Discussion
We issued a notice of proposed rulemaking (NPRM) to amend 14 CFR part 39 by adding an AD that would apply to certain PW PW4000-94 inch turbofan engines with HPC 10th stage disk, part number (P/N) 51H710 or 53H976-06, installed and certain PW4000-100 inch turbofan engines with HPC 10th stage disk, P/N 53H976-06, installed. The NPRM published in the Federal Register on December 9, 2015 (80 FR 76400). The NPRM was prompted by a report of a crack found in the HPC 10th stage disk. The root cause of the crack was a manual polishing procedure, previously used during manufacture, that caused surface scratches on the disk. The NPRM proposed to require a USI or ECI of the HPC 10th stage disk. We are issuing this AD to prevent failure of the HPC 10th stage disk, which could lead to an uncontained disk release, damage to the engine, and damage to the airplane.
Comments
We gave the public the opportunity to participate in developing this AD. The following presents the comments received on the NPRM (80 FR 76400, December 9, 2015) and the FAA's response to each comment.
Support for the NPRM
The Boeing Company and United Airlines expressed support for the NPRM (80 FR 76400, December 9, 2015).
Request To Use ECI as Follow-on to USI
American Airlines requested that we revise Compliance paragraph (e) of this AD to add a statement that the ECI can be used to confirm the presence of a crack if a USI is initially performed and the ECI is the final authority on whether or not a crack is present on the disk.
We agree. We revised Compliance paragraph (e) of this AD to allow a follow-on ECI.
Request To Allow Disk Replacement Repairs
Atlas Air requested that we revise the Compliance paragraph (e) of this AD to allow use of disk replacement repairs per the PW PW4000-94/ 100 Clean, Inspect, Repair (CIR) Manual Part No. 51A357, Section 72-35- 10, Repair 07.
We disagree. This AD requires removal ofthe 10th stage disk if it fails inspection. There are no FAA-approved repairs allowed on the 10th stage disk. The previously approved PW4000-94/100 CIR Manual Part No. 51A357, Section 72-35-07, Repair 04 to the drum rotor, replaces the disk, resulting in a part eligible for installation. We did not change this AD.
Request To Allow ECI at Overhaul
Air India Limited requested that Compliance paragraph (e) of this AD allow an ECI when the HPC is ``overhauled'' rather than when it is ``removed from the engine.'' Air India Limited indicated that ``overhauled'' is clearer than ``removed from the engine''.
We disagree. The intent of this AD is to inspect the 10th stage disk at exposure. The phrase, ``Whenever the HPC front drum rotor is removed from the engine . . .'' clearly describes the appropriate level of exposure for performing the ECI. We did not change this AD.
Request To Waive Repeat USI
Air India Limited requested that we revise Compliance paragraph (e) of this AD to indicate that a repeat USI should be waived to reduce the maintenance burden if the low-pressure turbine (LPT) is removed in less than 100 hours since the last USI.
We disagree. Our safety risk assessment assumed that a USI is performed whenever the high-pressure turbine (HPT) or LPT is removed from the engine and an ECI is performed whenever the HPC front drum rotor disk assembly is removed from the engine. We determined the inspection interval in the Compliance paragraph (e) of this AD provides an acceptable level of safety. We did not change this AD.
Request To Remove Compliance Statement
FedEx requested that we revise Compliance paragraph (e) of this AD to remove the statement, ``Comply with this AD within the compliance times specified, unless already done.'' FedEx stated that there are no compliance times specified and the compliance requires a repetitive inspection, so the statement does not apply.
We disagree. The statement ``. . . unless already done'' allows an operator who has performed an initial inspection before the effective date of the AD, but has not yet returned the part to service, to take credit for that action. While there is no calendar or cyclic time given, the requirements of this AD must be met when the HPT, LPT, or HPC front drum rotor disk assembly is removed from the engine. We did not change this AD.
Conclusion
We reviewed the relevant data, considered the comments received, and determined that air safety and the public interest require adopting this AD with the changes described previously. We have determined that these minor changes:
[Agr]re consistent with the intent that was proposed in the NPRM (80 FR 76400, December 9, 2015) for correcting the unsafe condition; and
Do not add any additional burden upon the public than was already
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proposed in the NPRM (80 FR 76400, December 9, 2015).
We also determined that these changes will not increase the economic burden on any operator or increase the scope of this AD.
Related Service Information Under 1 CFR Part 51
We reviewed PW Alert Service Bulletin (ASB) PW4G-100-A72-255, dated August 31, 2015 and PW ASB PW4ENG A72-833, dated August 20, 2015. The ASBs provide lists of affected HPC disks and describe procedures for USI and ECI of the HPC 10th stage disk. 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.
Costs of Compliance
We estimate that this AD affects 763 engines installed on airplanes of U.S. registry. We also estimate that it would take about 12 hours per engine to do the inspection. The average labor rate is $85 per hour. Based on these figures, we estimate the cost of this AD on U.S. operators to be $778,260.
Authority for This Rulemaking
Title 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.
We 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.
Regulatory Findings
This 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.
For the reasons discussed above, I certify that this AD:
(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),
(3) Will not affect intrastate aviation in Alaska to the extent that it justifies making a regulatory distinction, and
(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.
List of Subjects in 14 CFR Part 39
Air transportation, Aircraft, Aviation safety, Incorporation by reference, Safety.