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 all Boeing Model 727 series airplanes was published in the Federal Register on October 3, 1996 (61 FR 51621). That action proposed to require a one-time visual inspection of the manual extension gearbox assembly of the main landing gear (MLG) to detect whether this assembly contains the correct left and right gearbox housings/housing assemblies. If incorrect housings/housing assemblies are installed, that action also proposed to require repetitive dye penetrant inspections of these housings to determine whether cracking has occurred; and ultimately, replacement of these housings with correct housings.\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\nSupport for the Proposal \n\n\tThree commenters support the proposed rule. \n\nRequest to Withdraw the Proposal \n\n\tSeveral commenters state that the proposed AD is unnecessary because AD 79-04-01 R3, amendment 39-4000 (45 FR 84014, December 22, 1980), addresses the problem, thus the proposed AD only duplicates time and effort. One of these commenters points out that the "incomplete information . . ." of Boeing Overhaul Manual 32-35-01 (referred to in the Discussion Section of the preamble of the proposed AD) is "a very gray area." This commenter contends that almost all overhaul manuals contain "incomplete information," even when components are affected by AD's. The commenters assert that it is the responsibility of the operators and component vendors to determine which parts are affected by an AD. Two of these commenters state that all of their gearbox housings comply with the requirements of AD 79-01-04 R3. \n\n\tThe FAA does not concur that the proposed AD should be withdrawn. The FAA acknowledges that, even though an overhaul manual may contain incomplete information, operators are responsible for the overall airworthiness of the airplane. In addition, component vendors should be cognizant of AD's that affect parts they are overhauling. \n\n\tHowever, as explained in the Discussion section of the preamble of the proposed AD, the FAA has received a report indicating that a manual extension gearbox assembly for the MLG on a Boeing Model 727 series airplane had been replaced with a modified gearbox assembly that did not comply with AD 79-01-04 R3. In light of this report and the fact that the manufacturer's overhaul manual contained incomplete information for a period of time, the FAA finds that there currently may be other Model 727 series airplanes in service that are operating with incorrect gearbox housing/housing assemblies installed. Therefore, the FAA has determined that a one-time visual inspection of the manual extension gearbox assembly of the main landing gear (MLG) is necessary to detect whether or not these discrepant housings have been installed.\n\nRequest to Extend Compliance Time for One-Time Visual Inspection\n \n\tOne commenter requests that the compliance time for accomplishing the proposed one-time visual inspection be extended from the proposed 6 months to the first "C" check after the effective date of the AD. The commenter points out that it has found no cracked gearbox housing since accomplishment of AD 79-04-01 R3. \n\n\tThe FAA does not concur. In developing an appropriate compliance time for this action, the FAA considered the safety implications, parts availability, and normal maintenance schedules for timely accomplishment of the visual inspection. In consideration of these items, as well as the report indicating that a manual gearbox assembly containing an incorrect housing had been installed on an airplane in service, the FAA has determined that a 6-month compliance time is appropriate. \n\nRequest to Extend Compliance Time for Initial Dye PenetrantInspection \n\n\tOne commenter requests that the compliance time for accomplishing the dye penetrant inspection required by paragraph (c) of the proposed AD be extended from the proposed "prior to further flight" to "within 100 hours time-in-service or 50 landings, whichever occurs first." The commenter states that the proposed compliance time is not justified because the FAA has not received recent reports of incorrect housing/housing assemblies that have been cracked. This commenter also states that it would have to special schedule its fleet of airplanes to accomplish this inspection within the proposed compliance time; this would entail considerable additional expenses and schedule disruptions. \n\n\tThe FAA concurs partially. The FAA finds that stress corrosion cracking in the vertical support attaching lugs of the MLG manual extension-gearbox housing is caused by the combined action of corrosion and stress, either external (applied) or internal (residual). It is difficult to predict when stress corrosion cracking will occur because corrosion is influenced by unpredictable factors, such as the operating environment, maintenance, and the passage of time. If those housings/housing assemblies are still installed on airplanes more than 17 years after AD 79-04-01 R3 was issued, there is a greater likelihood that stress corrosion cracking exists; therefore, the FAA finds that accomplishment of a dye penetrant inspection prior to further flight following accomplishment of the initial visual inspection is warranted. \n\n\tHowever, the FAA's intent is that the dye penetrant inspection be conducted during a regularly schedule maintenance visit for the majority of the affected fleet, when the airplanes would be located at a base where special equipment and trained personnel would be readily available, if necessary. The FAA finds that in lieu of accomplishing a dye penetrant inspection, an operator may choose to replace the discrepant part with an updated part prior tofurther flight following accomplishment of the initial visual inspection. Therefore, paragraph (c) of the final rule has been revised to provide operators with this option. \n\nRequest to Revise Dye Penetrant Inspection Requirement \n\n\tOne commenter requests that operators be advised of where the incorrect gear boxes were found and of the source that obtained them. Subsequently, the discrepant gear boxes could be tracked and the proposed inspection requirements could be limited to those operators that received the discrepant housings from the suspect sources. The commenter also suggests that the initial visual inspection be accomplished within 300 landings and repeated at intervals not to exceed 6 months, and suggests that the replacement be accomplished within 18 months. \n\n\tThe FAA does not concur. The FAA is unable to determine all sources of discrepant housings. Therefore, the FAA finds that the proposed one-time visual inspection is necessary to determine whether certain gearbox housings have been installed. In addition, the FAA finds that a compliance time based on a number of landings is not acceptable because, as discussed previously, it is difficult to predict when stress corrosion cracking will occur. \n\nRequest to Revise Applicability of the Proposal \n\n\tTwo commenters request that the applicability of the proposed AD be revised to exclude airplanes whose operators are confident of their gearbox installations or have internal procedures to ensure that only correct housing/housing assemblies are installed in accordance with AD 79-04-01 R3. \n\n\tThe FAA does not concur. A one-time visual inspection to confirm the presence of correct housings should not pose an undue burden to operators. If an operator chooses to review its available records, however, to determine that incorrect manual extension gearbox assemblies have not been installed, the operator may request approval of an alternative method of compliance in accordance with the provisions ofparagraph (d) of this AD. \n\nRequest to Revise Table 2 of the Proposal \n\n\tOne commenter requests that Table 2 of the proposed AD, which lists the part numbers of correct replacement housings and housing assemblies, be revised to include the Boeing part number of the die forging from which these parts could be made. (Not all of the correct parts are made from this forging.) The commenter points out that the part number on this die forging is easily ascertained and permanent, unlike the numbers on the housings/housing assemblies currently listed in Table 2. For the reasons the commenter states, the FAA concurs and has revised Table 2, as requested. In addition, because all the incorrect housings/housing assemblies are made from a certain die forging, the FAA has added the Boeing part number of that forging to Table 1, which lists the part numbers of incorrect housing and housing assemblies.\n\nConclusion \n\n\tAfter careful review of the available data, including the comments notedabove, the FAA has determined that air safety and the public interest require the adoption of the rule with the changes previously described. The FAA has determined that these changes will neither increase the economic burden on any operator nor increase the scope of the AD. \n\nCost Impact \n\n\tThere are approximately 1,560 Boeing Model 727 series airplanes of the affected design in the worldwide fleet. The FAA estimates that 1,054 airplanes of U.S. registry will be affected by this AD. \n\n\tThe FAA estimates that it will take approximately 2 work hours per airplane to accomplish the required one-time visual inspection, at an average labor rate of $60 per work hour. Based on these figures, the cost impact of the AD on U.S. operators is estimated to be $126,480, or $120 per airplane. \n\n\tShould a dye penetrant inspection need to be performed, the FAA estimates that each inspection will take approximately 20 work hours per airplane, at an average labor rate of $60 per work hour. Based on these figures, the cost impact of the proposed dye penetrant inspection on U.S. operators is estimated to be $1,200 per airplane, per inspection. \n\n\tShould parts have to be replaced, the FAA estimates that it will take approximately 16 work hours per airplane to accomplish the replacement, at an average labor rate of $60 per work hour. Replacement parts will cost approximately $4,000 per housing. Based on these figures, the cost impact of replacement of parts on U.S. operators is estimated to be $4,960 per airplane if one housing is to be replaced, and $8,960 if both housings are to be replaced. \n\n\tThe cost impact figures discussed above are based on assumptions that no operator has yet accomplished any of the requirements of this AD action, and that no operator will accomplish those actions in the future if this AD were not adopted. \n\nRegulatory Impact \n\n\tThe regulations adopted herein will not have substantial direct effects on the States, on the relationship betweenthe 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\nList 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. The authority citation for part 39 continues to read as follows: Authority: 49 U.S.C. 106(g), 40113, 44701. § 39.13 - (Amended) \n\t2. Section 39.13 is amended by adding the following new airworthiness directive: