E6-22462. Airworthiness Directives; Boeing Model 737-200, -300, -400, and -500 Series Airplanes

AGENCY:

Federal Aviation Administration (FAA), Department of Transportation (DOT).

ACTION:

Final rule.

SUMMARY:

The FAA is adopting a new airworthiness directive (AD) for certain Boeing Model 737-200, -300, -400, and -500 series airplanes. This AD requires a one-time inspection of the frames between station 360 and station 907 to determine if a subject support bracket for the air conditioning outlet extrusion is installed, and related repetitive investigative actions and repair if necessary. This AD also provides an optional preventive modification that ends the repetitive investigative actions. This AD also requires a one-time post-modification/repair inspection for cracking of each repaired/modified frame. This AD results from numerous reports indicating that frame cracks have been found at the attachment holes for support brackets for the air conditioning outlet extrusion. We are issuing this AD to detect and correct such cracking, which, if the cracking were to continue to grow, could result in a severed frame. A severed frame, combined with existing multi-site damage at the stringer 10 lap splice, could result in rapid decompression of the airplane.

DATES:

This AD becomes effective February 8, 2007.

The Director of the Federal Register approved the incorporation by reference of a certain publication listed in the AD as of February 8, 2007.

ADDRESSES:

You may examine the AD docket on the Internet at http://dms.dot.gov or in person at the Docket Management Facility, U.S. Department of Transportation, 400 Seventh Street, SW., Nassif Building, Room PL-401, Washington, DC.

Contact Boeing Commercial Airplanes, P.O. Box 3707, Seattle, Washington 98124-2207, for the service information identified in this AD.

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; telephone [phone]; fax [phone].

SUPPLEMENTARY INFORMATION:

Examining the Docket

You may examine the airworthiness directive (AD) docket on the Internet at http://dms.dot.gov or in person at the Docket Management Facility office between 9 a.m. and 5 p.m., Monday through Friday, except Federal holidays. The Docket Management Facility office (telephone [phone]) is located on the plaza level of the Nassif Building at the street address stated in the ADDRESSES section.

Discussion

The FAA issued a notice of proposed rulemaking (NPRM) to amend 14 CFR part 39 to include an AD that would apply to certain Boeing Model 737-200, -300, -400, and -500 series airplanes. That NPRM was published in the Federal Register on October 6, 2005 (70 FR 58358). That NPRM proposed to require a one-time inspection of frames between station 360 and station 907 to determine if a subject support bracket for the air conditioning outlet extrusion is installed, and related repetitive investigative actions and repair if necessary. That NPRM also proposed to provide an optional preventive modification that would end the repetitive investigative actions. That NPRM also proposed to require a one-time post-modification/repair inspection for cracking of each repaired/modified frame.

Comments

We provided the public the opportunity to participate in the development of this AD. We have considered the comments received.

Request To Extend Certain Compliance Times

KLM Royal Dutch Airlines (KLM), and the Air Transport Association (ATA), on behalf of United Airlines (UAL) and US Airways, ask that the compliance time for the inspection be changed to coincide with scheduled maintenance checks.

UAL notes that the 6,000-flight-cycle interval for the post-modification/repair inspection (between 18,000 and 24,000 flight cycles) does not fall into a compatible maintenance opportunity. UAL states that, when given the opportunity by Boeing to review the preliminary service bulletin, the requirement for this inspection was “within 30,000 flight cycles.” UAL asks if there is an alternative inspection method, such as an open hole eddy current inspection, which would extend the 6,000-flight-cycle repetitive inspection interval to 9,000 flight cycles to align with a heavy maintenance check.

US Airways adds that the repeat inspection interval will have an adverse impact on operations. US Airways also adds that the repeat inspection interval seems to be arbitrary and unreasonable, and it imposes undue costs to the airline. US Airways has been addressing this issue since 1999, and notes that the existing maintenance program currently has a repeat inspection interval of 12,500 flight hours or approximately 9,375 flight cycles for the inspection for frame cracks in this location. US Airways adds that the inspection program has proven adequate to find and repair these cracks before they have an adverse impact on the structural integrity of the airplane. US Airways concludes that the increased inspection interval mentioned previously also minimizes impact to fleet operations, while still maintaining a sufficient level of safety. US Airways requests that the repeat inspection interval be increased to align with the existing scheduled heavy maintenance visits.

KLM states that page 3 of the NPRM, under “Relevant Service Information,” specifies a compliance time of 5,000 flight cycles after the date of the service bulletin for the initial inspection, and an interval of 6,000 flight cycles for the repetitive inspections. KLM adds that the inspection is applicable to all frames, which amounts to 35 frames on the left- and right-hand sides, for a total of 70 inspection areas on a Boeing Model 737-300 airplane. Due to the extent of this work, the inspection in the NPRM must be accomplished during a planned maintenance check, preferably a D-check when the support brackets are accessible. Based on the current inspection interval, the inspection must be accomplished during a C-check, which necessitates additional work. KLM asks if we have considered possible cycle interval changes in order to relieve the economic burden of this inspection.

We agree with the commenters' request to extend the inspection interval. We have worked with Boeing to expand the standard analysis methodology to better model service experience. The new analysis methodology allows for longer compliance times and longer grace periods for airplanes that did not have lower row lap splice cracking concerns.

The new compliance times are identified in paragraph 1.E., “Compliance,” of Revision 1 of Boeing Special Attention Service Bulletin 737-53-1216, dated June 8, 2006. The new compliance times for the initial general visual, medium frequency eddy current (MFEC) and high frequency eddy current (HFEC) inspections, as applicable, are prior to the accumulation of 40,000 total flight cycles, or within 5,000 or 9,000 flight cycles (depending on the airplane configuration) after issuance of the service bulletin, whichever occurs later. The service bulletin specifies a repetitive interval (for all subject frames) of 9,000 flight cycles. We have reviewed the procedures in Revision 1 and have determined that they are essentially the same as those in the original issue of the service bulletin (which was referenced in the NPRM). The effectivity section in Revision 01 shows changes of airplane operators; however, Revision 01 does not necessitate additional work. Therefore, we have revised this AD to refer to Revision 1 of the service bulletin as the appropriate source of service information for accomplishing the required actions at the new extended compliance times. We have also added a statement to paragraph (l) of this AD that gives credit for actions accomplished before the effective date of this AD in accordance with the original issue of the service bulletin.

Request To Adopt an Alternative Compliance/Inspection Schedule

Southwest Airlines (SWA) requests that we consider an alternative inspection method—an external detailed visual inspection—that would extend the grace period from 5,000 flight cycles to a total of 10,000 flight cycles, particularly for airplanes that are not susceptible to multi-site damage. SWA notes that the areas of inspection are not easily accessible as those areas are located behind the overhead bins. SWA adds that the majority of operators do not have convenient scheduled maintenance visits that result in access to the interior area behind the overhead bins within a span of 5,000 or 6,000 flight cycles. SWA suggests revising the repetitive inspection requirements (every 6,000 flight cycles) to longer thresholds (every 10,000 flight cycles) for airplanes over 30,000 flight cycles, provided that the external inspections are being accomplished. SWA proposes an alternative inspection option for those airplanes that are not susceptible to multi-site damage, as follows:

• Airplanes with less than 40,000 total flight cycles.
• Airplanes on which Boeing Service Bulletin 737-53A1177, Revision 6, has been done for lap joint repairs, including window belt replacements.
• Airplanes having line numbers 2553 and above, on which the lower row of fasteners of the stringer 10 lap joint is not susceptible to cracking.

SWA provided an example of an alternative compliance/inspection table, which could be used for airplanes having over 30,000 flight cycles.

We agree partially with the commenter's request. As stated previously under “Request to Extend Certain Compliance Times,” we have changed the compliance time in the AD to allow for better maintenance scheduling for operators. However, in order for operators to accomplish an inspection that is not specified in the AD, they must request and receive approval of an alternative method of compliance (AMOC) in accordance with paragraph (m) of this AD. This is necessary so that we can make a specific determination that an alternative inspection does or does not address the identified unsafe condition. If, after reviewing the changes included in this AD, SWA still wants to pursue the alternative inspection proposal, it can request an AMOC.

Request To Change Paragraph (f) of This AD

Boeing asks that the second sentence in paragraph (f) of the NPRM be changed to eliminate the reference to “part number (P/N) 65C7021.” Boeing reiterated the wording in that sentence and suggested it be changed to read, “Subject support brackets are attached to the frame with two rivets.” Boeing states that this change is required because the P/N may not be visible or even exist on the bracket, but the brackets can be easily identified by the number of fasteners attaching them to the frame. The structural detail of concern in the referenced service bulletin is the two fastener attachments. There are some air conditioning brackets (not having P/N 65C7021-( )) attached to the frame with three or more fasteners, but there is no known cracking at these locations.

We agree with the commenter's request for the reasons provided by the commenter. We have changed paragraph (f) of this AD accordingly.

Request To Clarify Which Frames Require Inspection

ATA, on behalf of Alaska Airlines, requests clarification of inspection requirements. Alaska states that the NPRM is not clear on the inspection requirements for the subject frames, and asks that clarification be provided in the final rule. Alaska also asks if access/identification of the brackets at the frame locations specified in the referenced service bulletin is required.

In addition, Alaska asks for clarification of the requirements for the optional preventive modification specified in paragraph (i) of the NPRM. Alaska states that the frames that do not require inspection may have two rivet attachments.

We agree that clarification is needed for the reason provided by the commenter. The frames between stations 360 and 907 that have a support bracket with a two-rivet configuration attached need to be identified and inspected. The specific bracket does not need to be identified by part number. Inspection of the frames at stations 540, 663.75, 685, and 727 is not necessary. In addition, inspection of the frames at stations 616 and 601 on Model 737-200/-300/-400/-500 airplanes and the frames at stations 578 and 601 on Model 737-400 airplanes is not necessary. These frames are not susceptible to cracking at the bracket attachment. The optional preventive modification is not necessary for frames not susceptible to cracking. We have revised paragraph (f) of this AD to clarify the frames that do require an inspection. The change for paragraph (f) of this AD also clarifies the provision for the optional preventive modification as specified in paragraph (i) of this AD.

Request To Include Previously Repaired Frames

United Airlines (UAL) states that neither the referenced service bulletin nor the NPRM addresses the disposition of a frame that has been repaired previously per the structural repair manual (SRM). UAL adds that inspection requirements are included in the service bulletin, but the corrective action necessary for cracking found during an inspection of a frame repaired previously per the SRM is not included. In addition, an option to install a new repair on a frame that was repaired previously per the SRM in order to end the repetitive inspection requirement is not included.

We agree partially with the commenter. We infer that the commenter wants further instruction on corrective action for discrepancies found in previously repaired frames and an option to install a new repair on those frames. We understand that installation of the generic frame repairs described in the SRM may vary extensively, depending on the original damage being repaired; however, guidelines do not exist to allow evaluation of these frame repairs for appropriate follow-on action. We agree that guidelines could be created that would allow the operator to evaluate the frame repair that is installed currently for appropriate follow-on actions. Such guidelines could be evaluated for issuance of an AMOC. Operators may request approval of an AMOC for repairs that are not identified in this AD under the provisions of paragraph (m)(1) of this AD. We have made no change to the AD in this regard.

Request for Credit for Previously Accomplished Actions

ATA, on behalf of Delta Airlines (DAL), states that on August 20, 2002, Boeing issued All Operator Message M-7200-02-01292. The message specifies accomplishing medium frequency eddy current inspections of affected brackets for airplanes with less than 30,000 total flight cycles, or within 5,000 flight cycles after issuance of the message, whichever occurred later. The inspections are to be repeated every 6,000 flight cycles (except where repairs or modifications were installed). The message also describes typical repairs and a terminating modification. DAL adds that neither the NPRM or the referenced service bulletin refer to the message or to the inspections and repairs accomplished per the message. DAL notes that this is a serious omission, as operators have been accomplishing inspections and repairs per the message during the twenty-eight months between issuance of the message and issuance of the referenced service bulletin. DAL states that credit for inspection/repairs and modifications accomplished in accordance with the message should be given in the AD.

We agree with the commenter's request for the reasons provided. We have reviewed Boeing Communication M-7200-02-01292, dated August 20, 2002, and find that the procedures therein are essentially the same as the procedures specified in the referenced service bulletin. Therefore, we have added a new paragraph (j) to the AD, and re-identified subsequent paragraphs, to give credit for actions accomplished before the effective date of this AD per the Boeing communication. The Boeing communication does not specify any post repair or modification inspection, therefore, operators are still required to accomplish those actions required by paragraph (k) of this AD.

Request To Increase Work Hours

KLM, and ATA, on behalf of UAL and U.S. Airways, ask that the work hours included in the Costs of Compliance section of the NPRM be increased.

UAL states that there is an enormous amount of open-up required to do the inspection that is not taken into account in the Costs of Compliance section of the NPRM.

US Airways states that the cost section does not accurately reflect the actual cost of the NPRM to the airline industry. U.S. Airways notes that the frames between station 360 and station 907 are affected by the subject inspection and encompass essentially all of section 43 and section 46 of the airplane. Passenger seats, passenger service units, overhead bins, and sidewall liners must be removed to accommodate the inspection. This excessive teardown of the interior passenger cabin will add considerable downtime to this inspection. These interior passenger cabin items are not routinely removed at the intervals required by the initial inspection, nor the repeat inspection intervals (6,000 flight cycles), identified by the NPRM. Additionally, the Costs of Compliance section does not reflect an accurate time required to perform repairs should any cracks be found. U.S. Airways requests that the Costs of Compliance section be revised to accurately reflect the impact this NPRM would have on the industry by including factors for interior tear down and assembly for the initial and repeat inspections, plus a more accurate downtime cost incurred to accomplish repairs.

KLM states that the work hours specified for the preventive modification and repair specified in the Costs of Compliance section are conservative. The estimated costs are based upon the inspection itself, while all activities to gain access to the support brackets are not taken into account. KLM adds that the work hours required to gain access in accordance with the referenced service bulletin are conservative when taking into account that passenger seats, service units, overhead stowage bins, and sidewall lining need to be removed. KLM requests that a more realistic number of work hours be specified in the Costs of Compliance section.

We do not agree with the commenters' requests. The cost information below describes only the direct costs of the specific actions required by this AD. Based on the best data available, the manufacturer provided the number of work hours (2 work hours per frame) necessary to do the required actions. This number represents the time necessary to perform only the actions actually required by this AD. We recognize that, in doing the actions required by an AD, operators may incur incidental costs in addition to the direct costs. The cost analysis in AD rulemaking actions, however, typically does not include incidental costs such as the time required to gain access and close up, time necessary for planning, or time necessitated by other administrative actions. Those incidental costs, which may vary significantly among operators, are almost impossible to calculate. We have not changed the AD in this regard.

We do not agree that the on-condition costs specified in the NPRM for time required to perform repairs if any cracks are found is inaccurate. As we noted above, the information provided by the manufacturer is the latest information we have, and that information has been used as the time required to perform repairs. We have not changed the AD in this regard.

Conclusion

We have carefully reviewed the available data, including the comments received, and determined that air safety and the public interest require adopting the AD with the changes described previously. These changes will neither increase the economic burden on any operator nor increase the scope of the AD.

Costs of Compliance

There are about 2,131 airplanes of the affected design in the worldwide fleet. This AD affects about 938 airplanes of U.S. registry. The inspection to identify subject support brackets, and subsequent MFEC and HFEC inspections take about 2 work hours per frame, with approximately 32 to 45 frames to be inspected per airplane, at an average labor rate of $65 per work hour. Based on these figures, the estimated cost of the AD for U.S. operators is between $3,902,080 and $5,487,300, or between $4,160 and $5,850 per airplane.

The following table provides the estimated costs for U.S. operators to comply with the inspections of each frame for cracking, the preventive modification, and the repair specified in this AD, at an average labor rate of $65 per work hour. Note that the estimated cost specified in the table is per frame, not per airplane, as it is unknown how many frames on each airplane will have a subject bracket installed.

Estimated On-Condition Costs

Action	Work hours	Parts	Cost per frame
Preventive modification	4	Operator-provided	$260
Repair	6	$608	998

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 have determined that this AD will not have federalism implications under Executive Order 13132. This AD will 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 this AD:

(1) Is not a “significant regulatory action” under Executive Order 12866;

(2) Is not a “significant rule” under DOT Regulatory Policies and Procedures (44 FR 11034, February 26, 1979); and

(3) Will not have a significant economic impact, positive or negative, on a substantial number of small entities under the criteria of the Regulatory Flexibility Act.

We prepared a regulatory evaluation of the estimated costs to comply with this AD and placed it in the AD docket. See the ADDRESSES section for a location to examine the regulatory evaluation.

List of Subjects in 14 CFR Part 39

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

Adoption of the Amendment

Accordingly, under the authority delegated to me by the Administrator, the FAA amends 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 Federal Aviation Administration (FAA) amends § 39.13 by adding the following new airworthiness directive (AD):

2006-26-09 Boeing: Amendment 39-14867. Docket No. FAA-2005-22629; Directorate Identifier 2005-NM-089-AD.

Effective Date

(a) This AD becomes effective February 8, 2007.

Affected ADs

(b) None.

Applicability

(c) This AD applies to Boeing Model 737-200, -300, -400, and -500 series airplanes; certificated in any category; as identified in Boeing Special Attention Service Bulletin 737-53-1216, Revision 1, dated June 8, 2006.

Unsafe Condition

(d) This AD results from numerous reports indicating that frame cracks have been found at the attachment holes for support brackets for the air conditioning outlet extrusion. We are issuing this AD to detect and correct such cracking, which, if the cracking were to continue to grow, could result in a severed frame. A severed frame, combined with existing multi-site damage at the stringer 10 lap splice, could result in rapid decompression of the airplane.

Compliance

(e) You are responsible for having the actions required by this AD performed within the compliance times specified, unless the actions have already been done.

Inspection to Determine Subject Support Brackets

(f) Perform a one-time general visual inspection of the frames between station 360 and station 907 to identify the support brackets for the air conditioning outlet extrusion attached with a two-rivet configuration, in accordance with Part I of the Accomplishment Instructions of Boeing Special Attention Service Bulletin 737-53-1216, Revision 1, dated June 8, 2006. Do this inspection at the applicable time specified in paragraph 1.E., “Compliance,” of the service bulletin, except, where the service bulletin specifies a compliance time after the issuance of the service bulletin, this AD requires compliance within the specified compliance time after the effective date of this AD.

Repetitive Inspections for Cracking

(g) For each frame with a subject support bracket identified during the inspection in accordance with paragraph (f) of this AD: Perform a medium-frequency eddy current inspection for cracking of the frame around the attachment rivets of the support bracket, and a high-frequency eddy current inspection for cracking of the frame adjacent to the inboard fastener hole, by doing all the actions specified in and in accordance with Part I of the Accomplishment Instructions of Boeing Special Attention Service Bulletin 737-53-1216, Revision 1, dated June 8, 2006, except for paragraph 3.B.2. of Part I (which was already done in accordance with paragraph (f) of this AD). Do the initial inspections at the applicable time specified in paragraph 1.E., “Compliance,” of the service bulletin, except, where the service bulletin specifies a compliance time after the issuance of the service bulletin, this AD requires compliance within the specified compliance time after the effective date of this AD. If no cracking is found, repeat the inspections thereafter at intervals not to exceed the repeat interval specified in paragraph 1.E., “Compliance,” of the service bulletin, until paragraph (h) or (i) of this AD is done.

Repair

(h) For any frame in which cracking is found during any inspection required by paragraph (g) of this AD: Before further flight, repair the cracking by doing all applicable actions in accordance with Part III of the Accomplishment Instructions of Boeing Special Attention Service Bulletin 737-53-1216, Revision 1, dated June 8, 2006. Then, do paragraph (k) of this AD, at the time specified in that paragraph. Doing this repair ends the repetitive inspections required by paragraph (g) of this AD for each modified frame.

Optional Preventive Modification

(i) For any frame on which a support bracket for the air conditioning outlet extrusion attached with a two-rivet configuration is installed: Doing all actions associated with the preventive modification in accordance with Part II of the Accomplishment Instructions of Boeing Special Attention Service Bulletin 737-53-1216, Revision 1, dated June 8, 2006, ends the repetitive inspections required by paragraph (g) of this AD for each modified frame. Do the requirements of paragraph (k) of this AD on each modified frame at the time specified in that paragraph.

Actions Accomplished According to Related Service Information

(j) Actions accomplished before the effective date of this AD according to Boeing Communication M-7200-02-01292, dated August 20, 2002; are considered acceptable for compliance with the corresponding actions specified in paragraphs (f), (g), (h), and (i) of this AD, as applicable.

Post-Modification/Repair Inspections

(k) For each frame repaired or modified in accordance with paragraph (h), (i), or (j) of this AD, as applicable: Within 24,000 flight cycles after doing the modification/repair, but after a minimum of 18,000 flight cycles after doing the modification/repair, do one-time detailed inspections for cracking of the repaired/modified frame, air conditioning attach brackets, and stringer clips, by doing all actions in accordance with Part IV of the Accomplishment Instructions of Boeing Special Attention Service Bulletin 737-53-1216, Revision 1, dated June 8, 2006. If any cracking is found during the post-modification/repair inspections, before further flight, repair the cracking using a method approved in accordance with paragraph (m) of this AD.

Actions Accomplished Previously

(l) Inspections/modifications/repairs done before the effective date of this AD in accordance with Boeing Special Attention Service Bulletin 737-53-1216, dated January 27, 2005, are acceptable for compliance with the corresponding actions required by this AD.

Alternative Methods of Compliance (AMOCs)

(m)(1) The Manager, Seattle Aircraft Certification Office (ACO), has the authority to approve AMOCs for this AD, if requested in accordance with the procedures found in 14 CFR 39.19.

(2) An AMOC that provides an acceptable level of safety may be used for any repair required by this AD, if it is approved by an Authorized Representative for the Boeing Commercial Airplanes Delegation Option Authorization Organization who 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.

(3) Before using any AMOC approved in accordance with 14 CFR 39.19 on any airplane to which the AMOC applies, notify the appropriate principal inspector in the FAA Flight Standards Certificate Holding District Office.

Material Incorporated by Reference

(n) You must use Boeing Special Attention Service Bulletin 737-53-1216, Revision 1, dated June 8, 2006, to perform the actions that are required by this AD, unless the AD specifies otherwise. The Director of the Federal Register approved the incorporation by reference of this document in accordance with 5 U.S.C. 552(a) and 1 CFR part 51. Contact Boeing Commercial Airplanes, P.O. Box 3707, Seattle, Washington 98124-2207, for a copy of this service information. You may review copies at the Docket Management Facility, U.S. Department of Transportation, 400 Seventh Street SW., Room PL-401, Nassif Building, Washington, DC; on the Internet at http://dms.dot.gov; or at the National Archives and Records Administration (NARA). For information on the availability of this material at the NARA, call [phone], or go to http://www.archives.gov/​federal_​register/​code_​of_​federal_​regulations/​ibr_​locations.html.

Issued in Renton, Washington, on December 21, 2006.

Ali Bahrami,

Manager, Transport Airplane Directorate, Aircraft Certification Service.