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###### Department of Transportation

###### Federal Aviation Administration 

          - 14 CFR Part 39 
          - [Docket No. 99-NM-380-AD] 
          - RIN 2120-AA64 

# 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 737-300, -400, and -500 series airplanes. This proposal would require repetitive inspections to detect cracking of certain areas of the forward pressure bulkhead, and repair, if necessary. This proposal also would require certain preventive modifications, which, when accomplished, would terminate the repetitive inspections for the affected areas. This action is necessary to prevent fatigue cracking on critical areas of the forward pressure bulkhead, which could result in rapid decompression of the airplane fuselage. This action is intended to address the identified unsafe condition. 

# DATES:

Comments must be received by December 4, 2000. 

# ADDRESSES:

Submit comments in triplicate to the Federal Aviation Administration (FAA), Transport Airplane Directorate, ANM-114, Attention: Rules Docket No. 99-NM-380-AD, 1601 Lind Avenue, SW., Renton, Washington 98055-4056. Comments may be inspected at this location between 9:00 a.m. and 3:00 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. 99-NM-380-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, Transport Airplane Directorate, 1601 Lind Avenue, SW., Renton, Washington. 

# FOR FURTHER INFORMATION CONTACT:

Nenita K. Odesa, Aerospace Engineer, Airframe Branch, ANM-120S, FAA, Seattle Aircraft Certification Office, 1601 Lind Avenue, SW., Renton, Washington 98055-4056; telephone (425) 227-2557; 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 notice 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 notice must submit a self-addressed, stamped postcard on which the following statement is made: “Comments to Docket Number 99-NM-380-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. 99-NM-380-AD, 1601 Lind Avenue, SW., Renton, Washington 98055-4056.                           ( print page 62314)    

## Discussion 

The FAA has received reports indicating that operators have found numerous fatigue cracks on the body station 178 forward pressure bulkhead on certain Boeing Model 737 series airplanes. The longest fatigue crack was approximately 25 inches in length. The fatigue cracks were found at three critical structural areas of the bulkhead, namely, at the side chord areas of the bulkhead, at certain vertical chords of the bulkhead; and on the bulkhead web itself between left and right buttock lines 17.0. Such fatigue cracking, if not corrected, could result in rapid decompression of the airplane fuselage. 

## Related Rulemaking 

On March 10, 2000, the FAA issued AD 2000-05-29, amendment 39-11639 (65 FR 14834, March 20, 2000), applicable to certain Boeing Model 737-100, -200, -300, -400, and -500 series airplanes, that requires repetitive inspections to detect cracking of various areas of the forward pressure bulkhead, and repair, if necessary. That action also provides for certain optional preventive modifications, which, if accomplished, would terminate the repetitive inspections for the affected areas. That action was prompted by reports indicating that numerous fatigue cracks were found on critical areas of the forward pressure bulkhead. The requirements of that AD are intended to prevent such fatigue cracking, which could result in rapid decompression of the airplane fuselage. 

In the preamble to AD 2000-05-29, the FAA specified that the actions required by that AD were considered interim action. The FAA indicated that it may consider further rulemaking action to mandate certain inspections and modifications to address fatigue cracking in the bulkhead of Model 737 series airplanes having line numbers 2738 through 3071, inclusive. The FAA has determined that further rulemaking action is indeed necessary; this proposed AD follows from that determination. 

## Explanation of Relevant Service Information 

The FAA has reviewed and approved Boeing Alert Service Bulletin 737-53A1208, dated May 6, 1999, which describes procedures for repetitive inspections to detect cracking of the vertical and side chord areas on the body station 178 forward pressure bulkhead; and repair, if necessary. The service bulletin lists several types of inspections to be performed on the vertical and side chord areas of the forward pressure bulkhead. The inspections applicable to these areas consist of detailed visual/borescope inspections, eddy current inspections, and ultrasonic inspections. 

The service bulletin also describes procedures for certain preventive modifications, which, if accomplished, would eliminate the need for the repetitive inspections. Specifically, these modifications consist of installing certain angles and straps to strengthen the vertical chord area at waterline 184, and the side chord area at waterline 207. 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 accomplishment of the actions specified in the service bulletin described previously, except as discussed below. 

## Differences Between Proposed Rule and Alert Service Bulletin 

Operators should note that the alert service bulletin refers to certain preventive modifications as optional. However, this proposed AD would make these preventive modifications mandatory, and would require accomplishment prior to the accumulation of 75,000 total flight cycles or within 12,000 flight cycles after the effective date of this AD, whichever occurs later. The proposed grace period of 12,000 flight cycles was developed to correspond with a typical operator's heavy maintenance check schedule in order to minimize disruption to scheduled operations. As with the compliance times proposed for the inspections, the FAA considered not only the manufacturer's recommendation, but the degree of urgency associated with addressing the subject unsafe condition, the average utilization of the affected fleet, and the high number of airplanes that have already been found to be affected by the unsafe condition. These mandatory preventive modifications, when accomplished, would constitute terminating action for the repetitive inspection requirements of this proposed AD. 

## Cost Impact 

There are approximately 330 Model 737 series airplanes of the affected design in the worldwide fleet. The FAA estimates that 115 airplanes of U.S. registry would be affected by this proposed AD. 

It would take approximately 2 work hours per airplane to accomplish the proposed inspection, 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 $13,800, or $120 per airplane, per inspection cycle. 

It would take approximately 38 work hours per airplane to accomplish the proposed modification of the vertical chords, at an average labor rate of $60 per work hour. Required parts would cost approximately $2,789 per airplane. Based on these figures, the cost impact of the proposed AD on U.S. operators is estimated to be $582,935, or $5,069 per airplane. 

It would take approximately 274 work hours per airplane to accomplish the proposed modification of the side chord areas, at an average labor rate of $60 per work hour. Required parts would cost approximately $6,629 per airplane. Based on these figures, the cost impact of the proposed AD on U.S. operators is estimated to be $2,652,935, or $23,069 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                           ( print page 62315)                        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. 

§ 39.13 

2. Section 39.13 is amended by adding the following new airworthiness directive: 

**Boeing: Docket 99-NM-380-AD.**

*Applicability:*                                 Model 737-300, -400, and -500 series airplanes, certificated in any category; as listed in Boeing Alert Service Bulletin 737-53A1208, dated May 6, 1999.                             

# Note 1:

This AD applies to each airplane identified in the preceding applicability provision, regardless of whether it has been modified, altered, or repaired in the area subject to the requirements of this AD. For airplanes that have been modified, altered, or repaired so that the performance of the requirements of this AD is affected, the owner/operator must request approval for an alternative method of compliance in accordance with paragraph (d) of this AD. The request should include an assessment of the effect of the modification, alteration, or repair on the unsafe condition addressed by this AD; and, if the unsafe condition has not been eliminated, the request should include specific proposed actions to address it.

*Compliance:*                                 Required as indicated, unless accomplished previously.                             

To detect fatigue cracking of the forward pressure bulkhead, which could result in rapid decompression of the airplane fuselage, accomplish the following: 

## Initial and Repetitive Inspections 

(a) Before the accumulation of 20,000 total flight cycles, or within 3,000 flight cycles after the effective date of this AD, whichever occurs later: Perform the applicable inspections of the vertical and side chord areas of the forward pressure bulkhead to detect cracking, in accordance with the Accomplishment Instructions of Boeing Alert Service Bulletin 737-53A1208, dated May 6, 1999. Thereafter, repeat the inspections at intervals not to exceed 6,000 flight cycles until the preventive modifications required by paragraph (c) of this AD have been accomplished. 

## Repair 

(b) If any cracking is detected during any inspection required by paragraph (a) of this AD, before further flight, repair the area in accordance with the Accomplishment Instructions of Boeing Alert Service Bulletin 737-53A1208, dated May 6, 1999. 

## Terminating Action 

(c) Before the accumulation of 75,000 total flight cycles, or within 12,000 flight cycles after the effective date of this AD, whichever occurs later: Accomplish preventive modifications of the vertical and side chord areas of the forward pressure bulkhead, in accordance with the Accomplishment Instructions of Boeing Alert Service Bulletin 737-53A1208, dated May 6, 1999. Accomplishment of these modifications constitutes terminating action for the repetitive inspections required by paragraph (a) of this AD. 

## Alternative Methods of Compliance 

(d) An alternative method of compliance or adjustment of the compliance time that provides an acceptable level of safety may be used if approved by the Manager, Seattle Aircraft Certification Office (ACO), FAA. Operators shall submit their requests through an appropriate FAA Principal Maintenance Inspector, who may add comments and then send it to the Manager, Seattle ACO. 

# Note 2:

Information concerning the existence of approved alternative methods of compliance with this AD, if any, may be obtained from the Seattle ACO.

## Special Flight Permit 

(e) Special flight permits may be issued in accordance with sections 21.197 and 21.199 of the Federal Aviation Regulations (14 CFR 21.197 and 21.199) to operate the airplane to a location where the requirements of this AD can be accomplished. 

Issued in Renton, Washington, on October 12, 2000. 

Donald L. Riggin, 

Acting Manager, Transport Airplane Directorate, Aircraft Certification Service. 

[FR Doc. 00-26711 Filed 10-17-00; 8:45 am] 

BILLING CODE 4910-13-P