AD 97-25-10

Effective January 12, 1998.

The service information referenced in this AD may be obtained from Pratt & Whitney, 400 Main St., East Hartford, CT 06108; telephone (860) 565-6600, fax (860) 565-4503. This information may be examined at the FAA, New England Region, Office of the Assistant Chief Counsel, 12 New England Executive Park, Burlington, MA. This information may be examined at the Federal Aviation Administration (FAA), New England Region, Office of the Assistant Chief Counsel, 12 New England Executive Park, Burlington, MA 01803-5299; or at the Office of the Federal Register, 800 North Capitol Street, NW., suite 700, Washington, DC.

Tara Goodman, Aerospace Engineer, Engine Certification Office, FAA, Engine and Propeller Directorate, 12 New England Executive Park, Burlington, MA 01803-5299; telephone (781) 238-7130, fax (781) 238-7199.

A proposal to amend part 39 of the Federal Aviation Regulations (14 CFR part 39) by superseding airworthiness directive (AD) 91-04-10, Amendment 39-6859 (56 FR 5343, February 11, 1991), applicable to Pratt & Whitney (PW) JT9D series turbofan engines, was published in the Federal Register on March 19, 1997 (62 FR 12979). That action proposed to require initial and repetitive fluorescent penetrant inspections (FPI) for cracks in cooling air holes of affected first stage high pressure turbine (HPT) disks, and, if necessary, replacement with serviceable parts. In addition, the action proposed to require initial and repetitive FPI for cracks in tie bolt holes of all affected second stage HPT disks. Finally, that action proposed to require reporting findings of cracked turbine disks.

Interested persons have been afforded an opportunity to participate in the making of this amendment. Due consideration has been given to the comments received.

One commenter states that since the root cause of the crack which was found on the cooling hole of the improved disk, part number (P/N) 840301, was caused by improper tooling application (use of reamer instead of a carbide insert) at the specified supplier, only the suspect supplier and lots should be affected by the AD. The commenter maintains that if the FAA suspects PW's qualified carbide insert machining process, every maintenance process requiring the manufacturer's qualification should always require FAA qualification in future. The Federal Aviation Administration (FAA) does not concur. The improved disk, P/N 840301, is manufactured with the new reamer tooling method; however, a disk of this P/N was found with cooling hole cracking. The reamer method was introduced by manufacturers to preclude abusive machining that was found using the carbide insert method. Investigation has shown that both the carbide insert method and the reamer method are subject to the same abusive machining phenomenon. The FAA approves manufacturing and maintenance processes, which are updated as necessary.

The same commenter requests that the FAA and PW develop an inspection procedure that can detect not only a crack but detect whether a severely worked layer of material exists or not, so that unnecessary repetitive inspections can be minimized by removing those disks. The FAA does not concur. It is not possible to detect 100% of the possible cracking conditions in the field using current inspection methods. Therefore, repetitive inspections are necessary for disks that are in service because cracking can propagate in fatigue from a layer of severely worked material resulting from the manufacturing process.

The same commenter requests the FAA extend the initial inspection requirement for disks that have been previously inspected, noting that the AD as proposed would allow 1,500 cycles in service (CIS) before initial inspection for disks never inspected while only 250 CIS for disks that have been inspected but that have accumulated more than 3,500 CIS since last inspection. The FAA does not concur. The current AD, effective in 1991, requires repetitive inspections of disks installed in JT9D-59A, -70A, -7Q, and -7Q3 engines at intervals not to exceed 3,500 CIS. Therefore, the example cited by the commenter should not occur, as disks installed in JT9D-59A or -7Q engines should never exceed 3,500 CIS since last inspection. Only if a disk installed in a JT9D-7R4 engine had exceeded 3,500 CIS since last inspection would the requirement to inspect no later than 250 CIS after the effective date of this AD apply. This inspection requirement was considered in the risk analysis and the FAA has determined that it is necessary. The commenter does not indicate how many JT9D-7R4 engines might be affected by the 250 CIS initial inspection requirement. Individual operators can apply for an adjustment to that compliance time under paragraph (d) of the AD.

One commenter states that the mandated use of FPI does not provide all possible assurance that defective HPT disks will be removed from service. The commenter believes that eddy current inspection, or a combination of the two methods, would clearly provide a greater probability of crack detection. The FAA does not concur. The first stage turbine cooling air holes and second stage tie bolt holes have low aspect ratios. The FAA has determined that FPI of low aspect ratio holes is adequate for detecting cracks in these locations.

One commenter states that there appears to be anomalies in the requirements for disks that have been in service for over 6,000 cycles since new (CSN), as stated in paragraph (b)(1)(i) of the compliance section. The commenter maintains that as the paragraph reads in the proposed rule, this inspection will always occur later than accumulating 8,000 CSN if CSN is greater than 6,000 on the effective date of the AD. The FAA concurs and has changed this paragraph to require inspection within 2,000 CIS if a disk has over 6,000 CSN on the effective date of the AD.

The same commenter suggests that at next engine shop visit would be sufficient as an interval for cooling hole and tie bolt hole inspections. The FAA does not concur. Since the timing of engine shop visits varies widely between operators, the use of shop visits to define inspection intervals in ADs does not provide adequate objectivity on which to assess the effectiveness of the required actions in addressing the unsafe condition. The FAA has determined that a maximum of 2,000 CIS interval is required.

Subsequent to the publication of the proposed rule, two JT9D-7R4D/E operators indicated that the 6,000 CIS re-inspection interval for second stage turbine hub tie bolt hole mismachining would require them to remove engines prematurely due to their high cycle utilization rate. The FAA has determined through a review of risk analysis that the additional risk involved in extending the re-inspection interval from 6,000 CIS to 8,000 CIS is sufficiently low and has changed the re-inspection interval accordingly.

In addition, the manufacturer has recommended that Special Process Operation Procedure (SPOP) 70 be used in lieu of SPOP 84 in order to permit the inspection of the second stage turbine hub tie bolt holes when the second stage turbine rotor is removed from the HPT module assembly without necessitating the removal of the second stage turbine blades. The FAA concurs and has changed the reference to the inspection procedure accordingly.

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.

The 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.

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 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.

Air transportation, Aircraft, Aviation safety, Incorporation by reference, Safety.

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:

1. The authority citation for part 39 continues to read as follows:

Authority: 49 U.S.C. 106(g), 40113, 44701.