06-8232. Airworthiness Directives; Boeing Model 747 Airplanes

AGENCY:

Federal Aviation Administration (FAA), Department of Transportation (DOT).

ACTION:

Supplemental notice of proposed rulemaking (NPRM); reopening of comment period.

SUMMARY:

The FAA is revising an earlier proposed airworthiness directive (AD) for certain Boeing Model 747 airplanes. The original NPRM would have required repetitive tests to detect hot air leaking from the trim air diffuser ducts or sidewall riser duct assemblies (collectively referred to in this proposed AD as “TADDs”), related investigative actions, and corrective actions if necessary. The original NPRM also would have provided an optional terminating action for the repetitive tests. The original NPRM resulted from reports of sealant deteriorating on the outside of the center wing fuel tank and analysis that sealant may deteriorate inside the tank due to excess heat from leaking TADDs. This action revises the original NPRM by referring to improved inspection procedures and extending the repetitive interval for certain related investigative actions. We are proposing this supplemental NPRM to prevent leakage of fuel or fuel vapors into areas where ignition sources may be present, which could result in a fire or explosion.

DATES:

We must receive comments on this supplemental NPRM by October 23, 2006.

ADDRESSES:

Use one of the following addresses to submit comments on this supplemental NPRM.

• DOT Docket Web site: Go to http://dms.dot.gov and follow the instructions for sending your comments electronically.
• Government-wide rulemaking Web site: Go to http://www.regulations.gov and follow the instructions for sending your comments electronically.
• Mail: Docket Management Facility; U.S. Department of Transportation, 400 Seventh Street, SW., Nassif Building, Room PL-401, Washington, DC 20590.
• Fax: [phone].
• Hand Delivery: Room PL-401 on the plaza level of the Nassif Building, 400 Seventh Street, SW., Washington, DC, between 9 a.m. and 5 p.m., Monday through Friday, except Federal holidays.

Contact Boeing Commercial Airplanes, P.O. Box 3707, Seattle, Washington 98124-2207, for service information identified in this proposed AD.

FOR FURTHER INFORMATION CONTACT:

Dan Kinney, Aerospace Engineer, Propulsion Branch, ANM-140S, FAA, Seattle Aircraft Certification Office, 1601 Lind Avenue, SW., Renton, Washington 98057-3356; telephone [phone]; fax [phone].

SUPPLEMENTARY INFORMATION:

Comments Invited

We invite you to submit any relevant written data, views, or arguments regarding this supplemental NPRM. Send your comments to an address listed in the ADDRESSES section. Include the docket number “Docket No. FAA-2004-19755; Directorate Identifier 2004-NM-23-AD” at the beginning of your comments. We specifically invite comments on the overall regulatory, economic, environmental, and energy aspects of this supplemental NPRM. We will consider all comments received by the closing date and may amend this supplemental NPRM in light of those comments.

We will post all comments submitted, without change, to http://dms.dot.gov, including any personal information you provide. We will also post a report summarizing each substantive verbal contact with FAA personnel concerning this supplemental NPRM. Using the search function of that Web site, anyone can find and read the comments in any of our dockets, including the name of the individual who sent the comment (or signed the comment on behalf of an association, business, labor union, etc.). You may review the DOT's complete Privacy Act Statement in the Federal Register published on April 11, 2000 (65 FR 19477-78), or you may visit http://dms.dot.gov.

Examining the Docket

You may examine the AD docket on the Internet at http://dms.dot.gov, or in person at the Docket Management Facility office between 9 a.m. and 5 p.m., Monday through Friday, except Federal holidays. The Docket Management Facility office (telephone [phone]) is located on the plaza level in the Nassif Building at the DOT street address stated in ADDRESSES. Comments will be available in the AD docket shortly after the Docket Management System receives them.

Discussion

We proposed to amend 14 CFR part 39 with a notice of proposed rulemaking (NPRM) for an AD (the “original NPRM”) for certain Boeing Model 747 airplanes. The original NPRM was published in the Federal Register on December 1, 2004 (69 FR 69844). The original NPRM proposed to require repetitive tests to detect hot air leaking from the trim air diffuser ducts or sidewall riser duct assemblies (collectively referred to in this supplemental NPRM as “TADDs”), related investigative actions, and corrective actions if necessary. The original NPRM also would have provided an optional terminating action for the repetitive tests.

Actions Since Original NPRM Was Issued

Since we issued the original NPRM, we have received reports indicating that the procedures referenced in Boeing Alert Service Bulletin 747-21A2418, Revision 2, dated March 4, 2004 (which we referenced in the original NPRM as the applicable source of service information for the proposed actions), are not sufficient to detect a damaged TADD in a timely manner.

Relevant Service Information

We have reviewed Boeing Service Bulletin 747-21A2418, Revision 4, dated November 17, 2005. Revision 4 of the service bulletin describes procedures that are similar to those in Revision 2. However, Revision 4 revises the part numbers for certain improved sidewall riser duct assemblies for installation on Boeing Model 747-400 series airplanes that are not freighters. This change is due to new environmental and flammability-resistance standards required under amendments 25-110, 91-279, 121-301, 125-43, and 135-90 of the Federal Aviation Regulations. (Refer to the final rule, docket no. FAA-2000-7909, “Improved Flammability Standards for Thermal/Acoustic Insulation Materials Used in Transport Category Airplanes” (68 FR 45046, July 31, 2003; with corrections published 68 FR 50054, August 20, 2003; and 69 FR 6532, February 11, 2004).) Revision 4 of the service bulletin also recommends increasing the initial inspection threshold from 27,000 flight hours to 32,000 flight hours, and the repetitive inspection interval from 7,000 flight hours to 12,000 flight hours, for the general visual inspection for damage or discrepancies of the TADDs.

Certain changes to the service information that were originally introduced in Boeing Service Bulletin 747-21A2418, Revision 3, dated December 21, 2004, are retained in Revision 4 of the service bulletin:

• Chapter 21-61-20 of the airplane maintenance manual (AMM) has been revised to contain more definitive pass/fail criteria for the repetitive tests and inspections of the TADDs. These revised criteria increase the chances of a defective TADD being detected in a timely manner.
• Chapter 21-61-21 of the AMM contains procedures for unwrapping insulation blankets as necessary before the general visual inspection to detect defective TADDs is done on Boeing Model 747-400 non-freighter series airplanes.

Accomplishing the actions specified in Revision 4 of the service information is intended to adequately address the unsafe condition. We have revised paragraphs (f), (g), (h), and (j) and Note 2 of this supplemental NPRM to refer to Revision 4 of the service information. We have also added a new paragraph (k) to this supplemental NPRM, and re-identified the subsequent paragraph, to give credit for actions done before the effective date of the AD in accordance with previous issues of the service bulletin.

With regard to extending compliance times for the general visual inspection, we have revised Table 1 of this supplemental NPRM to extend the repetitive interval for the general visual inspections from 7,000 flight hours to 12,000 flight hours. We have also revised Table 1 of this supplemental NPRM to extend the initial compliance threshold from 27,000 total flight hours to 32,000 total flight hours.

Comments

We have considered the following comments on the original NPRM.

Request To Relieve Testing Requirement

British Airways requests that we revise paragraph (f) of the original NPRM to relieve operators of the requirement to do a test to detect hot air leaking from the TADDs at the same time as the general visual inspection for damage or discrepancies of the TADDs. The commenter notes that, if the inspection is being accomplished, there is no need to do the test during the same maintenance check. The commenter assumes that the inspection exceeds the intent of the test in that the inspection would detect discrepancies of the TADDs that the test may not.

We concur with the commenter's request. We agree that it would be redundant to perform a hot air leak test at the same time as the general visual inspection when the repetitive intervals for these actions coincide. Therefore, we have revised paragraph (f) of this supplemental NPRM to clarify that, when the compliance times for a hot air leak test and a general visual inspection coincide, the hot air leak test is not required at that time.

Request To Allow Installation of Serviceable Improved TADDs

The Air Transport Association (ATA), on behalf of its member Northwest Airlines (NWA), and Boeing request that we revise paragraphs (h) and (j) and Note 3 of the original NPRM to allow installation of serviceable improved TADDs. Boeing states that the improved TADDs are expected to hold up well in service, and its customers are concerned about the proposed restriction on installing serviceable TADDs. In its comment submitted through ATA, NWA states that it does not believe that the failure rate of new TADDs is a significant improvement over properly repaired or serviceable used TADDs. NWA states that only a very small percentage of high-time TADDS have failed in service, and it believes that all duct leaks will be sufficiently addressed by the repetitive tests and inspections proposed in the original NPRM. NWA also disagrees that the TADDs deteriorate at a known rate in service, which was the justification stated in the original NPRM for not allowing installation of used TADDs. Similarly, Boeing comments that the deterioration rate is highly variable.

We agree with the commenters' request to allow installing serviceable improved TADDs. Our intent was to prohibit installing used ducts of the old type, not used ducts of the improved type. We have determined that installing serviceable improved parts will provide an acceptable level of safety. We have revised paragraphs (h) and (j) of this supplemental NPRM accordingly, and we have omitted Note 3 from this supplemental NPRM. However, as mentioned in the discussion of New Relevant Service Information, improved flammability standards may prohibit installing certain new, improved TADDs on non-freighter airplanes. Subsequent to the publication of the original NPRM, some of the improved TADDs failed a test of their insulation that is required by the improved flammability standards. Thus, under the requirements of that rule, certain improved TADDs that were listed in revisions of Boeing Service Bulletin 747-21A2418 prior to Revision 4, can no longer be installed (although they need not be removed if they were installed prior to September 2, 2003, the effective date of FAA-2000-7909).

Also, we do not agree with the commenters' statements that the rate of deterioration is unknown, although we acknowledge that there are many variables that contribute to the deterioration of the TADDs. The rate of deterioration is known to the extent that we know that TADDs having accumulated more than 20,000 total flight hours are suspect. Also, we do not know of an inspection process that would be adequate to ensure the integrity of a used duct of the old material. For these reasons, we have determined that it is not appropriate to allow installation of used TADDs made of the old material.

Request To Remove References to Deteriorated Sealant

Boeing requests that we revise the original NPRM to remove references to “reports of deteriorating sealants both inside and outside the center wing fuel tank due to heat damage from leaking TADDs.” Boeing states that it is not aware of reports of damaged fuel tanks caused by leaking TADDs.

We agree to revise the statement of what prompted the proposed AD to remove the references to reports of deterioration of the sealant inside the center wing fuel tank. We are unable to confirm direct observation of primary seal deterioration.

However, we disagree that primary or secondary seal deterioration is unlikely. Following reports of TADD leaks, Boeing analyzed the temperatures that the primary (inside) and secondary (outside) fuel barriers could reach. Analysis revealed that the secondary barrier could reach temperatures between 300 °F and 450 °F, and that internal tank temperatures could reach 378 °F. The sealants are not effective above 325 °F and are not qualified for prolonged exposure above 160 °F. In addition, FAA personnel observed deterioration of the secondary sealant in the center wing fuel tank. Therefore, if any damage or discrepancy of a TADD is found, we find it necessary to require a general visual inspection for damage of the primary and secondary fuel barriers of the center wing tank, and adjacent areas and items, as specified in paragraph (h) of this supplemental NPRM.

Based on this information, we have revised the Summary of this supplemental NPRM to state that the original NPRM “resulted from reports of sealant deteriorating on the outside of the center wing fuel tank and analysis that sealant may deteriorate inside the tank due to excess heat from leaking TADDs.” We have similarly revised paragraph (d) of this supplemental NPRM.

Request To Require Inspections Only on Affected Side

ATA, on behalf of NWA, requests that we revise paragraph (h) of the original NPRM to require an inspection for damage of the fuel barriers and adjacent areas only on the side of the airplane where a TADD failed. In its comment submitted through ATA, NWA states that the original NPRM does not acknowledge that the TADDs are located on both the left and right sides of the airplane. Neither ATA nor NWA state a justification for the request.

We infer that the commenter's request is intended to reduce the amount of work that needs to be accomplished to allow a quicker return of the airplane to service. We agree that it would be acceptable to inspect the fuel barriers and adjacent areas only on the side of the airplane where a TADD failed if no damage is found on the side of the airplane where a TADD failed. However, if any damage of the fuel barriers or adjacent areas is found on the side of the airplane where a TADD failed, both sides of the airplane must be inspected. Both sides must be inspected because the barrier damage is caused by hot air and if there is damage to one side, then there may be enough leakage to damage the other side.

We have revised paragraph (h)(1) of this AD to state that, “If no damage is found on the side of the airplane where the damaged or discrepant TADD is found, inspecting the other side of the airplane is not required.”

In addition, we have revised paragraph (j) of this AD to clarify the specific circumstances under which tests and inspections required by paragraph (f) of this AD are terminated. These changes better acknowledge that, as the commenter points out, there are TADDs on both the left and right sides of the airplane.

Request To Revise Repetitive Inspection Intervals

KLM Royal Dutch Airlines (KLM) requests that we extend the repetitive interval for the hot air leak test specified in paragraph (f)(1) of the original NPRM from 1,200 flight hours to 1,600 flight hours. The commenter states that the repetitive interval of 1,200 flight hours is not consistent with its maintenance intervals. KLM explains that its A-check is 770 flight hours, so it would have to perform this test either every A-check or in between A-checks. KLM states that either alternative would result in excessive cost. KLM notes that a repetitive interval of 1,600 flight hours would allow it to perform the test every second A-check. Boeing also commented that the interval for the hot air leak test should coincide with actual A-check intervals.

We do not agree with the request to extend the repetitive interval for the hot air leak test. The extension of the repetitive interval for the general visual inspections to 12,000 flight hours, as discussed previously, is contingent on the repetitive hot air leak tests being performed at intervals not to exceed 1,200 flight hours. We find that this repetitive interval is necessary to ensure that any discrepant TADD will be detected in a timely manner. We note that the 1,200-flight-hour repetitive interval is consistent with Boeing's recommendation in Revision 4 of the service bulletin and in its re-evaluation of compliance times. Further, since maintenance schedules vary among operators, it is not possible for us to revise the repetitive interval to meet the needs of a specific operator. In developing an appropriate repetitive interval for this action, we considered the manufacturer's recommendation, the degree of urgency associated with the subject unsafe condition, the average utilization of the affected fleet, and the time necessary to perform the test (estimated at 3 work hours). In light of all of these factors, we find that 1,200 flight hours is an appropriate interval of time for affected airplanes to continue to operate between repetitive tests without compromising safety. We have not changed the supplemental NPRM in this regard. However, paragraph (l) of the supplemental NPRM provides operators the opportunity to request an extension of the compliance time if data are presented to justify such an extension.

Request To Revise Compliance Time for Inspection of Replaced TADDs

ATA, on behalf of NWA, suggests that we revise the compliance time for the general visual inspection for damaged or replaced TADDs made of the original material. Paragraph (i) of the original NPRM specifies a compliance time of 27,000 flight hours after the TADD is replaced for this inspection. The commenter suggests that this compliance time be revised to “the next C-check after 21,200 flight hours.”

We partially agree with this request. We do not agree with the request to state the compliance time in relation to a C-check. We find that such a non-specific compliance time would not ensure that damaged TADDs are detected in a timely manner. However, we agree to extend the compliance time for inspecting replaced TADDs from 27,000 flight hours to 32,000 flight hours after replacement. We note that affected operators may elect to do the general visual inspection of the TADDs earlier than the stated compliance time, if it is more convenient to their maintenance schedules. We have revised paragraph (i) of this supplemental NPRM accordingly.

Request To Revise Compliance Time for Initial Leak Test

ATA, on behalf of NWA, requests that we revise the compliance time for the initial test specified in paragraph (f)(1) of the original NPRM. NWA states support for the test but believes that an equivalent level of safety can be achieved by doing the initial test at the compliance time specified in the referenced service bulletin, which the commenter interprets as 180 days or 2,000 hours, whichever is first. NWA states that a failed duct is often detected when floorboards or sidewalls become hot, or when the airplane crew has difficulty controlling cabin temperatures. Thus, a failed duct is often corrected by normal maintenance practices that limit exposure to high temperatures. For this reason, NWA states that compliance time for the initial inspection recommended in the service bulletin is sufficient to detect duct leaks that are not detected during normal operations.

We do not agree with the commenter's request. We note that 180 days or 2,000 flight hours (whichever is first) is the compliance time recommended by the referenced service bulletin for airplanes with 20,000 or more total flight hours. However, as we explained in the “Differences Between the Proposed AD and Service Information” section of the original NPRM, the compliance threshold of 21,200 total flight hours is the equivalent of the inspection threshold of 20,000 total flight hours specified in the service bulletin, plus one repeat interval (1,200 flight hours). In addition, the manufacturer has not requested that we revise the compliance time proposed in the original NPRM. In developing an appropriate compliance time for the initial test, we considered the manufacturer's recommendation, and the degree of urgency associated with the subject unsafe condition. In light of these factors, we find that the compliance time of 21,200 total flight hours, or 1,200 flight hours after the effective date of the AD, whichever is later, represents an appropriate interval of time for affected airplanes to continue to operate without compromising safety. We have not changed the supplemental NPRM in this regard.

Request To Ensure Adequate Supply of Replacement Parts

Lufthansa requests that we ensure that an adequate supply of replacement parts will be available for operators to comply with the proposed requirements. The commenter notes that there have been delays in obtaining material for planned modifications in accordance with Boeing Service Bulletin 747-21A2418. The commenter states that it anticipates that it will find TADDs that must be replaced.

We acknowledge the commenter's concerns and the delays it experienced. Parts availability is one of the factors that we consider when establishing a compliance time for an AD. In this case, we have determined through the manufacturer that an adequate supply of replacement parts will be available for operators to accomplish the proposed requirements within the proposed compliance time. We find that no additional changes to the supplemental NPRM are needed in this regard.

Request To Clarify Requirements of Paragraph (h)

Boeing requests that we revise paragraph (h) of the original NPRM to state that the actions in that paragraph apply if any discrepancy is found during either the hot air leak test or the general visual inspection for damage in accordance with paragraph (f) of the original NPRM.

We contacted Boeing for clarification of the meaning and intent of its comment. Upon further review of paragraph (h) of the original NPRM, Boeing concluded its comment was not necessary and could be withdrawn. We have not changed the supplemental NPRM in this regard.

Request To Allow Use of Later Revisions of Service Information

Air New Zealand (ANZ) requests that we revise paragraph (j) of the original NPRM, Optional Terminating Action, to allow use of later revisions of the referenced service information. ANZ notes that, when the AD refers to a specific revision of the service bulletin, e.g., Revision 2, operators may not use the later revisions without being out of compliance with the requirements of the AD when new service information is released that contains new part numbers for equivalent or better parts. ANZ suggests that we include language referring to “any subsequent documents, which list a new or equivalent part number or better, that does not have this unsafe condition.”

We do not agree with the request to refer to later revisions of the service information that have not yet been released. (As explained previously, we have revised this supplemental NPRM to refer to Boeing Service Bulletin 747-21A2418, Revision 4.) When we refer to a specific service bulletin in an AD, using a phrase such as that suggested by the commenter, or a phrase like “or later FAA-approved revisions,” violates Office of the Federal Register regulations for approving materials that are incorporated by reference. However, affected operators may request approval to use a later revision of the referenced service bulletin as an alternative method of compliance, under the provisions of paragraph (l) of this supplemental NPRM. We have not changed the supplemental NPRM further in this regard.

Request To Revise Cost Impact

Qantas Airways (QANTAS) requests that we revise the cost impact stated in the original NPRM. The commenter believes that the original NPRM underestimates the number of work hours necessary to do the general visual inspection for damage or discrepancies of the TADDs. QANTAS notes that significant time is necessary to gain access to the TADDs to perform the inspection and to close up after the inspection, in addition to testing the in-seat entertainment equipment. The commenter notes that the estimate of 43 work hours in Boeing Service Bulletin 747-21A2418 is more realistic.

We do not agree. The cost analysis in AD rulemaking actions typically does not include incidental costs such as the time required to gain access and close up, time necessary for planning, or time necessitated by other administrative actions. Those incidental costs, which may vary significantly among operators, are almost impossible to calculate. We have not changed the supplemental NPRM in this regard.

Requests for Editorial Changes

Boeing requests that we revise the Relevant Service Information section of the original NPRM as follows:

• Revise the statement, “The related investigative actions are repetitive general visual inspections for discrepancies or damage of the TADDs* * * ” to also refer to the hot air leak tests as related investigative actions.
• Revise the statement, “After a TADD is replaced with a new, improved TADD, the repetitive inspections are no longer needed for that TADD,” to note that neither the repetitive leak tests nor the repetitive inspections are needed after a new, improved TADD is installed.

Boeing's rationale for the first change is that the statement in the original NPRM implies that only the visual inspections constitute valid investigative actions. Boeing's rationale for the second change is to avoid questions (from operators) and misinterpretation.

We acknowledge the commenter's requests. However, we do not agree that any change is necessary. The Relevant Service Information section of the original NPRM states that the referenced service bulletin “describes procedures for repetitive tests to detect hot air leaking from the TADDs, related investigative actions, and corrective actions if necessary.” The statement to which the commenter refers defines what we mean by “related investigative actions.” We find that the contents of the Relevant Service Information section are sufficiently clear as written in the original NPRM. With regard to the commenter's second item, we agree with the statement as revised by the commenter. However, the Relevant Service Information section of the original NPRM is not restated in this supplemental NPRM. Thus, no change is possible in this regard.

Explanation of Additional Changes

We have reduced the estimated number of airplanes that would be affected by this supplemental NPRM to be consistent with the number of airplanes identified in the service bulletin.

After the original NPRM was issued, we reviewed the figures we have used over the past several years to calculate AD costs to operators. To account for various inflationary costs in the airline industry, we find it necessary to increase the labor rate used in these calculations from $65 per work hour to $80 per work hour. The cost impact information, below, reflects this increase in the specified hourly labor rate.

We have revised this action to clarify the appropriate procedure for notifying the principal inspector before using any approved AMOC on any airplane to which the AMOC applies.

FAA's Determination and Proposed Requirements of the Supplemental NPRM

Certain changes discussed above expand the scope of the original NPRM; therefore, we have determined that it is necessary to reopen the comment period to provide additional opportunity for public comment on this supplemental NPRM.

Costs of Compliance

There are about 1,081 airplanes of the affected design in the worldwide fleet. The following table provides the estimated costs for U.S. operators to comply with this supplemental NPRM.

Estimated Costs

Action	Work hours	Average labor rate per hour	Cost per airplane	Number of U.S.- registered airplanes	Fleet cost
Hot air leak test	3	$80	$240, per test cycle	216	$51,840, per test cycle.
General visual inspection	5	80	400, per inspection cycle	216	86,400, per inspection cycle.

Authority for This Rulemaking

Title 49 of the United States Code specifies the FAA's authority to issue rules on aviation safety. Subtitle I, Section 106, describes the authority of the FAA Administrator. Subtitle VII, Aviation Programs, describes in more detail the scope of the Agency's authority.

We are issuing this rulemaking under the authority described in Subtitle VII, Part A, Subpart III, Section 44701, “General requirements.” Under that section, Congress charges the FAA with promoting safe flight of civil aircraft in air commerce by prescribing regulations for practices, methods, and procedures the Administrator finds necessary for safety in air commerce. This regulation is within the scope of that authority because it addresses an unsafe condition that is likely to exist or develop on products identified in this rulemaking action.

Regulatory Findings

We have determined that this proposed AD would not have federalism implications under Executive Order 13132. This proposed AD would not have a substantial direct effect 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.

For the reasons discussed above, I certify that the proposed regulation:

1. Is not a “significant regulatory action” under Executive Order 12866;

2. Is not a “significant rule” under the 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.

We prepared a regulatory evaluation of the estimated costs to comply with this supplemental NPRM and placed it in the AD docket. See the ADDRESSES section for a location to examine the regulatory evaluation.

List of Subjects in 14 CFR Part 39

• Air transportation
• Aircraft
• Aviation safety
• Safety

The Proposed Amendment

Accordingly, under the authority delegated to me by the Administrator, the FAA proposes to amend 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. The Federal Aviation Administration (FAA) amends § 39.13 by adding the following new airworthiness directive (AD):

Boeing: Docket No. FAA-2004-19755; Directorate Identifier 2004-NM-23-AD.

Comments Due Date

(a) The FAA must receive comments on this AD action by October 23, 2006.

Affected ADs

(b) None.

Applicability

(c) This AD applies to Boeing Model 747-100, 747-100B, 747-100B SUD, 747-200B, 747-200C, 747-200F, 747-300, 747-400, 747-400D, 747-400F, 747SR, and 747SP series airplanes; certificated in any category; line numbers 1 through 1316 inclusive.

Unsafe Condition

(d) This AD results from reports of sealant deteriorating on the outside of the center wing fuel tank and analysis that sealant may deteriorate inside the tank due to excess heat from leaking trim air diffuser ducts or sidewall riser duct assemblies (collectively referred to in this AD as “TADDs”). We are issuing this AD to prevent leakage of fuel or fuel vapors into areas where ignition sources may be present, which could result in a fire or explosion.

Compliance

(e) You are responsible for having the actions required by this AD performed within the compliance times specified, unless the actions have already been done.

Repetitive Tests and Inspections

(f) Do the actions in Table 1 of this AD at the times specified in Table 1 of this AD, in accordance with the Accomplishment Instructions of Boeing Service Bulletin 747-21A2418, Revision 4, dated November 17, 2005. When the compliance times for a hot air leak test and a general visual inspection coincide, the hot air leak test is not required at that time, but is required within 1,200 flight hours (i.e., one repeat interval) after the general visual inspection.

Table 1.—Compliance Times

Do this action—	Initially at the later of—	Then repeat within this interval until paragraph (j) is done—
(1) Repetitive test to detect hot air leaking from TADDs	Prior to the accumulation of 21,200 total flight hours, or within 1,200 flight hours after the effective date of this AD	1,200 flight hours.
(2) General visual inspection for damage or discrepancies of the TADDs	Prior to the accumulation of 32,000 total flight hours, or within 12,000 flight hours after the effective date of this AD, except as provided by paragraph (g) of this AD	12,000 flight hours.

Note 1:

For the purposes of this AD, a general visual inspection is: “A visual examination of an interior or exterior area, installation, or assembly to detect obvious damage, failure, or irregularity. This level of inspection is made from within touching distance unless otherwise specified. A mirror may be necessary to ensure visual access to all surfaces in the inspection area. This level of inspection is made under normally available lighting conditions such as daylight, hangar lighting, flashlight, or droplight and may require removal or opening of access panels or doors. Stands, ladders, or platforms may be required to gain proximity to the area being checked.”

Note 2:

Boeing Service Bulletin 747-21A2418, Revision 4, refers to Chapters 21-61-20 and 21-61-21 of the 747 Airplane Maintenance Manual as an additional source for service information for the test and inspections of the TADDs.

(g) If any hot air leak is found during any test required by paragraph (f) of this AD: Before further flight, do the general visual inspection for damage or discrepancies of the TADDs, in accordance with the Accomplishment Instructions of Boeing Service Bulletin 747-21A2418, Revision 4, dated November 17, 2005.

Corrective Actions

(h) If any damage or discrepancy is found during any general visual inspection for damage required by paragraph (f) or (g) of this AD: Do the actions in paragraphs (h)(1), (h)(2), (h)(3), and (h)(4) of this AD, as applicable. Do all of these actions in accordance with the Accomplishment Instructions of Boeing Service Bulletin 747-21A2418, Revision 4, dated November 17, 2005.

(1) Before further flight: Perform a general visual inspection for damage of the primary and secondary fuel barriers of the center wing tank; structure adjacent to the discrepant TADD; and cables, cable pulleys, and raised cable seals in the over-wing area. If no damage is found on the side of the airplane where the damaged or discrepant TADD is found, inspecting the other side of the airplane is not required.

(2) Before further flight: Repair all damage or discrepancies found.

(3) Before further flight: Replace any damaged TADD with a new TADD having the same part number or a new or serviceable, improved TADD having a part number listed in the “New TADD Part Number” or “New Sidewall Riser Duct Assy Part Number” column, as applicable, of the tables in Section 2.C.2. of the service bulletin.

(4) Repeat the test and inspection required by paragraph (f) of this AD at the times specified in Table 1 of this AD, except as provided by paragraphs (i) and (j) of this AD.

(i) For any original-material TADD that is replaced with a new TADD having the same part number as the TADD being replaced: Within 21,200 flight hours after the TADD is replaced, do the test to detect hot air leaking from the replaced TADD, and within 32,000 flight hours after the TADD is replaced, do the general visual inspection for damage, as specified in paragraph (f) of this AD. Thereafter, repeat the test and inspection at the repetitive intervals specified in Table 1 of this AD, except when the times for a hot air leak test and a general visual inspection coincide, the leak test is not required.

Optional Terminating Action

(j) Replacing existing TADDs with new or serviceable, improved TADDs terminates repetitive test and inspection requirements as specified in paragraphs (j)(1), (j)(2), and (j)(3) of this AD. New or serviceable, improved TADDs are those having a part number listed in the “New TADD Part Number” or “New Sidewall Riser Duct Assy Part Number” column, as applicable, of the tables in Section 2.C.2. of Boeing Service Bulletin 747-21A2418, Revision 3, dated December 21, 2004; or Revision 4, dated November 17, 2005.

(1) The repetitive general visual inspections required by paragraph (f)(2) of this AD are terminated for each TADD that is replaced with a new or serviceable, improved TADD.

(2) Replacing all TADDs on one side of the airplane with new or serviceable, improved TADDs terminates all repetitive tests required by paragraph (f)(1) of this AD and all repetitive inspections required by paragraph (f)(2) of this AD only for the side of the airplane on which the improved TADDs are installed.

(3) Replacing all TADDs on both sides of the airplane with new or serviceable, improved TADDs terminates all repetitive tests required by paragraph (f)(1) of this AD and all repetitive inspections required by paragraph (f)(2) of this AD.

Previously Accomplished Actions

(k) Actions done before the effective date of this AD in accordance with Boeing Service Bulletin 747-21A2418, dated November 14, 2002; Revision 1, dated October 16, 2003; Revision 2, dated March 4, 2004; or Revision 3, dated December 21, 2004; are acceptable for compliance with the corresponding actions required by this AD.

Alternative Methods of Compliance (AMOCs)

(l)(1) The Manager, Seattle Aircraft Certification Office, FAA, has the authority to approve AMOCs for this AD, if requested in accordance with the procedures found in 14 CFR 39.19.

(2) Before using any AMOC approved in accordance with § 39.19 on any airplane to which the AMOC applies, notify the appropriate principal inspector in the FAA Flight Standards Certificate Holding District Office.

Issued in Renton, Washington, on September 14, 2006.

Kalene C. Yanamura,

Acting Manager, Transport Airplane Directorate, Aircraft Certification Service.