[Federal Register Volume 66, Number 134 (Thursday, July 12, 2001)]
[Proposed Rules]
[Pages 36509-36513]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 01-17431]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. 98-NM-196-AD]
RIN 2120-AA64


Airworthiness Directives; Boeing Model 737-200, -200C, -300, -
400, and -500 Series Airplanes

AGENCY: Federal Aviation Administration, DOT.

ACTION: Notice of proposed rulemaking (NPRM).

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SUMMARY: This document proposes the supersedure of an existing 
airworthiness directive (AD), applicable to certain Boeing Model 737 
series airplanes, that currently requires repetitive inspections to 
find cracking of the lower skin at the lower row of fasteners in the 
lap joints of the fuselage, and repair of any cracking found. That 
amendment also requires modification of the fuselage lap joints at 
certain locations, which constitutes terminating action for repetitive 
inspections of the modified areas. This proposed action would add 
repetitive inspections and would require replacement of the current 
preventive modification with an improved modification. This proposal is 
prompted by the FAA's determination that, in light of additional crack 
findings, certain modifications of the fuselage lap joints do not 
provide an adequate level of safety. The actions specified by the 
proposed AD are intended to find and fix cracking of the fuselage lap 
joints, which could result in sudden decompression of the airplane.

DATES: Comments must be received by August 27, 2001.

ADDRESSES: Submit comments in triplicate to the Federal Aviation 
Administration (FAA), Transport Airplane Directorate, ANM-114, 
Attention: Rules Docket No. 98-NM-196-AD, 1601 Lind Avenue, SW., 
Renton, Washington 98055-4056. Comments may be inspected at this 
location between 9:00 a.m. and 3:00 p.m., Monday through Friday, except 
Federal holidays. Comments may be submitted via fax to (425) 227-1232. 
Comments may also be sent via the Internet using the following address: 
[email protected]. Comments sent via fax or the Internet must 
contain ``Docket No. 98-NM-196-AD'' in the subject line and need not be 
submitted in triplicate. Comments sent via the Internet as attached 
electronic files must be formatted in Microsoft Word 97 for Windows or 
ASCII text.
    The service information referenced in the proposed rule may be 
obtained from Boeing Commercial Airplane Group, P.O. Box 3707, Seattle, 
Washington 98124-2207. This information may be examined at the FAA, 
Transport Airplane Directorate, 1601 Lind Avenue, SW., Renton, 
Washington.

FOR FURTHER INFORMATION CONTACT: Scott Fung, Aerospace Engineer, 
Airframe Branch, ANM-120S, FAA, Transport Airplane Directorate, Seattle 
Aircraft Certification Office (ACO), 1601 Lind Avenue, SW., Renton, 
Washington; telephone (425) 227-1221; fax (425) 227-1181.

SUPPLEMENTARY INFORMATION:

Comments Invited

    Interested persons are invited to participate in the making of the 
proposed rule by submitting such written data, views, or arguments as 
they may desire. Communications shall identify the Rules Docket number 
and be submitted in triplicate to the address specified above. All 
communications received on or before the closing date for comments, 
specified above, will be considered before taking action on the 
proposed rule. The proposals contained in this action may be changed in 
light of the comments received.
    Submit comments using the following format:
     Organize comments issue-by-issue. For example, discuss a 
request to change the compliance time and a request to change the 
service bulletin reference as two separate issues.
     For each issue, state what specific change to the proposed 
AD is being requested.
     Include justification (e.g., reasons or data) for each 
request.
    Comments are specifically invited on the overall regulatory, 
economic, environmental, and energy aspects of the proposed rule. All 
comments submitted will be available, both before and after the closing 
date for comments, in the Rules Docket for examination by interested 
persons. A report summarizing each FAA-public contact concerned with 
the substance of this proposal will be filed in the Rules Docket.
    Commenters wishing the FAA to acknowledge receipt of their comments 
submitted in response to this action must submit a self-addressed, 
stamped postcard on which the following statement is made: ``Comments 
to Docket Number 98-NM-196-AD.'' The postcard will be date stamped and 
returned to the commenter.

Availability of NPRMs

    Any person may obtain a copy of this NPRM by submitting a request 
to the FAA, Transport Airplane Directorate, ANM-114, Attention: Rules 
Docket No. 98-NM-196-AD, 1601 Lind Avenue, SW., Renton, Washington 
98055-4056.

Discussion

    On October 21, 1997, the FAA issued AD 97-22-07, amendment 39-10179 
(62 FR 55732, October 28, 1997), applicable to certain Boeing Model 737 
series airplanes, to require repetitive inspections to find cracking of 
the lower skin at the lower row of fasteners in the lap joints of the 
fuselage, and repair of any cracking found. That action also adds a 
requirement for modification of the fuselage lap joints at certain 
locations, which constitutes terminating action for repetitive 
inspections of the modified areas. That action was prompted by reports 
of numerous fatigue cracks in the lower skin of the fuselage lap joints 
at the lower row of fasteners. The requirements of that AD are intended 
to prevent such fatigue cracking, which could result in sudden 
decompression of the airplane.

Actions Since Issuance of Previous Rule

    Since the issuance of AD 97-22-07, the FAA has received additional 
reports of fatigue cracking in the lower skin of the lap joints of the 
fuselage on Model 737 series airplanes that had accumulated between 
57,000 and 84,400 flight cycles, and were previously inspected per that 
AD. Further investigation revealed additional cracking in various areas 
of the skin lap joints at the fastener locations that initiated away 
from the edge of the fastener hole in multiple locations. The majority 
of these cracks occurred at left and right stringers 4, 10, and 14. The

[[Page 36510]]

FAA finds that this damage can occur at those stringer locations 
between 40,000 and 50,000 flight cycles. These cracks are not always 
detectable using the open hole eddy current inspection procedures and 
can link up with adjacent cracks causing multiple site damage. In 
addition, cracking has been found in the window corners adjacent to the 
lap joints on certain airplanes.
    Based on these findings, the FAA has determined that the current 
inspection procedures specified in AD 97-22-07 are not adequate for 
detection of cracks in these locations, and that the preventive change 
that was required by that AD does not guarantee crack removal. 
Therefore, the FAA finds that additional rulemaking is necessary to 
require additional inspections for cracking, removal of the preventive 
change, and accomplishment of a lap joint modification.

Public Meeting

    A joint Federal Aviation Administration (FAA) and Boeing meeting 
was held on July 25-27, 2000, to inform industry of the activity on 
Boeing Model 727 and 737 fuselage lap joints. Others in attendance were 
representatives from air carriers and repair stations, as well as 
Principal Maintenance Inspectors (PMI) from FAA's Flight Standards 
Service. The objective of the meeting was to provide an overview of the 
FAA rulemaking process; discuss the recommendations of Boeing Service 
Bulletins 727-53A0222 and 737-53A1177, including background 
information; standardize the 727 and 737 service bulletins, where 
possible; and discuss the impact that the recommended service bulletin 
modifications would have on industry.
    During the meetings, holders of certain supplemental type 
certificates presented information pertaining to service bulletin 
activity for those airplanes that have been modified from a passenger 
to an all-cargo configuration. The meeting accomplished the objective 
of exchanging information between the FAA, Boeing, and industry on 
various aspects of Boeing Models 727 and 737 fuselage lap joints, 
including compliance planning. As a result of the meeting, attendees 
recognized the importance of modifying certain lap joints before 
reaching the point of widespread fatigue damage. Suggestions to improve 
the service bulletins and clarify AD compliance issues were made by 
operators and PMIs, and have been incorporated into the service 
bulletins and the proposed ADs discussed below. In addition, minutes of 
the public meeting are retained in the docket.

Other Relevant Proposed Rulemaking

    At this time, the FAA is considering two other separate rulemaking 
actions to address the remaining potential unsafe conditions relating 
to the cracking of the lap joints of the fuselage. Those two other 
actions would address:
     Replacement of certain repairs with improved repairs in 
certain fuselage lap joints done per the procedures described in the 
structural repair manual (SRM); and a high frequency eddy current 
inspection to find cracking of the SRM repairs of the lower skin at the 
lower row of fasteners in the lap joints of the fuselage, and repair of 
any cracking found on Model 737 series airplanes, line numbers 292 
through 2595 inclusive. And
     Repetitive inspections to find cracking of the lower skin 
at the lower row of fasteners in the lap joints of the fuselage, and a 
lap joint modification of Model 737-200 and -200C series airplanes, 
line numbers 1 through 291 inclusive.

Explanation of Relevant Service Information

    The FAA has reviewed and approved Boeing Service Bulletin 737-
53A1177, Revision 6, dated May 31, 2001, which describes, among other 
things, procedures for the following:
     Removal of the existing preventive modification and the 
installation of an improved lap joint cutout repair, which eliminates 
the need for certain repetitive inspections;
     Repetitive low frequency eddy current (LFEC) inspections 
to find cracking of the lower skin at the lower row of fasteners in the 
lap joints of the fuselage, and repair of any cracking;
     Post-mod inspections, installation of a lap joint repair, 
and follow-on LFEC inspections;
     Modification of the tearstrap splice straps; and
     Repetitive high frequency eddy current inspections of the 
fastener holes of the window corner of the lap joint area, and repair 
of any cracking.
    The service bulletin also specifies contacting the manufacturer for 
accomplishment of certain repairs. Accomplishment of the actions 
specified in the service bulletin is intended to adequately address the 
identified unsafe condition.

Explanation of Requirements of Proposed Rule

    Since an unsafe condition has been identified that is likely to 
exist or develop on other products of this same type design, the 
proposed AD would supersede AD 97-22-07 to continue to require 
repetitive inspections to find cracking of the lower skin at the lower 
row of fasteners in the lap joints of the fuselage, and repair of any 
cracking found. This proposed AD would add repetitive inspections, 
adjust inspection thresholds, and require an improved modification. 
Accomplishing the improved modification would terminate the inspections 
required by the existing AD. In addition, this proposed AD would 
require inspections of some airplanes on which the ``Preventive 
Change'' (NACA modification) specified in AD 97-22-07 already had been 
accomplished. This proposed AD also would require inspections following 
accomplishment of the improved modifications, and would require tear 
strap splice conditions for airplanes on which lap joints previously 
had been repaired per AD 97-22-07. This proposed AD also would require 
inspections on some airplanes in window corners in areas already being 
inspected per AD 91-07-04, amendment 39-6933 (56 FR 11355, March 18, 
1991), on other airplanes. The actions would be required to be 
accomplished per the service bulletin described previously, except as 
discussed below.

Differences Between Service Bulletin and Proposed Rule

    The FAA recognizes that the lap joint modification specified in 
this proposed AD would require jacking, shoring, removing interior 
components, and modifying certain lap joints, which would require 
taking the airplane out of service for as much as 22 days. This lengthy 
shop visit, as well as the relatively short compliance time required to 
accomplish this proposed AD, make it necessary for operators to engage 
in compliance planning to ensure that, when the deadline for compliance 
arrives, all of the required actions have been completed on all 
affected airplanes. Therefore, paragraph (f) of this proposed AD would 
require that operators submit to the FAA a compliance plan within 3 
months after the effective date of this AD. This will enable the FAA to 
verify that all operators will be able to meet the deadlines imposed by 
this proposed AD.
    Operators also should note that, in light of the complexity of the 
service bulletin, three separate rulemaking actions are being issued to 
address the potential unsafe conditions relating to the cracking of the 
lap joints of the fuselage. This proposed rule will address only Model 
737 series airplanes having line numbers (L/N) 292 through 2565 
inclusive.

[[Page 36511]]

    Paragraph (m) of this AD addresses only Model 737 series airplanes 
having L/N 520 through 2565 inclusive for accomplishment of the HFEC 
inspection to find cracking of the window corner fastener holes. Model 
737 series airplanes having L/N 1 through 519 inclusive were addressed 
in AD 91-07-04, amendment 39-6933. That AD requires ultrasonic and high 
frequency eddy current inspections for delamination of window belt skin 
doubler from fuselage skin.
    In addition, although the service bulletin specifies that the 
manufacturer may be contacted for disposition of certain repair/
modification conditions, this proposed AD requires the repair/
modification of those conditions to be done per a method approved by 
the FAA, or per data meeting the type certification basis of the 
airplane approved by a Boeing Company Designated Engineering 
Representative who has been authorized by the FAA to make such 
findings.

Cost Impact

    There are approximately 2,203 Model 737 series airplanes of the 
affected design in the worldwide fleet. The FAA estimates that 905 
airplanes of U.S. registry would be affected by this proposed AD.
    Cost estimates for the actions required by this proposed AD for 
U.S. operators over the life of the AD are included in the following 
table:

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                                                                                           Cost/
              Paragraph/AD action                   Number     Work Hours   Parts  ($)    airplane    Total cost
                                                   affected                                 ($)          ($)
----------------------------------------------------------------------------------------------------------------
(a) Lap joint inspection.......................          905          100            0        6,000    5,430,000
(f) Compliance planning........................          905           24            0        1,440    1,303,200
(g) Lap joint modification.....................          905        4,200       12,000      264,000  238,920,000
(h) Lap joint inspection.......................          905          100            0        6,000    5,430,000
(i) Post-NACA inspection.......................           25          100            0        6,000      150,000
(j) Post-NACA inspection.......................           10          100            0        6,000       60,000
(m) Window corner inspection...................          807           14            0          840      677,880
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    The cost estimates are based on the following criteria:
     Lap joint inspection cost estimates reflect costs for a 
single inspection cycle, and the work hours vary between groups of 
airplanes. Refer to paragraph 1.G. of Boeing Service Bulletin 737-
53A1177 for more detailed information. An average of 100 work hours was 
used in determining the cost estimates.
     An average of 24 work hours was used in estimating the 
costs for compliance planning.
     Lap joint modification work hours vary between groups of 
airplanes. Refer to paragraph 1.G of Boeing Service Bulletin 737-
53A1177 for more detailed information. An average of 4,200 work hours 
and $12,000 for parts were used in estimating these costs. Modification 
costs are spread over the estimated life of the AD, which is 
approximately 20 to 25 years.
     Window corner inspection work hours vary between groups of 
airplanes. Refer to paragraph 1.G of Boeing Service Bulletin 737-
53A1177 for more detailed information. An average of 14 work hours was 
used in estimating the costs of the inspections only.
    The FAA estimates that during the 10-year period after issuance of 
the proposed AD, worldwide operators would be required to modify 805 
Model 737 series airplanes. The new modification required by the 
proposed AD would take an average of approximately 4,200 work hours to 
accomplish, at an average labor rate of $60 per work hour. The 
worldwide cost impact of the required modification is estimated to be 
$212,701,000 over 10 years, or an average of $21,270,000 per year. The 
highest impact year is the third year after issuance of the AD: an 
estimated 155 Model 737 series airplanes would require modification in 
that year. Therefore, the worldwide cost impact of the modification is 
estimated to be $40,955,000 in that year. The affected Model 737 
airplanes operated by U.S. operators comprise approximately 41 percent 
of the total worldwide costs. Therefore, the highest cost impact in any 
given year for the modifications is estimated to be $16,791,000 for 
U.S. operators.
    The cost impact figures discussed above are based on assumptions 
that no operator has yet accomplished any of the current or proposed 
requirements of this AD action, and that no operator would accomplish 
those actions in the future if this AD were not adopted. The cost 
impact figures discussed in AD rulemaking actions represent only the 
time necessary to perform the specific actions actually required by the 
AD. These figures typically do not include incidental costs, such as 
the time required to gain access and close up, planning time, or time 
necessitated by other administrative actions.

Regulatory Impact

    The regulations proposed herein would not have a substantial direct 
effect 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, it 
is determined that this proposal would not have federalism implications 
under Executive Order 13132.
    For the reasons discussed above, I certify that this proposed 
regulation (1) is not a ``significant regulatory action'' under 
Executive Order 12866; (2) is not a ``significant rule'' under the DOT 
Regulatory Policies and Procedures (44 FR 11034, February 26, 1979); 
and (3) if promulgated, will not have a significant economic impact, 
positive or negative, on a substantial number of small entities under 
the criteria of the Regulatory Flexibility Act. A copy of the draft 
regulatory evaluation prepared for this action is contained in the 
Rules Docket. A copy of it may be obtained by contacting the Rules 
Docket at the location provided under the caption ADDRESSES.

List of Subjects in 14 CFR Part 39

    Air transportation, Aircraft, Aviation safety, Safety.

The Proposed Amendment

    Accordingly, pursuant to the authority delegated to me by the 
Administrator, the Federal Aviation Administration proposes to amend 
Part 39 of the Federal Aviation Regulations (14 CFR part 39) as 
follows:

PART 39--AIRWORTHINESS DIRECTIVES

    1. The authority citation for part 39 continues to read as follows:

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


Sec. 39.13  [Amended]

    2. Section 39.13 is amended by removing amendment 39-10179 (62 FR 
55732, October 28, 1997), and by adding

[[Page 36512]]

a new airworthiness directive (AD), to read as follows:

Boeing: Docket 98-NM-196-AD. Supersedes AD 97-22-07, amendment 39-
10179.

    Applicability: Model 737-200, -200C, -300, -400, and -500 series 
airplanes having line numbers 292 through 2565 inclusive, 
certificated in any category.

    Note 1: This AD applies to each airplane identified in the 
preceding applicability provision, regardless of whether it has been 
modified, altered, or repaired in the area subject to the 
requirements of this AD. For airplanes that have been modified, 
altered, or repaired so that the performance of the requirements of 
this AD is affected, the owner/operator must request approval for an 
alternative method of compliance per paragraph (n) of this AD. The 
request should include an assessment of the effect of the 
modification, alteration, or repair on the unsafe condition 
addressed by this AD; and, if the unsafe condition has not been 
eliminated, the request should include specific proposed actions to 
address it.

    Compliance: Required as indicated, unless accomplished 
previously.
    To find and fix cracking of certain fuselage lapjoints, which 
could result in sudden decompression of the airplane, accomplish the 
following:

Repetitive Low Frequency Eddy Current (LFEC) Inspections--Crown Areas

    (a) Do an LFEC inspection to find cracking of the lower skin at 
the lower row of fasteners in the lap joints of the fuselage, per 
PART I (``Inspection'') of the Accomplishment Instructions of Boeing 
Service Bulletin 737-53A1177, Revision 6, dated May 31, 2001; at the 
time specified in paragraph (b) or (c) of this AD, as applicable.
    (b) For airplanes that have accumulated more than 65,000 total 
flight cycles but not more than 70,000 total flight cycles as of the 
effective date of this AD: Do the inspection at the earlier of the 
times specified in paragraphs (b)(1) and (b)(2) of this AD. Repeat 
the inspection after that at intervals not to exceed 1,200 flight 
cycles until accomplishment of the lap joint repair required by 
paragraph (g) of this AD.
    (1) Within 1,200 flight cycles after the effective date of this 
AD.
    (2) Within 1,200 flight cycles after the last inspection, if 
any, accomplished in accordance with AD 97-22-07, amendment 39-
10179.
    (c) For airplanes that have accumulated at least 45,000 total 
flight cycles but not more than 65,000 total flight cycles as of the 
effective date of this AD: Do the inspection at the earlier of the 
times specified in paragraphs (c)(1) and (c)(2) of this AD. Repeat 
the inspection after that at intervals not to exceed 1,200 flight 
cycles until accomplishment of the lap joint repair required by 
paragraph (g) of this AD.
    (1) At the later of the times specified in paragraphs (c)(1)(i) 
and (c)(1)(ii) of this AD.
    (i) Before the accumulation of 50,000 total flight cycles.
    (ii) Within 1,200 flight cycles after the effective date of this 
AD.
    (2) Within 1,200 flight cycles after the last inspection, if 
any, accomplished in accordance with AD 97-22-07, amendment 39-
10179.

Crack Repair

    (d) Except as provided by paragraph (e) of this AD: If any 
cracking is found during any inspection required by this AD, before 
further flight, repair per PART II (``Crack Repair'') of the 
Accomplishment Instructions of Boeing Service Bulletin 737-53A1177, 
Revision 6, dated May 31, 2001.
    (e) If any cracking is found during any inspection required by 
this AD, and Boeing Service Bulletin 737-53A1177, Revision 6, dated 
May 31, 2001, specifies to contact Boeing for repair instructions: 
Repair any cracking, before further flight, per a method approved by 
the Manager, Seattle Aircraft Certification Office (ACO), FAA, 
Transport Airplane Directorate; or per data meeting the type 
certification basis of the airplane approved by a Boeing Company 
Designated Engineering Representative (DER) who has been authorized 
by the Manager, Seattle ACO, to make such findings. For a repair 
method to be approved by the Manager, Seattle ACO, as required by 
this paragraph, the approval letter must specifically reference this 
AD.

Compliance Plan

    (f) Within 3 months after the effective date of this AD, submit 
a plan to the FAA identifying a schedule for compliance with 
paragraph (g) of this AD. This schedule must include, for each of 
the operator's affected airplanes, the dates and maintenance events 
(e.g., letter checks) when the required actions will be 
accomplished. For the purposes of this paragraph, ``FAA'' means the 
Principal Maintenance Inspector (PMI) for operators that are 
assigned a PMI, or the cognizant Flight Standards District Office 
for other operators. Information collection requirements contained 
in this regulation have been approved by the Office of Management 
and Budget (OMB) under the provisions of the Paperwork Reduction Act 
of 1980 (44 U.S.C. 3501 et seq.) and have been assigned OMB Control 
Number 2120-0056.

    Note 2: Operators are not required to submit revisions to the 
compliance plan required by paragraph (f) of this AD to the FAA.

Lap Joint Modification (Repair)--Crown Areas

    (g) Install the lap joint repair per PART III or IV (``Lap Joint 
Repair'') of the Accomplishment Instructions of Boeing Service 
Bulletin 737-53A1177, Revision 6, dated May 31, 2001, as applicable; 
at the time specified in paragraph (g)(1), (g)(2), (g)(3), (g)(4), 
or (g)(5) of this AD, as applicable. Accomplishment of this repair 
terminates the repetitive inspections required by paragraphs (b) and 
(c) of this AD.
    (1) For airplanes that have accumulated 70,000 total flight 
cycles or more as of the effective date of this AD: Within 600 
flight cycles after the effective date of this AD, do the lap joint 
repair.
    (2) For airplanes that have accumulated 65,000 total flight 
cycles or more, but less than 70,000 total flight cycles as of the 
effective date of this AD: Do the repair at the later of the times 
specified in paragraphs (g)(2)(i) and (g)(2)(ii) of this AD.
    (i) Before the accumulation of 70,000 total flight cycles.
    (ii) Within 600 flight cycles after the effective date of this 
AD.
    (3) For airplanes that have accumulated 45,000 total flight 
cycles or more, but less than 65,000 total flight cycles as of the 
effective date of this AD: Within 5,000 flight cycles after the 
effective date of this AD.
    (4) For airplanes that have accumulated less than 45,000 total 
flight cycles as of the effective date of this AD: Before the 
accumulation of 50,000 total flight cycles.
    (5) Notwithstanding the times specified in paragraphs (g)(1), 
(g)(2), (g)(3), and (g)(4) of this AD, for airplanes on which the 
``Preventive Change'' has been accomplished per PART III of the 
Accomplishment Instructions of Boeing Alert Service Bulletin 737-
53A1177, Revision 3, dated September 18, 1997; (NACA modification): 
Within 18,000 flight cycles after accomplishment of the NACA 
modification.

Repetitive LFEC Inspections--Outside Crown Areas

    (h) Before the accumulation of 70,000 total flight cycles, or 
within 2,500 flight cycles after the effective date of this AD, 
whichever comes later: Perform an LFEC inspection to find cracking 
of the lap joints of the fuselage; as identified in Figures 2 
through 7 of the Accomplishment Instructions of Boeing Service 
Bulletin 737-53A1177, Revision 6, dated May 31, 2001. Do the 
inspection per the service bulletin. Repeat the inspection after 
that at intervals not to exceed 5,000 flight cycles.

Post-NACA Modification Inspections--Crown Areas

    (i) For airplanes that have the ``Preventive Change'' (NACA 
modification) of the crown lap joint stringers (``Crown Laps''): 
Within 12,000 flight cycles after accomplishment of the NACA 
modification, do either an external (Figure 8) or internal (Figure 
9) LFEC inspection to find cracking and corrosion per PART I 
(``Inspection'') of the Accomplishment Instructions of Boeing 
Service Bulletin 737-53A1177, Revision 6, dated May 31, 2001.
    (1) If the external inspection is done: Repeat the inspection 
after that at intervals not to exceed 1,500 flight cycles until 
accomplishment of the lap joint repair required by paragraph (g) of 
this AD.
    (2) If the internal inspection is done: Repeat the inspection 
after that at intervals not to exceed 4,500 flight cycles until 
accomplishment of the lap joint repair required by paragraph (g) of 
this AD.

Post-NACA Modification Inspection--Outside Crown Areas

    (j) For airplanes that have the ``Preventive Change'' (NACA 
modification) outside the crown areas: Before the accumulation of 
20,000 flight cycles after accomplishment of the NACA modification, 
do either an external (Figure 8) or internal (Figure 9) LFEC 
inspection to find cracking and corrosion per PART I 
(``Inspection'') of the Accomplishment Instructions of Boeing

[[Page 36513]]

Service Bulletin 737-53A1177, Revision 6, dated May 31, 2001.
    (1) If the external inspection is done: Repeat the external 
inspection after that at intervals not to exceed 1,500 flight 
cycles.
    (2) If the internal inspection is done: Repeat the internal 
inspection after that at intervals not to exceed 4,500 flight 
cycles.

Modification of Tear Strap Splice Straps

    (k) For airplanes that have the ``lap joint repair,'' as 
specified in Part IV of the Accomplishment Instructions of Boeing 
Service Bulletin 737-53A1177, Revision 2, dated July 24, 1997, or 
Revision 3, dated September 18, 1997: Within 45,000 flight cycles 
after accomplishment of this lap joint repair, modify the splice 
straps per Figures 10, 11, and 12 of the Accomplishment Instructions 
of Boeing Service Bulletin 737-53A1177, Revision 6, dated May 31, 
2001.

Follow-On LFEC Inspections

    (l) Within 45,000 flight cycles after accomplishment of the lap 
joint repair required by paragraph (g) of this AD: Do either an 
external (Figure 8) or internal (Figure 9) LFEC inspection to find 
cracking of the lap joint repair, per PART I (``Inspection'') of the 
Accomplishment Instructions of Boeing Service Bulletin 737-53A1177, 
Revision 6, dated May 31, 2001. Repeat the inspection after that at 
intervals not to exceed 2,800 flight cycles.

Repetitive High Frequency Eddy Current (HFEC) Inspections--Window 
Corners

    (m) For airplanes having line numbers 520 through 2565 
inclusive: Before the accumulation of 50,000 total flight cycles or 
within 1,200 flight cycles after the effective date of this AD, 
whichever comes later, do a HFEC inspection to find cracking, per 
PART V (``Window Corner Fastener Hole Cracking, Inspection and 
Repair'') of the Accomplishment Instructions of Boeing Service 
Bulletin 737-53A1177, Revision 6, dated May 31, 2001. Repeat the 
inspection after that at intervals not to exceed 4,500 flight 
cycles. Accomplishment of the modification per Part V of the 
Accomplishment Instructions of the service bulletin constitutes 
terminating action for the inspections required by this paragraph.

Alternative Methods of Compliance

    (n) 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 PMI, who may add comments 
and then send it to the Manager, Seattle ACO.

    Note 3: Information concerning the existence of approved 
alternative methods of compliance with this AD, if any, may be 
obtained from the Seattle ACO.

Special Flight Permits

    (o) Special flight permits may be issued per sections 21.197 and 
21.199 of the Federal Aviation Regulations (14 CFR 21.197 and 
21.199) to operate the airplane to a location where the requirements 
of this AD can be accomplished.

    Issued in Renton, Washington, on July 6, 2001.
Vi L. Lipski,
Manager, Transport Airplane Directorate, Aircraft Certification 
Service.
[FR Doc. 01-17431 Filed 7-11-01; 8:45 am]
BILLING CODE 4910-13-P