Airworthiness directives: Boeing,
[Federal Register: December 29, 1998 (Volume 63, Number 249)]
[Rules and Regulations]
[Page 71577-71580]
From the Federal Register Online via GPO Access [wais.access.gpo.gov]
[DOCID:fr29de98-3]
DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
14 CFR Part 39
[Docket No. 97-NM-309-AD; Amendment 39-10966; AD 98-26-23]
RIN 2120-AA64
Airworthiness Directives; Boeing Model 747 Series Airplanes
AGENCY: Federal Aviation Administration, DOT.
ACTION: Final rule.
SUMMARY: This amendment adopts a new airworthiness directive (AD), applicable to certain Boeing Model 747 series airplanes, that requires repetitive detailed visual inspections to detect
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corrosion on the rear spar web of the wing center section and adjacent bulkhead fittings at body station 1241; and corrective action, if necessary. This amendment is prompted by reports of corrosion found on the rear spar web and bulkhead fitting. The actions specified by this AD are intended to detect and correct such corrosion, which could cause cracking of the rear spar web, and result in a fuel leak and consequent fire/explosion in the wheel well of the main landing gear.
DATES: Effective February 2, 1999.
The incorporation by reference of certain publications listed in the regulations is approved by the Director of the Federal Register as of February 2, 1999.
ADDRESSES: The service information referenced in this AD may be obtained from Boeing Commercial Airplane Group, P.O. Box 3707, Seattle, Washington 98124-2207. This information may be examined at the Federal Aviation Administration (FAA), Transport Airplane Directorate, Rules Docket, 1601 Lind Avenue, SW., Renton, Washington; or at the Office of the Federal Register, 800 North Capitol Street, NW., suite 700, Washington, DC.
FOR FURTHER INFORMATION CONTACT: Bob Breneman, Aerospace Engineer, Airframe Branch, ANM-120S, FAA, Seattle Aircraft Certification Office, 1601 Lind Avenue, SW., Renton, Washington 98055-4056; telephone (425) 227-2776; fax (425) 227-1181.
SUPPLEMENTARY INFORMATION: A proposal to amend part 39 of the Federal Aviation Regulations (14 CFR part 39) to include an airworthiness directive (AD) that is applicable to certain Boeing Model 747 series airplanes was published in the Federal Register on March 27, 1998 (63 FR 14863). That action proposed to require repetitive detailed visual inspections to detect corrosion on the rear spar web of the wing center section and adjacent bulkhead fittings at body station 1241; and corrective action, if necessary.
Interested persons have been afforded an opportunity to participate in the making of this amendment. Due consideration has been given to the comments received.
Support for the Proposed Rule
One commenter supports the rule.
Request to Withdraw AD or Combine with Previous AD
Two commenters state that the existing corrosion prevention and control program (CPCP), which is mandated by AD 90-25-05, amendment 39- 6790 (55 FR 49268, November 27, 1990), would provide adequate repetitive inspection opportunities. One commenter requests that the proposed AD be revised to create a ``hybrid'' AD that comprises both the initial service bulletin inspections and the follow-on CPCP inspections. The commenter states that if a ``hybrid'' AD or a similar action is not accomplished, this AD would be redundant to the CPCP AD, and therefore unnecessary.
The FAA does not concur in the commenter's request to revise this AD to create such a hybrid. Contrary to the commenter's belief that the mandated CPCP already requires discrete inspections of this area, the FAA has determined that this is not so. AD 90-25-05 requires inspections in accordance with Boeing Document D6-36022, ``Aging Airplane Corrosion Prevention and Control Program, Model 747,'' Revision A, dated July 28, 1989. This mandated revision to the document does not explicitly require discrete inspections of the affected area. It is true that later revisions to the document do contain the subject inspections and that these revisions have been approved as alternative methods of compliance to AD 90-25-05. However, the FAA emphasizes that these approved alternative methods of compliance are optional; it is only the original revision to the Boeing document that is currently mandatory. Therefore, the FAA has determined that the issuance of this final rule is not redundant to the requirements of AD 90-25-05.
Request to Withdraw Proposed AD
One commenter, the manufacturer, opposes the proposed AD and requests that it be withdrawn. The commenter states that it does not believe that an unsafe condition exists. In addition, this commenter states that such an AD would be both redundant and unnecessary. Specifically, the commenter adduces from its review of past service history the following reasons for its comment. Since 1980, the commenter has received a total of 49 reports of corrosion (on 32 airplanes) at the affected area. In no case did the corrosion lead to any fuel leaking at the rear spar. In only one case was a crack found (and this crack was initiated at a fastener hole, not on the web away from the hole). This crack was found by a nondestructive test (NDT) inspection, not by the type of visual inspection required by the proposed AD. Due to the low stresses seen by the rear spar web at this fastener location, the crack growth at the affected fastener hole is so slow that the probability of detecting by visual means a crack that has grown beyond the fastener cap sealant is considered to be ``extremely remote.''
In addition, the commenter notes that a review of the corrosion data indicated that previous reports of corrosion reaching a depth of 0.25 inches were erroneous. In fact, the maximum depth of corrosion found at this location was only 0.20 inches. Furthermore, Boeing points out that the rear spar web thickness at this location is 0.40 inches, which is considerably thicker than the minimum thickness of 0.123 inches, for which the Model 747 has been certified. If one were to subtract from this value the 0.20 inches of maximum corrosion damage experienced to date, there would still be 0.20 inches of web thickness remaining to provide adequate fuel leak protection (and also static strength capability).
The FAA does not agree with the statement that there is no unsafe condition and does not concur that this AD should be withdrawn. While the FAA does not dispute the data presented by the commenter, it does not accept the conclusions that were made. There are, in general, two safety concerns that arise whenever corrosion is found on a piece of structure (including the rear spar web). First, there is a concern that a piece of structure, such as a rear spar web, could become so corroded that the remaining intact parent material would no longer be sufficiently thick to react applied loads. The FAA accepts the commenter's point that so far, none of the corrosion found to date has been sufficiently severe to put the static strength capability of the structure into doubt.
The second general concern (which is also the primary concern that the FAA has in this case) is that corrosion often leads to crack initiation in the parent material, and that this crack would eventually propagate through the entire thickness of the affected structure. Furthermore, such cracks are not usually detectable by visual means alone (as the crack in its early stages usually does not extend beyond the corrosion); instead, the cracks can only be detected by NDT methods, which, as the commenter points out, are not always reliable when corrosion is present. Furthermore, while it is true that cracks do propagate slowly when applied stress levels are low, it is well known that corrosion can accelerate the rate at which the crack grows. What all of this implies is that corrosion on the rear spar web could easily mask a crack that is propagating through the parent material; that such a crack cannot be reliably detected by visual or NDT inspections; and finally, that the
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structure could therefore fail before the crack is detected by any of the inspection programs that are now in place. This is the reason why the FAA concludes that the unsafe condition does exist.
Request to Revise Applicability of Proposed AD
Two commenters request that the proposed AD be revised to exclude those airplanes that have already accomplished the required actions. The FAA does not concur that a change to the AD is necessary. Operators are always given credit for work previously accomplished by means of the phrase in the compliance section of the AD that states ``required as indicated, unless accomplished previously.'' This statement serves the same purpose as the requested change.
Request to Reference Later Service Bulletin Revision
Several commenters request that the proposed rule be changed to refer to Revision 2 of Boeing Service Bulletin 747-57-2263, as this revision provides a better definition of the required inspections and corrective action than Revision 1 of the service bulletin does. (Revision 1 of the service bulletin was referenced in the proposed AD as the appropriate source of service information.)
The FAA concurs partially. Revision 2, which the FAA has reviewed and approved, does contain a better definition of the required actions. Specifically, the new revision to the service bulletin contains improved methods for removing corrosion and a new method for measuring the remaining spar web thickness. However, the FAA has determined that Revision 1 of the service bulletin also provides an acceptable level of safety. Therefore, the FAA has revised the final rule to include Revision 2 of the service bulletin as an additional source of service information for accomplishing the actions required by this AD.
Request to Extend Inspection Interval
One commenter requests that the repetitve inspection intervals specified in the proposed AD be changed from 2 years to 3 years. The FAA does not concur with the request. The 2-year intervals were developed by considering both the service history of this problem and the fact that there is a variance in the rate at which the structure can corrode (based upon different operation environments). No change to the final rule is necessary.
Request for Deferral of Repairs
One commenter requests that the proposed AD be changed to permit a two-year deferral for repairing any corrosion that is found, provided that repetitive ultrasonic inspections are performed to detect cracks that might be present. The FAA does not concur. Nondestructive inspections do not reliably detect cracking if active corrosion is present. Therefore, there would be no assurance that a corroded area is not also cracked. No change to the final rule has been made in this regard.
Request for Manufacturer Repair Approvals
One commenter requests that the proposed AD be revised to allow operators to contact the manufacturer in lieu of the FAA for certain repair approvals. The FAA concurs partially. Potential repairs to this area are likely to be complex and have not yet been defined for all cases. Therefore, the FAA needs to review such repairs until a complete method of repair has been defined and approved. However, the FAA has delegated such repairs in accordance with data meeting the type certification basis of the airplane approved by a Boeing Company Designated Engineering Representative (DER) who has been authorized by the FAA to make such findings. The final rule has been revised to provide for such repair approval.
Request to Revise Cost Estimates
Two commenters point out that the cost estimates contained in the proposed AD are unrealistic. One commenter asserts that accomplishment of the required inspections could take as many as 32 work hours. Also, the commenters note that the work hours to remove any corrosion could range from 74 to 320 work hours. One of the commenters points out that Revision 2 of the Boeing service bulletin contains more realistic work hour estimates.
The FAA concurs partially. The FAA agrees with the commenters that the 32-work hour figure is a more realistic estimate of the time to accomplish the required inspections; therefore, this rule has been changed accordingly. With respect to the request to change the work hours for accomplishing corrosion repair, the FAA does not concur. Corrosion repair is an ``on-condition'' action; such actions are not required to be considered in AD's because they are required to be accomplished quite apart from the AD, in order to maintain the airplane in an airworthy condition.
Conclusion
After careful review of the available data, including the comments noted above, the FAA has determined that air safety and the public interest require the adoption of the rule with the changes previously described. The FAA has determined that these changes will neither increase the economic burden on any operator nor increase the scope of the AD.
Cost Impact
There are approximately 816 Model 747 series airplanes of the affected design in the worldwide fleet. The FAA estimates that 236 airplanes of U.S. registry will be affected by this AD, that it will take approximately 32 work hours per airplane to accomplish the required actions, and that the average labor rate is $60 per work hour. Based on these figures, the cost impact of the AD on U.S. operators is estimated to be $453,120, or $1,920 per airplane, per inspection cycle.
The cost impact figure discussed above is based on assumptions that no operator has yet accomplished any of the requirements of this AD action, and that no operator would accomplish those actions in the future if this AD were not adopted.
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 final evaluation has been prepared for this action and it is contained in the Rules Docket. A copy of it may be obtained from 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.
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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
-
The authority citation for part 39 continues to read as follows:
Authority: 49 U.S.C. 106(g), 40113, 44701.
Sec. 39.13 [Amended]
-
Section 39.13 is amended by adding the following new airworthiness directive:
98-26-23 Boeing: Amendment 39-10966. Docket 97-NM-309-AD.
Applicability: Model 747 series airplanes, line positions 1 through 816 inclusive, certificated in any category.
Note 1: This AD applies to each airplane identified in the preceding applicability provision, regardless of whether it has been modified, altered, or repaired in the area subject to the requirements of this AD. For airplanes that have been modified, altered, or repaired so that the performance of the requirements of this AD is affected, the owner/operator must request approval for an alternative method of compliance in accordance with paragraph (b) of this AD. The request should include an assessment of the effect of the modification, alteration, or repair on the unsafe condition addressed by this AD; and, if the unsafe condition has not been eliminated, the request should include specific proposed actions to address it.
Compliance: Required as indicated, unless accomplished previously.
To detect and correct corrosion and consequent cracking of the rear spar web of the wing center section and adjacent bulkhead fittings at body station 1241, which could result in a fuel leak and consequent fire/explosion in the wheel well of the main landing gear, accomplish the following:
(a) Within 18 months after the effective date of this AD, perform a detailed visual inspection to detect corrosion of the rear spar web of the wing center section and adjacent bulkhead fittings at body station 1241, in accordance with Boeing Service Bulletin 747-57-2263, Revision 1, dated December 21, 1995, or Revision 2, dated March 26, 1998, including Appendix A. Thereafter, repeat the inspection at intervals not to exceed 2 years.
(1) If no corrosion is detected during the inspection: Prior to further flight, apply corrosion inhibitor in accordance with the service bulletin.
(2) If any corrosion is detected during the inspection, and the corrosion is within the limits specified by the service bulletin: Prior to further flight, accomplish the actions specified in paragraphs (a)(2)(i), (a)(2)(ii), and (a)(2)(iii).
(i) Remove the corrosion in accordance with the service bulletin. And
(ii) Perform a high frequency eddy current inspection to detect cracking in the area of removed corrosion in accordance with the service bulletin. If any crack is detected, prior to further flight, repair it in accordance with a method approved by the Manager, Seattle Aircraft Certification Office (ACO), FAA, Transport Airplane Directorate; or in accordance with data meeting the type certificate basis of the airplane approved by a Boeing Company Designated Engineering Representative who has been authorized by the Manager, Seattle ACO, to make such findings. And
(iii) Apply corrosion inhibitor in accordance with the service bulletin.
(3) If any corrosion is detected during the inspection, and the corrosion exceeds the limits specified by the service bulletin: Prior to further flight, repair the corroded area in accordance with a method approved by the Manager, Seattle ACO; or in accordance with data meeting the type certificate basis of the airplane approved by a Boeing Company Designated Engineering Representative who has been authorized by the Manager, Seattle ACO, to make such findings.
(b) An alternative method of compliance or adjustment of the compliance time that provides an acceptable level of safety may be used if approved by the Manager, Seattle ACO. Operators shall submit their requests through an appropriate FAA Principal Maintenance Inspector, who may add comments and then send it to the Manager, Seattle ACO.
Note 2: Information concerning the existence of approved alternative methods of compliance with this AD, if any, may be obtained from the Seattle ACO.
(c) Special flight permits may be issued in accordance with sections 21.197 and 21.199 of the Federal Aviation Regulations (14 CFR 21.197 and 21.199) to operate the airplane to a location where the requirements of this AD can be accomplished.
(d) Except for the repairs required by paragraphs (a)(2)(ii) and (a)(3), the actions shall be done in accordance with Boeing Service Bulletin 747-57-2263, Revision 1, dated December 21, 1995; or Boeing Service Bulletin 747-57-2263, Revision 2, dated March 26, 1998, including Appendix A, which contains the following list of effective pages:
Revision Page No.
level shown Date shown on page on page
1-54.............................
2 March 26, 1998.
Appendix A
1, 2.............................
2 March 26, 1998.
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 Boeing Commercial Airplane Group, P.O. Box 3707, Seattle, Washington 98124-2207. Copies may be inspected at the FAA, Transport Airplane Directorate 1601 Lind Avenue, SW., Renton, Washington; or at the Office of the Federal Register, 800 North Capitol Street, NW., suite 700, Washington, DC.
(e) This amendment becomes effective on February 2, 1999.
Issued in Renton, Washington, on December 17, 1998. Darrell M. Pederson, Acting Manager, Transport Airplane Directorate, Aircraft Certification Service.
[FR Doc. 98-34096Filed12-28-98; 8:45 am]
BILLING CODE 4910-13-P
