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 737-300, -400, and -500 series airplanes was published in the Federal Register on December 8, 1994 (59 FR 63277). That action proposed to require an inspection to determine the type of topcoat material currently on the insulation of the inner wall of the fan duct cowl (the firewall) of the thrust reversers, and application of an improved topcoat material, if necessary. \n\n\tInterested persons have been afforded an opportunity to participate in the making of this amendment. Due consideration has been given to the comments received. \n\n\tSeveral commenters request that the rule be revised so that, if the inspection reveals that the suspect topcoat material is present, operators would not be required to apply the improved topcoat material immediately prior to further flight. These commentersstate that the application of the improved topcoat material should be permitted at the operator's convenience after a positive inspection finding. This would encourage operators to conduct the inspection promptly, and then allow them to schedule the time and materials necessary for accomplishing the topcoat application at their subsequent heavy maintenance check. One commenter contends that in-service experience has shown that the risk of an engine fire resulting from the problems associated with the topcoat material is very low; in light of this, it is appropriate to allow an extended interval of time between conducting the inspection and applying the improved top coat. \n\n\tThe FAA does not concur with the request to permit application of the improved topcoat material at an extended interval after the inspection findings. It is the FAA's general policy that, once an unsafe condition has been determined to exist, that condition cannot be allowed to continue in the fleet. Therefore, it is essential that, if the inspection reveals that application of the improved topcoat is necessary, such application must be accomplished prior to further flight after the inspection. \n\n\tHowever, in light of the fact that there have been no in-service incidents associated with the addressed unsafe condition, and because the topcoat application procedures may be extensive for some operators, the FAA considers that the compliance time for the required actions can be extended somewhat. It is the FAA's intent that, if the application of the improved topcoat is necessary, it should be performed during a regularly scheduled maintenance interval when the airplane is at a base where special equipment, necessary parts, and trained personnel are available. If the compliance time for the action required by this AD is parallel to the operator's regular maintenance interval, the operator can easily schedule both the inspection and any necessary topcoat application to be performed during thesame maintenance hold. In consideration of these factors, the FAA finds that the compliance time may be extended from the proposed 24 months to 30 months without compromising safety. This extension will allow the majority of affected operators to accomplish the required actions during scheduled maintenance visits. \n\n\tAfter 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 with the change previously described. The FAA has determined that this change will neither increase the economic burden on any operator nor increase the scope of the AD. \n\n\tThere are approximately 135 Model 737-300, -400, and -500 series airplanes of the affected design in the worldwide fleet. The FAA estimates that 18 airplanes of U.S. registry will be affected by this AD, that it will take approximately 13 work hours per airplane to accomplish the required actions, and that the average labor rate is $60 per work hour. Required parts will be provided by the manufacturer at no charge to the operators. Based on these figures, the total cost impact of the AD on U.S. operators is estimated to be $14,040, or $780 per airplane. \n\n\tThe total 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. \n\n\tThe 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. \n\n\tFor 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."\n\n List of Subjects in 14 CFR Part 39 \n\n\tAir transportation, Aircraft, Aviation safety, Incorporation by reference, Safety.\n\nAdoption of the Amendment \n\n\tAccordingly, 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:\n\n PART 39 - AIRWORTHINESS DIRECTIVES \n\n\t1.\tThe authority citation for part 39 continues to read as follows: Authority: 49 U.S.C. App. 1354(a), 1421 and 1423; 49 U.S.C. 106(g); and 14 CFR 11.89. § 39.13 - (Amended) \n\n\t2.\tSection 39.13 is amended by adding the following new airworthiness directive: