Discussion \n\n\n\tWe issued a notice of proposed rulemaking (NPRM) to amend 14 CFR part 39 by adding an AD that would apply to certain The Boeing Company Model 777-200, -200LR, -300, and -300ER series airplanes. The NPRM published in the Federal Register on July 14, 2017 (82 FR 32507). The NPRM was prompted by reports of corrosion in the aft fuselage. The NPRM proposed to require a one-time review of the operator's maintenance procedures, repetitive detailed internal and external inspections for corrosion or cracking, and applicable on-condition actions. The NPRM also included an optional terminating action for the inspections. \n\tWe are issuing this AD to detect and correct untreated vacuum waste system spills or leaks, which could cause corrosion of the airplane structure, which could lead to fatigue cracks, and could ultimately result in rapid decompression and loss of structural integrity. \n\nComments \n\n\n\tWe gave the public the opportunity to participate in developing this final rule. The following presents the comments received on the NPRM and the FAA's response to each comment. \n\nSupport for the NPRM \n\n\n\tBoeing expressed its support for the NPRM. \n\nRequest To Extend the Inspection Compliance Time \n\n\n\tCathay Pacific expressed concern that it would not be able to demonstrate that it has performed an acceptable records review, which is required to demonstrate that all prior vacuum waste system spills or leaks were cleaned and neutralized using the acceptable procedure. Cathay Pacific noted that some airplanes in its fleet have been in service for more than 20 years, so an older record could easily be missed when doing this review. Cathay Pacific stated that because of this concern, it has opted to treat all airplanes as having inadequate records and perform inspections on them. Cathay Pacific stated that the applicable inspection compliance times do not allow waiting for the next scheduled maintenance check, leading toadditional downtime. \n\tWe infer that Cathay Pacific is requesting that we extend the compliance time for the initial and repetitive inspections. We disagree with the commenter's request. We have determined that the compliance times specified in this AD are necessary to address the identified unsafe condition. However, under the provisions of paragraph (j) of this AD, we will consider requests for approval of an alternative method of compliance (AMOC), including extension of the compliance times, if sufficient data is submitted to substantiate that a different compliance time will provide an acceptable level of safety. We have not changed this AD in this regard. \n\nRequest To Extend the Compliance Times for Certain Airplanes \n\n\n\tUnited Airlines (UAL) and Air France (AF) requested that we revise the compliance times for airplanes on which certain inspections have been done. UAL requested that the compliance time be extended for airplanes on which corrosion prevention and control program (CPCP) inspections have already been done. UAL noted that many operators have proven corrosion control programs that do not have corrosion findings greater than CPCP level 1, which mitigates the corrosion risk factor. UAL suggested that the initial inspection compliance time be extended for airplanes on which maintenance records show that no corrosion findings greater than CPCP level 1 have occurred in the inspection area in the 10 years prior to the effective date of the AD. \n\tAF requested that the compliance times be extended for airplanes on which maintenance planning document (MPD) inspections have been done. AF noted that existing MPD items require general visual inspections of certain areas below the aft and bulk cargo compartment floor panels. AF stated that because the majority of its fleet has already been inspected under the MPD items, the compliance times in the NPRM are too restrictive. AF noted that the initial compliance times cannot be accommodated into its 777 Cor heavy checks interval. AF suggested compliance times based on the number of days since the date of issuance of the original airworthiness certificate or date of issuance of the original export certificate of airworthiness instead of days after the effective date of the AD as specified in the proposed AD. \n\tWe disagree with the commenters' requests to extend the compliance times. The CPCP has three different levels of corrosion damage, as defined within the MPD, based on the severity and frequency of corrosion findings and requires operators to adjust their individual programs to limit corrosion findings to level 1 if they have level 2 or higher findings. However, operators have reported finding recurring corrosion damage in-between scheduled CPCP or MPD inspections that was due to untreated vacuum waste system residue. Additionally, we have reviewed the existing MPD inspections and have determined that the MPD inspections do not repeat at adequate intervals to address the unsafe condition. The determinations of the unsafe condition, mitigating actions, and compliance times were coordinated with the manufacturer. Under the provisions of paragraph (j) of this AD, we will consider requests for approval of AMOCs, including extensions of the compliance times, if sufficient data, such as an operator's individual CPCP and practices for treating vacuum waste system residue, is submitted to substantiate that a different compliance time will provide an acceptable level of safety. We have not changed this AD in this regard. \n\nRequest To Allow the Use of a Different Sodium Bicarbonate Compound \n\n\n\tJapan Airlines (JAL) and AF requested that we revise the NPRM to allow the use of a different sodium bicarbonate compound than the ASTM D928 specified in Boeing Alert Service Bulletin 777-53A0083, dated April 20, 2017. JAL noted it had difficulty finding the specified sodium bicarbonate compound, but could find an equivalent product. AF noted that is has a corresponding product. \n\tWe partially agree with the commenters' request. We agree that an equivalent sodium bicarbonate compound is acceptable. Boeing has issued Boeing Information Notice 777-53A0083 IN 01, dated September 1, 2017, to clarify that a commercially available sodium bicarbonate compound is acceptable for compliance. However, we do not agree to revise this AD because it does not require the use of ASTM D928 sodium bicarbonate compound. As indicated in the Accomplishment Instructions and Figure 2 of Boeing Alert Service Bulletin 777-53A0083, dated April 20, 2017, sodium bicarbonate must be used, but a specific compound type is not identified. \n\n((Page 5687)) \n\nRequest To Define a Neutral pH \n\n\n\tAmerican Airlines (AAL) requested that we revise the NPRM to define a neutral pH as one that has a value between 6.5 and 8.5, to account for natural variations in tap water. AAL stated that the NPRM does not define a tolerance from the common definition of neutral pH, which is a pH of 7. \n\tWedisagree with the commenter's request. Paragraph 3.A., General Information, Note 19, of Boeing Alert Service Bulletin 777-53A0083, dated April 20, 2017, defines neutralization as making the vacuum waste system spill or leak contents non-acidic or non-corrosive. No specific pH value is defined in the service information or required by this AD. Therefore, operators can include tolerances for a neutral pH. One way for operators to account for pH variances of their local clean water supply is to measure the pH level of their clean water supply in order to establish a baseline pH level, that can then be used to compare against samples taken from the fuselage structure. We have not changed this AD in this regard. \n\nRequest To Define a Standard Litmus Paper \n\n\n\tAAL and Cathay Pacific requested that we revise the NPRM to define a standard part number for the litmus paper to use in determining if the acid is neutralized. AAL noted that the NPRM does not specify a resolution or range for the litmus paper. Cathay Pacific claimed that because Boeing Alert Service Bulletin 777-53A0083, dated April 20, 2017, does not list a specific litmus paper, the instruction to ''use litmus paper'' is ambiguous and operators would not be able to determine if an acceptable litmus paper is used. \n\tWe disagree with the commenters' request. Litmus paper is a commonly available tool. Accomplishing the cleaning and neutralization steps in Boeing Alert Service Bulletin 777-53A0083, dated April 20, 2017, does not specify the use of a specific brand or type of litmus paper. We have not changed this AD in this regard. \n\nRequest To Define the Location and Quantity of Litmus Paper Testing Points \n\n\n\tAAL and Cathay Pacific requested that we revise the NPRM to define the locations where litmus paper testing must be done, as well as the number of samples that must be taken. AAL pointed out that the structural features that must be chemically neutralized are specified in Boeing Alert Service Bulletin 777-53A0083, dated April 20, 2017, while the litmus paper testing spots are not. Cathay Pacific suggested that Boeing Alert Service Bulletin 777-53A0083, dated April 20, 2017, implies that operators should do litmus paper testing on all the structural features in the inspection and neutralization area, but stated it does not believe this is the intent. \n\tWe agree to provide clarification on the number and location of litmus paper testing spots and confirm that paragraph (i) of this AD does not require testing with litmus paper at all structural features in the neutralization area. However, we do not agree that it is necessary to provide a specific number of samples or testing locations. The objective of the litmus paper testing is to verify that there are no remaining acidic or corrosive substances on the structure. The appropriate level of testing may vary between airplanes depending on factors such as maintenance records, previous spills or leaks, or repairs that obstruct access. Samples should be tested at enough locations within the affected area of the structure for the operator to determine that there are no residual acidic or corrosive contents on primary structural elements in the inspection area, including any locations where the sodium bicarbonate solution visibly reacted when applied, which indicates the presence of acidic or corrosive substances, and any locations where there are signs of corrosion damage. We have not changed this AD in this regard. \n\nRequest To Allow the Use of Alternative Corrosion Inhibiting Compounds \n\n\n\tAAL requested that we allow the use of alternative corrosion inhibiting compounds (which are applied to the cleaned and neutralized areas as part of the required restoration) as specified in Boeing Aircraft Maintenance Manual (AMM) Task 51-05-01-210-803. AAL noted that Boeing Alert Service Bulletin 777-53A0083, dated April 20, 2017, specifies BMS3-29 compound and does not allow the use of alternative compounds. \n\tWe agreewith the commenter's request. Boeing AMM Task 51-05-01- 210-803 specifies the application of a single coat of water displacing/ anti-corrosion compound BMS3-29 or BMS3-35 at a minimum, with an option to layer different compounds in areas with high potential for severe corrosion. We have added paragraph (h)(3) of this AD to specify acceptable alternative corrosion inhibiting compounds. \n\nRequest To Update the Costs of Compliance \n\n\n\tCathay Pacific requested that we update the work-hours estimate for cleaning and neutralization in the NPRM. Cathay Pacific stated that the area to be neutralized covers 13 frames and 15 stringers, so it will require more work-hours to complete this task. \n\tWe disagree with the commenter's request. The work-hours estimate is determined by Boeing and provided for informational and planning purposes only. In addition, Cathay Pacific did not provide any alternative estimates for the work-hours. We have not changed this AD in this regard. \n\nConclusionWe reviewed the relevant data, considered the comments received, and determined that air safety and the public interest require adopting this final rule with the changes described previously and minor editorial changes. We have determined that these minor changes: \n\tAre consistent with the intent that was proposed in the NPRM for correcting the unsafe condition; and \n\tDo not add any additional burden upon the public than was already proposed in the NPRM. \n\tWe also determined that these changes will not increase the economic burden on any operator or increase the scope of this final rule. \n\nRelated Service Information Under 1 CFR Part 51 \n\n\n\tWe reviewed Boeing Alert Service Bulletin 777-53A0083, dated April 20, 2017. The service information describes procedures for a one-time review of the operator's maintenance procedures, repetitive detailed internal and external inspections for corrosion or cracking, cleaning and neutralization of the internal inspection area (an optional terminating action), and applicable on-condition actions. This service information is reasonably available because the interested parties have access to it through their normal course of business or by the means identified in the ADDRESSES section. \n\nCosts of Compliance \n\n\n\tWe estimate that this AD affects 161 airplanes of U.S. registry. The cost to review an operator's maintenance procedures varies depending on the operator's recordkeeping system and fleet size so we did not include a specific estimate for that action. We estimate the following costs to comply with the remaining actions of this AD: \n\n((Page 5688)) \n\n\n\n\n\n\n\tEstimated Costs ---------------------------------------------------------------------------------------------------------------- \n\tCost on U.S. \n\tAction Labor cost Parts cost Cost per product operators ---------------------------------------------------------------------------------------------------------------- Inspections................... 75 work-hours x $85 $0 $6,375 per $1,026,375 per \n\tper hour = $6,375 inspection cycle. inspection cycle. \n\tper inspection \n\tcycle. ---------------------------------------------------------------------------------------------------------------- \n\n\n\n\n\tEstimated Costs for Optional Terminating Actions ---------------------------------------------------------------------------------------------------------------- \n\tCost per \n\tAction Labor cost Parts cost product ---------------------------------------------------------------------------------------------------------------- Cleaning and neutralization................ 30 work-hours x $85 per hour = $0 $2,550 \n\t$2,550. ---------------------------------------------------------------------------------------------------------------- \n\n\n\tWe have received no definitivedata that would enable us to provide cost estimates for the on-condition actions specified in this AD. \n\nAuthority for This Rulemaking \n\n\n\tTitle 49 of the United States Code specifies the FAA's authority to issue rules on aviation safety. Subtitle I, section 106, describes the authority of the FAA Administrator. Subtitle VII: Aviation Programs, describes in more detail the scope of the Agency's authority. \n\tWe are issuing this rulemaking under the authority described in Subtitle VII, Part A, Subpart III, Section 44701: ''General requirements.'' Under that section, Congress charges the FAA with promoting safe flight of civil aircraft in air commerce by prescribing regulations for practices, methods, and procedures the Administrator finds necessary for safety in air commerce. This regulation is within the scope of that authority because it addresses an unsafe condition that is likely to exist or develop on products identified in this rulemaking action. \n\tThis AD is issued in accordance with authority delegated by the Executive Director, Aircraft Certification Service, as authorized by FAA Order 8000.51C. In accordance with that order, issuance of ADs is normally a function of the Compliance and Airworthiness Division, but during this transition period, the Executive Director has delegated the authority to issue ADs applicable to transport category airplanes to the Director of the System Oversight Division. \n\nRegulatory Findings \n\n\n\tThis AD will not have federalism implications under Executive Order 13132. This AD will not have a substantial direct effect 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. \n\tFor the reasons discussed above, I certify that this AD: \n\t(1) Is not a ''significant regulatory action'' under Executive Order 12866, \n\t(2) Is not a ''significant rule'' under DOT Regulatory Policies and Procedures (44 FR 11034, February 26, 1979), \n\t(3) Will not affect intrastate aviation in Alaska, and \n\t(4) 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. \n\nList of Subjects in 14 CFR Part 39 \n\n\n\tAir transportation, Aircraft, Aviation safety, Incorporation by reference, Safety.