AD 94-22-08

This amendment supersedes an existing airworthiness directive (AD), applicable to certain Boeing Model 747 series airplanes, that currently requires inspections of the strut skin in the area of the precooler exhaust vent for cracks on the inboard and outboard struts, and repair, if necessary. This amendment requires inspections of an expanded area for certain airplanes, and inspections of airplanes on which a skin doubler has been installed as terminating action for the existing AD. This amendment is prompted by reports of strut skin fatigue cracks and heat damage found aft of the edges of skin doublers installed on certain Model 747 series airplanes. The actions specified by this AD are intended to prevent separation of an engine due to overheating and subsequent cracking of the engine strut.

Final rule

94-22-08 BOEING: Amendment 39-9057. Docket 94-NM-30-AD. Supersedes AD 87-04-21, Amendment 39-5543. \n\n\tApplicability: Model 747 series airplanes equipped with General Electric CF6 series engines, certificated in any category. \n\n\tCompliance: Required as indicated, unless accomplished previously. \n\n\tTo prevent separation of an engine due to overheating and subsequent cracking of the engine strut, accomplish the following: \n\n\t(a)\tFor airplanes listed in Boeing Service Bulletin 747-54-2091, Revision 1, dated October 22, 1984: Prior to the accumulation of 10,000 total hours time-in-service, or within the next 7-1/2 months after March 13, 1987 (the effective date of AD 87-04-21, Amendment 39-5543), whichever occurs later, perform a visual inspection to detect cracks of the strut skin in the area of the precooler exhaust vent on the inboard and outboard struts of Group 1 airplanes, and on the outboard struts of Group 2 airplanes, as defined in the service bulletin, in accordance with Boeing Service Bulletin 747-54-2091, Revision 1, dated October 22, 1984; Revision 2, dated March 24, 1988; Revision 3, dated July 27, 1989; Revision 4, dated December 14, 1989; or Revision 5, dated April 26, 1990. After the effective date of this AD, the inspection shall be accomplished in accordance with paragraph (b) of this AD. \n\n\t\t(1)\tIf no crack is found, repeat the inspection required by paragraph (a) of this AD thereafter at intervals not to exceed 15 months, until the inspection required by paragraph (b) or (c) of this AD, as applicable, is accomplished. \n\n\t\t(2)\tIf any crack is found, prior to further flight, repair in accordance with FAA- approved data, and repeat the inspection required by paragraph (a) of this AD thereafter at intervals not to exceed 15 months, until the inspection required by paragraph (b) or (c) of this AD, as applicable, is accomplished. \n\n\t(b)\tFor airplanes on which a frame stiffener and a skin doubler have not been installed during production or in accordance with Boeing Service Bulletin 747-54-2091, Revision 1, dated October 22, 1984; Revision 2, dated March 24, 1988; Revision 3, dated July 27, 1989; Revision 4, dated December 14, 1989; or Revision 5, dated April 26, 1990: Perform a visual inspection to detect cracks, heat discoloration, or wrinkles of the strut skin and internal structure in the area of the precooler exhaust vent from nacelle station (NAC STA) 230 to NAC STA 286 on the inboard and outboard struts of Group 1 airplanes and on the outboard struts of Group 2 airplanes, in accordance with the inspection procedures described in Figure 3 of Boeing Service Bulletin 747-54- 2091, Revision 5, dated April 26, 1990; at the time specified in paragraph (b)(1) or (b)(2) of this AD, whichever occurs later. Accomplishment of this inspection terminates the repetitive inspections required by paragraph (a) of this AD. \n\n\t\t(1)\tPrior to the accumulation of 10,000 total hours time-in-service on the airplane strut, or within 120 days after the effective date of this AD, whichever occurs later. Or \n\n\t\t(2)\tWithin 12 months after the immediately preceding inspection accomplished in accordance with paragraph (a) of this AD. \n\n\tNOTE 1: Paragraph (b) of this AD specifies an inspection zone that is expanded beyond the zone described in Revision 5 of the service bulletin to cover a 30-inch width from NAC STA 230 to NAC STA 286. \n\n\t(c)\tFor airplanes on which a frame stiffener and a skin doubler have been installed during production or in accordance with Boeing Service Bulletin 747-54-2091, Revision 1, dated October 22, 1984; Revision 2, dated March 24, 1988; Revision 3, dated July 27, 1989; Revision 4, dated December 14, 1989; or Revision 5, dated April 26, 1990: Within 120 days after the effective date of this AD, perform a visual inspection to detect cracks, heat discoloration, or wrinkles of the strut skin and internal structure in the area of the precooler exhaust vent from the edge of the doubler to NAC STA 286 on the inboard and outboard struts of Group 1 airplanes and on the outboard struts of Group 2 airplanes, in accordance with the inspection procedures described in Figure 3 of Boeing Service Bulletin 747-54-2091, Revision 5, dated April 26, 1990. \n\n\tNOTE 2: Paragraph (c) of this AD specifies an inspection zone that is expanded beyond the zone described in Revision 5 of the service bulletin to cover a 30-inch width from the doubler edge to NAC STA 286. \n\n\t(d)\tIf no crack, heat discoloration, or wrinkle is found during the inspection required by paragraph (b) or (c) of this AD, repeat that inspection thereafter at intervals not to exceed 15 months. \n\n\t(e)\tIf any crack, heat discoloration, wrinkle, or previously stop-drilled crack is found during the inspection required by paragraph (b) or (c) of this AD, prior to further flight, repair using either the small skin doubler and frame stiffener or the large skin doubler and frame stiffener specified in Boeing Service Bulletin 747-54-2901, Revision 5, dated April 26, 1990, in accordance with that service bulletin; or in accordance with a method approved by the Manager, Seattle Aircraft Certification Office (ACO), FAA, Transport Airplane Directorate. Thereafter, repeat that inspection at intervals not to exceed 15 months. \n\n\t(f)\tInstallation of a frame stiffener and a skin doubler referred to in Boeing Service Bulletin 747-54-2091 as "terminating action" does not constitute terminating action for the inspection requirements of this AD. \n\n\t(g)\tAn 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 Aircraft Certification Office (ACO), FAA, Transport Airplane Directorate. 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. \n\n\tNOTE 3: Information concerning the existence of approved alternative methods of compliance with this AD, if any, may be obtained from the Seattle ACO. \n\n\t(h)\tSpecial 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. \n\n\t(i)\tThe actions shall be done in accordance with the following Boeing service bulletins, which contain the specified effective pages: \n\n\nService Bulletin\nReferenced and Date\n\nPage Number\nRevision Level\nShown on Page\n Date\nShown on Page \n747-54-2091,\nRevision 1,\nOctober 22, 1984 \n1, 3-12\n\n2, 13-20\n1\n\nOriginal\nOctober 22, 1984 \n\nJanuary 27, 1984 \n747-54-2091,\nRevision 2,\nMarch 24, 1988\n1, 4-37\n\n2\n\n3\n2\n\nOriginal\n\n1\nMarch 31, 1988\n\nJanuary 27, 1984 \n\nOctober 22, 1984 \n747-54-2091,\nRevision 3,\nJuly 27, 1989\n1-24, 26-36\n\n25\n3\n\n2\nJuly 27,1989\n\nMarch 31, 1988 \n747-54-2091,\nRevision 4,\nDecember14, 1989\n1-5\n\n6-24, 26-36\n4\n\n3\nDecember 14, 1989\n\nJuly 27, 1989\n747-54-2091,\nRevision 5,\nApril 26, 1990 \n1-38\n5\nApril 26, 1990\n\nThe incorporation by reference of these documents 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. \n\n\t(j)\tThis amendment becomes effective on December 15, 1994.

A proposal to amend part 39 of the Federal Aviation Regulations (14 CFR part 39) by superseding AD 87-04-21, amendment 39-5543 (52 FR 3793, February 6, 1987), which is applicable to certain Boeing Model 747 series airplanes, was published in the Federal Register on June 14, 1994 (59 FR 30543). The action proposed to require, for certain airplanes, repetitive visual inspections to detect cracks, heat discoloration, or wrinkles of the strut skin and internal structure in the area of the precooler exhaust vent from nacelle station (NAC STA) 230 to NAC STA 300 in the inboard and outboard struts of certain airplanes, or in the area of the precooler exhaust vent from the edge of the skin doubler to NAC STA 300 in the inboard and outboard struts of certain other airplanes, and repair, if necessary. \n\n\tInterested persons have been afforded an opportunity to participate in the making of this amendment. Due consideration has been given to the comments received.Two commenters support the proposed rule. \n\n\tOne commenter, Boeing, requests that the aft limit of the inspection zones specified in paragraphs (b) and (c) of the proposal be changed from NAC STA 300 to NAC STA 286. The commenter indicates that expansion of the inspection zone to NAC STA 300 was based on one report of a blown duct. This incident was reported immediately. The commenter contends that the damage that occurred was the result of a system malfunction and, therefore, inspection should not be required beyond NAC STA 286. In light of this information, the FAA concurs with the commenter's request. Paragraphs (b) and (c) and NOTES 1 and 2 of the final rule have been revised to indicate that the aft limit of the expanded inspection zone only extends to NAC STA 286. \n\n\tThis commenter also requests that the FAA revise paragraph (c) of the proposed rule to exclude airplanes on which a large doubler was installed during production (airplane line numbers 587 and subsequent). The applicability of proposed paragraph (c) includes airplanes modified during production or in accordance with Boeing Service Bulletin 747-54-2091. The commenter notes that for 12 years there have been no in-service problems aft of the existing doubler to beyond NAC STA 286. The commenter also states that the Maintenance Review Board (MRB) report recommends a visual inspection of the strut side skin during regularly scheduled "C" checks. The commenter adds that, for airplanes retrofitted in service, data show that heat damage occurred in the area between NAC STA 270 and NAC STA 286 prior to installation of the doubler. The commenter states that one small crack has been reported aft of the large replacement doubler installed in accordance with Boeing Service Bulletin 747-54-2091; however, that crack is believed to have occurred as a result of previous damage. \n\n\tThe FAA does not concur with the commenter's request. This AD was prompted by reports of skin cracks and heat damage found aft of the edges of doublers installed in accordance with Boeing Service Bulletin 747-54-2091. The doubler configuration installed during production of these airplanes is identical to that installed in accordance with Boeing Service Bulletin 747-54-2091. The FAA finds that, regardless of whether the doubler was installed during production or in accordance with the Boeing service bulletin, cracking can occur in the subject area and the inspection of the expanded area specified in paragraph (c) of this AD must be accomplished on both groups of airplanes to ensure that the unsafe condition is positively addressed. \n\n\tThe commenter requests that the compliance times specified in paragraph (c) of the proposed rule be revised from 120 days to 15 months. Paragraph (c) of the proposal would require a visual inspection to detect cracks, heat discoloration, or wrinkles of the strut skin and internal structure in the area of the precooler exhaust vent from the edge of the doubler to NAC STA 300 on the inboard and outboard struts of certain airplanes and on the outboard struts of other airplanes. The commenter states that service data indicate that, for airplanes on which either no doubler or a temporary doubler has been installed, no stringer cracking has occurred in the presence of large skin cracks forward of NAC STA 270. The commenter explains that, for the outboard strut, skin stresses aft of NAC STA 270 are lower due to increased thickness of the skin and, if cracking should occur, the crack growth rate would be low. The commenter also notes that the MRB report recommends detailed visual inspections of the side skin at every "C" check interval, which is approximately every 15 months. In addition, the commenter states that service history amplifies the fact that zero time reinforcement of the side skin during the production stage forestalls primary damage to the adjacent skin. \n\n\tThe FAA does not concur with the commenter's request. The FAA has received reports of softening of the skin aft of the large doubler end at NAC STA 270 on both the inboard and the thicker-skinned outboard struts due to overheating. The FAA also has received reports of cracking in multiple fastener holes in that area of the struts. In light of these reports, the FAA has determined that 120 days represents the maximum interval of time allowable wherein the inspection can reasonably be accomplished and an acceptable level of safety can be maintained. \n\n\tFor the same reasons given as justification for its previous request, the commenter also requests that the compliance time specified in paragraph (b)(2) of the proposal be extended from 12 to 15 months. The commenter adds that the struts on these airplanes were designed to handle 3g side loads, which is well above normal operating loads. \n\n\tThe FAA does not concur with the commenter's request. AD 87-04-21 required inspections of the strut in accordance with Revision 1 of Boeing Service Bulletin 747-54-2091; however, that revision of the service bulletin does not specify procedures for inspections of the internal structure of the strut. The FAA has received numerous reports of cracked stiffeners due to heat damage; such cracks could only be seen during an inspection of the internal structure, as required by paragraph (b) of this AD. In light of these reports, and in consideration of the degree of urgency associated with addressing the subject unsafe condition, the FAA has determined that the compliance time, as proposed, represents the maximum interval in which the inspections can be accomplished in a timely manner within the fleet and still maintain an adequate level of safety. \n\n\tThe commenter requests that paragraphs (b) and (c) of the proposal be revised to remove the requirement to perform inspections of the internal structure of the strut from NAC STA 270 to NAC STA 286. The commenter references its justification for the previous two comments. The commenter adds that service history indicates that cracking will develop in the strut skin before it will develop in the strut stiffener or frame structure in this area. In addition, since service history demonstrates that stringer cracks are preceded by skin cracks, the commenter believes it is reasonable to expect that any sizable stringer damage will be found if a skin crack is observed during a visual inspection. The commenter adds that operators have the option to inspect internally at routine intervals. \n\n\tThe FAA does not concur. The FAA has received reports of skin cracking that has been associated with cracked stiffeners (internal structure) in the area forward of NAC STA 270. Although the skin is thicker aft of NAC STA 270 than forward of it, skin cracks have been found aft of NAC STA 270 that could likely be associated with stiffener cracks. Some skin cracking also initiated in multiple fastener holes in the area from NAC STA 270 to NAC STA 286 due to heat damage. In light of these reports,the FAA has determined that, in order to provide an acceptable level of safety, detailed visual inspections of the internal structure of the strut from NAC STA 270 to NAC STA 286 are necessary to detect possible damage in that area. \n\n\tThe same commenter requests that paragraph (e) of the proposed rule be revised to allow continued flight with a skin crack, heat discoloration, wrinkle, or previously stop-drilled crack in the precooler exhaust, provided that the length of the crack does not exceed one inch. The commenter indicates that service history demonstrates that some degree of damage is acceptable with continued monitoring inspections at an interval of 15 months. The commenter states that suspected heat damage of the side skin can be verified by conductivity checks. In addition, standard industry practice allows wrinkles to be two times the thickness (depending on design) and stop-drilling holes to slow crack growth as a time-limited repair. The commenter considers the time-limited repair acceptable for a maximum period of 18 months. \n\n\tThe FAA does not concur. The FAA has issued a number of AD's that address cracking and corrosion of lugs, links, fuse pins, and webs on the struts of Model 747 series airplanes (for example, AD 93-17-07, amendment 39-8678 (58 FR 45827, August 31, 1993); AD 92-24-51, amendment 39-8439 (57 FR 60118, December 18, 1992); and AD 86-07-06, amendment 39-5270 (51 FR 10821, March 31, 1986)). In some cases where the structure surrounding the previously stop-drilled crack, the skin crack, heat discoloration, or wrinkle is shown by service history to be free from cracks and corrosion, continued flight without repair of these items may have been acceptable. However, in light of the numerous problems associated with the strut on these airplanes, continued flight without repair of previously stop-drilled cracks, skin cracks, heat discoloration, or wrinkles cannot be allowed in the case of this AD without compromising the continuedoperational safety of the affected airplanes. However, the FAA would consider a request for an adjustment of the compliance time, in accordance with the provision of paragraph (g) of this AD, provided that appropriate justification accompanies the request. \n\n\tThe manufacturer has advised that it is currently developing a modification program for the engine strut that will positively address the unsafe condition addressed by this AD. Once this modification program is developed, approved, and available, the FAA may consider additional rulemaking. \n\n\tAfter 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. \n\n\tThere are approximately 250 Model 747 series airplanes of the affected design in theworldwide fleet. The FAA estimates that 4 airplanes of U.S. registry will be affected by this AD. \n\n\tThe inspections that were required previously by AD 87-04-21, and retained in this AD, take approximately 4 work hours per airplane to accomplish, at an average labor rate of $55 per work hour. Based on these figures, the total cost impact of that inspection requirement on U.S. operators is estimated to be $880, or $220 per airplane. \n\n\tThe new inspections that will be added by this new AD action will take approximately 4 work hours per airplane to accomplish, at an average labor rate of $55 per work hour. Based on these figures, the total cost impact of the new inspection requirements of this AD on U.S. operators is estimated to be $880, or $220 per airplane. \n\n\tThe total 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 ifthis AD were not adopted. \n\n\tThe 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. \n\n\tFor 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 copyof it may be obtained from the Rules Docket at the location provided under the caption "ADDRESSES."\n\n List of Subjects in 14 CFR Part 39 \n\n\tAir transportation, Aircraft, Aviation safety, Incorporation by reference, Safety. \n\nAdoption of the Amendment \n\n\tAccordingly, 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: \n\nPART 39 - AIRWORTHINESS DIRECTIVES \n\n\t1.\tThe authority citation for part 39 continues to read as follows: Authority: 49 U.S.C. App. 1354(a), 1421 and 1423; 49 U.S.C. 106(g); and 14 CFR 11.89. § 39.13 - (Amended) \n\n\t2.\tSection 39.13 is amended by removing amendment 39-5543 (52 FR 3793, February 6, 1987), and by adding a new airworthiness directive (AD), amendment 39-9057, to read as follows: