Airworthiness Directives; Boeing Model 747 Series Airplanes

DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. 93-NM-139-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 inspections to detect fatigue-

related skin cracks and corrosion of the skin lap joints in the

fuselage upper lobe, and repair, if necessary. This proposal would also

require modification of certain lap joints and inspections of modified

lap joints. This proposal is prompted by a structural review of Model

747 series airplanes. The actions specified by the proposed AD are

intended to prevent rapid decompression of the airplane and the

inability to carry fail-safe loads.

DATES: Comments must be received by April 19, 1994.

ADDRESSES: Submit comments in triplicate to the Federal Aviation

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

Attention: Rules Docket No. 93-NM-139-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 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-139-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-139-AD, 1601 Lind Avenue, SW., Renton, Washington

98055-4056.

Discussion

In April 1988, a transport category airplane managed to land after

tiny cracks in rivet holes in the upper fuselage linked together,

causing structural failure and explosive decompression. An 18-foot

section ripped from the fuselage. This accident focused greater

attention on the problem of aging aircraft.

In June 1988, the FAA sponsored a conference on aging airplane

issues, which was attended by representatives of the aviation industry

from around the world. It became obvious that, because of the

tremendous increase in air travel, the relatively slow pace of new

airplane production, and the apparent economic feasibility of operating

older technology airplanes rather than retiring them, increased

attention needed to be focused on this aging fleet and maintaining its

continued operational safety.

The Air Transport Association (ATA) of America and the Aerospace

Industries Association (AIA) of America committed to identifying and

implementing procedures to ensure continuing structural airworthiness

of aging transport category airplanes. An Airworthiness Assurance Task

Force, with representatives from the aircraft operators, manufacturers,

regulatory authorities, and other aviation representatives, was

established in August 1988. The objective of the Task Force was to

sponsor ``Working Groups'' to (1) select service bulletins, applicable

to each airplane model in the transport fleet, to be recommended for

mandatory modification of aging airplanes, (2) develop corrosion

directed inspections and prevention programs, and (3) review the

adequacy of each operator's structural maintenance program, (4) review

and update the Supplemental Structural Inspection Documents (SSID), and

(5) assess repair quality.

The Working Group assigned to review the Boeing Model 747 series

airplanes completed its work on Item (2) in July 1989 and developed a

baseline program for controlling corrosion problems that may jeopardize

the continued airworthiness of the Boeing Model 747 fleet. This program

is contained in Boeing Document Number D6-36022, ``Aging Airplane

Corrosion Prevention and Control Program--Model 747,'' dated July 1989.

The FAA issued AD 90-25-05, Amendment 39-6790 (55 FR 49268, November

27, 1990), which requires implementation of a corrosion prevention and

control program.

The Working Group completed a portion of its work on Item (1),

above. The Working Group's proposal is contained in Boeing Document

Number D6-35999, ``Aging Airplane Service Bulletin Structural

Modification Program--Model 747.'' The FAA issued AD 90-06-06,

Amendment 39-6490 (55 FR 8374, March 7, 1990), which requires the

installation of the structural modifications identified in the

document, and AD 92-27-04, Amendment 39-8437 (58 FR 8695, February 17,

1993), which requires structural inspections of older airplanes.

Additional structural inspections are required by AD 90-15-06,

Amendment 39-6653 (55 FR 28600, July 12, 1990), which references Boeing

Service Bulletin 747-53-2307, dated December 21, 1989. (A correction of

the rule was published in the Federal Register on July 30, 1990 (55 FR

31027).)

The action being proposed herein follows from the ongoing

activities of the Working Group relative to Item (1). The Working Group

has identified certain service difficulties that warrant mandatory

inspections following mandatory modification of these airplanes. The

Working Group considers that these service difficulties can be

controlled safely by repetitively inspecting following modification of

these airplanes, and that, because of the safety implications, the

inspections should be mandatory to assure that all operators perform

them. Typically, the addressed unsafe conditions have occurred

infrequently on older airplanes, and the Working Group has a very high

degree of confidence in the ability of an inspection program to detect

the damage before it impairs safety.

The Working Group has reviewed Boeing Service Bulletin 747-53-2307,

Revision 2, dated October 14, 1993, and has recommended it to the FAA

for mandating of the described inspections to detect fatigue-related

skin cracks and corrosion of the skin lap joints in the fuselage upper

lobe, and necessary repair. The SWG also recommends modification of

certain skin lap joints in the fuselage upper lobe that are fastened

with deep countersink rivets on airplanes that have accumulated 20,000

or more total landings. The upper skin panel in these particular lap

joints has a doubler that is hot-bonded to the skin. The countersink

depth of the rivet is greater than the basic skin gage. While this is

not a problem for a skin panel with the hot bond intact, disbonding

will result in a knife-edge condition in the skin layer. This condition

causes these lap joints to be more susceptible to fatigue-related

cracking. The SWG also recommends accomplishment of repetitive

inspections of these modified lap joints. Accomplishment of these

modifications and inspections will detect fatigue-related cracking and

corrosion in a timely manner and will ensure the structural integrity

of the skin lap joints beyond the design life of the airplane. The FAA

has concurred with the Working Group's recommendations and has

determined that AD action is warranted to mandate the inspections and

modifications to assure the continued airworthiness of the Model 747

fleet.

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

2307, Revision 2, dated October 14, 1993, that describes procedures for

inspections to detect fatigue-related skin cracks and corrosion of the

skin lap joints in the fuselage upper lobe, and repair, if necessary.

The service bulletin also describes procedures for ``full''

modification of certain skin lap joints in the fuselage upper lobe that

are fastened with deep countersink rivets on airplanes that have

accumulated 20,000 total landings. Such rivets are used in the lap

joints at the following locations:

1. Stringer 12, left and right, from station 520 to 1,000; and

2. Stringer 19, left and right, from station 520 to 740.

The ``full'' modification for deep countersink rivets at these

locations consists of opening the lap joint between circumferential

splices, removing adhesive, repairing all cracks and corrosion, and

reassembling the lap joint with \9/32\ inch protruding head rivets at

all rivet locations.

The service bulletin also describes procedures for repetitive

external high frequency eddy current (HFEC) inspections to detect

fatigue-related skin cracks of modified lap joints at the upper

fastener row, and repair, if necessary.

The service bulletin also describes procedures for an ``optional''

(partial) modification for certain lap joint areas that have an upper

skin panel thickness of 0.090 inch or less and that do not have cracks

or corrosion or an existing structural repair on the lap joint. The

``optional'' (partial) modification consists of removing the upper row

of rivets at a corrosion-free lap joint, performing an HFEC open hole

inspection to detect cracks in removed rivet locations, and installing

\1/4\-inch protruding head rivets.

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 inspections to detect fatigue-related skin

cracks and corrosion of the skin lap joints in the fuselage upper lobe,

and repair, if necessary. The proposed AD would also require ``full''

modification of certain skin lap joints in the fuselage upper lobe that

are fastened with deep countersink rivets on airplanes that have

accumulated 20,000 total landings. The inspections and ``full

modification'' would be required to be accomplished in accordance with

the service bulletin described previously.

The proposed AD would also require repetitive external HFEC

inspections to detect fatigue-related skin cracks of modified lap

joints, and repair, if necessary. After such repair, additional

repetitive external HFEC inspections would be required. The HFEC

inspections and repair would also be required to be accomplished in

accordance with the service bulletin described previously.

The proposed AD would also provide an ``optional'' (partial)

modification for certain skin panels that have a thickness that is

within specified limits, and that do not have cracks or corrosion or an

existing structural repair on the lap joint. For those skin panels,

this ``optional'' (partial) modification may be accomplished in lieu of

the ``full'' modification. However, the ``optional'' (partial)

modification may not be implemented at deep countersink fastener

locations. The ``optional'' (partial) modification would be required to

be accomplished in accordance with the service bulletin described

previously.

Accomplishment of the requirements of this proposed AD would

terminate the requirements of AD 90-15-06, Amendment 39-6653 (55 FR

28600, July 12, 1990). The FAA is considering further rulemaking action

to revise AD 90-15-06, Amendment 39-6653, accordingly once this

proposed AD becomes effective.

There are approximately 200 Model 747 series airplanes of the

affected design in the worldwide fleet. The FAA estimates that 116

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

The costs associated with the proposed inspections entail 100 work

hours per airplane per inspection, at an average labor rate of $55 per

work hour. (This figure does not include the time necessary for gaining

access and closing up.) Based on these figures, the total cost impact

of the proposed AD on U.S. operators, with regard to the proposed

inspections, is estimated to be $638,000, or $5,500 per airplane per

inspection.

The costs associated with the proposed ``full'' modification entail

approximately 96 work hours, for each 200-inch length of uncracked and

uncorroded lap joint, at an average labor rate of $55 per work hour.

(This figure does not include the time necessary for gaining access and

closing up.) There are 100 lap joint sections per airplane, each with a

length of 200 inches. The cost of required parts is expected to be

negligible. Based on these figures, the total cost impact of the

proposed AD on U.S. operators, with regard to the proposed ``full''

modification requirement, is estimated to be $61,248,000, or $528,000

per airplane.

The costs associated with the proposed initial HFEC inspection

following modification entails approximately 56 work hours, at an

average labor rate of $55 per work hour. Based on these figures, the

total cost impact of the proposed AD on U.S. operators, with regard to

the proposed initial HFEC inspection following modification, is

estimated to be $357,280, or $3,080 per airplane.

Based upon the figures discussed above, the total cost impact of

the proposed AD on U.S. operators is expected to be $62,243,280, or

$536,580 per airplane. This total cost figure includes the proposed

inspections and modification, for the first year of the average five-

year inspection cycle. This total cost figure assumes that no operator

has yet accomplished the proposed requirements of this proposed AD.

The FAA recognizes that the proposed ``full'' modification would

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

proposed compliance time specified in paragraph (e) of this proposed AD

should allow ample time for accomplishment of the ``full'' modification

coincidentally with scheduled major airplane inspection and maintenance

activities, thereby minimizing the costs associated with special

airplane scheduling.

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.

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-139-AD.

Applicability: Model 747 series airplanes, line positions 001

through 200 inclusive, certificated in any category.

Compliance: Required as indicated, unless accomplished

previously.

To prevent rapid decompression of the airplane and the inability

to carry fail-safe loads, accomplish the following:

(a) Within 1,000 landings after the effective date of this AD,

and thereafter at the intervals specified in paragraphs (a)(1),

(a)(2), and (a)(3) of this AD, perform inspections at the upper lobe

skin lap joints in accordance with Boeing Service Bulletin 747-53-

2307, Revision 2, dated October 14, 1993:

(1) Perform a detailed external visual inspection to detect

cracks and evidence of corrosion (bulging skin between fasteners,

blistered paint, dished fasteners, popped rivet heads, or loose

fasteners) in accordance with the service bulletin. Repeat that

inspection thereafter at intervals not to exceed 2,000 landings

until the inspection required by paragraph (f) of this AD is

accomplished.

(2) Perform a high frequency eddy current inspection (HFEC) to

detect cracks in the skin at the upper row of fasteners of the lap

joints forward of body station (BS) 1000 in accordance with the

service bulletin. Repeat that inspection thereafter at intervals not

to exceed 4,000 landings until the modification required by

paragraph (e) of this AD is accomplished.

(3) Perform a HFEC inspection to detect cracks in the skin at

the upper row of fastener holes of the lap joints aft of BS 1480 to

2360 in accordance with the service bulletin. Repeat that inspection

thereafter at intervals not to exceed 6,000 landings until the

inspection required by paragraph (f) of this AD is accomplished.

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

AD, or if any corrosion is found for which material loss exceeds 10

percent of the material thickness, accomplish paragraphs (b)(1) and

(b)(2) of this AD in accordance with Boeing Service Bulletin 747-53-

2307, Revision 2, dated October 14, 1993.

(1) Prior to further flight, repair any crack or corrosion

found, in accordance with the service bulletin.

(2) Within 15 months after accomplishing the repair, accomplish

the ``full'' modification described in the service bulletin for the

remainder of any skin panel lap joint in which a crack is found, or

in which corrosion is found that exceeds 10 percent of the material

thickness.

(c) If no crack is found during any inspection required by this

AD, but corrosion is found for which the material loss does not

exceed 10 percent of the material thickness: Accomplish paragraph

(c)(1) and (c)(2) of this AD for the entire affected skin panel lap

joint in accordance with Boeing Service Bulletin 747-53-2307,

Revision 2, dated October 14, 1993.

(1) Within 500 landings after accomplishing the inspection

during which the corrosion was found, and thereafter at intervals

not to exceed 500 landings until the ``full'' modification required

by paragraph (c)(2) of this AD is accomplished: Perform a HFEC

inspection to detect cracks of the corroded lap joint, in accordance

with the service bulletin.

(2) Within 30 months after accomplishing the inspection during

which the corrosion was found: Accomplish the ``full'' modification

required by paragraph (e) of this AD.

(d) The inspections required by this AD shall be performed by

removing the paint and using an approved chemical stripper; or by

ensuring that each fastener head is clearly visible.

(e) Except as provided in paragraph (g) of this AD, prior to the

accumulation of 20,000 total landings, or within the next 1,000

landings after the effective date of this AD, whichever occurs

later: Accomplish the modification described in the service bulletin

as a ``full'' modification of the skin lap joints at the locations

specified in paragraphs (e)(1) and (e)(2) of this AD, as applicable,

in accordance with Boeing Service Bulletin 747-53-2307, Revision 2,

dated October 14, 1993.

(1) For airplane line numbers 001 through 058, inclusive: Modify

the skin lap joints at Stringer 12 (left and right), station 520 to

1,000; and Stringer 19 (left and right), station 520 to 740.

(2) For airplane line numbers 59 through 200, inclusive: Modify

the skin lap joints at Stringer 12 (left and right), station 740 to

1,000; and Stringer 19 (left and right), station 520 to 740.

(f) Perform an external HFEC inspection to detect skin cracks of

any modified skin lap joints at the times specified in paragraphs

(f)(1), (f)(2), and (f)(3) of this AD, as applicable, in accordance

with Boeing Service Bulletin 747-53-2307, Revision 2, dated October

14, 1993. Repeat that inspection thereafter at intervals not to

exceed 3,000 landings. Accomplishment of this inspection terminates

the repetitive inspection requirements of paragraph (a) of this AD.

(1) For airplanes on which the ``full'' modification has been

accomplished: Within 10,000 landings after accomplishment of that

modification.

(2) For airplanes on which the ``optional'' (partial)

modification has been accomplished: Within 7,000 landings after

accomplishment of that modification.

(3) For airplanes having deep countersink fasteners located at

Section 42 on which the ``full'' modification, as described in the

original issue of the service bulletin, has been accomplished:

Within 5,000 landings after accomplishment of that modification.

(g) In lieu of the ``full'' modification required by paragraph

(e) of this AD, the ``optional'' (partial) modification described in

Boeing Service Bulletin 747-53-2307, Revision 2, dated October 14,

1993, may be accomplished for skin panels that have an outer

thickness of 0.090 inches or less, and that do not have any cracks,

corrosion, or an existing structural repair on the lap joint. The

``optional'' (partial) modification shall not be accomplished at

deep countersink fastener locations.

(h) Accomplishment of the requirements of this AD terminates the

requirements of AD 90-15-06, Amendment 39-6653.

(i) 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.

(j) Special flight permits may be issued in accordance with 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 17, 1994.

Darrell M. Pederson,

Acting Manager, Transport Airplane Directorate, Aircraft Certification

Service.

[FR Doc. 94-4128 Filed 2-23-94; 8:45 am]

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