[Federal Register Volume 62, Number 152 (Thursday, August 7, 1997)]
[Proposed Rules]
[Pages 42433-42439]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 97-20732]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. 96-NM-264-AD]
RIN 2120-AA64


Airworthiness Directives; Boeing Model 737-100 and -200 Series 
Airplanes

AGENCY: Federal Aviation Administration, DOT.

ACTION: Notice of proposed rulemaking (NPRM).

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SUMMARY: This document proposes the supersedure of an existing 
airworthiness directive (AD), applicable to certain Boeing Model 737-
100 and -200 series airplanes, that currently requires that the FAA-
approved maintenance inspection program be revised to include 
inspections that will give no less than the required damage tolerance 
rating for each Structural Significant Item, and repair of cracked 
structure. That AD was prompted by a structural re-evaluation by the 
manufacturer which identified additional structural elements where, if 
damage were to occur, supplemental inspections may be required for 
timely detection. This action would require additional and expanded 
inspections, and repair of cracked structure. This action also would 
expand the applicability of the existing AD to include additional 
airplanes. The actions specified by the proposed AD are intended to 
ensure the continued structural integrity of the Boeing Model 737-100 
and -200 fleet.

DATES: Comments must be received by September 16, 1997.

ADDRESSES: Submit comments in triplicate to the Federal Aviation 
Administration (FAA), Transport Airplane Directorate, ANM-103, 
Attention: Rules Docket No. 96-NM-264-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.
    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: Greg Schneider, Aerospace Engineer, 
Airframe Branch, ANM-120S, FAA, Seattle Aircraft Certification Office, 
1601 Lind Avenue, SW., Washington; telephone (425) 227-2028; fax (425) 
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,

[[Page 42434]]

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 96-NM-264-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. 96-NM-264-AD, 1601 Lind Avenue, SW., Renton, Washington 
98055-4056.

Determination To Develop the Supplemental Structural Inspection 
Program

    As part of its continuing work to maintain the structural integrity 
of older transport category airplanes, in the early 1980's, the FAA 
concluded that the incidence of fatigue cracking may increase as these 
airplanes reach or exceed their design service objective (DSO). A 
significant number of these airplanes were approaching or had exceeded 
the DSO on which the initial type certification approval was 
predicated. In light of this, and as a result of increased utilization, 
longer operational lives, and the high levels of safety expected of the 
currently operated transport category airplanes, the FAA determined 
that a supplemental structural inspection program (SSIP) was necessary 
to ensure a high level of structural integrity for all airplanes in the 
transport fleet.

Issuance of Advisory Circular

    As a follow-on from that determination, the FAA issued Advisory 
Circular (AC No. 91-56), ``Supplemental Structural Inspection Program 
for Large Transport Category Airplanes,'' dated May 6, 1981. The AC 
provides guidance material to manufacturers and operators for use in 
developing a continuing structural integrity program to ensure safe 
operation of older airplanes throughout their operational lives. This 
guidance material applies to large transport airplanes that were 
certified under the fail-safe requirements of Civil Air Regulations 4b 
or damage tolerance structural requirements of 14 CFR part 25, and that 
have a maximum gross weight greater than 75,000 pounds. The procedures 
set forth in this AC are applicable to the large transport category 
airplanes operated under subpart F of 14 CFR part 91 and parts 121, 
123, 125, and 135. The objective of the SSIP was to establish 
inspection programs to ensure timely detection of fatigue cracking.

Development of the Supplemental Structural Inspection Program

    In order to evaluate the effect of increased fatigue cracking with 
respect to maintaining fail-safe design and damage tolerance of the 
structure of Boeing Model 737-100 and -200 series airplanes, Boeing 
conducted a structural reassessment of those airplanes, using modern 
damage tolerance evaluation techniques. Boeing accomplished this 
reassessment using the criteria contained in AC No. 91-56, as well as 
14 CFR 25.571; Amdt. 25-45. During the reassessment, members of the 
airline industry participated with Boeing in working group sessions and 
developed the SSIP for Model 737-100 and -200 series airplanes. 
Engineers and maintenance specialists from the FAA also attended these 
sessions to observe these developments. Subsequently, based on the 
working group's recommendations, Boeing developed the Supplemental 
Structural Inspection Document (SSID) for Model 737-100 and -200 series 
airplanes.

Issuance of AD 91-14-20, Amendment 39-7061

    On August 9, 1991, the FAA issued AD 91-14-20, amendment 39-7061 
(56 FR 30680, July 5, 1991), which is applicable to certain Boeing 
Model 737 series airplanes. That AD currently requires that the FAA-
approved maintenance inspection program be revised to include 
inspections that will give no less than the required damage tolerance 
rating (DTR) for each Structural Significant Item (SSI), and repair of 
cracked structure. The AD references Boeing Document No. D6-37089, 
``Supplemental Structural Inspection Document'' (SSID), Revision B, 
dated February 18, 1987, and Revision C, dated January 1990, as the 
appropriate source of service information. That action was prompted by 
a structural re-evaluation that identified additional structural 
components where fatigue cracking is likely to occur. The requirements 
of that AD are intended to ensure the continued structural integrity of 
the entire Boeing Model 737 fleet.

Actions Since Issuance of Previous AD

    Since issuance of AD 91-14-20, the FAA has reconsidered the 
following four aspects of the existing SSID:

1. Classification of Fuselage Skin as ``Damage Obvious'' or 
``Malfunction Evident''

    AC No. 91-56, Change 2, dated April 15, 1983, recommends that the 
SSID should contain inspections of all critical parts or components for 
each airplane to ensure the continued safe operation of the existing 
fleet. The fuselage skin is an example of a critical component. 
Cracking in any critical part or component, if not detected and 
corrected in a timely manner, could result in reduced structural 
integrity of the airplane.
    Revisions B and C of the SSID excluded the fuselage skin from 
directed inspections, since it was classified as ``damage obvious'' or 
``malfunction evident.'' At the time of this classification, Revisions 
B and C of the SSID relied on venting or flapping to indicate cracks in 
the fuselage skin.
    Venting is a gradual loss of cabin pressure as a result of cracking 
in the pressurized area of the fuselage skin. Based on the design 
philosophy of flapping, these cracks in the fuselage skin would grow 
only to a specific length and then turn direction because of certain 
structural components. Because venting and flapping were considered to 
be readily apparent, Boeing considered that it was unnecessary to 
provide for additional inspections of the fuselage skin. Reliance also 
was placed on venting or flapping to allow for the safe operation of an 
airplane with such cracks. This technique worked well in ground tests 
and in some in-service incidents, but proved to be unreliable in other 
cases.
    In one such case, a large portion of Section 43 of the fuselage 
structure separated from a Boeing Model 737 series airplane. Results of 
a National Transportation Safety Board (NTSB) investigation revealed 
that this incident occurred as a result of the catastrophic failure of 
the fuselage skin at a lap joint. The results also revealed that, 
contrary to the design philosophy, controlled decompression of the 
structure (i.e., flapping or venting) did not occur due to the presence 
of widespread fatigue damage. As a result of this failure, the NTSB 
recommended that the SSID be revised to discontinue classification of 
the fuselage skin as ``damage obvious'' or ``malfunction evident.''
    The FAA concurs with the NTSB's recommendation. Therefore, the FAA 
has determined that additional inspections are necessary to ensure 
timely detection of cracks in the fuselage skin structure.

[[Page 42435]]

2. Deletions of Modified, Altered, or Repaired Structure from the SSIP

    Paragraph 1.4 of Appendix 1, ``Guidelines for Development of 
Supplemental Inspection Document,'' of AC No. 91-56, Change 2, dated 
April 15, 1983, states, ``the effect of repairs and modifications 
approved by the manufacturer should also be taken into account. In 
addition, it may be necessary to consider the effect of repairs and 
operator-approved modifications on individual airplanes. The operator 
has the responsibility for ensuring notification and consideration of 
any such aspects.''
    In addition, the FAA's current policy is that operators of 
transport category airplanes that are subject to AD's that mandate SSID 
programs should follow the guidelines of AC No. 91-56 and should 
continue to inspect any SSI that is modified, altered, or repaired in 
any way. Any modification that affects the loading spectrum, stress 
levels, or damage tolerance characteristics of the structure must be 
reassessed to determine its impact on the inspection program. Such a 
reassessment may require the development of additional inspection 
requirements for that modification.
    The FAA's policy also states that, ``* * * the [SSID] programs are 
based on type design crack growth data generated from analysis or 
structural tests using a realistic and conservative loading spectrum, 
material properties, part geometry, etc. For this reason, structural 
modifications that may increase stress levels in load carrying 
structures, including maximum weight limit increases, cargo door 
installations, and repairs to load carrying structures, must be 
reassessed for its impact on the structural inspection program.'' 
(Reference: Transport Airplane Directorate's Policy Letter, 
Information: Policy Regarding Impact of Modifications and Repairs on 
the Damage Tolerance Characteristics of Transport Category Airplanes, 
dated October 27, 1989. This letter will be retained in Rules Docket 
No. 96-NM-264-AD.)
    Section 5.0 of Revisions B and C of the SSID contains provisions 
that allow for the deletion of modified, altered, or repaired areas 
from the SSIP because Boeing considers these areas not to be 
``representative of the fleet.'' The FAA is aware that there have been 
a significant number of such deletions. As a result, contrary to the 
FAA's policy discussed above, operators are not following the 
guidelines of AC No. 91-56 and not continuing to inspect any SSI that 
is modified, altered, or repaired in any way.
    In addition, for Boeing Model 737-100 and -200 series airplanes 
that have been converted from a passenger configuration to an all-cargo 
configuration by the Supplemental Type Certification (STC) process, the 
FAA finds that Revisions B and C of the SSID do not include procedures 
for inspection of new SSI's created by this conversion, or unmodified 
SSI's affected by this conversion. (There are approximately 100 of 
these airplanes in the worldwide fleet of which several are listed in 
the effectivity listing of Revisions B and C of the SSID.) These 
conversions have the effect of removing SSI's from the SSIP and 
creating a large number of new SSI's that have not been assessed. 
Consequently, airplanes that have been converted to an all-cargo 
configuration do not have a SSID that specifies an inspection method 
and compliance time for each new SSI. Additionally, an unmodified SSI 
also could require a new inspection method and compliance time because 
the modification may increase the loads or change the load 
distributions in that SSI. These conditions would necessitate that the 
inspection interval for that affected, unmodified SSI be shorter than 
required in the Boeing SSID. Hence, the FAA finds that the objectives 
of the SSIP are not being met for these modified airplanes.
    Likewise, a design change (such as an increase in the maximum 
certified weight or in the center of gravity limits) also may cause an 
increase in the loads or change the load distributions in the affected, 
unmodified SSI's. The effect of this increase or change would be 
similar to the effect that a cargo conversion would have on an 
unmodified SSI. As a result, the inspection interval for an affected, 
unmodified SSI may need to be lower than required in the Boeing SSID. 
Thus, the DTR specified in the SSID for any SSI affected by a design 
change may no longer be applicable. Therefore, the FAA finds that the 
objectives of the SSIP are not being met for airplanes with such design 
changes.
    Furthermore, in consideration of AC No. 91-56 and current FAA 
policy, the FAA has determined that new inspection methods and 
compliance times are necessary for areas that have been modified, 
altered, or repaired to ensure timely detection of cracking in those 
areas. The FAA also has determined that new inspection methods and 
compliance times are necessary for those areas that were deleted from 
the SSIP by previously approved alternative methods of compliance, 
which includes those areas deleted in accordance with the requirements 
of Section 5.0 of the SSID. Furthermore, the new inspection methods and 
compliance times should meet the requirements of 14 CFR 25.1529, Amdt. 
25-45; 14 CFR 25.571, Amdt. 25-45; 14 CFR 25.571, Amdt. 25-54; 14 CFR 
25.571, Amdt. 25-72; or the guidelines of AC 91-56.

3. Candidate Fleet vs. Inspection Threshold Approach

    Paragraph 4.4 of AC No. 91-56, Change 2, dated April 15, 1983, 
states, ``Inspection thresholds for supplemental inspections should be 
established. These inspections would be supplemental to the normal 
inspection including the detailed internal inspections.'' Moreover, 
paragraph 4.4.2 of AC No. 91-56 states, ``* * * this threshold should 
be such as to include sufficient [high-cycle] airplanes in the 
inspection to develop added confidence in the integrity of the 
structure * * *.''
    A properly established inspection threshold ensures that: (1) The 
SSI inspections are accomplished; (2) fatigue cracks in SSI's are 
detected in a timely manner; (3) airplanes are automatically added to 
the SSIP; and (4) the SSIP includes a statistically valid number of 
airplanes.
    Among other things, Revisions B and C of the SSID define a 
candidate fleet approach to ensure that fatigue cracks in SSI's are 
detected in a timely manner in the entire fleet. The initial Boeing 
Model 737 candidate fleet consisted of a number of airplanes that had 
exceeded 37,500 flight cycles by April 30, 1983. In other words, Boeing 
considered 37,500 flight cycles to be the threshold for the airplanes 
in the candidate fleet. These airplanes were the most likely in the 
fleet to experience initial fatigue damage since they had the highest 
number of flight cycles. Boeing produced this SSID with the assumption 
that the airplanes in the candidate fleet would continue to represent 
the entire fleet and would have the highest number of flight cycles in 
the fleet.
    Under the existing SSIP, Boeing intended to periodically review the 
airplanes in the candidate fleet for significant changes in fleet 
distribution, composition, or utilization, and update the candidate 
fleet, if any significant change was detected. It was intended that the 
FAA would then mandate any change to the SSID through the rulemaking 
process.
    The FAA finds that the candidate fleet approach is deviating from 
Boeing's original philosophy in that the candidate fleet has not been 
updated to reflect changes (such as cargo conversions) in the fleet. 
This situation

[[Page 42436]]

could result in a statistically invalid number of airplanes in the SSIP 
and undetected fatigue cracks in SSI's. The candidate fleet approach 
also does not automatically account for non-candidate airplanes that 
eventually accumulate more flight cycles than that of certain candidate 
airplanes. High-cycle airplanes are more likely to experience initial 
fatigue damage in the fleet. The confidence in the structural integrity 
of the fleet of airplanes could be reduced if high-cycle airplanes are 
excluded from the SSIP.
    The FAA has reconsidered the candidate fleet approach described in 
Revisions B and C of the SSID, since it does not meet the guidelines of 
AC No. 91-56. The FAA has determined that the Boeing Model 737 SSIP 
must contain inspection thresholds for all Boeing Model 737-100 and -
200 series airplanes to ensure the timely detection of fatigue cracks 
in the SSI's. (The FAA is currently considering a separate rulemaking 
action to address the problems associated with fatigue cracking on all 
Boeing Model 737-300, -400, and -500 series airplanes.)
    The FAA has reviewed the thresholds derived from Boeing's 
reliability analysis. The analysis is based on a certain probability 
that cracks will be detected in the inspected fleet before they 
initiate on other airplanes that have not been inspected. The FAA has 
determined that the thresholds recommended in the analysis of past 
service experience of the Boeing Model 737 fleet are acceptable. 
Therefore, for Model 737-200C series airplanes, the FAA has determined 
that a threshold of 46,000 total flight cycles is necessary in order to 
produce a statistically valid assessment of the service history for 
these airplanes. For other Model 737-100 and -200 series airplanes, the 
FAA has determined that a threshold of 66,000 total flight cycles is 
necessary to produce a valid assessment. The threshold for Model 737-
200C series airplanes is lower than that of other Model 737 series 
airplanes since Model 737-200C series airplanes have a lower 
utilization rate and fewer airplanes in the fleet. Since the 
utilization rate is lower for Model 737-200C series airplanes, these 
airplanes have accumulated fewer flight cycles and have fewer airplanes 
with higher flight cycles than that of the remaining fleet.
    It should be noted that, although the proposed AD requires a 
threshold, the FAA may approve requests for adjustments to the 
compliance time [i.e., under paragraph (h)(1) of this proposed AD] 
provided that no cracking is detected in the airplane's SSI's. The 
request should include a new inspection threshold and must include data 
to substantiate that such an adjustment would provide an acceptable 
level of safety.
    Operators should note that the alternative inspection threshold may 
be based solely on the analysis of the data of the existing fleet. 
However, the FAA has determined that the analysis that derives the new 
inspection threshold must include: (1) Data relevant to a sufficient 
number of high-cycle airplanes, and (2) data that shows accomplishment 
of the inspections of the SSI's. An adequate statistical sampling size 
will provide confidence in the structural integrity of the fleet of 
airplanes. Therefore, additional airplanes may need to be added to the 
inspected fleet until a sufficient number of airplanes have been 
inspected with no crack findings.

4. Transferability of Airplanes

    Since issuance of the SSID and AD 91-14-20, the FAA has issued 
several AD's that implement Corrosion Prevention and Control Programs 
(CPCP) for aging airplanes. While developing the AD's that mandated the 
CPCP, the FAA recognized that an operator of an airplane that has been 
transferred from another operator could revise its maintenance program 
to restart the compliance times for the required corrosion tasks. This 
situation could lead to corrosion not being detected and corrected in a 
timely manner, which could reduce the structural integrity of the 
airplane.
    As a result, the CPCP AD's require that operators establish a 
program for accomplishment of the subject corrosion tasks before any 
airplane can be added to an air carrier's operations specification. 
Establishment of this program will ensure that airplanes transferred 
from operator to operator are inspected and that corrosion is detected 
in a timely manner.
    The FAA's intent in AD 91-14-20 was that operators of candidate 
fleet airplanes that have been previously operated under an FAA-
approved maintenance program accomplish the SSID inspections within the 
compliance time established by the previous operator. The FAA assumed 
that, under the existing SSID, these airplanes would be inspected in a 
manner similar to CPCP requirements. However, the SSID and AD 91-14-20 
do not address the transfer of airplanes in the candidate fleet from 
one operator to another.
    AD 91-14-20 currently requires that the revision to the maintenance 
program be included and be implemented in accordance with the 
procedures specified in Sections 5.0 and 6.0 of the SSID. However, the 
FAA finds that these sections do not provide explicit instructions to 
repetitively inspect airplanes that have been transferred from one 
operator to another. It also does not specify that new operators must 
continue the SSID inspections at the same frequency established by the 
previous operator.
    In addition, as AD 91-14-20 is currently worded, the FAA finds that 
operators that acquire candidate fleet airplanes that have been 
previously operated under a maintenance inspection program could revise 
their programs to restart the compliance times. This situation is 
contrary to standard AD requirements. An AD typically mandates an 
initial compliance time and a repetitive interval that remains 
unchanged for all operators of the affected airplanes.
    As a result of these omissions, the SSID inspections of a candidate 
fleet airplane could be deferred until it is required by the 
maintenance inspection program of the new operator. For airplanes that 
are transferred frequently, this situation could continue for the life 
of the airplane. As a result, fewer Boeing Model 737 candidate fleet 
airplanes are being inspected; thus, the size of the candidate fleet is 
in effect reduced. Even if airplanes are ultimately inspected under 
these circumstances, inspections would not be performed frequently 
enough to maintain the applicable DTR. The FAA has determined that such 
a reduction does not ensure the continued structural integrity of the 
entire Boeing Model 737 fleet.
    Implementation of procedures in the SSID that are similar to the 
CPCP will ensure that: (1) Airplanes transferred from operator to 
operator are inspected; (2) the SSIP includes a statistically valid 
number of airplanes; and (3) fatigue cracks are detected in a timely 
manner.
    Therefore, the FAA finds that, to ensure the continued structural 
integrity of the entire Model 737 fleet, the AD 91-14-20 must be 
revised to include provisions that address the transfer of airplanes. 
The FAA also finds that a program must be established to accomplish the 
inspections before any airplane that is subject to this proposal can be 
added to an air carrier's operations specifications.

FAA's Conclusions

    In light of all the factors discussed above, the FAA has determined 
that AD 91-14-20 does not adequately ensure timely detection of fatigue 
cracking in SSI's. Fatigue cracking in those items, if not detected and 
corrected in a timely manner, could result in reduced structural 
integrity of the airplane.

[[Page 42437]]

Explanation of New Relevant Service Information

    The FAA has reviewed and approved Boeing Document No. D6-37089, 
``Supplemental Structural Inspection Document'' (SSID), Revision D, 
dated June 1995, which describes procedures for revising the FAA-
approved maintenance inspection program for all Boeing Model 737-100 
and -200 series airplanes. This revision of the Model 737 SSID 
incorporates additional and expanded inspections from those that were 
contained in the previous version and mandated by AD 91-14-20. The 
fuselage skin structure that was the subject of an NTSB recommendation 
is included in these inspections. The FAA finds that accomplishment of 
these inspections will ensure the continuing structural integrity of 
the Boeing Model 737-100 and -200 fleet.

Explanation of Requirements of Proposed Rule

    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 supersede AD 91-14-20.
    Paragraph (a) of the proposed AD restates the requirements of AD 
91-14-20.
    Paragraph (b) of the proposed AD would require incorporation of a 
revision into the FAA-approved maintenance inspection program that 
provides no less than the required DTR for each SSI listed in Revision 
D of the SSID.
    Paragraph (c) of the proposed AD would establish specific 
compliance times for performing the initial inspection of the structure 
identified in Revision D of the SSID. Once the initial inspection has 
been performed, operators would be required to perform repetitive 
inspections at the intervals specified in the Document in order to 
remain in compliance with their maintenance inspection programs, which 
would have been revised in accordance with paragraph (b) of this 
proposed AD.
    Paragraph (d) of the proposed AD would require, for airplanes on 
which any design change or repair has been accomplished prior to the 
effective date of this proposed AD, a revision to the FAA-approved 
maintenance inspection program to include an inspection method for any 
new or affected SSI, and to include the compliance times for this 
inspection. This paragraph also would require that any new inspection 
method and the compliance times be approved by the FAA.
    Paragraph (e) of the proposed AD would require that the repair of 
any cracked structure is to be accomplished in accordance with an FAA-
approved method.
    Paragraph (f) of the proposed AD would require, for airplanes on 
which any design change or repair has been accomplished after the 
effective date of this proposed AD, a revision to the FAA-approved 
maintenance inspection program to include a new inspection method for 
any new or affected SSI, and to include the compliance times for this 
inspection. This paragraph also would require that any new inspection 
method and the compliance times be approved by the FAA.
    Before any airplane that is subject to this proposed AD can be 
added to an air carrier's operations specifications, a program for the 
accomplishment of the inspections required by this proposed AD must be 
established. Paragraph (g) of the proposed AD would require 
accomplishment of the following:
    1. For airplanes that have been inspected in accordance with this 
proposed AD, the inspection of each SSI must be accomplished by the new 
operator in accordance with the previous operator's schedule and 
inspection method, or the new operator's schedule and inspection 
method, whichever would result in the earlier accomplishment date for 
that SSI inspection. The compliance time for accomplishment of this 
inspection must be measured from the last inspection accomplished by 
the previous operator. After each inspection has been performed once, 
each subsequent inspection must be performed in accordance with the new 
operator's schedule and inspection method.
    2. For airplanes that have not been inspected in accordance with 
this proposed AD, the inspection of each SSI must be accomplished 
either prior to adding the airplane to the air carrier's operations 
specification, or in accordance with a schedule and an inspection 
method approved by the FAA. After each inspection has been performed 
once, each subsequent inspection must be performed in accordance with 
the new operator's schedule.
    Accomplishment of these inspections will ensure that: (1) 
Operators' newly acquired airplanes comply with its SSIP before being 
operated; and (2) frequently transferred aircraft are not permitted to 
operate without accomplishment of the inspections defined in the SSID.

Differences Between SSID and Proposed AD

    Operators should note the following differences between the 
procedures specified in Revision D of the SSID and the proposed 
requirements of this AD:
    1. Paragraphs 5.1.17 and 5.1.18 of the General Instructions of 
Revision D of the SSID permit deletions of modified, altered, or 
repaired structure from the SSIP. As described previously in Item 2 of 
the ``Actions Since Issuance of Previous AD'' section of this preamble, 
the FAA has determined that such deletions are unacceptable. Therefore, 
for airplanes on which the areas specified in the SSID have been 
modified, altered, or repaired, the proposed AD would require a 
revision to the operator's existing SSIP to include procedures for 
accomplishing a new FAA-approved inspection method that provides a new 
DTR for that SSI.
    2. Revision D of the SSID bases the supplemental inspections on 
specific high-cycle airplanes (i.e., candidate fleet airplanes) and 
does not include an inspection threshold for those airplanes. It also 
does not automatically add airplanes to the candidate fleet. Based on 
the discussion described previously in Item 3 of the ``Actions Since 
Issuance of Previous AD'' section of this preamble, the FAA has 
determined that the proposed AD would expand the applicability of this 
AD action to include all Model 737-100 and -200 series airplanes. In 
addition, for Model 737-200C series airplanes, the proposed inspection 
of all SSI's would be required to be accomplished prior to the 
accumulation of 46,000 total flight cycles, or within 18 months, 
whichever occurs later. For other Model 737-100 and -200 series 
airplanes, the proposed inspection of all SSI's would be required to be 
accomplished prior to the accumulation of 66,000 total flight cycles, 
or within 18 months, whichever occurs later.

Cost Impact

    There are approximately 1,021 Boeing Model 737-100 and -200 series 
airplanes of the affected design in the worldwide fleet. The FAA 
estimates that 404 airplanes of U.S. registry would be affected by this 
proposed AD.
    The actions that are proposed in this AD action would take 
approximately 1,200 work hours per airplane to accomplish, at an 
average labor rate of $60 per work hour. Based on these figures, the 
cost impact of the proposed requirements of this AD on U.S. operators 
is estimated to be $29,088,000, or $72,000 per airplane, per inspection 
cycle.
    The cost impact figure discussed above is based on assumptions that 
no operator has yet accomplished any of the current or proposed 
requirements of this AD action, and that no operator

[[Page 42438]]

would accomplish those actions in the future if this AD were not 
adopted.
    The number of required work hours, as indicated above, is presented 
as if the accomplishment of the actions proposed in this AD were to be 
conducted as ``stand alone'' actions. However, in actual practice, 
these actions for the most part would be accomplished coincidentally or 
in combination with normally scheduled airplane inspections and other 
maintenance program tasks. Therefore, the actual number of necessary 
additional work hours would be minimal in many instances. Additionally, 
any costs associated with special airplane scheduling would be minimal.

Regulatory Impact

    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 
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 removing amendment 39-7061 (56 FR 
30680, July 5, 1991), and by adding a new airworthiness directive (AD), 
to read as follows:

Boeing:  Docket 96-NM-264-AD. Supersedes AD 91-14-20, Amendment 39-
7061.

    Applicability: All Model 737-100 and -200 series airplanes, 
certificated in any category.
    Compliance: Required as indicated, unless accomplished 
previously.
    To ensure the continued structural integrity of the Boeing Model 
737-100 and -200 fleet, accomplish the following:

    Note 1: Where there are differences between the AD and the 
Supplemental Structural Inspection Document, the AD prevails.

    (a) For airplanes listed in Section 3.0 of Boeing Document No. D6-
37089, ``Supplemental Structural Inspection Document'' (SSID), Revision 
B, dated February 18, 1987, and Revision C, dated January 1990: Within 
12 months after August 9, 1991 (the effective date of AD 91-14-20, 
amendment 39-7061), incorporate a revision into the FAA-approved 
maintenance inspection program which provides no less than the required 
damage tolerance rating (DTR) for each Structural Significant Item 
(SSI) listed in that document. (The required DTR value for each SSI is 
listed in the document.) The revision to the maintenance program shall 
include and shall be implemented in accordance with the procedures in 
Sections 5.0 and 6.0 of the SSID. This revision shall be deleted 
following accomplishment of the requirements of paragraph (b) of this 
AD.

    Note 2: For the purposes of this AD, an SSI is defined as a 
principal structural element that could fail and consequently reduce 
the structural integrity of the airplane.

    (b) Within 12 months after the effective date of this AD, 
incorporate a revision into the FAA-approved maintenance inspection 
program that provides no less than the required DTR for each SSI listed 
in Boeing Document No. D6-37089, ``Supplemental Structural Inspection 
Document'' (SSID), Revision D, dated June 1995 (hereinafter referred to 
as ``Revision D''). (The required DTR value for each SSI is listed in 
the document.) The revision to the maintenance program shall include 
and shall be implemented in accordance with the procedures in Section 
5.0, ``Damage Tolerance Rating (DTR) System Application'' and Section 
6.0, ``SSI Discrepancy Reporting'' of Revision H. Upon incorporation of 
the revision required by this paragraph, the revision required by 
paragraph (a) of this AD may be deleted.
    (c) Except as provided in paragraph (d) or (f) of this AD, as 
applicable, perform an inspection to detect cracks in all structure 
identified in Revision D at the time specified in paragraph (c)(1) or 
(c)(2) of this AD, as applicable.
    (1) For Model 737-200C series airplanes: Inspect prior to the 
accumulation of 46,000 total flight cycles, or within 18 months 
after the effective date of this AD, whichever occurs later.
    (2) For Model 737-100 and -200 series airplanes, except for 
those airplanes identified in paragraph (c)(1) of this AD: Inspect 
prior to the accumulation of 66,000 total flight cycles, or within 
18 months after the effective date of this AD, whichever occurs 
later.

    Note 3: Once the initial inspection has been performed, 
operators are required to perform repetitive inspections at the 
intervals specified in Revision D in order to remain in compliance 
with their maintenance inspection programs, as revised in accordance 
with paragraph (b) of this AD.

    (d) For airplanes on which the structure identified in Revision 
D is affected by any design change or repair that was accomplished 
prior to the effective date of this AD: Within 18 months after the 
effective date of this AD, revise the FAA-approved maintenance 
inspection program to include an inspection method for any new or 
affected SSI, and to include the compliance times for initial and 
repetitive accomplishment of this inspection. For purposes of this 
section, an SSI is ``affected'' if it has been altered or repaired, 
or if the loads acting on the SSI have been increased or 
redistributed. Following accomplishment of the revision and within 
the compliance times established, perform an inspection to detect 
cracks in the structure affected by any design change or repair, in 
accordance with the new inspection method. The new inspection method 
and the compliance times shall be approved by the Manager, Seattle 
Aircraft Certification Office (ACO), FAA, Transport Airplane 
Directorate, 1601 Lind Avenue, SW., Renton, Washington 98055-4056, 
fax (425) 227-1181.
    Note 4: Notwithstanding the provisions of paragraphs 5.1.17 and 
5.1.18 of the General Instructions of Revision D, which would permit 
deletions of modified, altered, or repaired structure from the 
Supplemental Structural Inspection Program (SSIP), the inspection of 
SSI's that are modified, altered, or repaired shall be done in 
accordance with a method approved by the Manager, Seattle ACO.

    Note 5: For the purposes of this AD, a design change is defined 
as any modification, alteration, or change to operating limitations.

    (e) Cracked structure found during any inspection required by 
this AD shall be repaired, prior to further flight, in accordance 
with an FAA-approved method.
    (f) For airplanes on which the structure identified in Revision 
D is affected by any design change or repair that is accomplished 
after the effective date of this AD: Within 12 months after that 
modification, alteration, or repair for any new or affected SSI, 
revise the FAA-approved maintenance inspection

[[Page 42439]]

program to include an inspection method for any new or affected SSI, 
and to include the compliance times for initial and repetitive 
accomplishment of this inspection. For purposes of this section, an 
SSI is ``affected'' if it has been altered or repaired, or if the 
loads acting on the SSI have been increased or redistributed. 
Following accomplishment of the revision and within the compliance 
times established, perform an inspection to detect cracks in the 
structure affected by any design change or repair, in accordance 
with the new inspection method. The new inspection method and the 
compliance times shall be approved by the Manager, Seattle ACO.
    Note 6: Notwithstanding the provisions of paragraphs 5.1.17 and 
5.1.18 of the General Instructions of Revision D, which would permit 
deletions of modified, altered, or repaired structure from the SIP, 
the inspection of SSI's that are modified, altered, or repaired 
shall be done in accordance with a method approved by the Manager, 
Seattle ACO.

    (g) Before any airplane that is subject to this AD and that has 
exceeded the applicable compliance times specified in paragraph (c) 
of this AD can be added to an air carrier's operations 
specifications, a program for the accomplishment of the inspections 
required by this AD must be established in accordance with paragraph 
(g)(1) or (g)(2) of this AD, as applicable.
    (1) For airplanes that have been inspected in accordance with 
this AD, the inspection of each SSI must be accomplished by the new 
operator in accordance with the previous operator's schedule and 
inspection method, or the new operator's schedule and inspection 
method, whichever would result in the earlier accomplishment date 
for that SSI inspection. The compliance time for accomplishment of 
this inspection must be measured from the last inspection 
accomplished by the previous operator. After each inspection has 
been performed once, each subsequent inspection must be performed in 
accordance with the new operator's schedule and inspection method.
    (2) For airplanes that have not been inspected in accordance 
with this AD, the inspection of each SSI required by this AD must be 
accomplished either prior to adding the airplane to the air 
carrier's operations specification, or in accordance with a schedule 
and an inspection method approved by the Manager, Seattle ACO. After 
each inspection has been performed once, each subsequent inspection 
must be performed in accordance with the new operator's schedule.
    (h)(1) 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 7: Information concerning the existence of approved 
alternative methods of compliance with this AD, if any, may be 
obtained from the Seattle ACO.

    (2) Alternative methods of compliance, approved previously in 
accordance with AD 91-14-20, amendment 39-7061, are not considered 
to be approved as alternative methods of compliance with this AD.
    (i) 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.
    Issued in Renton, Washington, on July 31, 1997.
Darrell M. Pederson,
Acting Manager, Transport Airplane Directorate, Aircraft Certification 
Service.
[FR Doc. 97-20732 Filed 8-6-97; 8:45 am]
BILLING CODE 4910-13-O