Airworthiness Directives; Boeing Model 737-600, -700, -700C, -800 and -900 Series Airplanes

AGENCY:

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

ACTION:

Notice of proposed rulemaking (NPRM).

SUMMARY:

The FAA proposes to adopt a new airworthiness directive (AD) for certain Boeing Model 737-600, -700, -700C, -800 and -900 series airplanes. This proposed AD would require a one-time general visual inspection of frames between body station (BS) 360 and BS 907 to determine if certain support brackets of the air conditioning (A/C) outlet extrusions are installed; medium- and high-frequency eddy current inspections for cracking of the frames around the attachment holes of the subject brackets; and repair if necessary. This proposed AD would also require installing new, improved fittings for all support brackets of the A/C outlet extrusions between BS 360 and BS 907. This proposed AD results from numerous reports of multiple cracks in the frames around the attachment holes of certain support brackets of the A/C outlet extrusions. We are proposing this AD to detect and correct frame cracking, which, if not corrected, could lead to a severed frame that, combined with cracking of the skin lap splice above stringer 10, could result in rapid decompression of the airplane.

DATES:

We must receive comments on this proposed AD by July 9, 2007.

ADDRESSES:

Use one of the following addresses to submit comments on this proposed AD.

• DOT Docket Web site: Go to http://dms.dot.gov and follow the instructions for sending your comments electronically.
• Government-wide rulemaking Web site: Go to http://www.regulations.gov and follow the instructions for sending your comments electronically.
• Mail: Docket Management Facility, U.S. Department of Transportation, 400 Seventh Street, SW., Nassif Building, Room PL-401, Washington, DC 20590.
• Fax: (202) 493-2251.
• Hand Delivery: Room PL-401 on the plaza level of the Nassif Building, 400 Seventh Street, SW., Washington, DC, between 9 a.m. and 5 p.m., Monday through Friday, except Federal holidays.

Contact Boeing Commercial Airplanes, P.O. Box 3707, Seattle, Washington 98124-2207, for the service information identified in this proposed 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 (425) 917-6447; fax (425) 917-6590.

SUPPLEMENTARY INFORMATION:

Comments Invited

We invite you to submit any relevant written data, views, or arguments regarding this proposed AD. Send your comments to an address listed in the ADDRESSES section. Include the docket number “FAA-2007-28283; Directorate Identifier 2006-NM-254-AD” at the beginning of your comments. We specifically invite comments on the overall regulatory, economic, environmental, and energy aspects of the proposed AD. We will consider all comments received by the closing date and may amend the proposed AD in light of those comments.

We will post all comments we receive, without change, to http://dms.dot.gov , including any personal information you provide. We will also post a report summarizing each substantive verbal contact with FAA personnel concerning this proposed AD. Using the search function of that Web site, anyone can find and read the comments in any of our dockets, including the name of the individual who sent the comment (or signed the comment on behalf of an association, business, labor union, etc.). You may review DOT's complete Privacy Act Statement in the Federal Register published on April 11, 2000 (65 FR 19477-78), or you may visit http://dms.dot.gov .

Examining the Docket

You may examine the 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 (800) 647-5227) is located on the plaza level of the Nassif Building at the DOT street address stated in the ADDRESSES section. Comments will be available in the AD docket shortly after the Docket Management System receives them.

Discussion

We have received numerous reports of multiple cracks in the frame around the attachment holes of certain support brackets of the air conditioning (A/C) outlet extrusions on Model 737-200, -300, -400, and -500 series airplanes. Investigation has revealed that the frame cracks occur due to fatigue caused by a combination of forward and aft loads from fuselage expansion, and down loads from fuselage deflection on the attached structure. The subject frame cracks radiate from one side of the attachment hole, nearest the frame flanges; further, frame cracks up to 0.6 inch long have also been found on the inboard flange of the body station (BS) 907 frame adjacent to the support bracket. With continued fatigue cycling, frame cracking, if not corrected, could lead to a severed frame that, combined with cracking of the skin lap splice above stringer 10, could result in rapid decompression of the airplane.

The subject area on Model 737-600, -700, -700C, -800 and -900 series airplanes is almost identical to that on the affected Model 737-200, -300, -400, and -500 series airplanes. Therefore, certain Model 737-600, -700, -700C, -800 and -900 series airplanes are subject to the unsafe condition revealed on the Model 737-200, -300, -400, and -500 series airplanes.

The inspection threshold specified for the Model 737-600, -700, -700C, -900, and -900 series airplanes is later than the total flight cycles accumulated by some Model 737-200, -300, -400, and -500 series airplanes with reported cracks. We have determined that this is acceptable based on growth rate and cracking pattern of the cracks.

Relevant Service Information

We have reviewed Boeing Special Attention Service Bulletin 737-25-1544, dated October 4, 2006. The service bulletin describes procedures for doing a general visual inspection (GVI) of the frames between BS 360 and BS 907 to identify support brackets of the A/C outlet extrusions that have a two-rivet attachment fitting. The service bulletin also describes procedures for doing medium- and high-frequency eddy current (MFEC and HFEC) inspections for cracking of the frames around the attachment holes of the identified support brackets. The service bulletin also describes frame repair, if necessary, which includes installing reinforcing repair angles. The service bulletin also describes procedures for installing new, improved support fittings for all A/C outlet extrusions between BS 360 and BS 907. Accomplishing the actions specified in the service information is intended to adequately address the unsafe condition.

FAA's Determination and Requirements of the Proposed AD

We have evaluated all pertinent information and identified an unsafe condition that is likely to exist or develop on other airplanes of this same type design. For this reason, we are proposing this AD, which would require accomplishing the actions specified in the service information described previously.

Clarification of Purpose of GVI

Operators should note that, although the service bulletin indicates that the GVI is part of the procedure to detect cracking in the areas previously described, the primary intent of the GVI is to determine which support brackets between BS 360 and BS 907 are attached to the airframe with two rivets. Only those support brackets are subject to the inspections for cracking specified by the service bulletin. Therefore, this proposed AD would require a GVI to identify those support brackets between BS 360 and BS 907 that are attached to the airframe with two rivets.

Related Rulemaking

This unsafe condition may also exist in Boeing Model 737-200, -300, -400, and -500 series airplanes. Therefore, we have issued AD 2006-26-09, Amendment 39-14867 (72 FR 252, January 4, 2007), which has similar requirements, to address the unsafe condition in those airplane models. That unsafe condition, if uncorrected, could result in a severed frame that, combined with existing multi-site damage at the stringer 10 lap splice, could result in rapid decompression of the airplane.

Costs of Compliance

There are about 1,679 airplanes of the affected design in the worldwide fleet. This proposed AD would affect about 626 airplanes of U.S. registry. The following table provides the estimated costs for U.S. operators to comply with this proposed AD, at an average labor rate of $80 per work hour. Operators should note that special cold working tools and sleeves will be needed if any repair is required, which may increase costs.

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 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); 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 proposed 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
• 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.