Airworthiness Directives; Boeing Model 737-300, -400, and -500 Series Airplanes

AGENCY:

Federal Aviation Administration (FAA), DOT.

ACTION:

Notice of proposed rulemaking (NPRM).

SUMMARY:

We propose to adopt a new airworthiness directive (AD) for certain Boeing Model 737-300, -400, and -500 series airplanes. This proposed AD would require repetitive high frequency eddy current (HFEC) inspections for cracking of the 1.04-inch nominal diameter wire penetration hole in the frame and frame reinforcement, between stringers S-20 and S-21, on both the left and right sides of the airplane, and related investigative/corrective actions if necessary. This proposed AD results from reports of cracking in the frame, or in the frame and frame reinforcement, common to the 1.04-inch nominal diameter wire penetration hole intended for wire routing. We are proposing this AD to detect and correct cracking in the fuselage frames and frame reinforcements, which could reduce the structural capability of the frames to sustain limit loads, and result in cracking in the fuselage skin and subsequent rapid depressurization of the airplane.

DATES:

We must receive comments on this proposed AD by August 8, 2008.

ADDRESSES:

You may send comments by any of the following methods:

• Federal eRulemaking Portal: Go to http://www.regulations.gov . Follow the instructions for submitting comments.
• Fax: 202-493-2251.
• Mail: U.S. Department of Transportation, Docket Operations, M-30, West Building Ground Floor, Room W12-140, 1200 New Jersey Avenue, SE., Washington, DC 20590.
• Hand Delivery: U.S. Department of Transportation, Docket Operations, M-30, West Building Ground Floor, Room W12-140, 1200 New Jersey Avenue, SE., Washington, DC 20590, between 9 a.m. and 5 p.m., Monday through Friday, except Federal holidays.

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

Examining the AD Docket

You may examine the AD docket on the Internet at http://www.regulations.gov ; or in person at the Docket Management Facility between 9 a.m. and 5 p.m., Monday through Friday, except Federal holidays. The AD docket contains this proposed AD, the regulatory evaluation, any comments received, and other information. The street address for the Docket Office (telephone 800-647-5527) is in the ADDRESSES section. Comments will be available in the AD docket shortly after receipt.

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 send any written relevant data, views, or arguments about this proposed AD. Send your comments to an address listed under the ADDRESSES section. Include “Docket No. FAA-2008-0671; Directorate Identifier 2008-NM-017-AD” at the beginning of your comments. We specifically invite comments on the overall regulatory, economic, environmental, and energy aspects of this proposed AD. We will consider all comments received by the closing date and may amend this proposed AD because of those comments.

We will post all comments we receive, without change, to http://www.regulations.gov , including any personal information you provide. We will also post a report summarizing each substantive verbal contact we receive about this proposed AD.

Discussion

We have received reports of cracking in the frame, or in the frame and frame reinforcement, common to the 1.04-inch nominal diameter wire penetration hole intended for wire routing, between stringers S-20 and S-21, on both the left and right sides of the airplane. The cracked frames are located between station (STA) 500B and STA 520 on Model 737-300 and -400 series airplanes and between STA 482 and STA 520 on Model 737-500 series airplanes. The cracks at the 1.04-inch nominal diameter wire penetration hole are due to the effect of operating loads in combination with the stress concentration at the 1.04-inch nominal hole. The cracking initiated at the 1.04-inch nominal diameter wire penetration hole and grew towards the inner chord.

We have since received reports of more than fifty cracked frames at the 1.04-inch nominal diameter wire penetration hole on more than 20 airplanes, all either Model 737-300 or 737-500 series airplanes. The airplanes had accumulated between 35,832 and 66,694 total flight cycles.

This type of cracking has occurred at three frame stations on Model 737-300 series airplanes, at one frame station on Model 737-400 series airplanes, and at four stations on Model 737-500 series airplanes. Sixteen airplanes had cracking at multiple frames, and 10 frames had cracking at adjacent frames. Forty-three frames had cracking only at the inboard side of the 1.04-inch nominal diameter wire penetration hole in the frame inner chord or in the frames and frame reinforcement inner chord. Three of the frames had cracking in the outboard side of the 1.04-inch nominal diameter wire penetration hole in the frame and the frame reinforcement. Two of the frames were severed. Some of the frames had additional cracking at either the standoff/tooling holes or at the 0.50-inch diameter hole positioned below the 1.04-inch nominal diameter wire penetration hole.

Cracking in the fuselage frames at the wire penetration hole intended for wire routing will reduce the structural capability of the frames to sustain limit loads. Cracking in the frames could result in cracking in the fuselage skin and subsequent rapid depressurization of the airplane.

Relevant Service Information

We have reviewed Boeing Alert Service Bulletin 737-53A1279, dated December 18, 2007. The service bulletin describes procedures for doing either a high frequency eddy current (HFEC) surface inspection or HFEC hole/edge inspection for cracking of the 1.04-inch nominal diameter wire penetration hole in the frame and frame reinforcement, between stringers S-20 and S-21, on both the left and right sides of the airplane. If cracking is found, the service bulletin also describes procedures for related investigative and corrective actions. The related investigative action is doing an HFEC inspection for cracking in the 0.50-inch diameter hole and all standoff/tooling holes in the frame and frame reinforcement, between stringers S-19 and S-22. The corrective action is repairing any cracking found and repeating the HFEC inspections. If additional cracking is found, the service bulletin specifies contacting Boeing for repair instructions. The service bulletin further describes procedures for a preventative modification for frames on which either the initial or repetitive inspections have been done. The preventative modification terminates the repetitive inspections.

The initial compliance time for the initial inspection is either within 3,000 or 6,000 (but not to exceed 53,000 total flight cycles) flight cycles after release of the service bulletin, depending on the number of total flight cycles on the airplane. The repetitive interval for the HFEC inspection is 14,000 flight cycles. Corrective actions must be done before further flight.

FAA's Determination and Requirements of this Proposed AD

We are proposing this AD because we evaluated all relevant information and determined the unsafe condition described previously is likely to exist or develop in other products of the(se) same type design(s). This proposed AD would require accomplishing the actions specified in the service information described previously, except as discussed under “Differences Between the Proposed AD and the Service Bulletin.”

Differences Between the Proposed AD and the Service Bulletin

The service bulletin specifies to contact the manufacturer for instructions on how to remove damage and repair certain conditions, but this proposed AD would require removing damage and repairing 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 Commercial Airplanes Delegation Option Authorization Organization whom we have authorized to make those findings.

Costs of Compliance

We estimate that this proposed AD would affect 616 airplanes of U.S. registry. The following table provides the estimated costs for U.S. operators to comply with this proposed AD. The average labor rate is $80 per work hour.

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 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 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. 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.

You can find our regulatory evaluation and the estimated costs of compliance in the AD Docket.

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.