Events Leading to the Issuance of This AD
A proposal to amend part 39 of the Federal Aviation Regulations (14 CFR part 39) to include an AD that would apply to certain de Havilland DHC-6 series airplanes without Modification Nos. 6/1461 and 6/1462 incorporated was published in the Federal Register as a notice of proposed rulemaking (NPRM) on August 11, 1995 (60 FR 41030). The NPRM proposed to supersede AD 78-26-02 with a new AD that would (1) retain the current requirement of repetitively inspecting the fuselage side frame flanges at Fuselage Station (FS) 218.125 and FS 219.525, as applicable, and repairing or replacing any cracked part; and (2) require modifying the fuselage side frame flanges in the referenced FS areas (Modification Nos. 6/1461 and 6/1462), as terminating action for the repetitive inspections. Accomplishment of the proposed actions as specified in the NPRM would be in accordance with de Havilland Service Bulletin (SB) No. 6/371, dated June 2, 1978.
Modification No. 6/1461 introduces fuselage side frames manufactured from material having improved stress corrosion properties at FS 218.125, and Modification No. 6/1462 introduces fuselage side frames of this material at FS 219.525.
Interested persons have been afforded an opportunity to participate in the making of this amendment. No comments were received on the proposed rule or the FAA's determination of the cost to the public.
As written, the original NPRM (as does AD 78-26-02) allows continued flight if cracks are found in the fuselage side frames that do not exceed certain limits. Extensive analysis of the consequences of flying with known cracks in primary structure prompted the FAA to establish a policy that disallows airplane operation when known cracks exist in primary structure, unless the ability to sustain ultimate load with these cracks is proven. The fuselage structure is considered primary structure, and the FAA has not received any analysis to prove that ultimate load can be sustained with cracks in this area.
For this reason, the FAA determined that the crack limits contained in the original NPRM and AD 78-26-02 should be eliminated, and that the NPRM should be revised to propose immediate replacement of any cracked fuselage flanges. Since revising the proposed AD to require immediate replacement of any cracked fuselage flanges went beyond the scope of what was presented in the original NPRM, the FAA published a supplemental NPRM in the Federal Register on March 31, 1997 (62 FR 15129).
Interested persons were again afforded an opportunity to participate in the making of this amendment. No comments were received regarding the substance of the supplemental NPRM or the FAA's determination of the cost to the public.
The FAA's Determination
After careful review of all available information related to the subject presented above, the FAA has determined that air safety and the public interest require theadoption of the AD as proposed in the supplemental NPRM, except for minor editorial corrections. The FAA has determined that these minor corrections will not change the meaning of the AD and will not add any additional burden upon the public than was already proposed.
The FAA's Aging Commuter-class Aircraft Policy
The actions specified in this AD are part of the FAA's aging commuter class aircraft policy, which briefly states that, when a modification exists that could eliminate or reduce the number of required critical inspections, the modification should be incorporated. This policy is based on the FAA's determination that reliance on critical repetitive inspections on aging commuter-class airplanes carries an unnecessary safety risk when a design change exists that could eliminate or, in certain instances, reduce the number of those critical inspections. In determining what inspections are critical, the FAA considers (1) the safety consequences of the airplane if the known problem is not detected by the inspection; (2) the reliability of the inspection such as the probability of not detecting the known problem; (3) whether the inspection area is difficult to access; and (4) the possibility of damage to an adjacent structure as a result of the problem.
The alternative to modifying the fuselage side frames at FS 218.125 and FS 219.525 would be to rely on the repetitive inspections required by AD 78-26-02 to detect cracks in these areas.
Cost Impact
The FAA estimates that 94 airplanes in the U.S. registry will be affected by this AD, that it will take approximately 300 workhours per airplane to accomplish the required modification, and that the average labor rate is approximately $60 an hour. Parts cost approximately $16,200 (average) per airplane. Based on these figures, the total cost impact of the required modification on U.S. operators is estimated to be $3,214,800 or $34,200 per airplane. This cost figure is based upon the presumption that no affected airplane owner/operator has incorporated Modification Nos. 6/1461 and 6/1462.
The intent of the FAA's aging commuter airplane program is to ensure safe operation of commuter-class airplanes that are in commercial service without adversely impacting private operators. Of the approximately 94 airplanes in the U.S. registry that would be affected by this AD, the FAA has determined that approximately 45 percent are operated in scheduled passenger service. A significant number of the remaining 55 percent are operated in other forms of air transportation such as air cargo and air taxi.
This AD allows 4,800 hours time-in-service (TIS) after the effective date of the AD before mandatory accomplishment of the design modification. The average utilization of the fleet for those airplanes in commercial commuter service is approximately 25 to 50 hours TIS per week. Based on these figures, operators of commuter-class airplanes involved in commercial operation have to accomplish the required modification within 24 to 48 calendar months after this AD becomes effective. For private owners, who typically operate between 100 to 200 hours TIS per year, this allows 24 to 48 years before the required modification is required.
The following paragraphs present cost scenarios for airplanes where no cracks are found and where cracks are found during the inspections, and where the remaining airplane life is 15 years with an average annual utilization rate of 1,600 hours TIS. A copy of the full Cost Analysis and Regulatory Flexibility Determination for this action may be examined at the FAA, Central Region, Office of the Regional Counsel, Attention: Rules Docket No. 91-CE-45-AD, Room 1558, 601 E. 12th Street, Kansas City, Missouri.
- No Cracks Scenario: Under the provisions of AD 78-26-02, an owner/operator of an affected de Havilland DHC-6 series airplane in scheduled service who operates an average of 1,600 hours TIS annually will inspect every 400 hours TIS. This would amount to a remaining airplane life (estimated 15 years) cost of $18,420; this figure is based on the presumption that no cracks are found during the inspections. This AD requires the same inspections except at 600-hour TIS intervals until 4,800 hours TIS after the effective date of the AD when the operator has to replace the fuselage side frame flanges (eliminating the need for further repetitive inspections), which results in a present value cost of $31,433. The incremental cost of this AD for such an airplane is $13,013 or $4,959 annualized over the 3 years it will take to accumulate 4,800 hours TIS. An owner of a general aviation airplane who operates 800 hours TIS annually without finding any cracks during the 600-hour TIS inspections will incur a present value incremental cost of $7,598. This amounts to a per year amount of $1,594 over the 6 years it takes to accumulate 4,800 hours TIS.
- Excessive cracking scenario: AD 78-26-02 requires repairing or replacing the fuselage side frames if excessive cracking is found (as defined by SB No. 6/371), as will this AD. The difference is that AD 78-26-02 requires immediate crack repair and then replacement within 360 days after finding the crack, and this AD requires immediate repair and mandatory replacement of the fuselage side frames within 4,800 hours TIS after the effective date of the AD. This results in a present value total cost of $34,709 per airplane in scheduled service, which makes immediate replacement more economical ($32,400) than repetitively inspecting. With this scenario, this AD averages a present value cost savings over that required in AD 78-26-02 of $2,083 ($794 annualized over 3 years) for each airplane operated in scheduled service, and $6,607 ($1,386 annualized over 6 years) for each airplane operated in general aviation service.
Regulatory Flexibility Determination and Analysis
The Regulatory Flexibility Act of 1980 (RFA) was enacted by Congress to ensure that small entities are not unnecessarily or disproportionally burdened by government regulations. The RFA requires government agencies to determine whether rules could have a "significant economic impact on a substantial number of small entities," and, in cases where they could, conduct a Regulatory Flexibility Analysis in which alternatives to the rule are considered. FAA Order 2100.14A, Regulatory Flexibility Criteria and Guidance, outlines FAA procedures and criteria for complying with the RFA. Small entities are defined as small businesses and small not-for-profit organizations that are independently owned and operated or airports operated by small governmental jurisdictions. A "substantial number" is defined as a number that is not less than 11 and that is more than one-third of the small entities subject to a required rule, or any number of small entities judged to be substantial by the rulemaking official. A "significant economic impact" is defined by an annualized netcompliance cost, adjusted for inflation, which is greater than a threshold cost level for defined entity types. FAA Order 2100.14A sets the size threshold for small entities operating aircraft for hire at nine aircraft owned and the annualized cost thresholds, adjusted to 1994 dollars, at $69,000 for scheduled operators and $5,000 for unscheduled operators.
Of the 94 U.S.-registered airplanes affected by this AD, the federal government owns 4 airplanes. Of the other 90, one business owns 26 airplanes, two businesses own 7 airplanes each, one business owns 3 airplanes, seven businesses own 2 airplanes each, and thirty-three businesses own 1 airplane each.
Because the FAA has no readily available means of obtaining data on sizes of these entities, the economic analysis for this AD utilizes the worst case scenario using the lower annualized cost threshold of $5,000 for operators in unscheduled service instead of $69,000 for operators in scheduled service. With this in mind andbased on the above ownership distribution, the 33 entities owning two or fewer airplanes will not experience a "significant economic impact" as defined by FAA Order 2100.14A. Since the remaining 11 entities do not constitute a "substantial number" as defined in the Order, this AD will not have a "significant economic impact on a substantial number of small entities."
Regulatory Impact
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 copy of the final evaluation prepared for this action 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 USC 106(g), 40101, 40113, 44701.
39.13 [Amended]
2.Section 39.13 is amended by removing Airworthiness Directive (AD) 78-26-02, Amendment 39-3370, and adding the following new AD to read as follows: