The FAA proposed to amend 14 CFR part 39 with a proposed AD. The proposed AD applies to RRC models 250-C30, 250- 40, and 250-C47 series turboshaft engines. We published the proposed AD in the Federal Register on January 25, 2006 (71 FR 4065). That action proposed to add an additional life limit for third- and fourth-stage turbine wheels.
Examining the AD Docket
You may examine the docket that contains the AD, any comments received, and any final disposition in person at the Docket Management Facility Docket Office between 9 a.m. and 5 p.m., Monday through Friday, except Federal holidays. The Docket Office (telephone (800) 647-5227) is located on the plaza level of the Department of Transportation Nassif Building at the street address stated in ADDRESSES. Comments will be available in the AD docket shortly after the DMS receives them.
Comments
We provided the public the opportunity to participate in the development of this AD. We have considered the comments received.
Request to Correct Factual Errors
One commenter, RRC, requests that we correct factual errors in the NPRM and revise the Discussion paragraph, to state that:
Only third-stage turbine wheels actually failed in the past; and
Only the third-stage turbine wheel (not the third-and- fourth-stage turbine wheels) could prematurely fail if operated too many times in the transient overspeed region.
We agree with these factual corrections. We changed the AD by removing certain references to the fourth-stage turbine wheel and changing the AD to state that it results from analysis by RRC of failures of third-stage turbine wheels.
Final Rule Should Include the Lower-Speed Avoidance Range
A private citizen states that the final rule should include the lower-speed avoidance range (68.4% to 87.1%) in addition to the high- speed transient range, when counting speed excursions and retiring turbine wheels. The commenter gave three reasons for the request:First, that operation in the lower-speed avoidance range probably does more cumulative damage to the turbine wheel than operation in the high-speed excursion range. The commenter bases this on data that he claims shows higher stress levels at the low-speed transient range compared to the high-speed transient range.
Second, the fact that the engine control unit does not record operation in the low-speed avoid range, emphasizes the importance to inform operators about the danger of cumulative damage.
Third, the current commercial engine bulletin gives no information about cumulative damage in the low-speed avoid range. The commenter states that currently there is no warning to operators of the potential damage to turbine wheels operated for any length of time in the avoid range which may cause more cumulative damage than high-speed excursions.
We do not agree. The supporting data the commenter provided includes information from a manufacturer development configuration that does not represent the current production configuration. The data also does not represent the manufacturer's current, more detailed, knowledge of the stresses on the turbine airfoils. Transients in the low-speed range do not need to be tracked and used to retire turbine wheels. The low-speed range from 22,000 rpm to 28,045 rpm is a speed range that is normally passed through transiently, during the start up and shutdown procedures. The rate of speed change during the start up or shutdown is high enough that no significant time is spent at any resonant speed and no significant dynamic stresses are encountered that would lead to damage. In comparison to the low-speed excursions, if an operational situation occurs that results in a speed excursion above the maximum continuous speed, the rate of change of speed goes from positive to negative as it accelerates up to the maximum speed before returning back to the continuous operating range. During this transition, speed may hold close toconstant, or only change very slowly, for a number of seconds. In this case, there could be sufficient time for the dynamic stresses to build to their full resonant values and potentially cause some level of damage to a turbine wheel. This difference between a fast acceleration or deceleration through a resonant speed, and a potential slow motion or hold in a resonant speed is why counting of occurrences in the low speed range is unnecessary.
Finally, the inclusion of a specific "steady state operation prohibited" speed avoid zone denotes that failure could occur if operation outside of the defined continuous operating range was performed. RRC SB No. CEB A-72-3272, CEB A-72-5048, and CEB A-72-6054 (combined in one document), all Revision 2, dated June 27, 2006, clearly instruct operators to avoid the low-speed region. As stated above, this speed range is not of concern for normal transient operation of the engine. We did not change the AD.
Conclusion
We have carefully reviewed the available data, including the comments received, and determined that air safety and the public interest require adopting the AD with the changes described previously. We have determined that these changes will neither increase the economic burden on any operator nor increase the scope of the AD.
Costs of Compliance
We estimate that this AD will affect 1,300 engines installed on airplanes of U.S. registry. We also estimate that it will take about 42 work-hours per engine to replace the third- and fourth-stage turbine wheels, and that the average labor rate is $65 per work-hour. Required parts will cost about $25,000 per engine. We estimate that only 10% of all turbine wheel replacements will result from operators exceeding the new transient overspeed event limits. Based on these figures, we estimate the total potential maximum cost of the AD to U.S. operators to be $3,604,900.
Authority for This Rulemaking
Title 49 of the United States Code specifies the FAA'sauthority 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
We have determined that 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); 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.
We prepared a summary of the costs to comply with this AD and placed it in the AD Docket. You may get a copy of this summary at the address listed under ADDRESSES.
List of Subjects in 14 CFR Part 39
Air transportation, Aircraft, Aviation safety, Incorporation by reference, Safety.
Adoption of the Amendment
Accordingly, under the authority delegated to me by the Administrator, the Federal Aviation Administration amends 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. The FAA amends Sec. 39.13 by adding the following new airworthiness directive: