[Federal Register Volume 59, Number 219 (Tuesday, November 15, 1994)]
[Unknown Section]
[Page 0]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 94-26874]


[[Page Unknown]]

[Federal Register: November 15, 1994]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. 94-NM-30-AD; Amendment 39-9057; AD 94-22-08]

 

Airworthiness Directives; Boeing Model 747 Series Airplanes 
Equipped With General Electric CF6 Series Engines

AGENCY: Federal Aviation Administration, DOT.

ACTION: Final rule.

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SUMMARY: 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.

DATES: Effective December 15, 1994.
    The incorporation by reference of certain publications listed in 
the regulations is approved by the Director of the Federal Register as 
of December 15, 1994.

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: Tim Backman, Aerospace Engineer, 
Airframe Branch, ANM-120S, FAA, Transport Airplane Directorate, Seattle 
Aircraft Certification Office, 1601 Lind Avenue SW., Renton, Washington 
98055-4056; telephone (206) 227-2776; fax (206) 227-1181.

SUPPLEMENTARY INFORMATION: 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.
    Interested 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.
    One 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.
    This 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.
    The 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.
    The 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.
    The 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.
    For 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.
    The 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.
    The 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.
    The 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.
    The 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.
    The 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 continued operational 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.
    The 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.
    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.
    There are approximately 250 Model 747 series airplanes of the 
affected design in the worldwide fleet. The FAA estimates that 4 
airplanes of U.S. registry will be affected by this AD.
    The 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.
    The 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.
    The 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 if this AD were not adopted.
    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.

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. App. 1354(a), 1421 and 1423; 49 U.S.C. 
106(g); and 14 CFR 11.89.


Sec. 39.13  [Amended]

    2. Section 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:

94-22-08 Boeing: Amendment 39-9057. Docket 94-NM-30-AD. Supersedes 
AD 87-04-21, Amendment 39-5543.

    Applicability: Model 747 series airplanes equipped with General 
Electric CF6 series engines, certificated in any category.
    Compliance: Required as indicated, unless accomplished 
previously.
    To prevent separation of an engine due to overheating and 
subsequent cracking of the engine strut, accomplish the following:
    (a) For 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.
    (1) If 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.
    (2) If 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.
    (b) For 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.
    (1) Prior 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
    (2) Within 12 months after the immediately preceding inspection 
accomplished in accordance with paragraph (a) of this AD.

    Note 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.

    (c) For 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.

    Note 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.

    (d) If 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.
    (e) If 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.
    (f) Installation 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.
    (g) 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 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.

    Note 3: Information concerning the existence of approved 
alternative methods of compliance with this AD, if any, may be 
obtained from the Seattle ACO.

    (h) 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.
    (i) The actions shall be done in accordance with the following 
Boeing service bulletins, which contain the specified effective 
pages:

----------------------------------------------------------------------------------------------------------------
                                                                                Revision                        
           Service Bulletin referenced and date -              Page number    level shown    Date shown on page 
                                                                                on page                         
----------------------------------------------------------------------------------------------------------------
747-54-2091, Revision 1, October 22, 1984...................  1, 3-12-.....  1-...........  October 22, 1984.   
                                                              2, 13-20-....  Original-....  January 27, 1984.   
747-54-2091, Revision 2, March 24, 1988-....................  1, 4-37-.....  2-...........  March 31, 1988.     
-                                                             2-...........  Original-....  January 27, 1984.   
-                                                             3-...........  1-...........  October 22, 1984.   
747-54-2091, Revision 3, July 27, 1989-.....................  1-24, 26-36-.  3-...........  July 27, 1989.      
-                                                             25-..........  2-...........  March 31, 1988.     
747-54-2091, Revision 4, December 14, 1989-.................  1-5-.........  4-...........  December 14, 1989.  
-                                                             6-24, 26-36-.  3-...........  July 27, 1989.      
-                                                             25-..........  2-...........  March 31, 1988.     
747-54-2091, Revision 5, April 26, 1990-....................  1-38-........  5-...........  April 26, 1990.     
----------------------------------------------------------------------------------------------------------------

    The 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.
    (j) This amendment becomes effective on December 15, 1994.

    Issued in Renton, Washington, on October 25, 1994.
Darrell M. Pederson,
Acting Manager, Transport Airplane Directorate, Aircraft Certification 
Service.
[FR Doc. 94-26874 Filed 11-14-94; 8:45 am]
BILLING CODE 4910-13-U