AD 94-04-15 ACTIVE

This amendment adopts a new airworthiness directive (AD) that applies to certain Twin Commander Aircraft Corporation (Twin Commander) Models 500, 560A, 560E, 680, 680E, and 720 airplanes. This action requires inspecting the wing front spar lower cap (spar cap) for interference, fretting, or corrosion between the firewall flange and spar cap flange at Wing Station (WS) 96, and clearing any interference, repairing any fretting or corrosion damage, and replacing any cracked spar cap. Reports of two of the affected airplanes having cracked spar caps at WS 96 prompted this action. The actions specified by this AD are intended to prevent failure of the wing structure caused by a cracked spar cap at WS 96.

Final rule.

94-04-15 TWIN COMMANDER AIRCRAFT CORPORATION: Amendment 39-8835; Docket No. 92-CE-57-AD.
Applicability: The following model and serial number airplanes that do not have the wing front spar lower cap replaced in accordance with procedures specified in paragraph (a)(3) of this AD, certificated in any category:

Model
Serial Numbers
500
618 through 750
560A
231 though 450
560E
433 through 750
680
242 through 658
680E
623 through 750
720
501 through 750

Compliance: Required within the next 50 hours time-in-service after the effective date of this AD, unless already accomplished.

To prevent failure of the wing structure caused by a cracked wing front spar lower cap at Wing Station (WS) 96, accomplish the following:

(a) Inspect the wing front spar lower cap of each wing for corrosion or interference between the firewall flange and spar flange in accordance with the ACCOMPLISHMENT INSTRUCTIONS section of Twin Commander Service Bulletin (SB) No. 212, dated November 3, 1992.

(1) If any interference is found between the firewall flange and spar flange, prior to further flight, clear this interference in accordance with the ACCOMPLISHMENT INSTRUCTIONS section of Twin Commander SB No. 212, dated November 3, 1992.

(2) If any corrosion damage is found, prior to further flight, repair any corrosion damage to the wing front spar lower cap in accordance with the ACCOMPLISHMENT INSTRUCTIONS section of Twin Commander SB No. 212, dated November 3, 1992.

(3) If any cracks are found, prior to further flight, replace the wing front spar lower cap in accordance with replacement procedures obtained from the manufacturer through the Manager, Seattle Aircraft Certification Office (ACO), at the address specified in paragraph (c) of this AD.

(b) 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.

(c) An alternative method of compliance or adjustment of the compliance time that provides an equivalent level of safety may be approved by the Manager, Seattle 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: 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.

(d) The inspection and repair required by this AD shall be done in accordance with Twin Commander Service Bulletin No. 212, dated November 3, 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. 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.

(e) 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 500, 560A, 560E, 680, 680E, and 720 airplanes was published in the Federal Register on June 30, 1993 (58 FR 34950). The action proposed to require inspecting the spar cap for interference, fretting, or corrosion between the firewall flange and spar cap flange at WS 96, and clearing any interference, repairing any fretting or corrosion damage, and replacing any cracked spar cap. The proposed inspection and repair, if applicable, would be accomplished in accordance with Twin Commander SB No. 212, dated November 3, 1992.

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

Seven commenters concur with the proposal as written.

One commenter, the Civil Aviation Authority of Australia (CAA), states that the proposal should require an eddy current inspection of the spar cap. The CAA questions whether fretting and corrosion are the sole contributors to fatigue damage to WS 96, or if these merely aggravate what is already a fatigue-sensitive location. The CAA goes on to state that, if a crack can start from a corrosion pit as small as 0.006 inches (as happened on a Swedish airplane), then there is a good chance that even a clean airplane will crack over time. Also, the CAA states that, if a fatigue crack has started at the base of a corrosion pit, the blending of the corrosion will smear over the crack and conceal it from the dye penetrant inspection (which is called for in the SB), and that an eddy current inspection is the only method for detection of this crack. The FAA does not concur. The proposal relates to fretting that may occur because interference between a firewall stiffener and the forward flange of the lower spar cap. It would require action for fatigue cracks found as a result of this interference and fretting. Fretting does have a synergistic relationship with corrosion and fatigue, and its existence greatly accelerates fatigue crack growth. The FAA has received no reports of fatigue cracks in the affected area that were not a direct result of fretting. Also, the spar cap at WS 96 is extremely difficult to inspect using eddy current methods because of other structures in the area interfering with placement of the eddy current probe. Dye penetrant inspections are used extensively in the aviation industry and have proved very reliable in detecting cracks. The proposed AD is unchanged as a result of this comment.

Another commenter, the Twin Commander Aircraft Corporation, states that owners/operators should have their airplanes inspected regardless of whether the spar cap has been replaced because interference may even exist on a new spar. The FAA concurs that interference may exist if a new spar is installed, unless it was installed in accordancewith replacement procedures obtained from the manufacturer through the Manager, Seattle Aircraft Certification Office (as specified in paragraph (a)(3) of the proposed AD). The Seattle Aircraft Certification Office has not issued any of these procedures to airplane owners/operators; therefore, no airplane has the spar cap replaced in accordance with paragraph (a)(3) of the proposal and the proposal would affect all of the airplanes listed under the Applicability section. The proposed AD is unchanged as a result of this comment.

After careful review of all available information including the comments referenced above, the FAA has determined that air safety and the public interest require the adoption of the rule as proposed except for 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.

The FAA estimates that 332 airplanes in the U.S. registry will be affected by the proposed AD, that it will take approximately 20 workhours per airplane to accomplish the required inspection, and that the average labor rate is approximately $55 an hour. Parts to accomplish the required inspection cost approximately $20. Based on these figures, the total cost impact of the inspection specified in this AD on U.S. operators is estimated to be $371,840. One airplane owner has already accomplished the required inspection, and found no damage or cracks.

In addition, if the required inspection reveals cracks, or corrosion that exceeds certain limits, replacing the spar cap would be required at a cost of approximately $100,000, parts and labor included. An airplane affected by this AD could have a compliance cost as low as approximately $1,120 (labor + parts) if no cracks, interference, or corrosion damage is found during the inspection, and as high as approximately $101,120 (inspection + spar cap replacement) if the operator replaces the spar cap.

Based on these airplanes having an expected average remaining operating life of 10 years or 15 years, the annualized compliance cost would range between:

If only inspecting the spar cap at WS 96 is necessary for airplanes with an average remaining operating life of 10 years: $1,120 X 0.14349 (10-year capital recovery factor at a 7 percent interest rate) = $168 annualized cost;

If only inspecting the spar cap at WS 96 is necessary for airplanes with an average remaining operating life of 15 years: $1,120 X 0.11434 (15-year capital recovery factor at a 7 percent interest rate) = $126;

If replacing the spar cap is necessary for airplanes with an average remaining operating life of 10 years: approximately $101,120 X 0.15349 (capital recovery factor at a 7 percent interest rate) = $15,520; or

If replacing the spar cap is necessary for airplanes with an average remaining operating life of 15 years: approximately $101,120 X 0.11434 (capital recovery factor at a 7 percent interest rate) = $11,562.

The required AD's from Dockets No. 92-CE-43-AD (for Models 500, 560A, 560E, 680, 680E, and 720) and No. 92-CE-58-AD (for Models 685, 690, 690A, and 690B) will also affect certain airplanes included in this AD. The compliance costs of these other required AD's would 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 a one-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 andGuidance, 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 331 U.S.-registered airplanes affected by the required AD that have not complied with Twin Commander SB No. 212, dated November 3, 1992, are owned according to the following breakdown: 196 by individuals, 2 by U.S. government agencies, 7 by states or local governments, and 126 by other entities. Six entities own 2 airplanes 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 airplane to 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 adding the following new AD 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.