AD 98-20-20

On October 21, 1987, the FAA issued AD 87-23-10, amendment 39-5758 (52 FR 41551, October 29, 1987), applicable to certain Boeing Model 747 series airplanes, to require repetitive inspections for damage or cracking of the aft pressure bulkhead, and repair, if necessary. That action was prompted by analysis of inspection reports and the results of testing by the manufacturer. The actions required by that AD are intended to detect and correct fatigue cracking of the aft pressure bulkhead, which could result in rapid depressurization of the airplane.

Actions Since Issuance of Previous Rule

Since the issuance of that AD, the FAA has received a report indicating that one operator found a 7.5-inch-long crack in the upper portion of the web of the pressure bulkhead at Body Station 2360 on a Boeing Model 747 series airplane. Analysis of the cracked bulkhead web revealed a series of short cracks initiated at the fastener holes common to the outer chord of the Y-ring in multiple locations. These cracks propagated rapidly due to fatigue, and joined together to form the 7.5-inch-long crack.

That airplane had accumulated 25,777 total landings and 74,266 total flight hours at the time the crack was discovered. The upper portion of the web of the pressure bulkhead of that airplane had been inspected previously in accordance with AD 87-23-10, and the crack was discovered during a repeat detailed visual inspection performed approximately 7,000 landings after the initial inspection. These findings indicate that cracking of the upper portion of the web of the pressure bulkhead could develop on the affected airplanes in fewer landings than the repetitive inspection interval of 7,000 landings that is mandated by the existing AD.

Explanation of Relevant Service Information

The FAA has reviewed and approved Boeing Alert Service Bulletin 747-53A2275, Revision 6, dated August 27, 1998. That alert service bulletin describes procedures for, among other things, a detailed visual inspection performed from the forward side of the bulkhead to detect cracking of the upper segment of the bulkhead web at the attachment to the outer chord of the Y-ring. That alert service bulletin also describes procedures for follow-on corrective actions, if necessary, which include a surface probe eddy current inspection to detect cracking of the upper and lower segments of the bulkhead around the fasteners that attach the web to the outer chord of the Y-ring. The alert service bulletin also specifies that the manufacturer may be contacted for the disposition of certain repairs.

Explanation of Requirements of Rule

Since an unsafe condition has been identified that is likely to exist or develop on other airplanes of this same type design, this AD supersedes AD 87-23-10 to continue to require certain repetitive inspections for damage or cracking of the aft pressure bulkhead, and repair, if necessary. In addition, this AD removes repetitive detailed visual inspections for cracking of the bulkhead web to Y-ring lap joint area but retains the initial inspection for cracking in that area. This AD also adds a one-time detailed visual inspection from the forward side of the bulkhead to detect fatigue cracking of the upper segment of the bulkhead web, and follow-on corrective actions, if necessary. The actions are required to be accomplished in accordance with the alert service bulletin described previously, except as discussed below.

Interim Action

This is considered to be interim action. The FAA is currently considering requiring additional repetitive detailed visual inspections from the forward side of the bulkhead to detect cracking of the upper segment of the bulkhead web; repetitive surface probe high frequency eddy current inspections from the forward side of the bulkhead to detect cracking of the upper and lower segments of the bulkhead web; and repair, if necessary. However, the planned compliance time for these actions is sufficiently long so that notice and opportunity for prior public comment will be practicable.

Differences Between Alert Service Bulletin and This AD

Operators should note that, although the alert service bulletin specifies that the manufacturer may be contacted for disposition of certain repair conditions, this AD requires the repair of those conditions to be accomplished in accordance with a method approved by the FAA, or in accordance with 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.

Operators also should note that the alert service bulletin specifies accomplishment of the visual inspection prior to the accumulation of 20,000 total flight cycles (landings); within 250 flight cycles after receipt of Boeing Alert Service Bulletin 747-53A2275, Revision 6; or within 1,500 flight cycles after the last visualinspection from the forward side of the bulkhead; whichever occurs latest. The FAA has determined that such compliance options may not ensure that all affected airplanes are inspected in a timely manner.

In developing an appropriate compliance time for this AD, the FAA considered not only the manufacturer's recommendation, but also the time necessary to accomplish the inspection (4 hours), and the average utilization of the affected fleet. The FAA finds that, due to possible variances in average utilization among airplanes, a grace period of 90 days rather than 250 flight cycles (landings) will better ensure that the inspection is accomplished on all Boeing Model 747 series airplanes in a timely manner.

In light of all of these factors, the FAA finds that accomplishment of the inspection for all affected airplanes prior to the accumulation of 20,000 total landings, within 1,500 landings after the last visual inspection from the forward side of the bulkhead, or within 90 days after the effective date of this AD, whichever occurs latest, represents an appropriate interval of time allowable for affected airplanes to continue to operate without compromising safety.

Explanation of Applicability

AD 87-23-10 was applicable to Boeing Model 747 series airplanes, as listed in Boeing Service Bulletin 747-53-2275, Revision 1, dated August 13, 1987. This AD is applicable to Boeing Model 747 series airplanes, line positions 1 through 671. This change is being made to more precisely define the airplanes that are affected. The line positions are the same as those referenced in the effectivity of Boeing Service Bulletin 747-53-2275, Revision 1; no new airplanes are added as a result of this change.

Explanation of Disallowance of Adjustment Factor

Paragraph (g) of AD 87-23-10 specified that, based on continued mixed operation at lower cabin differential pressures, the compliance thresholds and intervals specified in that AD for Boeing Model 747 series airplanes could be multiplied by a 1.2 adjustment factor. Since the issuance of that AD, the FAA has determined that insufficient data exist to support such an adjustment to flight cycles. In fact, data are available that indicate that the use of a 1.2 adjustment factor provides inaccurate data and unjustified relief for inspection intervals. Consequently, this AD does not allow for such an adjustment factor, and the provisions of paragraph (g) of the existing AD have not been included in this AD.

Explanation of Disallowance of Modification

Paragraph (j) of AD 87-23-10 specifies that modification of Boeing Model 747 series airplanes by installing certain new, improved parts would constitute terminating action for the inspection requirements of that AD. Since the issuance of AD 87-23-10, the FAA has determined that the kit necessary for accomplishment of such modification was never made available by the manufacturer. Therefore, because it is not possible to comply with the actions described by paragraph (j), the provisions of that paragraph have not been included in this AD.

Determination of Rule's Effective Date

Since a situation exists that requires the immediate adoption of this regulation, it is found that notice and opportunity for prior public comment hereon are impracticable, and that good cause exists for making this amendment effective in less than 30 days.

Comments Invited

Although this action is in the form of a final rule that involves requirements affecting flight safety and, thus, was not preceded by notice and an opportunity for public comment, comments are invited on this rule. Interested persons are invited to comment on this 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 under the caption "ADDRESSES." All communications received on or before the closing date for comments willbe considered, and this rule may be amended in light of the comments received. Factual information that supports the commenter's ideas and suggestions is extremely helpful in evaluating the effectiveness of the AD action and determining whether additional rulemaking action would be needed.

Comments are specifically invited on the overall regulatory, economic, environmental, and energy aspects of the rule that might suggest a need to modify the 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 that summarizes each FAA-public contact concerned with the substance of this AD will be filed in the Rules Docket.

Commenters wishing the FAA to acknowledge receipt of their comments submitted in response to this rule must submit a self-addressed, stamped postcard on which the following statement is made: "Comments to Docket Number 98-NM-257-AD." The postcard willbe date stamped and returned to the commenter.

Regulatory Impact

The regulations adopted herein will not have substantial direct effects 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, in accordance with Executive Order 12612, it is determined that this final rule does not have sufficient federalism implications to warrant the preparation of a Federalism Assessment.

The FAA has determined that this regulation is an emergency regulation that must be issued immediately to correct an unsafe condition in aircraft, and that it is not a "significant regulatory action" under Executive Order 12866. It has been determined further that this action involves an emergency regulation under DOT Regulatory Policies and Procedures (44 FR 11034, February 26, 1979). If it is determined that this emergency regulation otherwise would be significant under DOT Regulatory Policies and Procedures, a final regulatory evaluation will be prepared and placed in the Rules Docket. A copy of it, if filed, may be obtained from the Rules Docket at the location provided under the caption "ADDRESSES."

List of Subjects in 14 CFR Part 39

Air transportation, Aircraft, Aviation safety, Incorporation by reference, Safety.

Adoption of the Amendment

Accordingly, pursuant to the authority delegated to me by the Administrator, the Federal Aviation Administration amends 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 (Amended) 2. Section 39.13 is amended by removing amendment 39-5758 (52 FR 41551, October 29, 1987), and by adding a new airworthiness directive (AD), amendment 39-10786, to read as follows: