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 British Aerospace (Jetstream) Model 4101 airplanes was published in the Federal Register on January 8, 1998 (63 FR 1074). That action proposed to require repetitive detailed visual inspections to detect cracking or other damage of certain diaphragm support structures of the forward equipment compartment; and repair, if necessary.
Interested persons have been afforded an opportunity to participate in the making of this amendment. Due consideration has been given to the comments received.
Request to Allow Flight With Known Cracks
One commenter, the manufacturer, requests that the proposed AD be revised to allow operators to continue operation of an unrepaired airplane for up to 300 flight cycles following detection of cracking of certain diaphragm support structures of the forward equipment compartment. The commenter states that, during full-scale fatigue testing, failure of both diaphragms occurred, and the test continued for another 24,000 flight cycles before either of the diaphragms was replaced. The commenter further states that, during the period between detection of the cracking and replacement of the diaphragms, no damage was detected that would cause concern regarding the structural integrity of the airplane. In light of these fatigue testing data, the commenter notes that the compliance time of 300 flight cycles after detection of cracking, as specified in the service bulletin, is already a very conservative threshold.
The FAA does not concur. It is the FAA s policy to require repair of known cracks prior to further flight, except in certain cases of unusual need, as discussed below. This policy is based on the fact that such damaged airplanes do not conform to the FAA-certificated type design and, therefore, are not airworthy until a properly approved repair is incorporated. The FAA s policy regarding flight with known cracks does allow deferral of repairs in certain cases, if there is an unusual need for a temporary deferral. Unusual needs include such circumstances as legitimate difficulty in acquiring parts to accomplish repairs. Because the FAA is not aware of any unusual need for repair deferral in regard to this AD, the FAA has determined that any subject diaphragm that is found to be cracked must be repaired prior to further flight in accordance with a method approved by the FAA. However, operators may request approval of an alternative method of compliance if data are provided to substantiate that such a method would provide an acceptable level of safety.
Request to Remove Requirement for Repetitive Inspections After Repair
One commenter, the manufacturer, requests that the requirement to continue the repetitive inspections following the installation of an improved diaphragm be removed from the proposal. The commenter states that, during full-scale fatigue testing, no new cracking of the diaphragms was detected following repair of the diaphragms until 65,700 total flight cycles. Based on these data, the commenter states that an inspection threshold of 20,000 landings after installation of a new diaphragm, and a repetitive inspection interval thereafter of 6,000 landings, would be adequate to ensure that any cracking would be detected in a timely manner. The commenter further states that such an inspection threshold and repetitive interval will be added to the Airworthiness Limitations specified in Chapter 5 of the Jetstream 4100 Airplane Maintenance Manual (AMM).
The FAA does not concur with the commenter s request to remove the requirement for repetitive inspections of the diaphragm following replacement. First, the commenter implies that an improved diaphragm is available; however, the FAA is not aware of any such improved part. Further, the lack of specific data in the servicebulletin regarding the repair prevents the FAA from determining whether elimination of the repetitive inspection requirement is warranted. Also, though the FAA acknowledges the manufacturer s intent to incorporate a program of repetitive inspections into the Airworthiness Limitations specified in Chapter 5 of the AMM, the FAA would have to engage in further rulemaking in order to require such an inspection program.
Although the FAA does not concur with the request to remove the repetitive inspection requirement following accomplishment of a repair, paragraph (b) of this AD contains a provision for requesting approval of an alternative method of compliance to address operators unique circumstances. In accordance with paragraph (b) of this AD, an operator may submit a repair method, along with a proposed repetitive inspection program or data to support elimination of the repetitive inspection requirement, for consideration by the FAA. No change to the final rule is necessary.
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.
Interim Action
This is considered to be interim action until final action is identified, at which time the FAA may consider further rulemaking.
Cost Impact
The FAA estimates that 55 airplanes of U.S. registry will be affected by this AD, that it will take approximately 1 work hour per airplane to accomplish the required inspection, 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 $3,300, or $60 per airplane, per inspection cycle.
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 maybe 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 adding the following new airworthiness directive: