Airworthiness Directives; Boeing Model 747 Series Airplanes Equipped With Pratt and Whitney JT9D-3 or JT9D-7 Series Engines, Excluding JT9D-70 Series Engines

DEPARTMENT OF TRANSPORTATION

14 CFR Part 39

[Docket No. 94-NM-05-AD; Amendment 39-9012; AD 94-17-17]

Airworthiness Directives; Boeing Model 747 Series Airplanes

Equipped With Pratt and Whitney JT9D-3 or JT9D-7 Series Engines,

Excluding JT9D-70 Series Engines

AGENCY: Federal Aviation Administration, DOT.

ACTION: Final rule.

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SUMMARY: This amendment adopts a new airworthiness directive (AD),

applicable to certain Boeing Model 747 series airplanes, that requires

inspections of the inboard and outboard strut chords, stiffeners, and

web to detect cracks and loose fasteners; repair of the chords,

stiffeners, or web, if necessary; and replacement of any loose

fastener. This amendment is prompted by reports of fatigue cracks and

loose fasteners found in the forward lower spar web of the inboard

strut on Model 747 series airplanes equipped with Pratt & Whitney JT9D-

3 and JT9D-7 series engines. The actions specified by this AD are

intended to prevent separation of the strut from the wing of the

airplane due to fatigue cracking.

DATES: Effective September 30, 1994.

The incorporation by reference of certain publications listed in

the regulations is approved by the Director of the Federal Register as

of September 30, 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) to include an airworthiness

directive (AD) that is applicable to certain Boeing Model 747 series

airplanes was published in the Federal Register on March 17, 1994 (59

FR 12560). That action proposed to require inspections of the inboard

and outboard strut chords, stiffeners, and web to detect cracks and

loose fasteners; repair of the chords, stiffeners, or web, if

necessary; and replacement of any loose fastener.

Interested persons have been afforded an opportunity to participate

in the making of this amendment. Due consideration has been given to

the comments received.

One commenter supports the proposed rule.

One commenter, Boeing, requests that the proposed rule be withdrawn

since only six cracks have been reported in the 20 years these

airplanes have been in service. In addition, these cracks were found

only on the inboard struts during normal maintenance. Boeing asserts

that the probability of another strut with a pre-existing firewall

crack experiencing loads that approach the ultimate design condition is

extremely remote. Boeing adds, however, that the airplane maintenance

manual is being revised to include an inspection of the lower spar web

in the case of an unusual overload event. Boeing also indicates that

the results of one operator's inspections revealed only one loose

fastener on one airplane out of 24 inspected. Boeing states that the

firewall webs will be reinforced during the strut modification program

referenced in the proposal.

The FAA does not concur with the commenter's request to withdraw

this AD. The FAA has determined that inspections mandated by the

issuance of this AD are necessary to correct the unsafe condition

presented by separation of the strut from the wing of the airplane due

to fatigue cracking in the web. These inspections are necessary until

the 0.025-inch inboard webs and the 0.032-inch outboard webs are

replaced as part of the strut modification program discussed in the

proposal. Sonic fatigue analysis performed by Boeing in support of the

decision to replace the webs has revealed that the existing inboard and

outboard webs on Model 747 series airplanes addressed in this AD are

inadequate. Recent findings from an investigation of an incident

involving a Model 747 series airplane indicate that fatigue cracking

found on the web of the airplane resulted from flexing or vibration of

the web material. This fatigue cracking resulted in the in-flight loss

of an engine during airplane operation in severe turbulence. Since the

FAA considers it probable for other airplanes to have cracked webs, and

since no requirements exist to restrict airplane operation in severe

turbulence, the FAA finds that this AD action is necessary in order to

prevent in-flight engine loss.

Several commenters request that inspections of the outboard webs be

removed from the proposed rule so that the AD is consistent with the

referenced service bulletin. The commenters indicate that no cracking

or loose fasteners have been found on the outboard webs. The commenters

also state that the design thickness of the webs (0.032 inch for the

outboard web; 0.025 inch for the inboard web) represents a 28 percent

difference, which should be considered significant.

The FAA does not concur. The thickness of the inboard web of the

incident airplane mentioned previously measured 0.027 inch. The FAA

finds that there is little difference between the inboard and outboard

webs with standard manufacturing tolerances. Each web is similar in

configuration, loading, and sonic environments. In light of these

considerations, the FAA concludes that the outboard strut web is

subject to the same unsafe condition as the inboard strut web and must

be included in this AD.

Several commenters request that the compliance times for

inspections of the outboard strut be extended. One commenter requests

that the compliance times be revised to align with those recommended in

the referenced service bulletin, less the amount of time provided for

public comment on the proposal. Another commenter requests that the

compliance times be extended to be consistent with a load analysis

based upon the actual web thickness of the outboard strut. This

commenter assumes that the proposed repetitive inspection interval is

based on a load analysis of the inboard strut structure. Another

commenter requests that the compliance times for the initial inspection

of the inboard strut be extended to 15 months. The commenter does not

provide any justification for this request, however.

The FAA concurs that the compliance times can be extended somewhat.

The compliance times specified in paragraphs (a)(1) and (a)(3) of this

AD have been revised to reflect the times recommended in the

manufacturer's service bulletin. The FAA finds that the proposed

compliance time of 6 months specified for airplanes that have

accumulated 6,000 or more total landings, but less than 15,000 total

landings [those airplanes applicable to paragraph (a)(2) of the final

rule] may be extended to 9 months. Although the service bulletin

specifies a recommended compliance time of 12 months for those

airplanes, the FAA finds that, based on the service history of affected

Model 747 series airplanes that fall within this group, 9 months

represents the maximum interval of time allowable for these airplanes

to continue to operate without compromising safety.

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 as proposed.

This is considered to be interim action. 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.

There are approximately 380 Model 747 series airplanes of the

affected design in the worldwide fleet. The FAA estimates that 140

airplanes of U.S. registry will be affected by this AD, that it will

take approximately 22 work hours per airplane to accomplish the

required actions, and that the average labor rate is $55 per work hour.

Based on these figures, the total cost impact of the AD on U.S.

operators is estimated to be $169,400, or $1,210 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 adding the following new

airworthiness directive:

94-17-17 Boeing: Amendment 39-9012. Docket 94-NM-05-AD.

Applicability: Model 747 series airplanes equipped with Pratt &

Whitney JT9D-3 or JT9D-7 series engines, excluding JT9D-70 series

engines; line numbers 001 through 510 inclusive; certificated in any

category.

Compliance: Required as indicated, unless accomplished

previously.

To prevent separation of the strut from the wing of the airplane

due to fatigue cracking, accomplish the following:

(a) Perform a detailed visual inspection of the inboard and

outboard strut forward lower spar chords, stiffeners, and web to

detect cracks and loose fasteners, in accordance with the procedures

described in Boeing Service Bulletin 747-54-2160, dated September 9,

1993, at the time specified in paragraph (a)(1), (a)(2), or (a)(3)

of this AD, as applicable. Repeat this inspection thereafter at

intervals not to exceed 2,000 landings or 8,000 hours time-in-

service, whichever occurs first.

(1) For airplanes that have accumulated less than 6,000 total

landings as of the effective date of this AD: Perform the initial

inspection at the later of the times specified in paragraphs

(a)(1)(i) and (a)(1)(ii) of this AD.

(i) Prior to the accumulation of 6,000 total landings on the

strut. Or

(ii) Within 12 months after the effective date of this AD.

(2) For airplanes that have accumulated 6,000 or more total

landings, but less than 15,000 total landings, as of the effective

date of this AD: Perform the initial inspection within 9 months

after the effective date of this AD.

(3) For airplanes that have accumulated 15,000 or more total

landings as of the effective date of this AD: Perform the initial

inspection within 6 months after the effective date of this AD.

(b) If any crack is found during any inspection required by

paragraph (a) of this AD, prior to further flight, repair in

accordance with procedures specified in Chapter 54-10-03 of the 747

Structural Repair Manual.

(c) If any loose fastener is found during any inspection

required by paragraph (a) of this AD, prior to further flight,

replace the fastener in accordance with procedures specified in

Chapter 51-30-02 of the 747 Structural Repair Manual.

(d) 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: Information concerning the existence of approved

alternative methods of compliance with this AD, if any, may be

obtained from the Seattle ACO.

(e) 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.

(f) The inspection shall be done in accordance with Boeing

Service Bulletin 747-54-2160, dated September 9, 1993. This

incorporation by reference was approved by the Director of the

Federal Register in accordance with 5 U.S.C. 552(a) and 1 CFR part

51. Copies may be obtained from Boeing Commercial Airplane Group,

P.O. Box 3707, Seattle, Washington 98124-2207. Copies may be

inspected at the FAA, Transport Airplane Directorate, 1601 Lind

Avenue, SW., Renton, Washington; or at the Office of the Federal

Register, 800 North Capitol Street, NW., suite 700, Washington, DC.

(g) This amendment becomes effective on September 30, 1994.

Issued in Renton, Washington, on August 17, 1994.

Darrell M. Pederson,

Acting Manager, Transport Airplane Directorate, Aircraft Certification

Service.

[FR Doc. 94-20594 Filed 8-30-94; 8:45 am]

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