A proposal to amend part 39 of the Federal Aviation Regulations (14 CFR part 39) to include an AD that is applicable to certain models of Pratt & Whitney (PW) PW4000 series turbofan engines was published in the Federal Register on March 22, 2001 (66 FR 16017). That action proposed to require operators to perform initial and repetitive inspections for cracking of high pressure compressor (HPC) front drum rotors based on cycle usage. The action also proposed to require the removal from service of any cracked HPC front drum rotors in accordance with PW Alert Service Bulletin (ASB) No. PW4ENG A72-722, dated September 29, 2000.
Comments
Interested persons have been afforded an opportunity to participate in the making of this amendment. Due consideration has been given to the comments received.
Five-Cycle Flyback Allowance
Six commenters express concern with the deletion of the five-cycle flyback allowance.
One commenter requests clarification of the difference between the NPRM and ASB 72-722 with respect to the deletion of the five-cycle flyback allowance for engines with indications requiring eddy current inspection (ECI) verification.
Another commenter states that the AD should provide the five-cycle flyback allowance because eight reported findings of HPC front drum cracking were discovered in the shop and that the cracks do not propagate quickly.
A third commenter expresses concern that in cases of suspect cracks, the airline would incur an undue economic burden waiting for confirmation as to whether the visual indication is a crack or not. Also, the commenter suggests an initial inspection threshold of 1,200 cycles-since-new and a 20-cycle window for ECI reinspection to verify a visual crack indication. The commenter suggests that it is preferable to inspect compressor drums at a more frequent inspection interval and allow an engine with a "suspect" crack to continue in service for a limited time to allow for planning a reinspection. The same commenter suggests a 1,500 cycle repetitive inspection.
Another commenter provided information on the nature of the crack findings to date. Fifteen engines were identified with crack indications through borescope inspection. Three were obvious visual cracks and 12 required further assessment using the ECI inspection procedure cited in the SB. These 12 indications were confirmed not to be cracks.
Two commenters state that the five-cycle allowance for additional nondestructive inspections is effective in eliminating false initial crack indications and should be reinstated.
The FAA agrees that the five-cycle flyback allowance for engines with suspect crack indications is acceptable. Since the publication of the NPRM, results of a metallurgical investigation were provided that substantiate the five-cycle flight allowance. This information also determined that if a suspect crack indication extends from the knife edges to the disk radius ofthe sixth or seventh stage, an ECI reinspection must be accomplished before further flight. Accordingly, a five-cycle flyback allowance has been added to the compliance section of the AD.
However, the FAA does not agree with the suggested 20-cycle reinspection window or with the 1,500 cycle repetitive inspection interval. The FAA based the initial inspection threshold, reinspect flight allowance, and repetitive inspection interval on a risk assessment that uses known inspection data results to date.
Initial Front Drum Rotor Inspection
Several commenters request clarification regarding initial and repetitive inspection of the HPC front drum rotors. Two commenters request clarification of the requirement stated in the NPRM that the initial inspection be performed before the drum accumulates 1,500 cycles-since-new (CSN). Other commenters express concern that the NPRM does not address front drum rotors that exceed 1,500 CSN.
The FAA agrees. It is the FAA's intent to beconsistent with the analytical data associated with HPC front drum rotor cracking. The ASB states that the initial borescope inspection should be performed on all HPC front drum rotors that have accumulated 1,000 cycles (total drum cycles), at the next "A" check, or 500 cycles after the receipt of the ASB. The FAA agrees with the inspection threshold of 500 cycles determined by the risk assessment and chose the upper limit of 1,500 CSN as the compliance threshold for the initial inspection. Rewording the initial inspection threshold inadvertently introduced confusion. The requirements of the ASB and the AD are equivalent because they are based on the same data. Therefore, the FAA will change the wording to be consistent with the ASB.
Repetitive Front Drum Rotor Inspection
One commenter requests that a drum rotor that was inspected in accordance with SB 72-722, having a total time beyond 1,500 CSN when inspected, be permitted to remain in service.
Another commenter asks ifthe AD has the same exemption as the ASB, which states that HPC front drum rotors previously inspected in accordance with the CIR Manual requirement at the last shop visit within 500 cycles, are exempt from the initial borescope inspection. That commenter also states that the requirement is not clear for the repetitive inspection requirement for HPC front drum rotors that were previously inspected in the shop.
Another commenter states that the provision in ASB PW4ENG A72-722 that exempts drums that were detail-part-inspected within 500 cycles from the initial inspection and fall into the 2,200 cycle reinspection interval should be included in the final rule.
The FAA agrees that if an HPC front drum rotor was inspected in accordance with On-Wing paragraphs of ASB 72-722, as required by the proposed rule, the initial inspection requirement is satisfied. However, the rotor is still subject to the repetitive On-Wing inspection requirements, within 2,200 cycles since last inspection.
The FAA also agrees that an HPC front drum rotor inspected in the shop utilizing the fluorescent penetrant inspection (FPI) procedure within the past 500 CIS is exempt from the initial borescope inspection. Again, the rotor is still subject to the repetitive On-Wing inspection requirements, within 2,200 cycles since last inspection.
Omission of ECI
One commenter requests that the ECI of ASB 72-722 be omitted when the drum rotor is FPI per SPOP82 (CIR P/N 51A357 72-35-00). Another commenter asks whether the HPC front drum knife edge spacer area between the sixth and seventh stage disks previously visually inspected with split case condition or during light maintenance also should be exempted from initial borescope inspection.
As noted above, the FAA agrees that an HPC front drum rotor that was fluorescent penetrant inspected in the shop, as cited in the compliance section of the ASB, within the past 500 cycles in service (CIS), satisfies the initial inspection requirement. The ECI inspection requirement that is provided to confirm a suspect crack indication does not apply to HPC front drum rotors that have been fluorescent penetrant inspected in the shop within the past 500 CIS. The in-shop FPI inspection is more rigorous than the on-wing inspection requirements.
Number of Confirmed Cracked Drum Rotors
One commenter provides new information that there are eleven confirmed cracked HPC front drum rotors to date as compared to the seven that were described in the Summary and Discussion Sections of the NPRM. The FAA agrees that the higher number is accurate.
Raise Inspection Requirement
One commenter expresses a preference to inspect after 3,000 CIS, given that operator's experience with HPC front drum rotor cracking.
The FAA does not agree with the suggested 3,000 cycle repetitive inspection interval. The FAA based the initial inspection threshold, reinspect flight allowance, and repetitive inspection interval on a 20-yearcumulative risk assessment that uses known inspection data results to date. The AD provides for individual operators to submit substantiating data that would support an increase in the repetitive inspection interval under the alternative method of compliance paragraph.
Inclusion of PW SB PW4ENG A72-693 in AD
One commenter states that the inspection procedure in Pratt & Whitney Service Bulletin PW4ENG A72-693 is the same as that required by the proposed rule; therefore, "credit" should be given for the inspections previously performed using this SB.
The FAA disagrees. PW ASB PW4ENG A72-693 was not referenced in the NPRM because that was a fleetwide campaign that has been completed and was not part of the 20-year risk assessment for which the start date was June 2000. However, credit will be given for inspections done prior to the issuance of this AD depending on when and how they were accomplished. These should be evaluated on an individual case basis within the context ofthe alternative method of compliance provision of the AD.
Exemption of PW4158 Engine Serial Numbers
One commenter notes that Revision 1 of the ASB does not exempt PW4158 engine serial numbers P728534 through P728546 from the inspection requirements and the AD does not need to include this information in the description of the differences between the manufacturer's service information and this AD. The FAA agrees and that statement has been removed from the AD.
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 with the changes described previously. The FAA has determined that these changes will neither increase the economic burden on any operator nor increase the scope of the AD.
Differences Between Manufacturer's Service Information and This AD
Since the publication of the NPRM, the manufacturer revised, and the FAA has approved the technical contents of alert service bulletin (ASB) No. PW4ENG A72-722, Revision 1, dated June 7, 2001. Although ASB No. PW4ENG A72-722, Revision 1, dated June 7, 2001, provides procedures for operators to perform off-wing initial and repetitive HPC drum rotor inspections, the off-wing requirements are not mandated by this AD. The FAA has evaluated a 20-year cumulative risk assessment and has determined that an acceptable level of safety will be met by requiring the on-wing inspections at the cyclic intervals detailed in the ASB.
Economic Analysis
The FAA estimates that there are 1,970 engines of the affected design in the worldwide fleet, and that 538 engines installed on aircraft of U.S. registry would be affected by this proposed AD. The FAA also estimates that it would take approximately 2.5 work hours per engine to accomplish the proposed on-wing inspection, and that the average labor rate is $60 per work hour. It is estimated that three engines would be found with cracked HPC front drum rotors in the time frame of one year. Approximately 269 engines will be inspected on average per year. The cost of removal and reinstallation of an engine is approximately $10,000, and the cost of replacing the HPC front drum rotor is approximately $750,000. Required replacement parts would cost $356,130 per engine. Based on these figures, the total cost impact per year of the proposed AD for accomplishing initial inspections and replacing HPC front drum rotors, on U.S. operators is estimated to be $3,388,730.
Regulatory Impact
This final rule does not have federalism implications, as defined in Executive Order 13132, because it would 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. Accordingly, the FAA has not consulted with state authorities prior to publication of this final rule.
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 the DOT Regulatory Policies and Procedures (44 FR 11034, February 26, 1979); and (3) if promulgated, 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 by contacting 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.
Sec. 39.13 [Amended]
2. Section 39.13 is amended adding a new airworthiness directive to read as follows: