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 Fairchild SA226 and SA227 series airplanes was published in the Federal Register as a notice of proposed rulemaking (NPRM) on July 31, 1998 (63 FR 40846). The NPRM proposed to require repetitively inspecting the wing spar center web cutout on both wings for cracks between WS 8 and WS 17.5, and immediately repairing any area found cracked. This repair would eliminate the need for the repetitive inspections on that particular wing spar. Accomplishment of the proposed action as specified in the NPRM would be required in accordance with the following documents:
- Fairchild Airframe Airworthiness Limitations Manual ST-UN-M001, Rev. No. C-6, dated April 7, 1998;
- Fairchild Airframe Inspection Manual ST-UN-M002, Rev. No. A-6, dated December 8, 1997;
- Fairchild Airframe Airworthiness Limitations Manual ST-UN-M003, Rev. No. 5, dated April 7, 1998;
- SA226/227 Series Structural Repair Manual, part number (P/N) 27-10054-079, pages 57 through 90; Initial Issue: March 1, 1983; Revision 28, dated June 24, 1998; and
- SA227 Series Structural Repair Manual, P/N 27-10054-127, pages 47 through 60; Initial Issue: December 1, 1991; Revision 7, dated June 24, 1998.
The NPRM was the result of reports of cracks in the wing spar center web cutout caused by fatigue due to airplane maneuvering and wind gusts.
Interested persons have been afforded an opportunity to participate in the making of this amendment. Due consideration has been given to the following comments.
Comment Issue No. 1:
Allow Flight When Cracks in the Wing Spar Center Web Do Not Exceed a Certain Length
Five commenters request that the FAA structure the proposed AD in a way that would allow continued flight if cracks were found in the wing spar center web cutout provided thecracks did not exceed a certain limit. One of these commenters states that, although requiring replacement of the wing spar center web if any crack if found is a good idea, many airplanes would be grounded while waiting for parts and that flight with a small crack is not necessarily unsafe.
The FAA does not concur that flight should be allowed with cracks in the wing spar center web cutout regardless of the size of the cracks. 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 these cracks exist. In certain circumstances, the FAA would allow flight with minor cracks provided an acceptable inspection and replacement schedule was submitted. Among the criteria for allowing flight with minor cracks are as follows:
Substantiation that the cracks are not in primary structure;
Substantiation that the cracks are in fail-safe structure. Various combinations of analysis and test, including that provided at the time of original certification, may be considered as ample substantiation. This must include the ability to sustain ultimate load with the maximum permissible crack. Other valid substantiations that may be considered include various combinations of fracture mechanics analysis, flight test, ground test. Temporary repairs such as "stop drilling" should be specified; or
Substantiation to verify that the single load path structure with the known cracks has the ability to carry ultimate loads.
Various combinations of fracture mechanics analysis, flight test, ground test, or proof test may be considered as ample substantiation. Only when unusual circumstances exist, such as the difficulty of an operator in obtaining replacement parts, will this be allowed.
Under no circumstances can any of these exceptions be considered as more than a temporary condition.
The FAA has not received information and documentation that meet any of the above criteria. Therefore, no changes are necessary to the final rule as a result of these comments.
Comment Issue No. 2:
Compliance Times in the Proposed AD Are Different Than Those Specified in the Applicable Service Information
Three commenters question why the FAA did not differentiate the compliance times of the SA226 series airplanes and the SA227 series airplanes. In particular, the Airframe Airworthiness Limitations Manuals specify an initial inspection time of 6,500 hours time-in-service (TIS) for the SA226 series airplanes and 10,600 hours TIS for the SA227 series airplanes. In addition, the Airframe Airworthiness Limitations Manuals specify repetitive inspection intervals of 3,000 hours TIS while the proposed AD specifies intervals of 2,000 hours TIS.
Individual commenters make the following points:
- The justification for the difference in compliance times is due to the design of the number 13 stringer cut-out in the wing spar center web being different in the SA226 series airplanes and the SA227 series airplanes.
- Experience shows that cracking in the affected area seems to be a problem on airplanes with over 10,000 hours TIS, but no cracks have been found by the individual commenter on airplanes with around 6,500 hours TIS. The commenter recommends that the FAA establish the initial inspection at 8,500 hours TIS.
- If the 2,000 hours TIS repetitive inspection interval is going to be used instead of 3,000 hours TIS, then the FAA needs to justify why 2,000 hours TIS is needed rather than what is already specified in the Airframe Airworthiness Limitations Manual.
The FAA does not concur that the compliance time of either the initial or repetitive inspection should be changed. Cracks do not always occur in all airplanes, nor do the cracks that develop on airplanes occur at the same time. Airplanes are operated in different environments and flight loads depending on the area of the country or world they are operated in or the type of operation they are routinely utilized for (e.g., commuter, cargo, general aviation, etc.), respectively. These factors contribute to the development of cracks and the crack growth rate of existing cracks. At the time that the Airframe Airworthiness Limitations Manuals were published, there were no cracks found in the wing spar center web cutout on in-service airplanes. The inspection intervals specified in these manuals were based on one full-scale fatigue test of an SA226 series airplane. The SA227 series airplanes have not been full-scale fatigue tested in the affected area. Based on analysis of all information on this subject received to date, the FAA has determined that the initial inspection compliance time of 6,500 hours TIS and the repetitive inspection interval of 2,000 hours TIS on all affected airplanes is justified.
No changes are necessary to the final rule as a result of these comments.
Comment Issue No. 3:
AD Concurrence
One commenter supports the AD as written. This commenter feels that the proposed AD would meet the safety intent of detecting and correcting fatigue cracking of the wing spar center web cutout area of Fairchild SA226 and SA227 series airplanes.
Comment Issue No. 4:
Remove the SA227 Series Airplanes From the Applicability of the Proposed AD
Two commenters state that the actions proposed in the AD are not necessary for the SA227 series airplanes because the Fairchild Airframe Airworthiness Limitations Manual ST-UN-M001 and ST-UN-M003 make these requirements mandatory for continued airworthiness. The commenters state that since these inspections are already required, the SA227 series airplanes should be removed from the Applicability of the proposed AD.
The FAA concurs that the proposed inspections are currently required, particularly by sections 135.411 and 135.425 of the Federal Aviation Regulations (14 CFR 135.411 and 14 CFR 135.425) for airplanes "type certificated for a passenger seating configuration, excluding any pilot seat, of ten seats or more," The SA227 series airplanes fall in this category. However, as discussed in the Comment Issue No. 2 section of this document, the inspection compliance times of the proposed AD differ from that specified in the Fairchild Airframe Airworthiness Limitations Manual ST-UN-M001 and ST-UN-M003. In addition, SA227 series airplanes that have been or are at a later date altered from the original 10 or more seat configuration (either through a supplemental type certificate or other FAA-approved method) may no longer be required by 14 CFR 135.411 and 14 CFR 135.425 to have the actions of the above-referenced Airframe Airworthiness Limitations Manuals accomplished. In this case, the only mechanism of assuring that the actions are accomplished is through the issuance of an AD.
The FAA has determined (1) that the compliance times specified in the proposed AD should take precedence over those specified in the Airframe Limitation Manuals (see Comment Issue No. 2 in this document); and (2) that the inspections should be required on any SA227 series airplane that has had the 10 or more seat configuration altered. For these reasons, the only change necessary to the final rule as a result of these comments is a statement that gives initial inspection credit to the owners/operators of those airplanes that are currently in compliance with the applicable Airframe Airworthiness Limitations Manual.
Comment Issue No. 5:
Account for Future Revisions to the Service Manuals
Two commenters recommend that the words "or later revision" be added to each reference to the Airworthiness Limitations Manuals and the Structural Repair Manual (SRM). This would allow any future revisions to automatically be incorporated into the AD.
The FAA does not concur. The FAA cannot approve data that does not exist. Approval of this nature could adversely affect aviation safety if documentation was included in thesubsequent service information that did not carry normal FAA review or was FAA-approved, but included information that did not accomplish the intent of the AD.
No changes have been made to the final rule as a result of these comments.
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 the adoption of the rule as proposed except for the addition of language that gives "already accomplished" credit for those owners/operators of those affected airplanes that are in compliance with the applicable Airframe Airworthiness Limitations Manual and minor editorial corrections. The FAA has determined that this addition and these minor editorial corrections will not change the meaning of the AD and will not add any additional burden upon the public than was already proposed.
Cost Impact
The FAA estimates that 490 airplanes in the U.S. registry will be affected by this AD, that it will take approximately 5 workhours per airplane to accomplish the initial inspection, and that the average labor rate is approximately $60 an hour. Based on these figures, the total cost impact of the initial inspection specified in this AD on U.S. operators is estimated to be $147,000, or $300 per airplane.
These figures only take into account the costs of the initial inspection and do not take into account the costs of repetitive inspections and the costs associated with any repair that will be necessary if cracks are found. The FAA has no way of determining the number of repetitive inspections an owner/operator will incur over the life of the airplane, or the number of airplanes that will need repairs.
If an affected airplane has cracks in both wing spar center webs, the repair will take approximately 400 workhours to accomplish at an average labor rate of $60 per hour. Parts to accomplish this repair cost approximately $400 per airplane. Based on these figures, the cost to repair cracked wing spar center webs on both sides of the airplane will be approximately $24,400 per airplane.
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 U.S.C. 106(g), 40113, 44701.
39.13 [Amended]
2. Section 39.13 is amended by adding a new airworthiness directive (AD) to read as follows: