05-2834. Airworthiness Directives; Boeing Model 737-100, -200, -200C, -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 all Boeing Model 737-100, -200, -200C, -300, -400, and -500 series airplanes. This proposed AD would require repetitive inspections of the stiffeners at left buttock line (LBL) and right buttock line (RBL) 6.15 for cracks; and replacement of both stiffeners with new, improved stiffeners if any stiffener is found cracked. This proposed AD would also allow replacement of both stiffeners at LBL and RBL 6.15 with new, improved stiffeners, which terminates the repetitive inspections. This proposed AD is prompted by reports of cracks in the stiffeners at LBL and RBL 6.15 on the rear spar of the wing center section. We are proposing this AD to detect and correct cracks in the stiffeners at LBL and RBL 6.15, which could result in damage to the keel beam structure and consequently reduce the capability of the airplane to sustain flight loads.

DATES:

We must receive comments on this proposed AD by April 1, 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.
• By fax: [phone].
• 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.

For service information identified in this proposed AD, contact Boeing Commercial Airplanes, P.O. Box 3707, Seattle, Washington 98124-2207.

You can examine the contents of this AD docket on the Internet at http://dms.dot.gov,, or at the Docket Management Facility, U.S. Department of Transportation, 400 Seventh Street SW., room PL-401, on the plaza level of the Nassif Building, Washington, DC. This docket number is FAA-2005-20356; the directorate identifier for this docket is 2004-NM-115-AD.

FOR FURTHER INFORMATION CONTACT:

Nancy Marsh, Aerospace Engineer, Airframe Branch, ANM-120S, FAA, Seattle Aircraft Certification Office, 1601 Lind Avenue, SW., Renton, Washington 98055-4056; telephone [phone]; fax [phone].

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 under ADDRESSES. Include “Docket No. FAA-2005-20356; Directorate Identifier 2004-NM-115-AD” in the subject line 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 submitted 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 can review DOT's complete Privacy Act Statement in the Federal Register published on April 11, 2000 (65 FR 19477-78), or you can visit http://dms.dot.gov.

Examining the Docket

You can examine the AD docket 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 DOT street address stated in the ADDRESSES section. Comments will be available in the AD docket shortly after the DMS receives them.

Discussion

We have received reports indicating that cracks have been found in the stiffeners at left buttock line (LBL) 6.15 and at right buttock line (RBL) 6.15 on the rear spar of the wing center section on several Boeing Model 737-300 series airplanes. On two of those airplanes, the stiffeners at LBL and RBL 6.15 were cracked all the way through, and the keel beam structure was damaged. These airplanes had accumulated between 20,697 and 47,496 total flight cycles. In another instance, on a Model 737-200 series airplane, the stiffener at RBL 6.15 was also cracked all the way through, just below the lower spar chord. That airplane had accumulated 40,888 total flight cycles.

The stiffeners on certain Model 737-100, -200C, -400, and -500 series airplanes are identical to those on the affected Model 737-200 and -300 series airplanes. Therefore, all of these models may be subject to the same unsafe condition.

The existing stiffeners are made from 7075-T6511 aluminum extrusion and have only one flange for attachment to the rear spar. These stiffeners do not provide the necessary strength to prevent cracks at LBL and RBL 6.15. This condition, if not detected and corrected, could result in damage to the keel beam structure and consequently reduce the capability of the airplane to sustain flight loads.

Relevant Service Information

We have reviewed Boeing Alert Service Bulletin 737-57A1269, Revision 1, dated September 16, 2004. The service bulletin describes procedures for doing repetitive detailed inspections of the stiffeners at LBL and RBL 6.15 for cracks; and replacing both stiffeners with new, improved stiffeners if any stiffener is found cracked. Replacement of a stiffener includes:

• Doing an eddy current inspection of all open fastener holes after removing the stiffener, after removing the gusset and grommet, and after removing the stiffener;
• Installing nutplates and ground studs; and
• Drilling holes, machining the spotface, and applying a primer for pre-installation of the stiffener.

Replacement of both stiffeners at LBL and RBL 6.15 with new, improved stiffeners eliminates the need for repetitive inspections. Accomplishing the actions specified in the service bulletin 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. Therefore, we are proposing this AD, which would require repetitive detailed inspections of the stiffeners at LBL and RBL 6.15 for cracks; and replacement of both stiffeners with new, improved stiffeners if any stiffener is found cracked. This proposed AD would also allow replacement of stiffeners at LBL and RBL 6.15 with new, improved stiffeners, which terminates the repetitive inspections. The proposed AD would require you to use the service information described previously to perform these actions, except as discussed under “Difference Between the Proposed AD and the Service Bulletin.”

Difference Between the Proposed AD and the Service Bulletin

The service bulletin specifies that you may contact the manufacturer for instruction on how to repair certain conditions, but this proposed AD would require you to repair those conditions 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 Delegation Option Authorization Organization who has been authorized by the FAA to make those findings.

Costs of Compliance

This proposed AD would affect about 3,132 airplanes worldwide. The following table provides the estimated costs, at an average labor rate of $65 per hour, for U.S. operators to comply with this proposed AD.

Estimated Costs

Action	Work hours	Parts	Cost per airplane	No. of U.S.-registered airplanes	Fleet cost
Inspection, per inspection cycle	1	None	$65, per inspection cycle	1,384	$89,960, per inspection cycle.

Authority for This Rulemaking

The FAA's authority to issue rules regarding aviation safety is found in Title 49 of the United States Code. 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.

This rulemaking is promulgated 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 AD.

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. 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, 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 adding the following new airworthiness directive (AD):

Boeing: Docket No. FAA-20356; Directorate Identifier 2004-NM-115-AD.

Comments Due Date

(a) The Federal Aviation Administration (FAA) must receive comments on this AD action by April 1, 2005.

Affected ADs

(b) None.

Applicability

(c) This AD applies to all Boeing Model 737-100, -200, -200C, -300, -400, and -500 series airplanes, certificated in any category.

Unsafe Condition

(d) This AD was prompted by cracks in the stiffeners at left buttock line (LBL) and right buttock line (RBL) 6.15 on the rear spar of the wing center section. We are issuing this AD to detect and correct cracks in the stiffeners at LBL and RBL 6.15, which could result in damage to the keel beam structure and consequently reduce the capability of the airplane to sustain flight loads.

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.

Service Bulletin Reference

(f) The term “service bulletin,” as used in this AD, means the Accomplishment Instructions of Boeing Alert Service Bulletin 737-57A1269, Revision 1, dated September 16, 2004.

Initial and Repetitive Inspections

(g) Before accumulating 15,000 total flights cycles, or within 180 days after the effective date of this AD, whichever occurs later: Do a detailed inspection of the stiffeners at LBL and RBL 6.15 for cracks, in accordance with Part I of the service bulletin. Thereafter at intervals not to exceed 4,500 flight cycles, repeat the detailed inspection until the stiffeners at LBL and RBL 6.15 have been replaced, in accordance with paragraph (h) or (i) of this AD.

Note 1:

For the purposes of this AD, a detailed inspection is: “An intensive examination of a specific item, installation, or assembly to detect damage, failure, or irregularity. Available lighting is normally supplemented with a direct source of good lighting at an intensity deemed appropriate. Inspection aids such as mirror, magnifying lenses, etc., may be necessary. Surface cleaning and elaborate procedures may be required.”

Replacement of Cracked Stiffener

(h) If any crack is found during any inspection required by this AD, before further flight, replace both stiffeners with new, improved stiffeners by doing all of the applicable actions in Part II through Part IX of the service bulletin; except where the service bulletin specifies to contact Boeing for appropriate action: Before further flight, repair according to a method approved by the Manager, Seattle Aircraft Certification Office (ACO), FAA, or according to data meeting the certification basis of the airplane approved by an Authorized Representative for the Boeing 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 approval must specifically reference this AD. Accomplishing the replacement terminates the repetitive inspections required by paragraph (g) of this AD.

Optional Terminating Action

(1) Replacement of both stiffeners at LBL and RBL 6.15 in accordance with paragraph (h) of this AD terminates the repetitive inspections required by this AD.

Credit for Previous Service Bulletin

(j) The actions done before the effective date of this AD in accordance with Boeing Alert Service Bulletin 737-57A1269, dated December 4, 2003, are acceptable for compliance with the corresponding actions required by this AD.

Alternative Methods of Compliance (AMOCs)

(k)(1) The Manager, Seattle ACO, FAA, 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 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.

Issued in Renton, Washington, on February 6, 2005.

Ali Bahrami,

Manager, Transport Airplane Directorate, Aircraft Certification Service.