[Federal Register Volume 63, Number 249 (Tuesday, December 29, 1998)]
[Rules and Regulations]
[Pages 71577-71580]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 98-34096]


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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.

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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

[[Page 71578]]

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

[[Page 71579]]

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.

[[Page 71580]]

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.


Sec. 39.13  [Amended]

    2. 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
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1-54.............................            2  March 26, 1998.
 
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                               Appendix A
 
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1, 2.............................            2  March 26, 1998.
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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. 98-34096 Filed 12-28-98; 8:45 am]
BILLING CODE 4910-13-P