AD 94-02-01

This amendment adopts a new airworthiness directive (AD) that is applicable to Textron Lycoming Model T5508D turboshaft engines. This action requires a cyclic life reduction for the T5508D impeller, a more conservative method for determining low cycle fatigue (LCF) damage to the impeller, and a method for prorating past impeller usage, based on the new LCF counting factors. This amendment is prompted by a report of a rotorcraft accident found to have been caused by an uncontained impeller failure. A subsequent field campaign inspection of high-time impellers utilized by heavy lift operators confirmed 12 more impellers with similar distress. The actions specified in this AD are intended to prevent an impeller failure, which can result in an uncontained engine failure, inflight shutdown, or possible rotorcraft damage.

Final rule, request for comments.

94-02-01 Textron Lycoming: Amendment 39-8798. Docket 93-ANE-56.
Applicability: Textron Lycoming Model T5508D turboshaft engines, installed on but not limited to Bell 214B helicopters.
Compliance: Required as indicated, unless accomplished previously.
To prevent an impeller failure, which can result in an uncontained engine failure, inflight shutdown, or possible rotorcraft damage, accomplish the following:
(a) Within 7 days after the effective date of this AD, conduct a revised impeller operating cycle count (prorate), in accordance with paragraph E of Textron Lycoming Service Bulletin (SB) No. T5508D-0002, Revision 7, dated June 25, 1993.
(b) Following the implementation of the revised cycle count methodology, specified in paragraph (a) of this AD, replace those impellers installed in aircraft that exceed the new life limit, on the effective date of this AD, in accordance with the drawdown requirements defined in Table 1 of Textron Lycoming SB T5508D-0040, dated June 25, 1993.
(c) For those impellers not installed in aircraft on the effective date of this AD, with 8,000 or more cycles in service (CIS), replace with a new or serviceable impeller prior to further flight.
(d) Thereafter, Textron Lycoming Model T5508D impeller, part numbers 2-100-180-13 and 2-100-180-19, are life limited to 8,000 CIS, as defined in Table 1 of Textron Lycoming SB No. T5508D-0002, Revision 7, dated June 25, 1993.
(e) An alternative method of compliance or adjustment of the compliance time that provides an acceptable level of safety may be used if approved by the Engine Certification Office. The request should be forwarded through an appropriate FAA Principal Maintenance Inspector, who may add comments and then send it to the Engine Certification Office. NOTE: Information concerning the existence of approved alternative methods of compliance with this airworthiness directive, if any, may be obtained from the Engine Certification Office.
(f) The operationalcyclic counting and replacement of impellers shall be done in accordance with the following service bulletins:

Document No.
Pages
Revision
Date
Textron Lycoming
SB No. T5508D-0002
1-12
7
June 25, 1993
Total pages: 12.

Textron Lycoming SB No. T5508D-0040
1-2
Original
June 25, 1993
Total pages: 2.

This incorporation by reference was approved by the Director of the Federal Register in accordance with 5 U.S.C. 552(a) and 1 CFR part 51. Copies may be obtained from Textron Lycoming, 550 Main Street, Stratford, CT 06497. Copies may be inspected at the FAA, Office of the Assistant Chief Counsel, Attention: Rules Docket No. 93-ANE-56, 12 New England Executive Park, Burlington, Massachusetts; or at the Office of the Federal Register, 800 North Capitol Street, NW., suite 700, Washington, DC.
(g) This amendment becomes effective on February 4, 1994.

The Federal Aviation Administration (FAA) received a report of a rotorcraft accident caused by an uncontained failure of a Textron Lycoming T5508D compressor impeller. The impeller failure was caused by a low cycle fatigue (LCF) crack which initiated and propagated to failure in the impeller aft face cooling air holes. Based on this accident, Textron Lycoming issued a Service Bulletin (SB) requiring all operators to inspect impellers with greater than 5,000 cycles in service (CIS). To date, 12 impellers have been discovered with distress in the impeller aft face cooling air holes similar to the accident aircraft but of lesser magnitude. Subsequent analysis and testing of this current design impeller, as well as service experience, has revealed a lower LCF life than originally calculated. This lower LCF life is based on a new engineering analysis using different, improved component geometry and LCF material properties than were used in the original engineering lifing analysis for the impeller. In addition to arriving at a lower LCF life, the new engineering analysis also derived the updated impeller cyclic counting factors. Configuration-specific material testing, combined with updated operator mission profiles, revealed a need to update the impeller cyclic counting factors because minor cyclic LCF damage was greater than previously calculated. This condition, if not corrected, can result in impeller failure, which can result in an uncontained engine failure, inflight shutdown, or possible rotorcraft damage.

This amendment requires a cyclic life reduction for the impeller, a more conservative sub- cycle counting factors table, and a method for prorating past impeller usage based on the new cyclic counting factors. For those impellers that exceed the new life limit, a drawdown schedule will be implemented for safe removal of time-expired impellers. This program is substantiated by the demonstrated correlation between spinpit testing of actual cracked parts, engineering analysis, configuration specimen fatigue testing, and field service experience. A safety assessment has also been performed to substantiate the drawdown schedule for time-expired impellers to conservatively manage these those impellers beyond the new life limits.

The FAA has reviewed and approved the technical contents of Textron Lycoming SB No. T5508D-0040, dated June 25, 1993, that describes the removal schedule necessary for those impellers that exceed the new life limits, and Textron Lycoming SB No. T5508D-0002, Revision 7, dated June 25, 1993, that reduces the cyclic life limit of the impeller from 16,600 to 8,000 cycles, revises the minor cycle counting factors table used for cyclic computation, and provides a method for prorating past impeller usage based on the new cycle counting factors.

Since an unsafe condition has been identified that is likely to exist or develop on other products of this same type design, this ADis being issued to prevent impeller failure, which can result in an uncontained engine failure, inflight shutdown, or possible rotorcraft damage. This AD requires a cyclic life reduction for the impeller, a more conservative sub-cycle counting factors table, and a method for prorating past impeller usage based on the new cycle counting factors. For those impellers that exceed the new life limit, the FAA has established a drawdown schedule for safe removal. These actions are required to be accomplished in accordance with the service bulletins described previously.

Since a situation exists that requires the immediate adoption of this regulation, it is found that notice and opportunity for prior public comment hereon are impracticable, and that good cause exists for making this amendment effective in less than 30 days.

Comments Invited

Although this action is in the form of a final rule that involves requirements affecting flight safety and, thus, was not preceded by notice and an opportunity for public comment, comments are invited on this rule. Interested persons are invited to comment on this rule by submitting such written data, views, or arguments as they may desire. Communications should identify the Rules Docket number and be submitted in triplicate to the address specified under the caption "ADDRESSES." All communications received on or before the closing date for comments will be considered, and this rule may be amended in light of the comments received. Factual information that supports the commenter's ideas and suggestions is extremely helpful in evaluating the effectiveness of the AD action and determining whether additional rulemaking action would be needed.

Comments are specifically invited on the overall regulatory, economic, environmental, and energy aspects of the rule that might suggest a need to modify the rule. All comments submitted will be available, both before and after the closing date for comments, in the Rules Docket for examination by interested persons. A report that summarizes each FAA-public contact concerned with the substance of this AD will be filed in the Rules Docket.

Commenters wishing the FAA to acknowledge receipt of their comments submitted in response to this notice must submit a self-addressed, stamped postcard on which the following statement is made: "Comments to Docket Number 93-ANE-56." The postcard will be date stamped and returned to the commenter.

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.

The FAA has determined that this regulation is an emergency regulation that must be issued immediately to correct an unsafe condition in aircraft, and is not a "significant regulatory action" under Executive Order 12866. It has been determined further that this action involves an emergency regulation under DOT Regulatory Policies and Procedures (44 FR 11034, February 26, 1979). If it is determined that this emergency regulation otherwise would be significant under DOT Regulatory Policies and Procedures, a final regulatory evaluation will be prepared and placed in the Rules Docket. A copy of it, if filed, may be obtained from 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 14 CFR part 39 of the Federal Aviation Regulations as follows:

PART 39 - AIRWORTHINESS DIRECTIVES
1. The authority citation for part 39 continues to read as follows: Authority: 49 U.S.C. App. 1354(a), 1421 and 1423; 49 U.S.C. 106(g); and 14 CFR 11.89.

39.13 - [AMENDED]
2. Section 39.13 is amended by adding the following new airworthiness directive: