AD 98-26-13

A proposal to amend part 39 of the Federal Aviation Regulations (14 CFR part 39) to include an airworthiness directive (AD) that is applicable to certain Boeing Model 747 series airplanes was published in the Federal Register on March 20, 1998 (63 FR 13566). That action proposed to require a one-time inspection to determine the material type of the stop support fittings of the main entry doors. That action also proposed to require repetitive visual inspections to detect cracks of certain stop support fittings of the main entry doors, and replacement of any cracked stop support fitting with a certain new stop support fitting.

Interested persons have been afforded an opportunity to participate in the making of this amendment. Due consideration has been given to the comments received.

Support for the Proposal

Several commenters support the proposed rule.

Request to Include a Threshold for Initial Inspection

Two commenters request that the proposed compliance time for the initial high frequency eddy current (HFEC) inspection to determine the material type of the stop support fittings of the main entry doors be revised from 18 months after the effective date of this AD, as stated in the proposal, to 6 years after delivery of the airplane or 18 months after the effective date of the AD, whichever occurs later. One of the commenters points out that cracking of the fittings has been attributed to stress corrosion and that, when corrosion prevention is performed properly (i.e., in accordance with the Corrosion Prevention and Control Program (CPCP)), the growth of corrosion cracking is very slow. The commenter notes that corrosion and stress corrosion cracking is unlikely to occur on younger airplanes.

The FAA does not concur with the commenters' request to include a threshold for the initial inspection. As stated previously in the notice of proposed rulemaking (NPRM), the FAA has determined that all affected airplanes are older than 6 years since the date of manufacture of the airplane. The youngest airplane has been in service for more than seven years. Therefore, all operators are required to perform the initial inspection of the affected airplanes within 18 months after the effective date of this AD. No change to the final rule is necessary in this regard.

Request to Limit the Area of Inspection

One commenter requests that the proposed HFEC inspection to determine the material type of the stop support fittings of the main entry doors should be required only if the material of the stop support fittings is unknown, as specified in Figure 3, Table 1, of the referenced service bulletin.

The FAA concurs with the commenter that the HFEC inspection required by this AD should be required only for those stop support fittings. The FAA's intent is that the HFEC inspection be accomplished only at the locations specified in the referenced service bulletin, where the material type is unknown. The visual inspection must be accomplished only on those stop support fittings of the main entry doors that are made from either 7079-T651 or 7075-T651 material. The FAA has revised paragraph (a) of the final rule to clarify this point.

Request to Extend Repetitive Inspection Intervals

Several commenters request that the repetitive interval for accomplishment of the visual inspections to detect cracks of certain stop support fittings of the main entry doors be extended from the proposed 18 months to 36 months, as specified in Boeing Service Bulletin 747-53-2358, dated August 26, 1993 (which was referenced as the appropriate source of service information in the NPRM). One of the commenters notes that the cracks on the affected stop support fittings are attributed to stress corrosion, which is a function of environment and time. As such, the inspection interval specified in the service bulletin is based on results of inspections of the fleet of Model 747 series airplanes, and on the degree of corrosion or cracking found during those inspections. Another commenter notes that the growth rate of stress corrosion cracks depends mainly on the environment and the age of the airplane, and that growth of such cracks is relatively slow when corrosion prevention measures are accomplished properly in accordance with the CPCP.

One of these commenters also requests that the repetitive interval for the visual inspections be extended from the proposed 18 months to 2,000 flight cycles or 36 months, whichever occurs first. That commenter points out that the 18-month intervals specified in the proposal are not consistent with the inspection intervals of 2,000 flight cycles that are specified for inspections of similar fittings at main entry door 5 that are required by AD 92-02-01, amendment 39-8137 (57 FR 5373, February 14, 1992).

The FAA concurs with the commenters' requests to extend the repetitive visual inspection intervals. As a result of these comments, the FAA has reviewed results from inspections of similar fittings of main entry door 5 that were accomplished in accordance with AD 92-02-01. Based on this review, the FAA has determined that repetitive inspections of fittings that are accomplished at 2,000-flight-cycle intervals are sufficient to detect cracked fittings in a timely manner. Therefore, the FAA has revised paragraph (a)(2)(i) of the final rule to state, ". . . repeat the visual inspection thereafter at intervals not to exceed 36 months or 2,000 flight cycles, whichever occurs first."

Request to Allow Continued Use of Subject Stop Support Fittings

One commenter requests that the proposal be revised to allow cracked stop support fittings of the main entry doors to be replaced with new stop support fittings that are made from either 7079-T651 or 7075-T651 material, provided that repetitive inspections of the replacement parts are performed at intervals of 36 months. The commenter states that a non-crackedstop support fitting made from 7079-T651 or 7075-T651 material provides the required strength capability. The commenter also notes that discarding all spares of stop support fittings made from 7079-T651 or 7075-T651 material is a waste of resources.

The FAA infers that the commenter is requesting that paragraph (a)(2)(ii) of the proposal be revised to allow installation of new parts made from either 7079-T651 or 7075-T651 material, or parts made from 7075-T73 material, and that paragraph (c) of the proposal not be included in the final rule. The FAA does not concur with the commenter's request to allow continued use of the subject stop support fittings. The FAA has determined that the cracking of the stop support fittings of the main entry doors is caused by a combination of internal residual stress resulting from the manufacturing process, clamp-up stress from the installation of the fittings, operational stress due to pressurization of the airplane, and stress corrosion. Other parts made from 7079-T651 or 7075-T651 material previously have been found to crack while in storage, due to internal residual stress. While the FAA is not requiring the replacement of uncracked stop support fittings of the main entry doors, the FAA will not promote long-term inspections of the stop support fittings by approving the installation of replacement parts that are subject to the same unsafe condition. No change to the final rule is necessary in this regard.

Request to Amend Aging Fleet Inspection and Modification Program

One commenter suggests that Boeing Service Bulletin 747-53-2358 be reviewed by the 747 Structures Task Group (STG) for possible inclusion in the aging aircraft inspection or modification program.

The FAA infers that the commenter is requesting that the FAA delay issuance of the final rule until the STG has reviewed Boeing Service Bulletin 747-53-2358 and considered including that service bulletin in Boeing Document No. D6-35999, dated March1989, "Aging Airplane Service Bulletin Structural Modification Program, Model 747." (The FAA previously issued AD 90-06-06, amendment 39-6490 (55 FR 8374, March 7, 1990), which requires incorporation of certain structural modifications in accordance with Boeing Document No. D6-35999.)

The FAA does not concur. The FAA has determined that rulemaking is necessary to address the unsafe condition (stress corrosion cracking on certain stop support fittings of the main entry doors, which could result in failure of the stop support fittings, loss of a main entry door, and consequent rapid decompression of the airplane). By issuing this new rule, the FAA has taken action to ensure that the stop support fittings of the main entry doors on the affected Boeing Model 747 series airplanes are inspected and replaced, if necessary, in a timely manner. This action does not preclude a review of Boeing Service Bulletin 747-53-2358 by the STG for possible inclusion in Boeing Document No. D6-35999. However, the FAA finds that to delay this action would be inappropriate in light of the identified unsafe condition. Therefore, no change to the final rule is necessary in this regard.

Explanation of Additional Changes Made to This Final Rule

In the proposal, paragraph (a)(1) reads, "If the fitting is made from 7075-T73 material, no further action is required by this AD." Since the issuance of the NPRM, the FAA has determined that such language could be misleading to operators, because follow-on actions are required for any stop support fitting of the main entry door that is made from 7079-T651 or 7075-T651 material, regardless of whether other stop support fittings are made from 7075-T73 material. Therefore, paragraph (a)(1) of the final rule has been revised to read, "...no further action is required by this AD for that fitting."

Conclusion

After careful review of the available data, including the comments noted above, the FAA has determined that air safety andthe public interest require the adoption of the rule with the changes described previously. The FAA has determined that these changes will neither increase the economic burden on any operator nor increase the scope of the AD.

Cost Impact

There are approximately 515 Boeing Model 747 series airplanes of the affected design in the worldwide fleet. The FAA estimates that 164 airplanes of U.S. registry will be affected by this AD.

It will take approximately 1 work hour per door to accomplish the required HFEC inspection, at an average labor rate of $60 per work hour. Based on these figures, the cost impact of the HFEC inspection required by this AD on U.S. operators is estimated to be $60 per door.

The cost impact figure discussed above is based on assumptions that no operator has yet accomplished any of the requirements of this AD action, and that no operator would accomplish those actions in the future if this AD were not adopted.

Should an operator be requiredto accomplish the required visual inspection, it will take approximately 2 work hours per door to accomplish the required actions, at an average labor rate of $60 per work hour. Based on these figures, the cost impact of the visual inspection required by this AD on U.S. operators is estimated to be $120 per door.

Should an operator elect to accomplish the optional terminating action that is provided by this AD action, the number of hours required to accomplish it would be approximately 124 work hours per door, at an average labor rate of $60 per work hour. Required parts would cost approximately $13,000 per door. Based on these figures, the cost impact of the optional terminating action on U.S. operators is estimated to be $20,440 per door.

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 amongthe 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.

For the reasons discussed above, I certify that this action (1) is not a "significant regulatory action" under Executive Order 12866; (2) is not a "significant rule" under 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. A final evaluation has been prepared for this action and it is contained in the Rules Docket. A copy of it 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 theAmendment 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 adding the following new airworthiness directive: