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

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:

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Revision

Service Bulletin referenced and date - Page number level shown Date shown on page

on page

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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.

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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]

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