[Federal Register Volume 66, Number 161 (Monday, August 20, 2001)]
[Rules and Regulations]
[Pages 43475-43477]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 01-20807]



[[Page 43475]]

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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. 2001-NM-236-AD; Amendment 39-12393; AD 2001-17-02]
RIN 2120-AA64


Airworthiness Directives; Boeing Model 737-600, -700, and -800 
Series Airplanes

AGENCY: Federal Aviation Administration, DOT.

ACTION: Final rule; request for comments.

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SUMMARY: This amendment adopts a new airworthiness directive (AD) that 
is applicable to certain Boeing Model 737-600, -700, and -800 series 
airplanes. This action requires repetitive inspections for corrosion or 
cracking of the keel beam splices, and corrective action, if necessary. 
This action also provides an optional terminating action for the 
repetitive inspections. This action is necessary to find and fix 
corrosion or cracking of the keel beam splices, which could result in 
failure of the keel beam and consequent failure of the forward fuselage 
of the airplane. This action is intended to address the identified 
unsafe condition.

DATES: Effective September 4, 2001.
    Comments for inclusion in the Rules Docket must be received on or 
before October 19, 2001.

ADDRESSES: Submit comments in triplicate to the Federal Aviation 
Administration (FAA), Transport Airplane Directorate, ANM-114, 
Attention: Rules Docket No. 2001-NM-236-AD, 1601 Lind Avenue, SW., 
Renton, Washington 98055-4056. Comments may be inspected at this 
location between 9:00 a.m. and 3:00 p.m., Monday through Friday, except 
Federal holidays. Comments may be submitted via fax to (425) 227-1232. 
Comments may also be sent via the Internet using the following address: 
[email protected]. Comments sent via fax or the Internet must 
contain ``Docket No. 2001-NM-236-AD'' in the subject line and need not 
be submitted in triplicate. Comments sent via the Internet as attached 
electronic files must be formatted in Microsoft Word 97 for Windows or 
ASCII text.
    Information related to this AD may be examined at the FAA, 
Transport Airplane Directorate, 1601 Lind Avenue, SW., Renton, 
Washington.

FOR FURTHER INFORMATION CONTACT: James Blilie, Aerospace Engineer, 
Airframe Branch, ANM-120S, FAA, Seattle Aircraft Certification Office, 
1601 Lind Avenue, SW., Renton, Washington 98055-4056; telephone (425) 
227-2131; fax (425) 227-1181.

SUPPLEMENTARY INFORMATION: The FAA has received a report that severe 
corrosion was found on a keel beam splice on two Boeing Model 737-700 
series airplanes. At the time the severe corrosion was found, the 
airplanes had been in service for approximately 22 months since date of 
manufacture. This corrosion has been attributed to the material of the 
keel beam splice plates that were installed during production of 
certain Boeing Model 737-600, -700, and -800 series airplanes. The 
material, 7150-T6511, is known to be highly susceptible to corrosion. 
Such corrosion, if not found and fixed, could cause cracking of the 
keel beam splices, which in turn could lead to rapid degradation of the 
strength of the keel beam splices, and result in failure of the keel 
beam and consequent failure of the forward fuselage of the airplane.
    This unsafe condition may exist or develop on Model 737-600 and -
700 series airplanes up to and including line number 908; and on Model 
737-800 series airplanes up to and including line number 455. The keel 
beam splices on airplanes after those line numbers are made of a more 
corrosion-resistant material.

Other Relevant Rulemaking

    On November 5, 1990, we issued AD 90-25-01, amendment 39-6789 (55 
FR 49263, November 27, 1990). That AD applies to all Boeing Model 737 
series airplanes and requires implementation of a corrosion prevention 
and control program (CPCP) specified in Boeing Document Number D6-38528 
``Aging Airplane CPCP, Model 737,'' Revision A, dated July 28, 1989.
    The airplanes subject to this new AD are also subject to AD 90-25-
01. However, we have previously approved an alternative method of 
compliance (AMOC) to paragraph (a) of that AD for Boeing Model 737 
``Next Generation'' airplanes (which includes Model 737-600, -700, and 
-800 series airplanes). This AMOC allows certain inspection thresholds 
and repetitive intervals listed in Section 8 (``Structural Maintenance 
Program'') of Boeing Document Number D626A001, dated June 2000 (the 
``Maintenance Planning Document'' (MPD) for the Boeing 737 Next 
Generation airplanes), to be used as an alternative to the thresholds 
and intervals listed in Boeing Document Number D6-38528, Revision A.

FAA's Determination

    We have determined that existing inspections of the keel beam 
splices included in the CPCP required by the existing AD and in Boeing 
Document Number D626A001 are not sufficient to ensure that the splices 
are inspected for corrosion and cracking in a timely manner. This 
determination is based on the following information:
     Task Number 53-210-00 of Boeing Document Number D626A001, 
Section 8, dated June 2001, includes repetitive general visual 
inspections for any discrepancy of the keel beam under the wing-to-body 
fairing, including the keel beam splice (among other areas). We find 
that the procedures involved in this inspection are sufficient to 
ensure that corrosion and cracking of the keel beam splices are found. 
However, the compliance time for this inspection is 12 years since the 
airplane's date of manufacture or 36,000 total flight cycles, whichever 
occurs first, and the repetitive interval is 8 years or 24,000 flight 
cycles, whichever occurs first. We have determined that the compliance 
threshold is not early enough and the repetitive interval is too long 
to ensure that corrosion and cracking of the keel beam splices is found 
and fixed in a timely manner. (As stated above, severe corrosion of the 
keel beam splice plates has been found on two Model 737-700 series 
airplanes within 22 months after the date of manufacture of those 
airplanes.)
     Task 53-828-00 of Boeing Document Number D626A001, Section 
7 (``Zonal Inspection Program''), dated June 2001, contains 
instructions for an optional general visual inspection for 
discrepancies in a specific area aft of the keel beam at a suggested 
repetitive interval of 18 months. However, the procedures do not 
specifically state that the keel beam splices should be inspected.

Determination of Compliance Time

    Although this action is in the form of a final rule that involves 
requirements affecting flight safety and, thus, was not preceded by 
notice and an opportunity for public comment, the FAA and 
representatives of the airplane manufacturer met on July 23 and 25, 
2001. (Records of these meetings are available in the Rules Docket for 
examination by interested persons.) The purpose of these meetings was 
to allow the manufacturer to provide revised engineering data that 
could potentially affect the compliance time for the actions required 
by this AD. Although the manufacturer concurs with our determination 
that the corrosion

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addressed by this AD represents a safety concern, it asserts that the 
compliance times could be increased over the times we planned to 
require for the actions in this AD.
    The revised data presented primarily consisted of a finite-element 
analysis (FEA) of the wing and fuselage of the Boeing Model 737 ``Next 
Generation'' airplanes. The FEA included results of a ``splice plates 
intact'' model, and an analysis of the model with the keel beam chords 
rendered ineffective (due to corroded or cracked splice plates) at the 
body station (BS) 540 location. The manufacturer asserted that the FEA 
was necessary to properly analyze this area because this area, the 
lower wing-to-fuselage connection, is highly complex and redundant.
    The manufacturer asserted that the buttock line 41 fuel beam and 
the under-wing longeron would be adequate to react limit load in the 
case of failure of the keel beam chord splices at BS 540. (Limit load 
is defined as the highest application of load that is expected to occur 
in service.) Based on these data, the manufacturer suggested that an 
18-month repetitive inspection interval, similar to the MPD inspection 
of an adjacent area which was described previously, would provide an 
adequate level of safety.
    We have reviewed the revised data provided by the manufacturer and 
concur that the area is structurally very complex and difficult to 
analyze, due to the structural interactions of the fuselage and wing. 
We accept that the alternate load paths shown by the manufacturer's 
analysis are adequate to react limit load in the event of failure of 
the keel beam splices at BS 540. However, the fatigue life of the 
alternate load paths is unknown and is expected to be reduced due to 
the significant increase in loads.
    Given the level of risk, we conclude that urgent airworthiness 
action continues to be necessary and requires the immediate adoption of 
this AD without notice and opportunity for prior public comment. 
However, we have determined that the manufacturer's analysis allows for 
an increase in the initial inspection threshold and repetitive 
inspection interval over what we planned to require, as well as an 
increase in the planned grace period (for airplanes over the initial 
inspection threshold).
    The initial reports of severe corrosion were received in July 2000, 
on airplanes with line numbers 73 and 90. As described previously, at 
that time, these airplanes had been in service for approximately 22 
months. It was not until May 2001, that we determined the actual extent 
of the corrosion of the splice plate. As a result, it is possible that 
there are approximately 400 airplanes at present that are at two years 
or more since date of manufacture, with some airplanes being as old as 
4 years since date of manufacture. Inspection of these airplanes may 
reveal corrosion considerably in excess of the severe corrosion 
observed on line numbers 73 and 90.
    We originally intended to set a compliance threshold of 12 months 
since date of manufacture for the initial inspection, with a repetitive 
inspection interval of 12 months. We intended to allow a grace period 
of 30 days after the effective date of this AD for airplanes older than 
12 months since date of manufacture. As discussed above, due to the 
revised data provided by the manufacturer, we have determined that the 
following changes to the compliance times for this AD will provide an 
acceptable level of safety:
     For airplanes at less than 18 months since date of 
manufacture as of the effective date of this AD, extension of the 
initial inspection threshold to the later of 18 months since date of 
manufacture or 90 days after the effective date of this AD.
     For airplanes at 18 months or more since date of 
manufacture as of the effective date of this AD, extension of the 
initial inspection threshold to the later of 24 months since date of 
manufacture or 30 days after the effective date of this AD.
     For all airplanes, extension of the repetitive inspection 
interval to 18 months.

Explanation of the Requirements of the Rule

    Since an unsafe condition has been identified that is likely to 
exist or develop on other airplanes of the same type design, this AD is 
being issued to find and fix corrosion or cracking of the keel beam 
splices, which could lead to rapid degradation of the strength of the 
keel beam splices, and result in failure of the keel beam and 
consequent failure of the forward fuselage of the airplane. This AD 
requires repetitive detailed visual inspections for corrosion or 
cracking of the keel beam splices, and repair or replacement of splice 
plates and bolts with new, improved parts, if necessary. This action 
also provides an optional terminating action for the repetitive 
inspections.

Interim Action

    This is considered to be interim action. We are currently 
considering requiring the replacement of existing splice plates and 
bolts with new, improved parts, which is included in this AD as an 
optional terminating action that terminates the repetitive inspections 
required by this AD action. However, the planned compliance time for 
the replacement is sufficiently long so that notice and opportunity for 
prior public comment will be practicable.

Difference Between This AD and Service Document

    Task Number 53-210-00 of Boeing Document Number D626A001 describes 
a general visual inspection for discrepancies of the keel beam under 
the wing-to-body fairing, including the keel beam splice. However, we 
have determined that it is necessary for this AD to require a detailed 
visual inspection for corrosion or cracking of the keel beam splice 
only.

Determination of Rule's Effective Date

    Since a situation exists that requires the immediate adoption of 
this regulation, it is found that notice and opportunity for prior 
public comment hereon are impracticable, and that good cause exists for 
making this amendment effective in less than 30 days.

Comments Invited

    Although this action is in the form of a final rule that involves 
requirements affecting flight safety and, thus, was not preceded by 
notice and an opportunity for public comment, comments are invited on 
this rule. Interested persons are invited to comment on this 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 under the caption ADDRESSES. All 
communications received on or before the closing date for comments will 
be considered, and this rule may be amended in light of the comments 
received. Factual information that supports the commenter's ideas and 
suggestions is extremely helpful in evaluating the effectiveness of the 
AD action and determining whether additional rulemaking action would be 
needed.
    Submit comments using the following format:
     Organize comments issue-by-issue. For example, discuss a 
request to change the compliance time and a request to change the 
service bulletin reference as two separate issues.
     For each issue, state what specific change to the AD is 
being requested.
     Include justification (e.g., reasons or data) for each 
request.
    Comments are specifically invited on the overall regulatory, 
economic,

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environmental, and energy aspects of the rule that might suggest a need 
to modify the 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 that summarizes each FAA-
public contact concerned with the substance of this AD will be filed in 
the Rules Docket.
    Commenters wishing the FAA to acknowledge receipt of their comments 
submitted in response to this rule must submit a self-addressed, 
stamped postcard on which the following statement is made: ``Comments 
to Docket Number 2001-NM-236-AD.'' The postcard will be date-stamped 
and returned to the commenter.

Regulatory Impact

    The regulations adopted herein will not have a substantial direct 
effect 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, it 
is determined that this final rule does not have federalism 
implications under Executive Order 13132.
    The FAA has determined that this regulation is an emergency 
regulation that must be issued immediately to correct an unsafe 
condition in aircraft, and that it is not a ``significant regulatory 
action'' under Executive Order 12866. It has been determined further 
that this action involves an emergency regulation under DOT Regulatory 
Policies and Procedures (44 FR 11034, February 26, 1979). If it is 
determined that this emergency regulation otherwise would be 
significant under DOT Regulatory Policies and Procedures, a final 
regulatory evaluation will be prepared and placed in the Rules Docket. 
A copy of it, if filed, 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, 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. 106(g), 40113, 44701.


Sec. 39.13  [Amended]

    2. Section 39.13 is amended by adding the following new 
airworthiness directive:

2001-17-02  Boeing: Amendment 39-12393. Docket 2001-NM-236-AD.

    Applicability: Model 737-600 and -700 series airplanes, line 
numbers 1 through 908 inclusive; and Model 737-800 series airplanes, 
line numbers 1 through 455 inclusive; certificated in any category.

    Note 1: This AD applies to each airplane identified in the 
preceding applicability provision, regardless of whether it has been 
modified, altered, or repaired in the area subject to the 
requirements of this AD. For airplanes that have been modified, 
altered, or repaired so that the performance of the requirements of 
this AD is affected, the owner/operator must request approval for an 
alternative method of compliance in accordance with paragraph (e) of 
this AD. The request should include an assessment of the effect of 
the modification, alteration, or repair on the unsafe condition 
addressed by this AD; and, if the unsafe condition has not been 
eliminated, the request should include specific proposed actions to 
address it.

    Compliance: Required as indicated, unless accomplished 
previously.
    To prevent rapid degradation of the strength of the keel beam 
splices, which could result in failure of the keel beam and 
consequent failure of the forward fuselage, accomplish the 
following:

Repetitive Inspections

    (a) Perform a detailed visual inspection for corrosion or 
cracking of the keel beam splices, according to Boeing Document 
D626A001 (the ``Maintenance Planning Data Document''), Task Number 
53-210-00, dated June 2001. Do the initial inspection at the 
compliance time specified in paragraph (a)(1) or (a)(2) of this AD; 
as applicable; and repeat the inspection at least every 18 months, 
until the requirements of paragraph (c) of this AD have been done.
    (1) For airplanes at less than 18 months since date of 
manufacture as of the effective date of this AD: Inspect within 18 
months since date of manufacture, or 90 days after the effective 
date of this AD, whichever comes later.
    (2) For airplanes at 18 months or more since date of manufacture 
as of the effective date of this AD: Inspect within 24 months since 
date of manufacture, or 30 days after the effective date of this AD, 
whichever comes later.

    Note 2: For the purposes of this AD, a detailed visual 
inspection is defined as: ``An intensive visual examination of a 
specific structural area, system, installation, or assembly to 
detect damage, failure, or irregularity. Available lighting is 
normally supplemented with a direct source of good lighting at 
intensity deemed appropriate by the inspector. Inspection aids such 
as mirror, magnifying lenses, etc., may be used. Surface cleaning 
and elaborate access procedures may be required.''

Repair or Replacement

    (b) If any corrosion or cracking is found during the inspection 
required by paragraph (a) of this AD, before further flight, repair 
or replace the splice plates and bolts with new, improved parts, 
according to a method approved by the Manager, Seattle Aircraft 
Certification Office (ACO), FAA; or per data meeting the type 
certification basis of the airplane approved by a Boeing Company 
Designated Engineering Representative (DER) who has been authorized 
by the Manager, Seattle ACO, to make such findings. For a repair 
method to be approved by the Manager, Seattle ACO, as required by 
this paragraph, the Manager's approval letter must specifically 
reference this AD.

Optional Terminating Action

    (c) Replacement of splice plates and bolts with new, improved 
parts not made from 7150-T6511 material; according to a method 
approved by the Manager, Seattle ACO, or per data meeting the type 
certification basis of the airplane approved by a Boeing Company DER 
who has been authorized by the Manager, Seattle ACO, to make such 
findings; constitutes terminating action for the repetitive 
inspections required by paragraph (a) of this AD.

Spares

    (d) As of the effective date of this AD, no person shall install 
a splice plate made from 7150-T6511 material, or with part number 
144A7155-1 or 143A7812-1, on any airplane.

Alternative Methods of Compliance

    (e) 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 ACO. 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.

Special Flight Permits

    (f) 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.

Effective Date

    (g) This amendment becomes effective on September 4, 2001.

    Issued in Renton, Washington, on August 13, 2001.
Vi L. Lipski,
Manager, Transport Airplane Directorate, Aircraft Certification 
Service.
[FR Doc. 01-20807 Filed 8-17-01; 8:45 am]
BILLING CODE 4910-01-P