AD 94-04-14 ACTIVE

This amendment supersedes Airworthiness Directive (AD) 91-08-09, which currently requires the following on certain Twin Commander Aircraft Corporation (Twin Commander) 500, 680, 681, 685, and 690 series airplanes: repetitively inspecting the wing front spar lower cap (spar cap) for corrosion; and replacing the spar cap if corrosion exceeds certain limits. This action incorporates updated and more detailed inspection procedures, extends the repetitive inspection intervals, and provides the option of incorporating one of three modifications as terminating action for the repetitive inspections. The actions specified by the AD are intended to prevent wing structural damage that, if not detected, could progress to the point of failure.

Final rule.

94-04-14 TWIN COMMANDER AIRCRAFT CORPORATION: Amendment 39-8834; Docket No. 92-CE-58-AD. Supersedes AD 91-08-09, Amendment 39-6965.

Applicability: The following model and serial number airplanes that do not have the wing front spar lower cap replaced in accordance with the procedures of one of the three modifications specified in paragraph (b) of this AD, certificated in any category:

Models
Serial Numbers
500U, 680FL, 680FL(P), and 680W
1731 through 1854
500S
1755 through 3323
681
6001 through 6072
685
12000 through 12066
690, 690A, and 690B
11001 though 11566

Compliance: Required within the next 90 calendar days after the effective date of this AD or within 12 calendar months after the last inspection required by AD 91-08-09, whichever occurs later, unless already accomplished, and thereafter as indicated.

To prevent wing structural damage that, if not detected, could progress to the point of failure, accomplish the following:

NOTE1: The serial number of the Model 685 airplanes differs from that specified in Twin Commander Service Bulletin (SB) No. 208A, dated November 9, 1992. This AD takes precedence over that service information.

(a) Ultrasonically inspect each area of the wing front spar lower cap for corrosion in accordance with the instructions in Twin Commander SB No. 208A, dated November 9, 1992.

(1) If no corrosion is found, reinspect within the next 36 calendar months.

(2) If corrosion is found that is less than 50 percent of the allowable service limits referenced in Table 1 of Twin Commander SB No. 208A, dated November 9, 1992, reinspect within the next 30 calendar months.

(3) If corrosion is found to be between 50 to 100 percent of the allowable service limits referenced in Table 1 of Twin Commander SB No. 208A, dated November 9, 1992, reinspect within the next 12 calendar months.

(4) If corrosion is found to be greater than 100 percent of the allowable service limits referenced in Table 1 of Twin Commander SB No. 208A, dated November 9, 1992, prior to further flight, replace the wing front spar lower cap in accordance with one of the replacement modifications referenced in paragraph (b) of this AD.

(b) The repetitive inspection requirement of this AD may be eliminated by replacing the wing front lower spar cap in accordance with the instructions in one of the following, as applicable:

(1) For Models 685, 690, 690A, and 690B: Twin Commander Custom Kit No. CK-144, Revision A, dated November 12, 1992;

(2) For Models 500S, 500U, 680W, 681, 680FL, and 680FL(P): Twin Commander Custom Kit No. CK-145, dated August 21, 1992; or

(3) For Models 690, 690A, and 690B: AVIADESIGN, Inc. Supplemental Type Certificate SA5740NM.

(c) Special flight permits may be issued in accordance with 14 CFR 21.197 and 21.199 to operate the airplane to a location where the requirements of this AD can be accomplished.

(d) An alternativemethod of compliance or adjustment of the initial and repetitive compliance times that provides an equivalent level of safety may be approved by the Manager, Seattle Aircraft Certification Office (ACO), FAA, Northwest Mountain Region, 1601 Lind Avenue S.W., Renton, Washington 98055-4056. The request shall be forwarded through an appropriate FAA Maintenance Inspector, who may add comments and then send it to the Manager, Seattle ACO, FAA, Northwest Mountain Region.

NOTE 2: Information concerning the existence of approved alternative methods of compliance with this AD, if any, may be obtained from the Seattle ACO, FAA, Northwest Mountain Region.

(e) The inspections required by this AD shall be done in accordance with Twin Commander Service Bulletin No. 208A, dated November 9, 1992. 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. The replacement (as applicable) required by this AD shall be done in accordance with the instructions to either Twin Commander Custom Kit No. CK-144, Revision A, dated November 12, 1992; or Twin Commander Custom Kit No. CK-145, dated August 21, 1992, whichever is applicable. This incorporation by reference was previously approved by the Director of the Federal Register in accordance with 5 U.S.C. 552(a) and 1 CFR Part 51 as of April 11, 1994. Copies may be obtained from Twin Commander Aircraft Corporation, 19003 59th Drive, NE, Arlington, Washington 98223. Copies may be inspected at the FAA, Central Region, Office of the Assistant Chief Counsel, Room 1558, 601 E. 12th Street, Kansas City, Missouri, or at the Office of the Federal Register, 800 North Capitol Street, NW., suite 700, Washington, DC.

(f) This amendment (39-8834) supersedes AD 91-08-09, Amendment 39-6965.

(g) This amendment becomes effective on April 12, 1994.

A proposal to amend part 39 of the Federal Aviation Regulations to include an AD that applies to certain Twin Commander Models 685, 690, 690A, and 690B airplanes was published in the Federal Register on June 30, 1993 (58 FR 34952). The proposed AD would supersede AD 91-08-09 with a new AD that would (1) retain the requirement of repetitively inspecting the spar cap, and replacing the spar cap if certain corrosion limits are exceeded; (2) incorporate Twin Commander SB No. 208A, dated November 9, 1992; and (3) incorporate spar cap replacement procedures specified in Twin Commander Custom Kit No. CK-144, Revision A, dated November 12, 1992; Twin Commander Custom Kit No. CK-145, dated August 21, 1992; and AVIADESIGN, Inc. Supplemental Type Certificate (STC), dated July 16, 1992.

Interested persons have been afforded an opportunity to participate in the making of this amendment. Due consideration has been given to the 10 comments received.

Seven commenters concur with the proposal as written.

Twin Commander states that the proposal should be changed to reflect that Custom Kit - 145, which specifies procedures for replacing the spar cap for certain airplanes, also affects Models 680W and 681. The FAA concurs and has added these models to paragraph (b)(2) of the proposal, and has deleted paragraph (b)(4).

Another commenter, the Civil Aviation Authority of Australia (CAA), recommends replacing the wing spar cap before the next 500 hours time-in-service (TIS) if corrosion is found, instead of treating any corrosion found. The CAA states that, if a corroded spar is not replaced, then the small fatigue crack formed as a result of the corrosion will grow to weaken the spar to a level below its design strength. The FAA is currently conducting a study to determine the fatigue crack growth rates from irregular corroded surfaces. Part of this study includes the consideration of a replacement time period for spar caps found corroded. When this study is complete, the FAA will consider taking further AD action to cover this issue. The proposed AD is unchanged as a result of this comment.

A third commenter requests an extension of the time period for reinspection of a spar cap found not corroded from 36 calendar months to 60 calendar months. This commenter states that a well-maintained airplane that is always hangared and removed from a coastal environment should only have these inspections every 60 months. The FAA does not concur that this compliance time should be extended. The FAA has determined that the majority of airplanes affected by this proposal are not maintained and operated in the conditions referenced above, and that the 36-month inspection interval is suitable for the typical affected airplane when found corrosion-free. In addition, establishing a reliable corrosion growth rate for these airplanes is extremely difficult, especially considering the diverse environment and operating conditions these airplanes are approved for. The proposed AD is unchanged as a result of this comment.

After careful review of all available information, the FAA has determined that air safety and the public interest require the adoption of the rule as proposed except for the minor change discussed above and minor editorial corrections. The FAA has determined that these minor corrections will not change the meaning of the AD nor add any additional burden upon the public than was already proposed.

Of the 562 affected airplanes registered in the United States, 10 have already accomplished the required inspection in accordance with Twin Commander SB No. 208A, dated November 9, 1992. None of these airplanes required spar cap replacement.

The compliance time for this AD is presented in calendar time instead of hours time-in- service (TIS). The FAA has determined that a calendar time for compliance would be the most desirable method because the unsafe condition described by the AD is caused by corrosion. Corrosion can occur on airplanes regardless of whether the airplane is in service.

The FAA estimates that 562 airplanes in the U.S. registry would be affected by this AD, that it would take between 110 and 162 workhours per airplane (varies by model; a weighted average of 146 workhours) to accomplish the required inspection, and that the average labor rate is approximately $55 an hour. Based on these figures, the total initial cost impact of the inspection specified by this AD on U.S. operators is estimated to be $4,688,980 ($8,343 per airplane). As previously discussed, 10 of the 562 affected airplanes have already accomplished the required inspection. This reduces the initial cost impact of the inspection specified by this AD on U.S. operators to $4,432,560.

The inspections currently required by AD 91-08-09 carry an earlier FAA-estimated cost impact on U.S. operators of $880 (16 workhours X $55 per hour) per airplane. In actuality, those inspections range from $1,100 (20 workhours X $55 per hour) to $1,980 (36 workhours X $55 per hour) depending on the airplane model. Because of the more comprehensive inspection procedures specified in Twin Commander SB No. 208A, the initial cost impact of the inspections of this AD carries an additional cost impact of between $4,950 to $6,930 per airplane over that which is already required by AD 91-08-09. This AD provides owners of the affected airplanes some relief from some of the provisions of AD 91-08-09. For example:

If no corrosion is found, this AD would require repetitive inspections at 36-calendar month intervals instead of 12-calendar month intervals, which is a savings of between $2,200 to $3,960 over the next 24 calendar months depending on the airplane model.

If corrosion is less than 50 percent of the allowable service limits referenced in Twin Commander SB 208A, this AD would require reinspection at 30-calendar month intervals instead of 12-calendar month intervals, which is a savings of between $2,750 to $4,950 over the next 30 calendar months depending on the airplane model.

Replacing the spar cap terminates this repetitive inspection requirement, which is a savings of $1,100 to $1,980 per year depending on the airplane model.

In addition, if the required inspection revealed corrosion in excess of certain established limits, then replacing the spar cap would be required at a cost of approximately $100,000, parts and labor included. If the spar cap did not reveal corrosion in excess of certain established limits, then an airplane affected by this AD could have an incremental compliance cost as low as $4,950 (proposed inspection cost at 110 workhours minus the cost of the inspection required by AD 91-08-09) or $6,930 (required inspection cost at 162 workhours minus the cost of the inspection required by AD 91-08-09). If replacing the spar cap would be necessary, an airplane affected by this AD could have an incremental compliance cost as high as approximately $104,950 (required inspection at 110 workhours minus the cost of the inspection required by AD 91-08-09 plus spar cap replacement) to $106,930 (required inspection at 162 workhours minus the cost of the inspection required by AD 91-08-09 plus spar cap replacement).

The FAA cannot predict the results of the required inspections, especially as the airplanes continue to age. Approximately 61 percent of the airplanes inspected in accordance with Twin Commander SB No. 208 (AD 91-08-09) had no significant spar cap corrosion, 24 percent had less than 50 percent of the established corrosion service limits, and 13 percent had between 50 to 100 percent of the established corrosion service limits. Less than 3 percent of the airplanes exceeded the established corrosion service limits.

Based on an expected average remaining operating life of 20, 25, or 30 years per affected airplane (depending on the model), the incremental annualized compliance cost would be:If only repetitively inspecting the spar cap is necessary for airplanes with an average operating life of 20 years: $904 to $7,239 incremental annualized cost (depending on the airplane model and frequency of inspection, and using a 7 percent interest rate);

If only repetitively inspecting the spar cap is necessary for airplanes with an average operating life of 25 years: $1,195 to $7,176 incremental annualized cost (depending on the airplane model and frequency of inspection, and using a 7 percent interest rate);

If only repetitively inspecting the spar cap is necessary for airplanes with an average operating life of 30 years: $1,290 to $7,495 incremental annualized cost (depending on the airplane model and frequency of inspection, and using a 7 percent interest rate);

If replacing the spar cap is necessary for airplanes with an average operating life of 20 years: approximately $9,675 incremental annualized cost (using a 7 percent interest rate) if the spar cap isreplaced during the first inspection;

If replacing the spar cap is necessary for airplanes with an average operating life of 25 years: approximately $8,719 incremental annualized cost (using a 7 percent interest rate) if the spar cap is replaced during the first inspection; and

If replacing the spar cap is necessary for airplanes with an average operating life of 30 years: approximately $8,145 incremental annualized cost (using a 7 percent interest rate) if the spar cap is replaced during the first inspection.

The required AD's from Dockets No. 92-CE-26-AD (for Model 500S and 690B), No. 92-CE- 38-AD (for Models 685, 690, 690A, and 690B), and No. 92-CE-43-AD (for Models 500S, 500U, 680FL, 680FL(P), 680W, and 681) will also affect certain airplanes included in this AD. The compliance costs of these other required AD's will add to the cost discussed above. However, replacing the spar cap would only be required once, so the $100,000 replacement cost, if required, would be aone-time action.

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 would have a "significant economic impact on a substantial number of small entities," and, in cases where they would, 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.

The 562 U.S.-registered airplanes affected by this AD are owned according to the following breakdown: 92 by individuals, 7 by U.S. government agencies, 20 by states or local governments, and 443 by other entities. Twenty seven entities each own more than one of the affected airplanes: one owns 7, four own 5 each, three own 3 each, and nineteen own 2 each.

The FAA cannot determine the sizes of all the affected non-individual owner entities nor the relative significance of the costs estimated above. Because of these uncertainties, no cost thresholds for significant economic impact can be reasonably determined. Based on the possibility that this AD could have a significant impact on a substantial number of small entities, the FAA conducted a regulatory flexibility analysis. A copy of this analysis may be obtained by contacting the Rules Docket at the location provided under the caption "ADDRESSES".

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 significant under DOT Regulatory Policies and Procedures (44 FR 11034, February 26, 1979) because of substantial public interest; and, (3) may have a significant economic impact on a substantial number of small entities. The FAA has conducted an Initial Regulatory Flexibility Determination and Analysis and has considered alternatives to this action that could minimize the impact on small entities. A copy of this analysis
may be obtained by contacting the Rules Docket at the location provided under the caption "ADDRESSES". After careful consideration, the FAA has determined that the required action is the best course to achieve the safety objective of returning the airplaneto its original certification level of safety.

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.

Section 39.13 - [AMENDED]

2. Section 39.13 is amended by removing AD 91-08-09, Amendment 39-6965 (56 FR 14307, April 9, 1991), and by adding the following new airworthiness directive to read as follows:

Service information that applies to this AD may be obtained from the Twin Commander Aircraft Corporation, 19003 59th Drive, NE, Arlington, Washington 98223. This information may also be examined at the Federal Aviation Administration (FAA), Central Region, Office of the Assistant Chief Counsel, Room 1558, 601 E. 12th Street, Kansas City, Missouri 64106; or at the Office of the Federal Register, 800 North Capitol Street, NW., suite 700, Washington, DC.

Mr. Mike Pasion, Aerospace Engineer, FAA, Northwest
Mountain Region, 1601 Lind Avenue S.W., Renton, Washington 98055-4056; telephone (206) 227-2594; facsimile (206) 227-1181.