AD 96-17-04

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 737-100 and -200 series airplanes was published in the Federal Register on March 13, 1996 (61 FR 10294). That action proposed to require inspections to detect cracking of the support fittings of the Krueger flap actuator and, if necessary, replacement of existing fittings with new steel fittings and modification of the aft attachment of the actuator.

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

One commenter supports the proposal.

Request to Revise Proposed Inspection Requirements

The Air Transport Association (ATA), on behalf of its member operators, requests that the proposed requirement to perform repetitive eddy current inspections be replaced with a requirement to perform close visual inspections at 3,000-flight hour intervals, followed by an eddy current inspection or replacement of the fitting within a 4-year period. This commenter maintains that this alternative inspection program is:

1. more consistent with the recommendations of the airframe manufacturer; 2. equivalent in safety to that proposed in the notice; and 3. more cost effective.

Further, this commenter states that, while the proposed eddy current inspection may be viewed as a more critical inspection process, it is not necessary to respond to the airworthiness concern. This commenter contends that, in order to determine whether a more stringent process is required (i.e., more stringent than the manufacturer's recommendations), the FAA should review service history data to determine whether cracking of the subject support fittings has actually become a fleet-wide problem. The commenter maintains that, while the one incident described in the preamble to the notice was certainly of concern, there is insufficient data to indicate that cracked support fittings is an industry problem.

The FAA does not concur. As explained in the preamble to the notice, the subject cracking in the fittings is attributed to stress corrosion combined with fatigue. The crack growth rate for such cracking is not known; however, it is known that material that the fitting is made from, 7075-T6 aluminum, is highly susceptible to stress corrosion cracking and has low toughness. It is also known that the critical crack size for this fitting is 0.165 inch. Cracks of this small size cannot be found with a high degree of confidence using a visual inspection technique. An eddy current inspection is a much more reliable method of finding such small cracks.

As for the service history of the subject problem, there have been several reports of cracking found in actuator attach support fitting assemblies on a number of in-service Model 737 series airplanes. There also have been two accidents involving hydraulic system failures that were associated with the failure of the actuator attach lugs on the support fittings. The FAA considers this a sufficient amount of service history to demonstrate that a potential unsafe condition associated with the subject cracking exists in airplanes equipped with the subject fittings.

In light of the small critical crack size, the high susceptibility to stress corrosion cracking of 7075-T6 material, and the ample service history relative to the addressed unsafe condition, the FAA does not find that the commenter's suggested alternative inspection program would provide an acceptable level of safety compared to that required by this final rule.

Request to Revise Proposed Inspection Intervals

One commenter requests that the proposed inspections be required in terms of flight cycles, rather than in terms of time-in-service. The commenter states that, because fatigue cracking of the actuator support fitting is caused by cycling of the Krueger flap, the maximum inspection intervals should be limited by flight cycles, not flight hours.

The FAA does not concur. The cracking mechanism associated with the addressed problem is stress corrosion cracking combined with fatigue. Although the commenter is correct that fatigue is cycle-driven, stress corrosion cracking is time- or flight hour-driven, since it is caused by a sustained tensile stress in a corrosive environment. Therefore, the FAA finds that a flight hour (time-in-service) inspection interval is appropriate for these inspections. Conclusion

After careful review of the available data, including the comments noted above, the FAA has determined that air safety and the public interest require the adoption of the rule as proposed.

Cost Impact

There are approximately 727 Model 737-100 and -200 series airplanes of the affected design in the worldwide fleet. The FAA estimatesthat 270 airplanes of U.S. registry will be affected by this AD, that it will take approximately 12 work hours per airplane (6 work hours per wing) to accomplish the required actions, and that the average labor rate is $60 per work hour. Based on these figures, the cost impact of the AD on U.S. operators is estimated to be $194,400, or $720 per airplane, per inspection.

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.

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.

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 the Amendment

Accordingly, pursuant to the authority delegated to me by the Administrator, the Federal Aviation Administration amends part39 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: