[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:
    <bullet> For McDonnell Douglas Model DC-8 series airplanes: AD 93-
01-15, amendment 39-8464 (58 FR 5576, January 22, 1993);
    <bullet> For McDonnell Douglas Model DC-9 series airplanes: AD 96-
13-03, amendment 39-9671 (61 FR 31009, June 19, 1996); and
    <bullet> 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