability to
adjust the SSIP threshold. The FAA concurs. In evaluating requests for
extension of thresholds, the FAA would consider whether identified
cracking has been addressed in accordance with the philosophy described
by the commenters.
One commenter expresses concern that eventually all Model 737
airplanes would be subject to the Boeing SSIP. This commenter suggests
that the threshold be defined in the SSID and managed by the STG. The
FAA does not concur. As discussed previously, if data are submitted
substantiating extension of the threshold, the FAA will approve such
extensions, which may have the effect of excepting relatively low-time
airplanes. The FAA would be receptive to proposals of threshold
extensions from any source that submits sufficient data, including the
STG. Because the thresholds are specified in the AD itself, there is no
need for the SSID to be revised to incorporate the threshold.
Compliance Time for Initial Inspection
One commenter requests that the compliance time for the initial
inspection requirements of paragraphs (c), (d), and (f) of the proposed
AD be clarified. The commenter asks if there is anything in the
proposed AD that would establish a threshold for inspections other than
the 46,000-flight cycle compliance time specified in paragraph (c)(1)
of the proposed AD. The commenter states that it has Model 727-100C
series airplanes that have accumulated less than 27,000 total flight
cycles, but are more than 30 years old. (This comment specifically
addresses Model 727 airplanes identified in NPRM Docket No. 96-NM-263-
AD; however, the comment also applies to this AD.)
The FAA finds that no change to the final rule is necessary. The
age of an airplane is irrelevant to the inspection threshold. Because
the inspections are related to fatigue, only the number of flight
cycles that have accumulated on an airplane are relevant to the
inspection threshold. If an airplane has been modified, altered, or
repaired, such as an STC cargo conversion, the results of an assessment
in accordance with either paragraph (d) or (g) of the AD could indicate
that the initial inspections are required prior to the thresholds
specified in paragraph (c) of the AD.
Limit Applicability of the Transferability Requirement
One commenter concurs with paragraph (g) of the proposed AD, which
addresses the inspection schedule for transferred airplanes, provided
that it is limited to airplanes that have exceeded the threshold
established by paragraph (c)(1) or (c)(2). Paragraph (h) of the final
rule [proposed paragraph (g)] is limited as stated by the commenter,
and paragraph (h) is adopted as proposed.
Clarification of FAA-Approved Method
One commenter requests that paragraph (e) of the proposed AD be
clarified so that there is no confusion regarding the level of FAA
approval required for repairs to SSI's. The commenter states that it
interprets paragraph (e) to mean that any Designated Engineering
Representative (DER) with delegated authority would still have the
authority to approve repairs to SSI's based on a static strength
analysis. The commenter also interprets that an operator would have 12
months after the repair to develop an alternative inspection plan, or
to demonstrate that the existing inspection program provides an
acceptable level of safety.
The commenter is correct that DER's still have the authority to
approve repairs to SSI's based on a static strength analysis. Except as
discussed under the heading ``Evaluation of Existing STC Design
Changes,'' paragraph (f) of the final rule [proposed paragraph (e)] is
unchanged from the corresponding paragraph of AD 91-14-20. The
commenter also is correct that operators are allowed 12 months after
installation of the repair to revise their FAA-approved maintenance or
inspection program to include new inspections for the affected SSI's.
The new inspection method and compliance times must be approved by the
Manager of the Seattle ACO.
Delegate Approval Authority to DER's
Several commenters request that the FAA delegate approval authority
to the DER's to approve new inspections and compliance times specified
in paragraphs (d) and (f) of the proposed AD. These commenters state
that this delegation would decrease the time required to obtain such
approvals. These commenters question whether the FAA will be able to
process a substantial number of requests that will be generated because
of the proposed AD. This question arises from one commenter's past
experience with the
[[Page 27471]]
CPCP in which the approval process took a long period of time.
In the broader context of delegation of AD required approvals, the
FAA has recently issued guidance on this subject and will be
implementing this guidance in the near future. Because this request may
be accommodated through FAA management of designees, no revision to the
final rule is needed.
Credit for Previous Inspections
Several commenters request that paragraph (c) of the proposed AD
positively reflect that an operator is in compliance if inspections
have been accomplished in accordance with Revision D of Boeing Document
No. D6-37089 prior to the effective date of the AD. These commenters
state that paragraph (c) of the proposed AD is not clear with regard to
whether or not credit is to be given and when the next inspection would
be required. These commenters point out that the phrase ``Compliance:
Required as indicated, unless accomplished previously,'' as stated in
the proposed AD, allows the necessary credit for previously
accomplished inspections.
The FAA does not consider that a change to the final rule is
necessary. Operators are given credit for work previously performed by
means of the phrase in the AD that was referenced by the commenters. In
the case of this AD, if the initial inspection has been accomplished
prior to the effective date of this AD, this AD does not require that
it be repeated. However, the AD does require that repetitive
inspections be conducted thereafter at the intervals specified in the
Boeing SSID, and that other follow-on actions be accomplished when
indicated.
Further FAA/Industry Discussions
Several commenters request that the FAA have further discussions
with Boeing, operators, and other regulatory agencies prior to issuing
the final rule because the proposed AD reflects a major change in FAA
policy and extends well beyond the original concept of the Boeing SSIP.
The FAA does not concur. As discussed in the NPRM and the preceding
discussion of comments, this AD is consistent with the FAA's long-
standing policy, as expressed in AC No. 91-56. As demonstrated by the
breadth and depth of comments received, the public has had an ample
opportunity to comment on the merits of the proposal.
Cost Estimate
Several commenters request that the FAA revise the Cost Impact
information of NPRM Docket No. 96-NM-263-AD (for Model 727 airplanes)
and NPRM Docket No. 96-NM-264-AD (for Model 737 airplanes) to
accurately reflect the costs associated with accomplishing the
requirements of both proposed AD's.
One commenter states that all affected 737 airplanes worldwide
should be included in the cost estimate in NPRM Docket No. 96-NM-264-
AD. The FAA does not concur. Airworthiness directives that are issued
by the FAA directly affect only U.S.-registered airplanes; therefore,
the cost estimate in an AD is limited only to U.S.-registered
airplanes.
Several commenters to NPRM Docket No. 96-NM-263-AD (applicable to
Model 727 series airplanes) state that 1,030 Model 727 airplanes (U.S.-
registered) are affected by the proposed AD, not just 74 airplanes, as
specified in NPRM. One of these commenters states that the cost
estimate in the NPRM does not reflect the cost for all 727 operators to
incorporate Revision H of the SSID into an FAA-approved maintenance or
inspection program. Similarly, several commenters also state that the
cost estimate in NPRM Docket No. 96-NM-264-AD does not reflect
comparable costs for all 737-100 and -200 airplanes.
The FAA concurs with the commenters in that the NPRM proposed that
every affected U.S. operator must revise their maintenance or
inspection programs to incorporate Revision H (for Model 727 airplanes)
or Revision D (for Model 737 airplanes) of the SSID within 12 months
after the effective date of the applicable AD. As discussed previously
under the heading ``Extend Compliance Time for Revising the Maintenance
or Inspection Program,'' the FAA has revised both final rules so that
the maintenance or inspection program revision is only required for any
airplane prior to its reaching the applicable threshold.
In addition, the FAA has revised the Cost Impact information of
Final Rule Docket No. 96-NM-263-AD to address a total of 1,001
airplanes, which includes 223 airplanes (35 operators) that are
estimated to exceed the thresholds specified in the AD within the next
10 years. For this final rule, the FAA also has revised the Cost Impact
information to address a total of 404 airplanes, which includes 158
airplanes (39 operators) that are estimated to exceed the thresholds
specified in the AD within the next 10 years. As discussed previously
under the heading ``Modify Criteria for Adjusting the Threshold,'' if
sufficient substantiating data are submitted to justify extending the
threshold, the FAA will grant such extensions so that the operators of
some relatively low utilization airplanes may never be required to
revise their maintenance or inspection program to incorporate the SSIP.
One commenter estimates that it will take 1,700 work hours per
airplane (for Model 727 airplanes) to identify previously installed
repairs, which will require at least 10 days of downtime to survey each
airplane at a total cost to the commenter of $8.9 million. This
commenter also estimates that its cost due to lost revenue would be
$10.2 million, for a total cost of $19.1 million over 6 months
(identification and lost revenue). This commenter further estimates
that it will cost $110.5 million to survey existing repairs on all 727
airplanes.
Another commenter estimates that it will cost $240 million to
accomplish the initial inspection to determine if there are existing
repairs on the 727 airplanes. This task will take over 4,000 work hours
per airplane to accomplish (2,000 work hours to open and close; 500
work hours to inspect, map, assess, etc.; and 1,500 work hours to
complete non-routines generated by this special inspection).
Similar comments were submitted to NPRM Docket No. 96-NM-264-AD;
however, the commenters did not provide specific cost figures for
performing assessments on existing repairs.
As discussed under the heading ``Extend Compliance Time for
Assessing Existing Repairs and Boeing Modifications,'' the FAA has
revised both final rules to postpone the requirement to assess existing
repairs of SSI's until after the applicable SSID inspection. This
revision eliminates the need for any special inspection in order to
comply with the requirement to assess repairs.
Several commenters also state that the cost estimate in the NPRM's
did not reflect the costs of developing inspection programs for repairs
and Boeing modifications that are installed prior to the effective date
of the AD. The FAA concurs and has revised the Cost Impact information
of both final rules to include (within the total costs) $258,000 per
airplane over the next 10 years to account for these costs.
Several commenters assert that the cost of the proposed AD is over
$100 million, which is more than 20 times the FAA's estimate in the
NPRM Docket No. 96-NM-263-AD (for Model 727 airplanes). As discussed
below in the Cost Impact information, the FAA estimates that the total
cost over the next 10 years associated with this final rule is
$98,044,800, or an average of $9,804,480 per year. The FAA also
[[Page 27472]]
estimates that the highest total cost during any one of the next 10
years associated with this final rule is $23,916,000. The difference
between these estimates is at least in part attributable to the changes
in the final rule discussed previously, which provide significant
relief to operators. (Similar comments were submitted to NPRM Docket
No. 96-NM-264-AD; however, the commenters did not provide a total cost
estimate for these actions.)
Additional Clarifications
In reviewing the comments submitted to the NPRM, questions arose
regarding the relationship of the inspection threshold requirements of
paragraph (c) of the proposed AD and the provisions of the Boeing SSID
that allow for sampling of specified percentages of the affected fleet.
As explained in the NPRM, the FAA's intent in paragraph (c) is to
require that all airplanes that exceed the threshold be inspected in
accordance with the Boeing SSID. To the extent that there is any
potential for conflict between paragraph (c) and the Boeing SSID, the
provisions specified in this AD would prevail. Therefore, even if
Revision D would permit operators to omit inspections of SSI's based on
a sampling approach, this AD requires that those inspections be
performed on all airplanes exceeding the specified thresholds. The FAA
has revised the final rule to include a new NOTE following paragraph
(c) to clarify this point.
Similarly, the FAA notes that paragraph (b) of the proposed AD
would have required that the revision to the maintenance or inspection
program include certain SSID provisions that were proposed to be
overridden by other paragraphs of the proposed AD. The FAA has revised
the requirements of paragraph (b) to clarify that the AD overrides
these SSID provisions.
The FAA has added a parenthetical clarification in the
applicability of the final rule to point out that Model 737-200C series
airplanes also are subject to the requirements of the AD.
Conclusion
After careful review of the available data, including the comments
noted above, the FAA has determined that air safety and the public
interest require the adoption of the rule with the changes previously
described. The FAA has determined that these changes will neither
increase the economic burden on any operator nor increase the scope of
the AD.
Cost Impact
There are approximately 1,007 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 and 43 U.S. operators
(over 10 years) will be affected by this AD.
Incorporation of the SSID program into an operator's maintenance or
inspection program, as required by AD 91-14-20, takes approximately
1,000 work hours per airplane (6 affected airplanes) to accomplish, at
an average labor rate of $60 per work hour. Based on these figures, the
cost of incorporating the revised procedures (specified in Revisions B
and C of the SSID) into the maintenance or inspection program is
estimated to be $360,000, or $60,000 per airplane.
The recurring inspections, as required by AD 91-14-20, take
approximately 500 work hours per airplane to accomplish, at an average
labor rate of $60 per work hour. Based on these figures, the recurring
inspection cost to the 6 U.S.-registered candidate fleet airplanes is
estimated to be $180,000, or $30,000 per airplane, per inspection
cycle.
The incorporation of Revision D of the SSID into an operator's
maintenance or inspection program, as required by this new AD, takes
approximately 1,200 work hours (per operator) to accomplish, at an
average labor rate of $60 per work hour. The FAA estimates that within
10 years, 39 operators will be required to incorporate Revision D of
the SSID. Based on these figures, the cost of incorporating the revised
procedures (specified in Revision D of the SSID) into the maintenance
or inspection program is estimated to be $2,808,000, or $72,000 per
operator.
The recurring inspections, as required by this new AD, take
approximately 600 work hours per airplane to accomplish, at an average
labor rate of $60 per work hour. The FAA estimates that after 10 years,
39 operators will be required to inspect 146 airplanes and assess the
damage tolerance characteristics of each repaired SSI or each SSI that
is physically altered by an existing design change other than an STC.
The cost impact of this inspection and assessment required by this AD
on U.S. operators is estimated to be $71,328,000 over 10 years, or an
average of $48,855 per airplane, per year. During the 10 years, the FAA
also conservatively estimates that 43 operators of 404 airplanes will
be required to assess the damage tolerance characteristics of each SSI
on which the structure identified in Revision D of the SSID has been
physically altered in accordance with an STC prior to the effective
date of this AD. The cost impact of this assessment required by this AD
on U.S. operators is estimated to be $21,000,000 over 10 years, or an
average of $5,198 per airplane, per year.
In summary, the FAA estimates that the actions, as required by this
new AD, will cost $98,044,800 over 10 years, or an average of
$9,804,480 per year. The FAA also estimates that the average cost per
airplane over 10 years is $242,685, or an average of $24,269 per year.
The highest total cost during any one of the 10 years is $23,916,000.
(The FAA has included in the Rules Docket a detailed description of
cost estimates related to the actions required by this AD.)
The cost impact figures discussed above are based on assumptions
that no operator has yet accomplished any of the requirements of this
AD action, and that no operator would accomplish those actions in the
future if this AD were not adopted. However, it can reasonably be
assumed that the majority of the affected operators have already
initiated the original SSID program (as required by AD 91-14-20), and
many may have already initiated the additional inspections required by
this new AD action.
Regulatory Impact
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 final
rule does not have sufficient federalism implications to warrant the
preparation of a Federalism Assessment.
For the reasons discussed above, I certify that this action (1) is
not a ``significant regulatory action'' under Executive Order 12866;
(2) is not a ``significant rule'' under DOT Regulatory Policies and
Procedures (44 FR 11034, February 26, 1979); and (3) 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 final evaluation has been prepared for this action
and it is contained in the Rules Docket. A copy of it may be obtained
from the Rules Docket at the location provided under the caption
ADDRESSES.
List of Subjects in 14 CFR Part 39
Air transportation, Aircraft, Aviation safety, Incorporation by
reference, Safety.
[[Page 27473]]
Adoption of the Amendment
Accordingly, pursuant to the authority delegated to me by the
Administrator, the Federal Aviation Administration amends 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),
amendment 39-10531, to read as follows:
98-11-04 Boeing: Amendment 39-10531. Docket 96-NM-264-AD.
Supersedes AD 91-14-20, Amendment 39-7061.
Applicability: All Model 737-100 and -200 series airplanes
(including Model 737-200C series airplanes), certificated in any
category.
Compliance: Required as indicated, unless accomplished
previously.
To ensure the continued structural integrity of the total Boeing
Model 727 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) Prior to reaching the threshold specified in paragraph (c)
of this AD, or within 12 months after the effective date of this AD,
whichever occurs later, incorporate a revision into the FAA-approved
maintenance or 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.) Except
as provided to the contrary in paragraphs (c), (d), and (g) of this
AD, the revision to the maintenance or inspection 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 D. 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), (e), or (g) of this AD,
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 4,000 flight
cycles measured from the date 12 months after the effective date of
this AD, whichever occurs later.
Note 3: The requirements specified in paragraph (c)(1) of this
AD only apply to airplanes listed as 737-200C on the type
certificate data sheet. Paragraph (c)(1) does not apply to airplanes
that have been modified from a passenger configuration to an all-
cargo configuration by supplemental type certificate (STC).
Paragraphs (c)(2) and (d) apply to those airplanes.
(2) For all 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 4,000 flight cycles measured
from the date 12 months after the effective date of this AD,
whichever occurs later.
Note 4: Notwithstanding the provisions of paragraphs 5.1.1,
5.1.2, 5.1.6(e), .1.11, 5.1.12, 5.1.13, 5.2, 5.2.1, 5.2.2, 5.2.3,
and 5.2.4 of the General Instructions of Revision D, which would
permit operators to perform fleet and rotational sampling
inspections, to perform inspections on less than whole airplane
fleet sizes and to perform inspections on substitute airplanes, this
AD requires that all airplanes that exceed the threshold be
inspected in accordance with Revision D.
Note 5: 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 or inspection programs, as revised in
accordance with paragraph (b) of this AD.
(d) For airplanes on which the structure identified in Revision
D has been physically altered in accordance with an STC prior to the
effective date of this AD: Accomplish the requirements specified in
paragraph (d)(1) or (d)(2) of this AD.
(1) Within 18 months after the effective date of this AD, assess
the damage tolerance characteristics of each SSI created or affected
by each STC to determine the effectiveness of the applicable
Revision D inspection for each SSI and, if not effective, revise the
FAA-approved maintenance or inspection program to include an
inspection method for each new or affected SSI, and to include the
compliance times for initial and repetitive accomplishment of each
inspection. 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.
Note 6: For purposes of this AD, an SSI is ``affected'' if it
has been physically altered or repaired, or if the loads acting on
the SSI have been increased or redistributed. The effectiveness of
the applicable inspection method and compliance time should be
determined based on a damage tolerance assessment methodology, such
as that described in FAA Advisory Circular AC No. 91-56, Change 2,
dated April 15, 1983.
(2) Accomplish paragraphs (d)(2)(i), (d)(2)(ii), and (d)(2)(iii)
of this AD.
(i) Within 18 months after the effective date of this AD, submit
a plan that describes a methodology for accomplishing the
requirements of paragraph (d)(1) of this AD to the Manager, Seattle
ACO, 1601 Lind Avenue, SW., Renton, Washington 98055-4056; fax (425)
227-1181.
Note 7: The plan should include a detailed description of the:
STC; methodology for identifying new or affected SSI's; method for
developing loads and validating the analysis; methodology for
evaluating and analyzing the damage tolerance characteristics of
each new or affected SSI; and proposed inspection method. The plan
would not need to include all of these elements if the operator can
otherwise demonstrate that its plan will enable the operator to
comply with paragraph (d)(2)(iii) of this AD.
(ii) Within 18 months after the effective date of this AD,
perform a detailed visual inspection in accordance with a method
approved by the Manager, Seattle ACO to detect cracks in all
structure identified in Revision D that has been altered by an STC.
(A) If no crack is detected, repeat the detailed visual
inspection thereafter at intervals not to exceed 18 months.
(B) If any crack is detected, prior to further flight, repair it
in accordance with a method approved by the Manager, Seattle ACO.
(iii) Within 48 months after the effective date of this AD,
revise the FAA-approved maintenance or inspection program to include
an inspection method for each new or affected SSI, and to include
the compliance times for initial and repetitive accomplishment of
each inspection. The inspection methods and the compliance times
shall be approved by the Manager, Seattle ACO. Accomplishment of the
actions specified in this paragraph constitutes terminating action
for the repetitive
[[Page 27474]]
inspection requirements of paragraph (d)(2)(ii)(A) of this AD.
Note 8: 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 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.
(e) For airplanes on which the structure identified in Revision
D has been repaired or physically altered by any design change other
than an STC identified in paragraph (d), prior to the effective date
of this AD: At the time of the first inspection of each SSI after
the effective date of this AD in accordance with Revision D,
identify each repair or design change to that SSI. Within 12 months
after such identification, assess the damage tolerance
characteristics of each SSI created or affected by each repair or
design change to determine the effectiveness of the applicable SSID
inspection for each SSI and, if not effective, revise the FAA-
approved maintenance or inspection program to include an inspection
method and compliance times for each new or affected SSI. The new
inspection method and the compliance times shall be approved by the
Manager, Seattle ACO.
Note 9: For the purposes of this AD, a design change is defined
as any modification, alteration, or change to operating limitations.
(f) Except as provided in paragraph (d)(2)(ii)(B) of this AD,
cracked structure found during any inspection required by this AD
shall be repaired, prior to further flight, in accordance with an
FAA-approved method.
(g) For airplanes on which the structure identified in Revision
D is affected by any design change (including STC's) or repair that
is accomplished after the effective date of this AD: Within 12
months after that modification, alteration, or repair, revise the
FAA-approved maintenance or inspection program to include an
inspection method and compliance times for each new or affected SSI,
and to include the compliance times for initial and repetitive
accomplishment of each inspection. The new inspection method and the
compliance times shall be approved by the Manager, Seattle ACO.
Note 10: 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.
(h) 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
(h)(1) or (h)(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.
(i)(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 11: Information concerning the existence of approved
alternative methods of compliance with this AD, if any, may be
obtained from the Seattle ACO.
(i)(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.
(j) 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.
(k) The actions specified in paragraphs (b) and (c) shall be
done in accordance with Boeing Document No. D6-37089, ``Supplemental
Structural Inspection Document'' (SSID), Revision D, dated June
1995, which contains the following list of effective pages:
------------------------------------------------------------------------
Revision level shown on
Page number shown on page page
------------------------------------------------------------------------
List of Effective Pages Pages 1 thru 10.... D.
------------------------------------------------------------------------
Note: The issue date of Revision D is indicated only on the
title page; no other page of the document is dated.) This
incorporation by reference was approved by the Director of the
Federal Register in accordance with 5 U.S.C. 552(a) and 1 CFR part
51. Copies may be obtained from Boeing Commercial Airplane Group,
P.O. Box 3707, Seattle, Washington 98124-2207. Copies may be
inspected at the FAA, Transport Airplane Directorate, 1601 Lind
Avenue, SW., Renton, Washington; or at the Office of the Federal
Register, 800 North Capitol Street, NW., suite 700, Washington, DC.
(l) This amendment becomes effective on June 23, 1998.
Issued in Renton, Washington, on May 12, 1998.
D.L. Riggin,
Acting Manager, Transport Airplane Directorate, Aircraft Certification
Service.
[FR Doc. 98-13078 Filed 5-18-98; 8:45 am]
BILLING CODE 4910-13-U
