2012-24805. Airworthiness Directives; The Boeing Company Airplanes

AGENCY:

Federal Aviation Administration (FAA), DOT.

ACTION:

Supplemental notice of proposed rulemaking (NPRM); reopening of comment period.

SUMMARY:

We are revising an earlier proposed airworthiness directive (AD) for certain Model 737-200, -200C, -300, -400, and -500 series airplanes. That proposed AD would have superseded an existing AD that applies to certain The Boeing Company Model 737-200, -200C, -300, -400, and -500 series airplanes. The existing AD currently requires repetitive inspections to find fatigue cracking of certain upper and lower skin panels of the fuselage, and follow-on and corrective actions if necessary. The existing AD also includes a terminating action for the repetitive inspections of certain modified or repaired areas only. That NPRM proposed to add new inspections for cracking of the fuselage skin along certain chem-milled lines, and corrective actions if necessary. That NPRM also proposed to reduce certain thresholds and intervals required by the existing AD. This action revises that NPRM by reducing the proposed repetitive inspection intervals. We are proposing this supplemental NPRM to detect and correct fatigue cracking of the skin panels, which could result in sudden fracture and failure of the skin panels of the fuselage, and consequent rapid decompression of the airplane. Since these actions impose an additional burden over that proposed in the previous NPRM, we are reopening the comment period to allow the public the chance to comment on these proposed changes.

DATES:

We must receive comments on this supplemental NPRM by November 26, 2012.

ADDRESSES:

You may send comments, using the procedures found in 14 CFR 11.43 and 11.45, by any of the following methods:

• Federal eRulemaking Portal: Go to http://www.regulations.gov. Follow the instructions for submitting comments.
• Fax: [phone].
• Mail: U.S. Department of Transportation, Docket Operations, M-30, West Building Ground Floor, Room W12-140, 1200 New Jersey Avenue SE., Washington, DC 20590.
• Hand Delivery: U.S. Department of Transportation, Docket Operations, M-30, West Building Ground Floor, Room W12-140, 1200 New Jersey Avenue SE., Washington, DC 20590, between 9 a.m. and 5 p.m., Monday through Friday, except Federal holidays.

For service information identified in this proposed AD, contact Boeing Commercial Airplanes, Attention: Data & Services Management, P.O. Box 3707, MC 2H-65, Seattle, Washington 98124-2207; telephone [phone], extension 1; fax [phone]; Internet https://www.myboeingfleet.com. You may review copies of the referenced service information at the FAA, Transport Airplane Directorate, 1601 Lind Avenue SW., Renton, Washington. For information on the availability of this material at the FAA, call 425-227-1221.

Examining the AD Docket

You may examine the AD docket on the Internet at http://www.regulations.gov; or in person at the Docket Management Facility between 9 a.m. and 5 p.m., Monday through Friday, except Federal holidays. The AD docket contains this proposed AD, the regulatory evaluation, any comments received, and other information. The street address for the Docket Office (phone: [phone]) is in the ADDRESSES section. Comments will be available in the AD docket shortly after receipt.

FOR FURTHER INFORMATION CONTACT:

Wayne Lockett, Aerospace Engineer, Airframe Branch, ANM-120S, FAA, Seattle Aircraft Certification Office, 1601 Lind Avenue SW., Renton, Washington 98057-3356; phone: [phone]; fax: [phone]; email: [e-mail].

SUPPLEMENTARY INFORMATION:

Comments Invited

We invite you to send any written relevant data, views, or arguments about this proposed AD. Send your comments to an address listed under the ADDRESSES section. Include “Docket No. FAA-2011-0155; Directorate Identifier 2009-NM-141-AD” at the beginning of your comments. We specifically invite comments on the overall regulatory, economic, environmental, and energy aspects of this proposed AD. We will consider all comments received by the closing date and may amend this proposed AD because of those comments.

We will post all comments we receive, without change, to http://www.regulations.gov, including any personal information you provide. We will also post a report summarizing each substantive verbal contact we receive about this proposed AD.

Discussion

We issued an NPRM to amend 14 CFR part 39 to include an AD that would apply to certain Model 737-200, -200C, -300, -400, and -500 series airplanes. That NPRM published in the Federal Register on March 8, 2011 (76 FR 12619). That NPRM proposed to supersede AD 2004-18-06, Amendment 39-13784 (69 FR 54206, September 8, 2004), to continue to require repetitive inspections to find fatigue cracking of certain upper and lower skin panels of the fuselage, and follow-on and corrective actions if necessary. That NPRM also included a terminating action for the repetitive inspections of certain modified or repaired areas only. That NPRM proposed to add new inspections for cracking of the fuselage skin along certain chem-milled lines, and corrective actions if necessary. That NPRM also proposed to reduce certain thresholds and intervals required by the existing AD. That NPRM resulted from reports indicating new findings of cracks were found along the edges of the chem-milled pockets in the upper skin at certain stringers.

Actions Since Previous NPRM Was Issued

Since we issued the previous NPRM (76 FR 12619, March 8, 2011), extensive continued evaluation of the chem-mill step cracking has been done, which resulted in a determination that the repetitive inspection intervals and the compliance time for the inspections of the crown area and other known areas of fuselage skin cracking must be reduced in order to adequately address the identified unsafe condition.

Comments

We gave the public the opportunity to comment on the previous NPRM (76 FR 12619, March 8, 2011). The following presents the comments received on the previous NPRM and the FAA's response to each comment.

Supportive Comments

The National Transportation Safety Board and Jonathan W. Ketron support the content of the previous NPRM (76 FR 12619, March 8, 2011).

Request To Supersede AD 2003-14-06, Amendment 39-13225 (68 FR 42956, July 21, 2003)

Alaska Airlines (ASA) asked that we change the previous NPRM (76 FR 12619, March 8, 2011) to also supersede AD 2003-14-06, Amendment 39-13225 (68 FR 42956, July 21, 2003). ASA explained that paragraph (g) of AD 2004-18-06, Amendment 39-13784 (69 FR 54206, September 8, 2004), allows accomplishing one-time internal inspections in accordance with paragraph (b) or paragraph (c) of AD 2003-14-06 in order to terminate the repetitive inspections required by paragraph (a) of AD 2004-18-06 (paragraph (g) of the previous NPRM). ASA added that the previous NPRM restates these required external inspections. ASA noted that AD 2003-14-06 was initially released to address inadequate phosphoric anodizing of the skin panels, leading to disbonding of internal doublers. ASA stated further that subsequent events showed that fatigue cracking of chem-milled skins cannot be solely attributed to improper anodizing or disbonded doublers. ASA added that the previous NPRM does not clearly address the requirements in AD 2003-14-06, which will remain in effect until after the final rule is released.

We do not agree to supersede AD 2003-14-06, Amendment 39-13255 (68 FR 42956, July 21, 2003). The one-time internal inspection in that AD is required for safety and is not related to the chem-milled step cracking addressed by this supplemental NPRM. In addition, the one-time inspection required by AD 2003-14-06 only terminates the external eddy current inspections required by paragraph (a) of AD 2004-18-06, Amendment 39-13784 (69 FR 54206, September 8, 2004), which is restated in paragraph (g) of this supplemental NPRM. All external detailed inspections are still required by paragraph (g) of this supplemental NPRM. We have made no change to this supplemental NPRM in this regard.

Request for Clarification of Language in Paragraphs (g) and (h) of the Previous NPRM (76 FR 12619, March 8, 2011)

Boeing requested that paragraphs (g) and (h) of the previous NPRM (76 FR 12619, March 8, 2011) be revised to better distinguish between existing and new requirements. Specifically, Boeing stated that the language used in paragraphs (g) and (h) of the previous NPRM could be interpreted to mean that using Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009, is optional for accomplishing the required actions at the inspection thresholds and intervals required by AD 2004-18-06, Amendment 39-13784 (69 FR 54206, September 8, 2004). Boeing requested clarification of the wording to indicate that the inspection, methods, thresholds, and “repeats” in accordance with Revision 3 of this service bulletin are now required.

We agree that clarification is necessary. Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009, can be used for accomplishing the required actions in AD 2004-18-06, Amendment 39-13784 (69 FR 54206, September 8, 2004). However, it is not required to use Revision 3 of this service bulletin to accomplish the requirements of the existing AD; rather, it is required to use that revision for the new actions required by paragraphs (p), (q), and (s) of this supplemental NPRM.

Consequently, we have revised paragraphs (g) and (h) of this supplemental NPRM to remove the sentence beginning “As of the effective date of this AD * * *” to clarify that using Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009, is not required for accomplishing the requirements of the existing AD.

Request To Change “Internal” Detailed Inspection to “External” Detailed Inspection

Boeing asked that we change the “internal” detailed inspection specified in paragraph (h)(1) of the previous NPRM (76 FR 12619, March 8, 2011) to an “external” detailed inspection. Boeing stated that this is a typographical error because it is a restatement of the requirements of AD 2004-18-06, Amendment 39-13784 (69 FR 54206, September 8, 2004).

We agree that the reference to an “internal” detailed inspection is a typographical error. We have changed paragraph (h)(1) of this supplemental NPRM to specify an “external” detailed inspection.

Request To Remove a Paragraph Reference

Boeing asked that we remove the reference to “paragraph (m)” in the third sentence of paragraph (g) of the previous NPRM (76 FR 12619, March 8, 2011). Boeing stated that paragraph (m) of the previous NPRM addresses disbond inspections, which should not terminate the detailed or eddy current inspection, as specified.

We find that clarification is necessary. Paragraph (m) of this supplemental NPRM states that accomplishing the optional terminating action “before the effective date of this AD” terminates the eddy current inspections required by paragraph (g) of this supplemental NPRM. In addition, paragraph (m) of this supplemental NPRM states that the detailed inspections specified in paragraph (g) of this supplemental NPRM are not terminated. Paragraph (m) of this supplemental NPRM states further that, “as of the effective date of this AD,” the optional terminating action does not terminate the repetitive eddy current inspections required by paragraph (g) of this supplemental NPRM.

We find it necessary to retain the reference to paragraph (m) of this supplemental NPRM in paragraph (g) to indicate that accomplishing the terminating action in paragraph (m) of this supplemental NPRM “before the effective date of this AD” terminates the eddy current inspections required by paragraph (g) of this supplemental NPRM. However, we have revised paragraph (m) of this supplemental NPRM to indicate that neither detailed nor eddy current inspections can be terminated by accomplishing the optional terminating action “on or after the effective date of this AD.”

Request To Clarify Terminating Action

Boeing asked that the preventative modification language specified in paragraph (w) of the previous NPRM (76 FR 12619, March 8, 2011) be clarified to specify that doing the modification terminates only the repetitive inspections of the chem-milled steps common to stringer 12. Boeing stated that Note (c) of Table 2, paragraph 1.E., “Compliance,” of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009, specifies that accomplishing the preventative modification only terminates the inspections at stringer 12. Boeing added that there have been post-modification cracks reported at the chem-milled steps common to stringer 10, even though the modification doubler covers the area. Boeing noted that the internal modification doubler does not span the stringer 10 or stringer 13 chem-milled steps by three rows of fasteners; therefore, inspections should continue at those locations even though the modification doubler is common to them.

We agree that clarification is necessary. Doing the preventive modification of the chem-milled areas in the skin at stringer 12, as specified in Part 7 of the Accomplishment Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009, ends the repetitive inspections for the modified areas common to stringer 12 only. Cracking of the chem-milled steps has been found at locations where doublers do not extend a minimum of three fastener rows beyond the chem-milled step. We have changed paragraph (w) of this supplemental NPRM accordingly.

Requests To Revise Certain Compliance Times

Boeing asked that the compliance time specified in paragraph (p)(1) of the previous NPRM (76 FR 12619, March 8, 2011) be changed. Boeing stated that it is unclear why airplanes that have accomplished paragraphs (i), (j)(1)(ii), (k), (l), or (m) of the previous NPRM (which are repairs and disbond inspections) must be inspected within 1,800 flight cycles after the effective date of the AD. Boeing added that airplanes identified in paragraph (p)(2) of the previous NPRM that have not accomplished those paragraphs are allowed to wait until 4,500 flight cycles after the last inspection, or 1,800 flight cycles after the effective date of the AD, whichever is later. Boeing questioned why the airplanes on which repairs have not been installed or a disbond inspection has not been accomplished have a different compliance time than airplanes that do have these repairs and inspections. Boeing noted that by using “whichever occurs later” in paragraph (p)(2) of the previous NPRM, an operator may accumulate up to 6,299 flight cycles (4,499 + 1,800) between inspections that were previously required at 4,500-flight-cycle intervals in accordance with AD 2004-18-06, Amendment 39-13784 (69 FR 54206, September 8, 2004).

We agree that the compliance times required by paragraph (p) of this supplemental NPRM should be clarified. After further review of the new inspection requirements in paragraph (p) of this supplemental NPRM, we have determined that those inspections are not related to existing installed repairs or the disbond inspection. The compliance times should not be based on local repairs that may be installed on the airplanes. For areas that have repairs installed in accordance with paragraphs (i), (j)(1)(ii), (k), (l), and (m) of this AD, the inspection is already terminated for these areas only. Therefore, we have deleted paragraphs (p)(1) and (p)(2) of the previous NPRM (76 FR 12619, March 8, 2011), and included the following reduced compliance time in paragraph (p) of this supplemental NPRM: “Within 4,500 flight cycles after doing the most recent inspection required by paragraph (g) of this AD, or within 1,800 flight cycles after the effective date of this AD, whichever is earlier.”

Boeing also asked that the compliance time specified in paragraph (s) of the previous NPRM (76 FR 12619, March 8, 2011) be changed. Boeing stated that the repetitive interval should be changed to 1,800 flight cycles for inspections in areas of known cracking in the lower lobe and Section 41, and when both eddy current and detailed inspections are required, in accordance with Part 4 of the Accomplishment Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009.

Boeing also noted that for inspections in areas with no known cracking, the threshold should be at the latest of 35,000 total flight cycles, or at the earliest of 4,500 flight cycles from the release of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009, or 9,000 flight cycles after the previous inspection. Boeing stated that, based on additional crack growth data, the repetitive inspections in areas of known cracking should be reduced to 1,800 flight cycles. Boeing added that the inspection threshold should be at the latest of 35,000 total flight cycles, or at the earliest of 1,800 flight cycles from the release of that service bulletin or 4,500 flight cycles after the previous inspection. Boeing concluded that in areas of no known cracking, the inspections are in place to monitor for new cracking; therefore, the repetitive inspection intervals can be extended.

We agree that the compliance times should be changed based on the data provided by the manufacturer. We have moved the compliance times for the initial and repetitive inspections identified in paragraph (s) of the previous NPRM (76 FR 12619, March 8, 2011) to paragraphs (s)(1) and (s)(2) of this supplemental NPRM. We also reduced the repetitive inspection interval for known cracking in paragraph (s)(2) of this supplemental NPRM to 1,800 flight cycles.

Boeing also asked that the compliance times and repetitive intervals specified in paragraphs (t), (u)(1), and (u)(2) of the previous NPRM (76 FR 12619, March 8, 2011) be changed. Boeing requested that all compliance times (initial and repetitive) that specify 4,500 flight cycles be changed to 1,800 flight cycles. Boeing stated that the window belt skin area is equivalent to the crown skin area, and the initial threshold and repetitive interval should be the same as required for the crown area.

We agree that the compliance times should be changed based on the information provided. We have changed the compliance times and repetitive intervals in paragraphs (t), (u)(1), and (u)(2) of this supplemental NPRM accordingly.

Request To Provide Certain Clarifications

Southwest Airlines (SWA) asked that we change paragraph (h) of the previous NPRM (76 FR 12619, March 8, 2011) to provide clarification whether the inspections identified in Figures 26, 27, and 31 of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009, are required. SWA asked whether an eddy current or external detailed inspection is necessary for certain locations if those inspection figures are required. SWA stated that paragraph (h) of the previous NPRM specifies an external detailed inspection of the lower lobe area and Section 41 of the fuselage, in accordance with Part 4 of the Accomplishment Instructions of that service bulletin. SWA added that Part 4 also specifies eddy current inspections every 1,800 flight cycles.

We agree that clarification is necessary. The subject inspections identified in Figures 26, 27, and 31 of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009, are specified in this supplemental NPRM; however, the inspections in those figures are required by paragraph (s), not paragraph (h), of this supplemental NPRM. No change to either paragraph (h) or (s) of this supplemental NPRM is necessary.

SWA also asked how paragraphs (s) and (u) of the previous NPRM (76 FR 12619, March 8, 2011) are related, and if paragraph (u) of the previous NPRM applies to airplane groups 4, 11, and 16. SWA noted that the inspections identified in paragraph (s) of the previous NPRM are applicable to Groups 1 through 21 and are done in accordance with Part 4 of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009; except as required by paragraph (u) of the previous NPRM. SWA also noted that the inspections identified in paragraph (u) of the previous NPRM are applicable to Groups 3, 5, 9, 10, 12, 14, 15, 17, 18, 19, 20, and 21, and are done in accordance with Part 2 of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009. SWA stated that it is unsure how Part 2 and Part 4 are related. SWA added that Part 2 addresses Groups 4, 11, and 16; however, those groups are not specified in paragraph (u) of the previous NPRM.

We agree that clarification is necessary because the exception in paragraph (s) of the previous NPRM (76 FR 12619, March 8, 2011) is a typographical error and should have identified paragraph (x) instead of paragraph (u) of the previous NPRM. We have changed paragraph (s) of this supplemental NPRM accordingly, which also addresses the commenter's concern about Groups 4, 11, and 16.

Request To Exclude Certain Inspection Areas

SWA asked that, to avoid mandating additional inspections in the previous NPRM (76 FR 12619, March 8, 2011), we exclude the inspection areas identified in AD 2009-21-01, Amendment 39-16038 (74 FR 52395, October 13, 2009). SWA stated that the inspection threshold and repetitive intervals required by AD 2009-21-01 are more restrictive for certain areas of the lower lobe skins than those in the previous NPRM.

We agree that the inspection threshold and repetitive intervals required by AD 2009-21-01, Amendment 39-16038 (74 FR 52395, October 13, 2009), are more restrictive for certain areas. Therefore, we have added a new paragraph (y)(5) to this supplemental NPRM (and reidentified subsequent paragraphs) to specify that inspections and corrective actions required by paragraph (g) of AD 2009-21-01 meet the corresponding provisions of paragraph (s) of this supplemental NPRM.

Request To Require Eddy Current Inspections Only

SWA asked that only nondestructive inspections (NDI) be used in areas where the previous NPRM (76 FR 12619, March 8, 2011) requires both external detailed and eddy current inspections every 1,800 flight cycles. SWA stated that paragraph (f) of AD 2008-19-03, Amendment 39-15670 (73 FR 56958, October 1, 2008), requires repetitive inspections along the chem-milled steps at stringers S-1 and S-2R, between STA 400 and STA 460. SWA added that for line numbers 1001 through 2552, the operator has the option of accomplishing repetitive external detailed inspections every 2,300 flight cycles, or NDI every 4,500 flight cycles. SWA noted that the previous NPRM reduces the repetitive inspection interval specified in Tables 1 through 5 of paragraph 1.E., “Compliance,” of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009, from 4,500 flight cycles to 1,800 flight cycles. SWA made its request due to this reduced inspection interval.

We do not agree that only NDI can be used where both external detailed and eddy current inspections are required. The overall safety concern related to Model 737 fuselage skin panels addressed in this supplemental NPRM is mitigated by using detailed inspections in conjunction with eddy current inspections. We have made no change to the supplemental NPRM in this regard.

Request To Use Ultrasonic Phased Array (UTPA) Inspection

SWA asked that it be allowed to use a UTPA inspection to inspect areas where the previous NPRM (76 FR 12619, March 8, 2011) requires external NDI methods. SWA stated Boeing confirmed that it is satisfactory to inspect areas covered by the dorsal fin by internally using the UTPA inspection in lieu of the NDI methods, using information in the nondestructive test manual, in lieu of the external NDI methods.

We agree that a UTPA inspection might be able to be used instead of the external NDI method. However, we do not agree to include the UTPA inspection in this supplemental NPRM because we cannot include individual operators' methods of compliance. Under the provisions of paragraph (y) of this supplemental NPRM, we will consider requests for approval of a change to the inspection type if sufficient data are submitted to substantiate that the inspection would provide an acceptable level of safety. We have made no change to the supplemental NPRM in this regard.

Request To Allow a General Visual Inspection (GVI) Instead of Detailed Inspection

SWA and British Airways asked that we allow a GVI in lieu of the detailed inspection specified in paragraphs (p) and (q) of the previous NPRM (76 FR 12619, March 8, 2011). SWA stated that both paragraphs specify doing external detailed and eddy current inspections of the crown area and other known areas of fuselage skin cracking in accordance with Part 1 of the Accomplishment Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009. SWA noted that Part 1 provides doing a GVI with a reduced inspection interval of 1,000 flight cycles in non-lap splice areas as an alternative to the detailed inspection. SWA and British Airways stated that the previous NPRM does not identify the reference in Note (b) of Table 1 of paragraph 1.E., “Compliance,” of that service bulletin, which allows performing a GVI at a reduced inspection interval in lieu of the detailed inspection in areas away from the lapjoints. SWA also asked if the GVI applies to Figures 32 through 37 of this service bulletin.

We agree with the commenters' requests because we have determined that doing the GVI at a reduced inspection interval provides an acceptable level of safety. In addition, the GVI does apply to Figures 32 through 37 of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009, because the figures are part of the Accomplishment Instructions, which is the part of the service bulletin referred to in the requirements. We have revised paragraphs (p), (q), and (s) of this supplemental NPRM to allow this option.

Request To Allow Use of Corrosion Inhibiting Compound

US Airways asked that we allow the use of an alternative corrosion inhibiting compound (CIC) during repairs. US Airways stated that paragraphs (u) and (w) of the previous NPRM (76 FR 12619, March 8, 2011) refer to Parts 7 and 8 of the Accomplishment Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009; Parts 7 and 8 refer to Figures 10 and 24, respectively. US Airways added that the figures specify applying CIC BMS 3-23, Type 2, when completing repairs. US Airways noted that Boeing has approved newer CIC BMS 3-35, which is compatible with CIC BMS 3-23, and is identified in the applicable corrosion prevention manual. US Airways added that Boeing does not plan to add CIC BMS 3-35 to its FAA-approved repair documents. US Airways stated that adding this to the previous NPRM would avoid requests for approval of AMOCs.

We agree that CIC BMS 3-35 can be used as an alternative to CIC BMS 3-23. CIC BMS 3-35 has been qualified and approved as an AMOC for other repair situations that specify CIC BMS 3-23. We have added a new statement to paragraphs (u) and (w) of this supplemental NPRM to include the option of using CIC BMS 3-35 as an equivalent CIC.

Changes to Supplemental NPRM

The exception in paragraph (o) of this supplemental NPRM was included in the requirements of AD 2004-18-06, Amendment 39-13784 (69 FR 54206, September 8, 2004); however, it was inadvertently removed from the proposed requirements in the previous NPRM (76 FR 12619, March 8, 2011). We have included the exception in paragraphs (g) and (h) of this supplemental NPRM.

We have removed the reference to the Boeing 737 Non-Destructive Test Manual specified in paragraph (o) of this supplemental NPRM; the inspection identified in that paragraph may be done using Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009.

We have removed the definition of a “detailed inspection” in Note 1 of the previous NPRM (76 FR 12619, March 8, 2011) from this supplemental NPRM. That definition is provided in Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009.

Due to a typographical error in paragraph (w) of the previous NPRM (76 FR 12619, March 8, 2011), we have changed the reference in paragraph (w) of this supplemental NPRM to paragraph (u), instead of paragraph (p) or (q), respectively.

We have revised the optional repair method specified in paragraph (k) of this supplemental NPRM to remove the reference to repairing any cracking per the applicable structural repair manual (SRM) identified in Table 1 of the previous NPRM (76 FR 12619, March 8, 2011). Instead, we have added a statement to paragraph (k) of this supplemental NPRM to specify that guidance on repairing any cracking can be found in the applicable SRM identified in paragraphs (k)(1) through (k)(4) of this supplemental NPRM.

FAA's Determination

We are proposing this supplemental NPRM because we evaluated all the relevant information and determined the unsafe condition described previously is likely to exist or develop in other products of the same type design. Certain changes described above expand the scope of the previous NPRM (76 FR 12619, March 8, 2011). As a result, we have determined that it is necessary to reopen the comment period to provide additional opportunity for the public to comment on this supplemental NPRM.

Proposed Requirements of the Supplemental NPRM

This supplemental NPRM would require the actions proposed in the previous NPRM (76 FR 12619, March 8, 2011). However, this supplemental NPRM reduces the proposed repetitive inspection intervals and the compliance time for the inspections of the crown area and other known areas of fuselage skin cracking in the previous NPRM (76 FR 12619, March 8, 2011).

Costs of Compliance

There are about 903 airplanes of U.S. registry affected by AD 2004-18-06, Amendment 39-13784 (69 FR 54206, September 8, 2004).

The inspections of the crown area that are required by AD 2004-18-06, Amendment 39-13784 (69 FR 54206, September 8, 2004), take about 94 work hours per airplane to accomplish, at an average labor rate of $85 per work hour. Based on these figures, the estimated cost of the currently required inspections is $7,990 per airplane, per inspection cycle.

The inspections of the lower lobe area that are required by AD 2004-18-06, Amendment 39-13784 (69 FR 54206, September 8, 2004), take about 96 work hours per airplane to accomplish, at an average labor rate of $85 per work hour. Based on these figures, the estimated cost of the currently required inspections is $8,160 per airplane, per inspection cycle.

Should an operator elect to install the preventive modification specified in AD 2004-18-06, Amendment 39-13784 (69 FR 54206, September 8, 2004), it will take about 108 work hours per airplane to accomplish, at an average labor rate of $85 per work hour. Based on these figures, the estimated cost of the modification is $9,180 per airplane.

The new proposed inspections would affect about 701 airplanes of U.S. registry.

The new proposed inspections would take about 27 work hours per airplane, at an average labor rate of $85 per work hour. Based on these figures, the estimated cost of the new actions specified in this proposed AD for U.S. operators is $1,608,795, or $2,295 per airplane, per inspection cycle.

Authority for This Rulemaking

Title 49 of the United States Code specifies the FAA's authority to issue rules on aviation safety. Subtitle I, section 106, describes the authority of the FAA Administrator. “Subtitle VII: Aviation Programs” describes in more detail the scope of the Agency's authority.

We are issuing this rulemaking under the authority described in Subtitle VII, Part A, Subpart III, Section 44701: “General requirements.” Under that section, Congress charges the FAA with promoting safe flight of civil aircraft in air commerce by prescribing regulations for practices, methods, and procedures the Administrator finds necessary for safety in air commerce. This regulation is within the scope of that authority because it addresses an unsafe condition that is likely to exist or develop on products identified in this rulemaking action.

Regulatory Findings

We determined that this proposed AD would not have federalism implications under Executive Order 13132. This proposed AD 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.

For the reasons discussed above, I certify that the 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),

(3) Will not affect intrastate aviation in Alaska, and

(4) 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.

List of Subjects in 14 CFR Part 39

• Air transportation
• Aircraft
• Aviation safety
• Incorporation by reference
• Safety

The Proposed Amendment

Accordingly, under the authority delegated to me by the Administrator, the FAA proposes to amend 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.

§ 39.13

[Amended]

2. The FAA amends § 39.13 by removing AD 2004-18-06, Amendment 39-13784 (69 FR 54206, September 8, 2004), and adding the following new AD:

The Boeing Company: Docket No. FAA-2011-0155; Directorate Identifier 2009-NM-141-AD.

(a) Comments Due Date

We must receive comments by November 26, 2012.

(b) Affected ADs

(1) This AD supersedes AD 2004-18-06, Amendment 39-13784 (69 FR 54206, September 8, 2004).

(2) AD 2002-07-08, Amendment 39-12702 (67 FR 17917, April 12, 2002); and AD 2003-14-06, Amendment 39-13225 (68 FR 42956, July 21, 2003); and AD 2009-21-01, Amendment 39-16038 (74 FR 52395, October 13, 2009); affect this AD.

(c) Applicability

This AD applies to The Boeing Company Model 737-200, -200C, -300, -400, and -500 series airplanes, certificated in any category, as listed in Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009.

(d) Subject

Joint Aircraft System Component (JASC)/Air Transport Association (ATA) of America Code 53, Fuselage.

(e) Unsafe Condition

This AD was prompted by new findings of vertical cracks along chem-milled steps adjacent to the butt joints. We are issuing this AD to detect and correct fatigue cracking of the skin panels, which could result in sudden fracture and failure of the skin panels of the fuselage, and consequent rapid decompression of the airplane.

(f) Compliance

Comply with this AD within the compliance times specified, unless already done.

(g) Retained External Detailed and Eddy Current Inspections

This paragraph restates the requirements of paragraph (a) of AD 2004-18-06, Amendment 39-13784 (69 FR 54206, September 8, 2004) with revised service information. For Groups 1 through 5 airplanes identified in Boeing Alert Service Bulletin 737-53A1210, Revision 1, dated October 25, 2001: Before the accumulation of 35,000 total flight cycles, or within 4,500 flight cycles after October 13, 2004 (the effective date of AD 2004-18-06), whichever is later, do external detailed and eddy current inspections of the crown area and other known areas of fuselage skin cracking, in accordance with Part 1 and Figure 1 of the Work Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 1, dated October 25, 2001; or in accordance with Part 1 of the Accomplishment Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009; except as provided by paragraph (o) of this AD. Repeat the external detailed and eddy current inspections at intervals not to exceed 4,500 flight cycles until paragraph (i), (j)(1)(ii), (k), (l), or (m) of this AD has been done, as applicable. Although paragraph 1.D. of Boeing Alert Service Bulletin 737-53A1210, Revision 1, dated October 25, 2001, references a reporting requirement, such reporting is not required by this AD. Accomplishing the actions required by paragraph (p) or (q) of this AD ends the repetitive requirements in this paragraph.

(h) Retained External Detailed Inspection With Reduced Compliance Time

This paragraph restates the requirements of paragraph (b) of AD 2004-18-06, Amendment 39-13784 (69 FR 54206, September 8, 2004) with revised service information. For all airplanes identified in Boeing Alert Service Bulletin 737-53A1210, Revision 1, dated October 25, 2001, do an external detailed inspection of the lower lobe area and section 41 of the fuselage for cracking, in accordance with Part 2 and Figure 2 of the Work Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 1, dated October 25, 2001; or in accordance with Part 4 of the Accomplishment Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009; except as provided by paragraph (o) of this AD. At the earlier of the times specified in paragraphs (h)(1) and (h)(2) of this AD, do the inspection specified in this paragraph, and repeat the inspection thereafter at intervals not to exceed 4,500 flight cycles until paragraph (j)(2) or paragraph (k), as applicable, of this AD has been done. Accomplishing the actions required by paragraph (s) of this AD ends the requirements in this paragraph.

(1) Within 9,000 flight cycles after doing the most recent external detailed inspection.

(2) Within 4,500 flight cycles after October 13, 2004 (the effective date of AD 2004-18-06, Amendment 39-13784 (69 FR 54206, September 8, 2004)), or before the accumulation of 40,000 total flight cycles, whichever occurs later.

(i) Retained Preventive Modification at Stringer 12

This paragraph restates the requirements of paragraph (c) of AD 2004-18-06, Amendment 39-13784 (69 FR 54206, September 8, 2004) with revised service information. For Groups 3 and 5 airplanes identified in Boeing Alert Service Bulletin 737-53A1210, Revision 1, dated October 25, 2001: If no cracking is found during any inspection required by paragraph (g) of this AD, doing the preventive modification of the chem-milled pockets in the upper skin as specified in Part 5 of the Work Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 1, dated October 25, 2001; or as specified in Part 7 of the Work Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009, except as required by paragraph (x) of this AD; ends the repetitive external detailed and eddy current inspections required by paragraph (g) of this AD for the modified area only. As of the effective date of this AD, use only Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009.

(j) Retained Corrective Actions

This paragraph restates the requirements of paragraph (d) of AD 2004-18-06, Amendment 39-13784 (69 FR 54206, September 8, 2004), with revised service information. If any cracking is found during any inspection required by paragraph (g), (h), (p), (q), or (s) of this AD, before further flight, do the actions specified in paragraphs (j)(1) and (j)(2) of this AD, as applicable, in accordance with the Work Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 1, dated October 25, 2001; or in accordance with the Work Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009. As of the effective date of this AD, use only Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009. Where Boeing Alert Service Bulletin 737-53A1210, Revision 1, dated October 25, 2001; or Revision 3, dated July 16, 2009; specify to contact Boeing for repair instructions, before further flight, repair in accordance with a method approved by the Manager, Seattle Aircraft Certification Office (ACO), FAA; or in accordance with data meeting the type certification basis of the airplane if it is approved by the Boeing Commercial Airplanes Organization Designation Authorization (ODA) or any other person authorized by the Manager, Seattle ACO, to make those findings. For a repair method to be approved the repair must meet the certification basis of the airplane and the approval must specifically refer to this AD.

(1) Except as provided by paragraph (k) of this AD, for cracking of the crown area, do the repair specified in either paragraph (j)(1)(i) or (j)(1)(ii) of this AD.

(i) Do a time-limited repair in accordance with Part 4 of the Work Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 1, dated October 25, 2001; or in accordance with Part 6 of the Work Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009, except as required by paragraph (x) of this AD, then do the actions required by paragraph (l) of this AD at the times specified in that paragraph.

(ii) Do a permanent repair in accordance with Part 3 of the Work Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 1, dated October 25, 2001; or in accordance with Part 5 of the Work Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009. Installation of a permanent repair ends the repetitive inspections required by paragraph (g) of this AD for the repaired area only. Installation of the lap joint repair specified in paragraph (g) of AD 2002-07-08, Amendment 39-12702 (67 FR 17917, April 12, 2002), is considered acceptable for compliance with the corresponding permanent repair specified in this paragraph for the repaired areas only.

(2) Except as provided by paragraph (k) of this AD, for cracking of the lower lobe area and Section 41, repair in accordance with Part 2 of the Work Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 1, dated October 25, 2001; or in accordance with paragraph (j)(2)(i) or (j)(2)(ii) of this AD. Accomplishment of this repair ends the repetitive inspections required by paragraph (h) of this AD for the repaired area only. As of the effective date of this, do the repair specified in paragraph (j)(2)(i) or (j)(2)(ii) of this AD.

(i) Do a time-limited repair in accordance with Part 6 of the Work Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009, except as required by paragraph (x) of this AD, then do the actions required by paragraph (l) of this AD at the times specified in that paragraph.

(ii) Do a permanent repair in accordance with Part 5 of the Accomplishment Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009.

(k) Retained Optional Repair Method

This paragraph restates the requirements of paragraph (e) of AD 2004-18-06, Amendment 39-13784 (69 FR 54206, September 8, 2004) with revised service information. For cracking in any area specified in paragraphs (j)(1) and (j)(2) of this AD within the limitations of the applicable structural repair manual (SRM) specified in paragraphs (k)(1) through (k)(4) of this AD, repair any cracks, in accordance with a method approved by the Manager, Seattle ACO; or in accordance with data meeting the type certification basis of the airplane if it is approved by the Boeing Commercial Airplanes ODA or any other person authorized by the Manager, Seattle ACO, to make those findings. For a repair method to be approved the repair must meet the certification basis of the airplane and the approval must specifically refer to this AD. Accomplishment of the applicable repair terminates the repetitive inspections required by paragraphs (g) and (h) of this AD for the repaired area only. Guidance on repairing the cracking can be found in the applicable SRM specified in paragraphs (k)(1) through (k)(4) of this AD.

(1) For Model 737-100, -200 series airplanes, Subject 53-30-3, Figure 48, of Boeing 737-100/-200 SRM D6-15565, Revision 102, dated September 10, 2010.

(2) For Model 737-300 series airplanes, Subject 53-00-01, Figure 229, of Boeing 737-300 SRM D6-37635, Revision 92, dated November 10, 2010.

(3) For Model 737-400 series airplanes, Subject 53-00-01, Figure 231, of Boeing 737-400 SRM, D6-38246, Revision 75, dated November 10, 2010.

(4) For Model 737-500 series airplanes, Subject 53-00-01, Figure 229, of Boeing 737-500 SRM, D6-38441, Revision 70, dated November 10, 2010.

(l) Retained Follow-on and Corrective Actions

This paragraph restates the requirements of paragraph (f) of AD 2004-18-06, Amendment 39-13784 (69 FR 54206, September 8, 2004) with revised service information. If a time-limited repair is done, as specified in paragraph (j)(1)(i) or (j)(2)(i) of this AD: Do the actions specified in paragraphs (l)(1), (l)(2), and (l)(3) of this AD, at the times specified in paragraphs (l)(1), (l)(2), and (l)(3) of this AD, in accordance with the Work Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 1, dated October 25, 2001; or in accordance with the Work Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009.

(1) Within 3,000 flight cycles after doing the repair: Do the actions specified in paragraph (l)(1)(i) or (l)(1)(ii) of this AD. Then repeat the applicable inspection specified in paragraph (l)(1)(i) or (l)(1)(ii) of this AD at intervals not to exceed 3,000 flight cycles until permanent rivets are installed in the repaired area, which ends the repetitive inspections for this paragraph. As of the effective date of this AD, do only the inspections specified in paragraph (l)(1)(ii) of this AD.

(i) For repairs done before the effective date of this AD: Do a detailed inspection of the repaired area for loose fasteners in accordance with Part 4 of the Work Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 1, dated October 25, 2001, or do the actions specified in paragraph (l)(1)(ii) of this AD. If any loose fastener is found, before further flight, replace with a new fastener, in accordance with the Work Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 1, dated October 25, 2001.

(ii) For repairs done after the effective date of this AD: Do a detailed inspection of the repaired area for loose, damaged, and missing fasteners, in accordance with Part 6 of the Work Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009. If any loose, missing, or damaged fastener is found, before further flight, replace with a new fastener, in accordance with Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009.

(2) At the applicable time specified in paragraph (l)(2)(i) and (l)(2)(ii) of this AD: Do inspections of the repaired area for cracking in accordance with Part 4 of the Work Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 1, dated October 25, 2001; or in accordance with Part 6 of the Work Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009. If any cracking is found, before further flight, repair in accordance with a method approved by the Manager, Seattle ACO, or in accordance with data meeting the type certification basis of the airplane if it is approved by the Boeing Commercial Airplanes ODA or any other person authorized by the Manager, Seattle ACO, to make those findings. For a repair method to be approved the repair must meet the certification basis of the airplane and the approval must specifically refer to this AD.

(i) For repairs done before the effective date of this AD: Within 4,000 flight cycles after doing the repair, do the inspections.

(ii) For repairs done on or after the effective date of this AD: Within 3,000 flight cycles after doing the repair, do the inspections.

(3) At the earlier of the times specified in paragraphs (l)(3)(i) and (l)(3)(ii) of this AD: Make the repair permanent in accordance with Part 4 and Figure 20 of the Work Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 1, dated October 25, 2001, or do the permanent repair, in accordance with Part 5 of the Work Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009, which ends the repetitive inspections for the repaired area only. As of the effective date of this AD, only Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009, can be used to make the repair permanent.

(i) Within 10,000 flight cycles after doing the repair in accordance with Boeing Alert Service Bulletin 737-53A1210, Revision 1, dated October 25, 2001.

(ii) At the later of the times specified in paragraphs (l)(3)(ii)(A) and (l)(3)(ii)(B) of this AD.

(A) Within 6,000 flight cycles after doing the repair.

(B) Within 1,000 flight cycles after the effective date of this AD.

(m) Retained Optional Terminating Action for Repetitive Eddy Current Inspections

This paragraph restates the requirements of paragraph (g) of AD 2004-18-06, Amendment 39-13784 (69 FR 54206, September 8, 2004) with revised service information. Accomplishment of paragraph (b) or (c), as applicable, of AD 2003-14-06, Amendment 39-13225 (68 FR 42956, July 21, 2003), before the effective date of this AD ends the repetitive eddy current inspections required by paragraph (g) of this AD for that skin panel only; however, the repetitive external detailed inspections required by paragraph (g) of this AD are still required for all areas. Accomplishing paragraph (b) or (c), as applicable, of AD 2003-14-06, on or after the effective date of this AD, does not end either the repetitive detailed or eddy current inspections required by paragraph (g) of this AD.

(n) Retained Credit for Previous Actions

This paragraph restates the requirements of paragraph (h) of AD 2004-18-06, Amendment 39-13784 (69 FR 54206, September 8, 2004). Inspections, repairs, and preventive modifications done before October 13, 2004 (the effective date of AD 2004-18-06), in accordance with Boeing Alert Service Bulletin 737-53A1210, dated December 14, 2000, are acceptable for compliance with the corresponding actions required by paragraphs (g), (h), (i), (j), (k), and (l) of this AD.

(o) Retained Exception to Service Bulletin Procedures

This paragraph restates the requirements of paragraph (i) of AD 2004-18-06, Amendment 39-13784 (69 FR 54206, September 8, 2004) with revised service information. For airplanes subject to the requirements of paragraphs (g) and (h) of this AD: Inspections are not required in areas that are spanned by an FAA-approved repair that has a minimum of 3 rows of fasteners above and below the chem-milled step. If an external doubler covers the chem-milled step, but does not span it by a minimum of 3 rows of fasteners above and below, in lieu of requesting approval for an alternative method of compliance (AMOC), one option to comply with the inspection requirement of paragraphs (g) and (h) of this AD is to inspect all chem-milled steps covered by the repair in accordance with the Work Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009.

(p) New Requirements of This AD: Repetitive External Detailed and Eddy Current Inspections of the Crown Area and Other Known Areas of Fuselage Skin Cracking, and Corrective Actions

For Groups 1 through 5 and Groups 9 through 21 airplanes identified in Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009, on which the inspections required by paragraph (g) of this AD have been done before the effective date of this AD: Within 4,500 flight cycles after doing the most recent inspection required by paragraph (g) of this AD, or within 1,800 flight cycles after the effective date of this AD, whichever is earlier; do external detailed and eddy current inspections of the crown area and other known areas of the fuselage skin cracking, in accordance with Part 1 of the Accomplishment Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009; except as provided by paragraph (o) of this AD. Repeat the external detailed and eddy current inspections thereafter at intervals not to exceed 1,800 flight cycles. Accomplishing the inspections required by this paragraph ends the repetitive inspections required by paragraph (g) of this AD. Before further flight, do all applicable corrective actions as specified in paragraph (j) of this AD. For the locations specified in Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009; in lieu of doing detailed inspections, operators may do general visual inspections, provided that the general visual inspections are done at intervals not to exceed 1,000 flight cycles.

(q) New Requirements of This AD: Repetitive External Detailed and Eddy Current Inspections of the Crown Area and Other Known Areas of Fuselage Skin Cracking, and Corrective Actions

For Groups 1 through 5 and 9 through 21 airplanes identified in Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009; on which the inspections required by paragraph (g) of this AD have not been done before the effective date of this AD: Before the accumulation of 28,000 total flight cycles, or within 1,800 flight cycles after the effective date of this AD, whichever is later, do external detailed and eddy current inspections of the crown area and other known areas of fuselage skin cracking, in accordance with Part 1 of the Accomplishment Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009, except as provided by paragraph (o) of this AD. Repeat the external detailed and eddy current inspections thereafter at intervals not to exceed 1,800 flight cycles. Accomplishing the inspections required by this paragraph ends the repetitive inspections required by paragraph (g) of this AD. Before further flight, do all applicable corrective actions as specified in paragraph (j) of this AD. For the locations specified in Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009; in lieu of doing detailed inspections, operators may do general visual inspections, provided that the general visual inspections are done at intervals not to exceed 1,000 flight cycles.

(r) New Requirements of This AD: Repetitive External Detailed and Eddy Current Inspections of the Fuselage Skin Along the Chem-Milled Steps of the Butt Joints, and Corrective Actions

For Group 1 through 5, and 9 through 21 airplanes identified in Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009: At the later of the times specified in paragraphs (r)(1) and (r)(2) of this AD, do external detailed and eddy current inspections for vertical cracks in the fuselage skin along the chem-milled steps of the butt joints, in accordance with Part 3 of the Accomplishment Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009. Repeat the inspections thereafter at intervals not to exceed 1,800 flight cycles or 1,800 flight hours, whichever occurs first. If any cracking is found, before further flight, repair in accordance with Part 5 of the Accomplishment Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009. Doing the repair terminates the repetitive inspections specified in this paragraph for the repaired area only.

(1) Before the accumulation of 55,000 total flight cycles or 55,000 total flight hours, whichever occurs first.

(2) Within 1,800 flight cycles or 1,800 flight hours after the effective date of this AD, whichever occurs first.

(s) New Requirements of This AD: Repetitive Detailed and Eddy Current Inspections Along the Chem-Milled Lines of the Fuselage Skin of the Lower Lobe Area and Section 41, and Corrective Actions

For Groups 1 through 21 airplanes identified in Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009: At the applicable time specified in paragraph (s)(1) or (s)(2) of this AD, do external detailed and eddy current inspections, as applicable, for horizontal cracks along the chem-milled lines of the fuselage skin of the lower lobe area and section 41, in accordance with Part 4 of the Accomplishment Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009; except as required by paragraph (x) of this AD. Repeat the inspections thereafter at intervals not to exceed 1,800 flight cycles. Accomplishing the inspections required by this paragraph ends the repetitive inspections required by paragraph (h) of this AD. Before further flight, do all applicable corrective actions as specified in paragraph (j) of this AD. For the locations specified in Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009; in lieu of doing detailed inspections, operators may do general visual inspections, provided that the general visual inspections are done at intervals not to exceed 1,000 flight cycles.

(1) In areas of no known cracking where only a detailed inspection is accomplished: Do the inspection at the later of the times specified in paragraphs (s)(1)(i) and (s)(1)(ii) of this AD.

(i) Before the accumulation of 35,000 total flight cycles.

(ii) Within 9,000 flight cycles after the most recent inspection required by paragraph (h) of this AD, or within 1,800 flight cycles after the effective date of this AD, whichever is earlier.

(2) In areas of known cracking where both a detailed and eddy current inspection is accomplished: Do the inspection at the latest of the times specified in paragraphs (s)(2)(i) and (s)(2)(ii) of this AD.

(i) Before the accumulation of 35,000 total flight cycles.

(ii) Within 4,500 flight cycles after the most recent inspection required by paragraph (h) of this AD, or within 1,800 flight cycles after the effective date of this AD, whichever is earlier.

(t) New Requirements of This AD: Repetitive External Detailed and Eddy Current Inspections Along the Chem-Milled Lines of the Fuselage Skin of the Window Belt Area, and Corrective Actions

For Groups 4, 11, and 16 airplanes identified in Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009: Before the accumulation of 25,000 total flight cycles or within 1,800 flight cycles after the effective date of this AD, whichever is later, do external detailed and eddy current inspections for horizontal cracks along the chem-milled lines of the fuselage skin of the fuselage window belt area, in accordance with Part 2 of the Accomplishment Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009. Repeat the inspections thereafter at intervals not to exceed 1,800 flight cycles. If any cracking is found, before further flight, repair using a method approved in accordance with paragraph (y) of this AD. Doing the repair terminates the repetitive inspections specified in this paragraph for the repaired area only.

(u) New Requirements of This AD: Repetitive External Detailed and Eddy Current Inspections Along the Chem-Milled Lines of the Fuselage Skin of the Fuselage Window Belt Area, and Corrective Actions

For Groups 3, 5, 9, 10, 12, 14, 15, 17, 18, 19, 20, and 21 airplanes identified in Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009: Do the actions specified in paragraph (u)(1) or (u)(2) of this AD, as applicable. Part 7 (Figure 10) of the Accomplishment Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009, specifies applying corrosion inhibiting compound (CIC) BMS 3-23 to the surfaces of the repaired area. As an option to using CIC BMS 3-23, operators may use CIC BMS 3-35, which is equivalent to CIC BMS 3-23.

(1) For airplanes on which the inspections required by paragraph (g) of this AD have been done before the effective date of this AD: Within 4,500 flight cycles after doing the most recent inspection required by paragraph (g) of this AD, or within 1,800 flight cycles after the effective date of this AD, whichever is earlier, do external detailed and eddy current inspections for horizontal cracks along the chem-milled lines of the fuselage skin of the fuselage window belt area, in accordance with Part 2 of the Accomplishment Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009. Repeat the inspections thereafter at intervals not to exceed 1,800 flight cycles. If any cracking is found, before further flight, repair in accordance with Part 8 of the Accomplishment Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009, except as required by paragraph (x) of this AD.

(2) For airplanes on which the inspections required by paragraph (g) of this AD have not been done before the effective date of this AD: Before the accumulation of 25,000 total flight cycles or within 1,800 flight cycles after the effective date of this AD, whichever is later, do external detailed and eddy current inspections for horizontal cracks along the chem-milled lines of the fuselage skin of the fuselage window belt area, in accordance with Part 2 of the Accomplishment Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009. Repeat the inspections thereafter at intervals not to exceed 1,800 flight cycles. If any cracking is found, before further flight, repair in accordance with Part 8 of the Accomplishment Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009, except as required by paragraph (x) of this AD.

(v) New Optional Repair

For airplanes on which cracking is found during any inspection required by paragraph (p), (q), (r), or (s) of this AD, as applicable, doing the repair of the chem-milled area in the skin, as specified in Part 5 or Part 6 of the Accomplishment Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009, ends the repetitive external detailed and eddy current inspections required by paragraph (p), (q), (r), or (s) of this AD, as applicable, for the repaired area only.

Note 1 to paragraph (v) of this AD: Part 8 of the Accomplishment Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009, specifies a post-repair inspection of the skin chem-milled crack repair at stringer 12; that inspection is not required by this AD. The damage tolerance inspections specified in Table 7 of paragraph 1.E., “Compliance,” of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009, may be used in support of compliance with section 121.1109(c)(2) or 129.109(c)(2) of the Code of Federal Regulations (14 CFR 121.1109(c)(2) or 14 CFR 129.109(c)(2)).

(w) New Optional Preventive Modification at Stringer 12

For airplanes on which no cracking is found during any inspection required by paragraph (u) of this AD, doing the preventive modification of the chem-milled areas in the skin at stringer 12, as specified in Part 7 of the Accomplishment Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009, except as required by paragraph (x) of this AD, ends the repetitive external detailed and eddy current inspections required by paragraph (u) of this AD, for the modified areas common to stringer 12 only. Part 7 (Figure 10) of the Accomplishment Instructions of Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009, specifies applying CIC BMS 3-23 to the surfaces of the repaired area. As an option to using CIC BMS 3-23, operators may use CIC BMS 3-35, which is equivalent to CIC BMS 3-23.

(x) Exception to Service Information

Where Boeing Alert Service Bulletin 737-53A1210, Revision 3, dated July 16, 2009, specifies to contact Boeing for repair instructions, before further flight, repair using a method approved in accordance with paragraph (y) of this AD.

(y) Alternative Methods of Compliance (AMOCs)

(1) The Manager, Seattle ACO, FAA, has the authority to approve AMOCs for this AD, if requested using the procedures found in 14 CFR 39.19. In accordance with 14 CFR 39.19, send your request to your principal inspector or local Flight Standards District Office, as appropriate. If sending information directly to the manager of the ACO, send it to the attention of the person identified in the Related Information section of this AD. Information may be emailed to: [e-mail].

(2) Before using any approved AMOC, notify your appropriate principal inspector, or lacking a principal inspector, the manager of the local flight standards district office/certificate holding district office.

(3) An AMOC that provides an acceptable level of safety may be used for any repair required by this AD if it is approved by the Boeing Commercial Airplanes ODA that has been authorized by the Manager, Seattle ACO, to make those findings. For a repair method to be approved, the repair must meet the certification basis of the airplane, and the approval must specifically refer to this AD.

(4) AMOCs approved previously in accordance with AD 2004-18-06, Amendment 39-13784 (69 FR 54206, September 8, 2004), are approved as AMOCs for the corresponding provisions of this AD.

(5) Inspections and corrective actions required by paragraph (g) of AD 2009-21-01, Amendment 39-16038 (74 FR 52395, October 13, 2009), are approved as AMOCs for the corresponding provisions of paragraph (s) of this AD; but only for the areas of the lower lobe skin identified in AD 2009-21-01.

(z) Related Information

(1) For more information about this AD, contact Wayne Lockett, Aerospace Engineer, Airframe Branch, ANM-120S, FAA, Seattle Aircraft Certification Office, 1601 Lind Avenue SW., Renton, Washington 98057-3356; phone: [phone]; fax: [phone]; email: [e-mail].

(2) For service information identified in this AD, contact Boeing Commercial Airplanes, Attention: Data & Services Management, P.O. Box 3707, MC 2H-65, Seattle, Washington 98124-2207; telephone [phone], extension 1; fax [phone]; Internet https://www.myboeingfleet.com.

(3) You may review copies of the referenced service information at the FAA, Transport Airplane Directorate, 1601 Lind Avenue SW., Renton, Washington. For information on the availability of this material at the FAA, call 425-227-1221.

Issued in Renton, Washington, on September 28, 2012.

Ali Bahrami,

Manager, Transport Airplane Directorate, Aircraft Certification Service.