Airworthiness Directives; Boeing Model 737-200, -300, -400, and -500 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 737-200, -300, -400, and -500 series airplanes. This proposed AD would 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. This proposed AD also provides an optional preventive modification that would end the repetitive investigative actions. This proposed AD would also require a one-time post-modification/repair inspection for cracking of each repaired/modified frame. This proposed 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 proposing 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:

We must receive comments on this proposed AD by November 21, 2005.

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:

Sue Lucier, Aerospace Engineer, Airframe Branch, ANM-120S, FAA, Seattle Aircraft Certification Office, 1601 Lind Avenue, SW., Renton, Washington 98055-4056; telephone (425) 917-6438; fax (425) 917-6590.

SUPPLEMENTARY INFORMATION:

Comments Invited

We invite you to submit any relevant written data, views, or arguments regarding this proposed AD. Include the docket number “FAA-2005-22629; Directorate Identifier 2005-NM-089-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 indicating that frame cracks have been found at the attachment holes for support brackets for the air conditioning outlet extrusion on Boeing Model 737-200, -300, -400, and -500 series airplanes. The subject support brackets have a certain part number and are attached to the frame with two rivets. Subject support brackets may be installed on frames between station 360 and station 907. Investigation has revealed that the frame cracks occur due to fatigue and grow in a circumferential direction. The circumferential growth of the cracks is not likely to lead to a severed frame; however, with continued fatigue cycling, a crack could potentially turn in a direction that would lead to a severed frame. Also, frame cracks have been found on multiple adjacent frames, and at the lower row of fasteners of the stringer 10 lap joint, which is susceptible to multi-site damage. Therefore, frame cracks at the attachment holes for the support bracket of the air conditioning outlet extrusion, if not corrected, could eventually lead to a severed frame, which, combined with existing multi-site damage at the stringer 10 lap splice, could result in rapid decompression of the airplane.

Relevant Service Information

We have reviewed Boeing Special Attention Service Bulletin 737-53-1216, dated January 27, 2005. Part I of the service bulletin describes procedures for a general visual inspection to identify where subject support brackets (defined previously) may be installed on frames between station 360 and station 907. Part I of the service bulletin also describes procedures for related investigative actions following identification of subject support brackets. The related investigative actions consist of a medium-frequency eddy current (MFEC) inspection for cracking of the frame around the attachment rivets of the support bracket, and a high-frequency eddy current (HFEC) inspection for cracking of the frame adjacent to the inboard fastener hole.

For any subject support bracket on which no cracking is found, the service bulletin specifies to perform these inspections repetitively, or to do a preventive modification. Part II of the service bulletin describes procedures for the preventive modification, which involves performing an open-hole HFEC inspection of the frame holes for the support bracket, and repairing any cracks in accordance with the repair procedures (in Part III of the service bulletin). If no crack is found during the inspection of the frame holes, the modification procedures involve installing a doubler and cold-working fastener holes, as applicable.

For any subject frame on which cracking is found, Part III of the service bulletin specifies procedures for repair. The repair involves cutting out the frame web, doing a dye penetrant or HFEC inspection of the cutout to ensure it is free from cracks, installing repair angles, and cold working fastener holes as applicable.

Part IV of the service bulletin describes procedures for performing a one-time post-repair/modification inspection of any modified or repaired frame, which involves the following:

• Performing a detailed inspection for cracking of the modification doubler or repair angle, as applicable.
• Performing a detailed inspection for cracking of the frame, two stringers above and two stringers below the support bracket.
• Performing a detailed inspection for cracking of the air conditioning attach brackets.
• Performing a detailed inspection for cracking of the frame at the stringer clips.
• Reporting any cracking to Boeing.

Accomplishing the general visual inspection, repetitive MFEC and HFEC inspections, and any necessary corrective actions specified in the service information is intended to adequately address the unsafe condition.

Section 1.E., Compliance, of the service bulletin specifies compliance times for the actions in the service bulletin. The service bulletin specifies that the initial general visual, MFEC, and HFEC inspections, as applicable, are required prior to the accumulation of 30,000 total flight cycles, or within 5,000 flight cycles after the date of the service bulletin (or after a frame repair was made), whichever occurs later. The service bulletin specifies a repetitive interval (for all subject frames) of 6,000 flight cycles.

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, except as discussed under “Differences Between the Proposed AD and Service Information.” If no cracking is found, this proposed AD would also provide for optional accomplishment of the preventive modification, which would end the repetitive inspections for each modified frame.

Consistent with the service information, the proposed AD would allow repetitive inspections to continue in lieu of the preventive modification for any frame on which no cracking is found. In making this determination, we considered that long-term continued operational safety in this case will be adequately ensured by repetitive inspections to detect cracking before it represents a hazard to the airplane.

Differences Between the Proposed AD and Service Information

Part IV of the Accomplishment Instructions of the referenced service bulletin does not specify what corrective action is necessary if cracking is found during a post-modification/repair inspection. We find that any cracking found during a post-modification/repair inspection must be repaired in one of the following ways:

• Using a method that we approve; or
• Using data that meet the certification basis of the airplane, and that have been approved by an Authorized Representative for the Boeing Commercial Airplanes Delegation Option Authorization Organization whom we have authorized to make those findings.

Also, Part IV of the Accomplishment Instructions of the referenced service bulletin specifies reporting to Boeing any damage found during the post-modification/repair inspections. This proposed AD would not require that action. We do not need this information from operators.

The service bulletin specifies a compliance time relative to the date of the service bulletin; however, this proposed AD would require compliance before the specified compliance time after the effective date of this AD.

Costs of Compliance

There are about 2,131 airplanes of the affected design in the worldwide fleet. This proposed AD would affect about 938 airplanes of U.S. registry. The proposed inspection to identify subject support brackets, and subsequent MFEC and HFEC inspections would 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 proposed 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 proposed 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.

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.