Airworthiness Directives; Boeing Model 747 Series Airplanes

DEPARTMENT OF TRANSPORTATION

14 CFR Part 39

[Docket No. 93-NM-212-AD]

Airworthiness Directives; Boeing Model 747 Series Airplanes

AGENCY: Federal Aviation Administration, DOT.

ACTION: Notice of proposed rulemaking (NPRM).

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SUMMARY: This document proposes the adoption of a new airworthiness

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

airplanes. This proposal would require inspection to detect cracking of

certain fuselage longitudinal lap joints, repair of any cracking found,

and replacement of the countersunk fasteners in those laps joints with

protruding head fasteners. This proposal is prompted by a structural

reassessment of Model 747 series airplanes. The actions specified by

the proposed AD are intended to prevent skin cracking in the

longitudinal lap joints of certain stringers, which can lead to rapid

decompression of the airplane.

DATES: Comments must be received by April 29, 1994.

ADDRESSES: Submit comments in triplicate to the Federal Aviation

Administration (FAA), Transport Airplane Directorate, ANM-103,

Attention: Rules Docket No. 93-NM-212-AD, 1601 Lind Avenue, SW.,

Renton, Washington 98055-4056. Comments may be inspected at this

location between 9 a.m. and 3 p.m., Monday through Friday, except

Federal holidays.

The service information referenced in the proposed rule may be

obtained from Boeing Commercial Airplane Group, P.O. Box 3707, Seattle,

Washington 98124-2207. This information may be examined at the FAA,

Transport Airplane Directorate, 1601 Lind Avenue, SW., Renton,

Washington.

FOR FURTHER INFORMATION CONTACT: Steven C. Fox, Aerospace Engineer,

Airframe Branch, ANM-120S, FAA, Transport Airplane Directorate, Seattle

Aircraft Certification Office, 1601 Lind Avenue, SW., Renton,

Washington 98055-4056; telephone (206) 227-2777; fax (206) 227-1181

SUPPLEMENTARY INFORMATION:

Comments Invited

Interested persons are invited to participate in the making of the

proposed rule by submitting such written data, views, or arguments as

they may desire. Communications shall identify the Rules Docket number

and be submitted in triplicate to the address specified above. All

communications received on or before the closing date for comments,

specified above, will be considered before taking action on the

proposed rule. The proposals contained in this notice may be changed in

light of the comments received.

Comments are specifically invited on the overall regulatory,

economic, environmental, and energy aspects of the proposed rule. All

comments submitted will be available, both before and after the closing

date for comments, in the Rules Docket for examination by interested

persons. A report summarizing each FAA-public contact concerned with

the substance of this proposal will be filed in the Rules Docket. -

Commenters wishing the FAA to acknowledge receipt of their comments

submitted in response to this notice must submit a self-addressed,

stamped postcard on which the following statement is made: ``Comments

to Docket Number 93-NM-212-AD.'' The postcard will be date stamped and

returned to the commenter.

Availability of NPRMs

Any person may obtain a copy of this NPRM by submitting a request

to the FAA, Transport Airplane Directorate, ANM-103, Attention: Rules

Docket No. 93-NM-212-AD, 1601 Lind Avenue, SW., Renton, Washington

98055-4056.

Discussion

Recently, the Boeing Commercial Airplane Group, manufacturer of

Model 747 series airplanes, conducted a structural reassessment of

these airplanes. The FAA has reviewed the results of this reassessment

and has determined that the longitudinal lap joints of Stringer (S-)12

are critical to the structural integrity of these airplanes. -

The upper skin in the S-12 longitudinal lap joints are made of

waffle doublers that are hot bonded to skin panels. The S-12

longitudinal lap joints have three rows of countersunk fasteners. The

depth of the countersink exceeds the thickness of the skin panel. If

the waffle doubler disbonds from the skin panel, the resultant sharp

edges at the longitudinal lap joint fasteners can cause fatigue

cracking at the fastener holes.

Skin cracking in the longitudinal lap joints of S-12L and S-12R, if

not detected and corrected, could result in rapid decompression of the

airplane.

The FAA has reviewed and approved Boeing Service Bulletin 747-53-

2366, dated August 6, 1992, that describes procedures for repetitive

high frequency eddy current (HFEC) inspections to detect cracking of

the skin around the fasteners in the upper row of the longitudinal lap

joints of S-12L and S-12R from station 520 to station 741.1, and

replacement of the countersunk fasteners in these longitudinal lap

joints with protruding head fasteners.

Since an unsafe condition has been identified that is likely to

exist or develop on other products of this same type design, the

proposed AD would require repetitive external HFEC inspections to

detect cracking of the skin around the fasteners in the upper row of

the longitudinal lap joints of S-12L and S-12R from station 520 to

station 741.1, and replacement of the countersunk fasteners in these

longitudinal lap joints with protruding head fasteners. The actions

would be required to be accomplished in accordance with the service

bulletin described previously. This proposal also would require repair

of all cracking in accordance with procedures specified in the 747

Structural Repair Manual.

This proposal is applicable only to airplanes having line numbers

201 through 230. Since the tolerance specifications for the bonding

process for airplanes manufactered after line number 230 were more

stringent, those airplanes are not subject to the unsafe condition

addressed by this proposed AD.

There are approximately 30 Model 747 series airplanes of the

affected design in the worldwide fleet. The FAA estimates that 12

airplanes of U.S. registry would be affected by this proposed AD, that

it would take approximately 17 work hours per airplane to accomplish

the proposed inspections, and that the average labor rate is $55 per

work hour. Based on these figures, the total cost impact of the

proposed inspections on U.S. operators is estimated to be $11,220, or

$935 per airplane.

The FAA estimates that it would take approximately 302 work hours

per airplane to accomplish the proposed requirement to replace

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

Required parts would be nominal in cost. Based on these figures, the

total cost impact of the proposed requirement to replace fasteners on

U.S. operators is estimated to be $199,320, or $16,610 per airplane.

Based on the figures, above, the total cost impact of the proposed

actions (cost of inspections added to the cost of replacement of

fasteners) on U.S. operators is estimated to be $210,540, or $17,545

per airplane.

The FAA recognizes that the proposed replacement of fasteners would

require a large number of work hours to accomplish. However, the 4-year

compliance time specified in paragraph (b) of this proposed AD should

allow ample time for the replacement of the fasteners to be

accomplished coincidentally with scheduled major airplane inspection

and maintenance activities, thereby minimizing the costs associated

with special airplane scheduling. -

The total cost impact figure discussed above is based on

assumptions that no operator has yet accomplished any of the proposed

requirements of this AD action, and that no operator would accomplish

those actions in the future if this AD were not adopted.

The regulations proposed herein would 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

proposal would not have sufficient federalism implications to warrant

the preparation of a Federalism Assessment.

For the reasons discussed above, I certify that this 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) if promulgated, 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 copy of the draft

regulatory evaluation prepared for this action is contained in the

Rules Docket. A copy of it may be obtained by contacting the Rules

Docket at the location provided under the caption ADDRESSES.

List of Subjects in 14 CFR Part 39

Air transportation, Aircraft, Aviation safety, Safety.

The Proposed Amendment

Accordingly, pursuant to the authority delegated to me by the

Administrator, the Federal Aviation Administration proposes to amend 14

CFR part 39 of the Federal Aviation Regulations 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:

Boeing: Docket 93-NM-212-AD.

Applicability: Model 747 series airplanes, having line numbers

201 through 230 inclusive; certificated in any category. -

Compliance: Required as indicated, unless accomplished

previously.

To prevent rapid decompression of the airplane, accomplish the

following:

(a) Prior to the accumulation of 15,000 total flight cycles or

within 1,000 flight cycles after the effective date of this AD,

whichever occurs later, perform an external high frequency eddy

current (HFEC) inspection to detect cracking of the skin at the

upper row of countersunk fasteners in the longitudinal lap joints of

Stringer (S-)12L and S-12R from station 520 to station 741.1, in

accordance with Boeing Service Bulletin 747-53-2366, dated August 6,

1992.

(1) If no cracking is found, repeat the inspection thereafter at

intervals not to exceed 4,000 flight cycles until the replacement of

fasteners required by paragraph (b) of this AD is accomplished.

(2) If any cracking is found, prior to further flight, repair in

accordance with the 747 Structural Repair Manual. After repair,

repeat the inspection at intervals not to exceed 4,000 flight cycles

until the replacement of fasteners required by paragraph (b) of this

AD is accomplished.

(b) Prior to the accumulation of 20,000 total flight cycles or

within 4 years after the effective date of this AD, whichever occurs

later, replace the countersunk fasteners in the upper row of the

longitudinal lap joints of S-12L and S-12R from station 520 to

station 741.1 with protruding head fasteners in accordance with

Boeing Service Bulletin 747-53-2366, dated August 6, 1992.

Replacement of these fasteners constitutes terminating action for

the repetitive inspections required by paragraph (a) of this AD.

(c) Prior to the accumulation of 10,000 total flight cycles

after replacement of the countersunk fasteners required by paragraph

(b) of this AD, perform an external HFEC inspection to detect

cracking of the skin at the upper row of protruding head fasteners

in the longitudinal lap joints of S-12L and S-12R from station 520

to station 741.1 in accordance with Boeing Service Bulletin 747-53-

2366, dated August 6, 1992.

(1) If no cracking is found, repeat the inspection thereafter at

intervals not to exceed 3,000 flight cycles.

(2) If any cracking is found, prior to further flight, repair in

accordance with the 747 Structural Repair Manual. After repair,

repeat the inspection at intervals not to exceed 3,000 flight

cycles.

(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

Federal Aviation Regulations (FAR) 21.197 and 21.199 to operate the

airplane to a location where the requirements of this AD can be

accomplished.

Issued in Renton, Washington, on February 28, 1994.

Darrell M. Pederson,

Acting Manager, Transport Airplane Directorate, Aircraft Certification

Service.

[FR Doc. 94-4954 Filed 3-3-94; 8:45 am]

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