Airworthiness Directives; Boeing Model 737-200 and -200C Series Airplanes

AGENCY:

Federal Aviation Administration, DOT.

ACTION:

Notice of proposed rulemaking (NPRM).

SUMMARY:

This document proposes the adoption of a new airworthiness directive (AD) that is applicable to certain Boeing Model 737-200 and -200C series airplanes. This proposal would require repetitive inspections to find cracking of certain fuselage lap joint areas, and repair of any cracking found. This proposal also would require eventual modification of those areas, which would constitute terminating action for the repetitive inspections. This action is necessary to find and fix cracking of certain fuselage lap joint areas, which could result in rapid decompression of the airplane. This action is intended to address the identified unsafe condition.

DATES:

Comments must be received by August 27, 2001.

ADDRESSES:

Submit comments in triplicate to the Federal Aviation Administration (FAA), Transport Airplane Directorate, ANM-114, Attention: Rules Docket No. 2000-NM-74-AD, 1601 Lind Avenue, SW., Renton, Washington 98055-4056. Comments may be inspected at this location between 9 a.m. and 3 p.m., Monday through Friday, except Federal holidays. Comments may be submitted via fax to (425) 227-1232. Comments may also be sent via the Internet using the following address: 9-anm-nprmcomment@faa.gov. Comments sent via fax or the Internet must contain “Docket No. 2000-NM-74-AD” in the subject line and need not be submitted in triplicate. Comments sent via the Internet as attached electronic files must be formatted in Microsoft Word 97 for Windows or ASCII text.

The service information referenced in the proposed rule may be obtained from Boeing Commercial Airplane Group, P.O. Box 3707, Seattle, Washington 98124-2207. This information may be examined at the FAA, 1601 Lind Avenue, SW., Renton, Washington.

FOR FURTHER INFORMATION CONTACT:

Scott Fung, Aerospace Engineer, Airframe Branch, ANM-120S, FAA, Seattle Aircraft Certification Office, 1601 Lind Avenue, SW., Renton, Washington; telephone (425) 227-1221; fax (425) 227-1181.

SUPPLEMENTARY INFORMATION:

Comments Invited

Interested persons are invited to participate in the making of the proposed rule by submitting such written data, views, or arguments as they may desire. Communications shall identify the Rules Docket number and be submitted in triplicate to the address specified above. All communications received on or before the closing date for comments, specified above, will be considered before taking action on the proposed rule. The proposals contained in this action may be changed in light of the comments received.

Submit comments using the following format:

• Organize comments issue-by-issue. For example, discuss a request to change the compliance time and a request to change the service bulletin reference as two separate issues.
• For each issue, state what specific change to the proposed AD is being requested.
• Include justification (e.g., reasons or data) for each request.

Comments are specifically invited on the overall regulatory, economic, environmental, and energy aspects of the proposed rule. All comments submitted will be available, both before and after the closing date for comments, in the Rules Docket for examination by interested persons. A report summarizing each FAA-public contact concerned with the substance of this proposal will be filed in the Rules Docket.

Commenters wishing the FAA to acknowledge receipt of their comments submitted in response to this action must submit a self-addressed, stamped postcard on which the following statement is made: “Comments to Docket Number 2000-NM-74-AD.” The postcard will be date stamped and returned to the commenter.

Availability of NPRMs

Any person may obtain a copy of this NPRM by submitting a request to the FAA, Transport Airplane Directorate, ANM-114, Attention: Rules Docket No. 2000-NM-74-AD, 1601 Lind Avenue, SW., Renton, Washington 98055-4056.

Structural Airworthiness of Aging Transport Category Airplanes

On April 28, 1988, a Boeing Model 737 series airplane was involved in an accident in which a 15-foot long section of fuselage structure peeled open during flight. In light of this, the FAA initiated an Aging Fleet Program. The objective of that program is to identify and implement procedures to ensure the continuing structural airworthiness of aging transport category airplanes.

As part of the Aging Fleet Program, the airplane manufacturer conducted cyclic pressure (fatigue) tests to evaluate the performance of the various fuselage skin panel lap joint configurations. The fuselage skin panel joint consists of two adjacent panels that overlap each other longitudinally and are joined together by three rows of fasteners at the overlap (hence, lap joint). Cracks in the upper skin of the lap joint led to the structural failure that occurred in the 1988 accident discussed previously. These lap joints, installed on early Model 737 series airplanes having line numbers 1 through 291, were modified by replacing the countersunk fasteners in the upper fastener row of the lap joint with protruding head fasteners to correct and prevent cracking in the upper skin of the lap joint. To date, no cracking has been detected in the lower fastener row of these (modified) lap joints.

However, at some locations on these same airplanes, the lap joint has a different configuration that includes a doubler, and cracking has been found in the lower row of fasteners in the lower skin of these joints. This type of joint was used extensively on subsequent airplanes (line numbers 292 through 2565 inclusive) to improve the lap joint and to prevent cracks in the upper skin. In 1994, tests were conducted on lap joints that incorporate doublers; test results revealed cracks in the lower skin of this lap joint. The airplane manufacturer determined that these cracks were caused by increased stresses in this area due to the increased bending stresses associated with the installation of the doubler on the upper skin.

In light of these test results, the manufacturer inspected this type of lap joint on five aging airplanes and detected a total of 273 fatigue cracks. The use of eddy current inspection techniques was required as the cracks in the lower skin are not detectable visually due to the positioning of the lower skin between the upper skin and the circumferential tear strap. Many of these cracks were found to have occurred simultaneously at adjacent fastener hole locations in the lower skin of the fuselage lap joint.

This type of cracking of the lap joint is known as multiple site damage (MSD). MSD is characterized by the simultaneous presence of fatigue cracks in the same structural element (such as the lower skin panel of the lap joint). Coalescence of cracks at adjacent fastener holes in the lower skin can lead to sudden fracture and failure of the lap joint, which could result in rapid decompression of the airplane due to the reduction in the residual strength of a lap joint in the presence of MSD. This reduction of the structural integrity of the fuselage may occur at loads significantly below those that would be expected for structure having a single large crack. The accident discussed previously has demonstrated dramatically that small cracks acting together can have a significant effect on the residual strength of the aircraft structure.

Related Rulemaking

On October 21, 1997, the FAA issued AD 97-22-07, amendment 39-10179 (62 FR 55732, October 28, 1997), applicable to Boeing Model 737 series airplanes, line numbers 292 through 2565 inclusive, which requires repetitive inspections to detect cracking of the lower skin at the lower row of fasteners in the lap joints of the fuselage that have this doubler, and repair of any cracking detected. That action also requires modification of the fuselage lap joints at stringers S10 and S14, located between body stations (BS) 360 and BS 540, and located between BS 727 and BS 908.

Public Meeting

A joint Federal Aviation Administration (FAA) and Boeing meeting was held on July 25-27, 2000, to inform industry of the activity on Boeing Model 727 and 737 fuselage lap joints. Others in attendance were representatives from air carriers and repair stations, as well as Principal Maintenance Inspectors (PMI) from the FAA Flight Standards Service. The objective of the meeting was to provide an overview of the FAA rulemaking process; discuss the recommendations of Boeing Service Bulletins 727-53A0222 and 737-53A1177, including background information; standardize the 727 and 737 service bulletins, where possible; and discuss the impact that the recommended service bulletin modifications would have on industry.

During the meetings, holders of certain supplemental type certificates presented information pertaining to service bulletin activity for those airplanes that have been modified from a passenger to an all-cargo configuration. The meeting accomplished the objective of exchanging information between the FAA, Boeing, and industry on various aspects of Boeing Models 727 and 737 fuselage lap joints, including compliance planning. As a result of the meeting, attendees recognized the importance of modifying certain lap joints before reaching the point of widespread fatigue damage. Suggestions to improve the service bulletins and clarify AD compliance issues were made by operators and PMIs, and have been incorporated into the service bulletins and the proposed ADs discussed below. In addition, minutes of the public meeting are retained in the docket.

Other Relevant Rulemaking

At this time, the FAA is considering two other separate rulemaking actions to address the remaining potential unsafe conditions relating to the cracking of the lap joints of the fuselage. Those two other actions would address:

• Additional repetitive inspections to find cracking of the lower skin at the lower row of fasteners in the lap joints of the fuselage, and replacement of the preventive modification with an improved modification on Model 737 series airplanes, line numbers 292 through 2565 inclusive. And
• Replacement of certain repairs with improved repairs in certain fuselage lap joints done per the procedures described in the structural repair manual (SRM); and a high frequency eddy current inspection to find cracking of the SRM repairs of the lower skin at the lower row of fasteners in the lap joints of the fuselage, and repair of any cracking found on Model 737 series airplanes, line numbers 292 through 2565 inclusive.

Explanation of Relevant Service Information

The FAA has reviewed and approved Boeing Service Bulletin 737-53A1177, Revision 6, dated May 31, 2001, which describes, among other things, procedures for repetitive low frequency eddy current inspections to find cracking of the left and right stringer S-10 and S-14 lap joint areas, between BS 360 and BS 540, and BS 727 and BS 908, and repair, if necessary. The service bulletin also describes procedures for the installation of a lap joint modification, which, when accomplished, would eliminate the need for the repetitive inspections. Accomplishment of the actions specified in the service bulletin is intended to adequately address the identified unsafe condition.

Explanation of Requirements of Proposed Rule

Since an unsafe condition has been identified that is likely to exist or develop on other products of this same type design, the proposed AD would require repetitive inspections to find cracking of certain fuselage lap joint areas, and repair of any cracking found. The proposed AD also would require eventual modification of those areas, which would constitute terminating action for the repetitive inspections. The actions would be required to be accomplished per the service bulletin described previously, except as discussed below.

Differences Between Service Bulletin and Proposed Rule

The FAA recognizes that the lap joint modification specified in this proposed AD would require jacking, shoring, removing interior components, and modifying certain lap joints, which would require taking the airplane out of service for as much as 22 days. This lengthy shop visit, as well as the relatively short compliance time required to accomplish this proposed AD, make it necessary for operators to engage in compliance planning to ensure that, when the deadline for compliance arrives, all of the required actions have been completed on all affected airplanes. Therefore, paragraph (d) of this proposed AD would require that operators submit to the FAA a compliance plan within 3 months after the effective date of this AD. This will enable the FAA to verify that all operators will be able to meet the deadlines imposed by this proposed AD.

Operators should note that, in light of the complexity of the service bulletin, three separate rulemaking actions are being issued to address the potential unsafe conditions relating to the cracking of the lap joints of the fuselage. This proposed rule will address only Model 737-200 and -200C series airplanes having line numbers 1 through 291 inclusive.

Operators also should note that, although the service bulletin specifies that the manufacturer may be contacted for disposition of certain repair/modification conditions, this proposed AD requires the repair/modification of those conditions to be accomplished per a method approved by the FAA, or per data meeting the type certification basis of the airplane approved by a Boeing Company Designated Engineering Representative who has been authorized by the FAA to make such findings.

Cost Impact

There are approximately 159 Model 737-200 and -200C series airplanes of the affected design in the worldwide fleet. The FAA estimates that 55 airplanes of U.S. registry (over 10 years) would be affected by this proposed AD.

It would take approximately 16 work hours per airplane to accomplish the proposed inspections, at an average labor rate of $60 per work hour. Based on these figures, the cost impact of the proposed AD on U.S. operators is estimated to be $52,800, or $960 per airplane, per inspection cycle.

It would take approximately 75 work hours per airplane to accomplish the proposed modifications, at an average labor rate of $60 per work hour. Required parts would cost approximately $1,500 per airplane. Based on these figures, the cost impact of the proposed AD on U.S. operators is estimated to be $330,000, or $6,000 per airplane.

The compliance plan that is proposed in this AD action would take approximately 24 work hours per airplane to accomplish, at an average labor rate of $60 per work hour. Based on these figures, the cost impact of the compliance plan on U.S. operators is estimated to be $79,200, or $1,440 per airplane.

The cost impact figures discussed above are based on assumptions that no operator has yet accomplished any of the proposed requirements of this AD action, and that no operator would accomplish those actions in the future if this proposed AD were not adopted. The cost impact figures discussed in AD rulemaking actions represent only the time necessary to perform the specific actions actually required by the AD. These figures typically do not include incidental costs, such as the time required to gain access and close up, planning time, or time necessitated by other administrative actions.

Regulatory Impact

The regulations proposed herein would not have a substantial direct effect on the States, on the relationship between the national Government and the States, or on the distribution of power and responsibilities among the various levels of government. Therefore, it is determined that this proposal would not have federalism implications under Executive Order 13132.

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

List of Subjects in 14 CFR Part 39

• Air transportation
• Aircraft
• Aviation safety
• Safety

The Proposed Amendment

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

PART 39—AIRWORTHINESS DIRECTIVES

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

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