AD Amdt-39-11018

[Federal Register Volume 64, Number 21 (Tuesday, February 2, 1999)]
[Rules and Regulations]
[Pages 4959-4964]
From the Federal Register Online via the Government Publishing Office [<a href="http://www.gpo.gov">www.gpo.gov</a>]
[FR Doc No: 99-2272]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. 98-NM-50-AD; Amendment 39-11018; AD 99-03-04]
RIN 2120-AA64


Airworthiness Directives; Boeing Model 737-100, -200, -300, -400, 
and -500 Series Airplanes

AGENCY: Federal Aviation Administration, DOT.

ACTION: Final rule.

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SUMMARY: This amendment adopts a new airworthiness directive (AD), 
applicable to all Boeing Model 737-100, -200, -300, -400, and -500 
series airplanes, that requires installation of

[[Page 4960]]

components to provide shielding and separation of the fuel system 
wiring (that is routed to the fuel tanks) from adjacent wiring. This 
amendment also requires installation of flame arrestors and pressure 
relief valves in the fuel vent system. This amendment is prompted by 
testing results, obtained in support of an accident investigation, and 
by re-examination of possible causes of a similar accident. The actions 
specified by this AD are intended to prevent possible ignition of fuel 
vapors in the fuel tanks, and external ignition of fuel vapor exiting 
the fuel vent system and consequent propagation of a flame front into 
the fuel tanks.

EFFECTIVE DATE: March 9, 1999.

ADDRESSES: This information may be examined at the Federal Aviation 
Administration (FAA), Transport Airplane Directorate, Rules Docket, 
1601 Lind Avenue, SW., Renton, Washington; or at the Office of the 
Federal Register, 800 North Capitol Street, NW., suite 700, Washington, 
DC.

FOR FURTHER INFORMATION CONTACT: Chris Hartonas, Aerospace Engineer, 
Systems and Equipment Branch, ANM-130S, FAA, Transport Airplane 
Directorate, Seattle Aircraft Certification Office, 1601 Lind Avenue, 
SW., Renton, Washington 98055-4056; telephone (425) 227-2864; fax (425) 
227-1181; or Dorr Anderson, Aerospace Engineer, Propulsion Branch, ANM-
140S, FAA, Transport Airplane Directorate, Seattle Aircraft 
Certification Office, 1601 Lind Avenue, SW., Renton, Washington 98055-
4056; telephone (425) 227-2684; fax (425) 227-1181.

SUPPLEMENTARY INFORMATION: A proposal to amend part 39 of the Federal 
Aviation Regulations (14 CFR part 39) to include an airworthiness 
directive (AD) that is applicable to all Boeing Model 737-100, -200, -
300, -400, and -500 series airplanes was published in the Federal 
Register on April 22, 1998 (63 FR 19852). [An action to reopen the 
comment period for the proposal was issued on July 8, 1998 (63 FR 
38524, July 17, 1998).] That action proposed to require installation of 
components for the suppression of electrical transients, and/or 
installation of components to provide shielding and separation of the 
fuel system wiring (that is routed to the fuel tanks) from adjacent 
wiring. That action also proposed to require installation of flame 
arrestors and pressure relief valves in the fuel vent system.
    Interested persons have been afforded an opportunity to participate 
in the making of this amendment. Due consideration has been given to 
the comments received.

Support for the Proposal

    Two commenters support the proposed rule.

Request To Withdraw or Delay the Release of the AD

    Two commenters, the airplane manufacturer and a supplier of fuel 
quantity indication system (FQIS) components, indicate that the current 
fuel system wiring configuration is safe when properly maintained, and 
that modifications are not necessary or, at the very minimum, should be 
delayed until further testing can be completed. Both commenters stress 
that the safety record regarding the existing FQIS for Boeing Model 
737-100 through -500 series airplanes is excellent and exceeds all 
regulatory requirements. In addition, the commenters note there is no 
proof that the FQIS contributed to the center fuel tank explosions on a 
Model 737-300 series airplane in 1990 and on a Model 747-100 series 
airplane in 1996. The commenters further note that the data gathered to 
date relative to electromagnetic interference (EMI) testing of the FQIS 
do not clearly support the contention that an unsafe condition exists. 
The airplane manufacturer also states that additional data should be 
gathered on potential ignition threats, in order to reach a regulatory 
and industry consensus regarding the adequacy of the current FQIS. The 
features of the existing FQIS that are intended to prevent an ignition 
source from entering the fuel tank are also extensively discussed by 
the airplane manufacturer.
    The FAA does not concur with the request to withdraw or delay the 
release of the final rule. The FAA has determined that sufficient data 
currently are available to support a requirement to incorporate 
shielding and separation of the fuel system wiring on Model 737-100 
through -500 series airplanes to protect against hot shorts or EMI 
transients, which may result in in-tank energy levels of sufficient 
magnitude to ignite fuel vapor. Therefore, the current fuel system 
wiring configuration on Model 737-100 through -500 series airplanes 
must be modified. In addition, the FAA has determined that delaying 
publication of the final rule to accommodate further testing is not in 
the best interest of the public or industry. No change to the AD in 
this regard is necessary.
    Regarding safety of the existing FQIS and compliance with 14 CFR 
part 25 (``Airworthiness Standards: Transport Category Airplanes''), 
the FAA notes that the current regulations do not explicitly address 
the unsafe condition that is or may be present in the fuel tanks of 
Model 737-100 through -500 series airplanes. Therefore, the fact that 
the existing FQIS was determined to be in compliance with part 25 when 
these airplane models were certificated is not relevant. In addition, 
the FAA is currently working on a proposal to amend part 25 that would 
explicitly require demonstrating that ignition sources could not be 
present in fuel tanks when failure conditions and aging are considered. 
The FAA agrees with the commenters that no conclusive evidence exists 
to indicate that the FQIS contributed to the two accidents referred to 
by the commenters. However, it is the nature of such accidents that 
they often destroy the evidence that could lead to a conclusive 
identification of the cause of the accident. Even without the 
destruction caused by the accident, there often is no specific physical 
evidence of low energy electrical arcing.
    The FAA does not concur that the final rule should be delayed until 
further EMI testing and data gathering can be completed. The FAA 
recognizes the value of further testing; however, the final rule should 
not be delayed for this purpose. Though further testing may be used to 
better understand possible scenarios that may lead to excessive voltage 
reaching the fuel tanks, the FAA has determined that separation and 
shielding is the most practical and reliable method to eliminate or 
minimize this hazard. An explanation of how the FAA reached this 
determination follows.
    The FAA has developed the requirement for fuel system wiring 
separation and shielding as a result of investigation into the 1996 
accident referred to by the commenter. During the investigation, the 
National Transportation Safety Board (NTSB) used systems analysis 
methods to determine what systems on the Model 747 series airplane are 
most likely to have been the source of ignition energy in the center 
fuel tank. That analysis included examinations of system failure modes 
and effects, service history, and similar airplanes.
    The FAA notes that more than one failure would be required to 
create an ignition source inside the tank. The fact that fuel tank 
explosions on Model 737 and 747 series airplanes are rare would seem to 
support a claim that single failures have not been causing fuel tank 
explosions. However, during the 1996 Model 747 accident investigation, 
the fuel system wiring safety analysis and the examinations of Model 
747 series airplanes performed by the NTSB revealed several scenarios 
in which a

[[Page 4961]]

combination of a latent failure or aging condition within the fuel tank 
and a subsequent single failure or electrical interference condition 
outside the tank can cause an ignition source to occur inside a fuel 
tank.
    Examples of these in-tank and out-of-tank conditions that can 
contribute to a multiple failure ignition scenario were found in 
airplane service records and on Model 747 series airplanes that were 
inspected by the FAA and the NTSB. Various center wing fuel tanks were 
found to have conductive debris in the tanks, damaged FQIS wire 
insulation at the fuel probes, and contamination of probes and in-tank 
wiring by conductive copper/sulfur or silver/sulfur films. Each of 
these conditions can create latent potential ignition sources inside 
the fuel tank.
    During the investigation into the 1990 accident involving a Model 
737-300 series airplane, examination of the fuel system float switch 
wiring revealed damaged insulation and exposed conductor material of 
several wires. Further examination of wire bundles for other systems 
revealed numerous areas in which wire insulation had been damaged. The 
wire insulation damage may have resulted during a modification after 
the airplane was delivered to the airline. However, because other wires 
were found to have damage not related to any post-delivery 
modifications, the wire insulation damage may have resulted from the 
installation of the wire bundle at the factory. Recent inspections of 
the final assembly revealed wiring damage during out-of-sequence 
production on Model 737 series airplanes.
    In addition, several conditions have been identified that can lead 
to sufficient energy in the fuel system wiring to create an ignition 
source if combined with one of the latent conditions described above. 
For example, direct short circuit conditions can occur in wire bundles 
containing FQIS wiring. Model 737 series airplanes have recently been 
observed with aluminum drill shavings on and inside various wire 
bundles in several locations between the flight deck and the fuel tank. 
Such shavings can, with vibration or other motion, cut through wire 
insulation and provide a conductive path between wires in a bundle. 
Service history contains records of wire bundle fires, which may have 
been due to such conditions. Also, electromagnetic coupling can occur 
between systems routed together in bundles.
    When the fuel system wiring practices used on other manufacturers' 
transport airplanes certificated in the same time period as the Model 
737 series airplane are examined, the FAA finds that those other 
airplanes incorporated wiring features (shielding and separation from 
other systems) that preclude the multiple failure scenarios discussed 
above. An examination of the service history for those other airplane 
manufacturers' models also shows that significantly fewer fuel tank 
fire/explosion events have occurred (a tabulation of transport airplane 
fuel tank fires was included in the FAA Notice of Request for Comments 
on NTSB Safety Recommendations published in the Federal Register on 
April 3, 1997). The two most recent fuel tank explosion accidents (in 
1990 and 1996, as referred to previously) remain unsolved, and both 
airplane types involved in those accidents follow the wiring practices 
addressed by this AD. Therefore, the FAA has determined that, to 
address the potential for fuel tank ignition due to a latent failure 
plus one subsequent failure, the type design of Model 737-100 through -
500 series airplanes must be brought up to the same wiring standards as 
other transport airplanes certificated during the same time period the 
Model 737 was certificated. No change to the final rule is necessary in 
this regard.

Request To Extend Compliance Time

    Five commenters, comprising the airplane manufacturer, a supplier 
of FQIS components, two operators of Model 737 series airplanes, and an 
association of airlines operating in the U.S., request an extension of 
the compliance period for incorporation of fuel system wiring 
modifications and installation of fuel vent system flame arrestors. In 
general, the commenters consider the 12-month compliance period to be 
too short.
    One commenter recommends a 24-month compliance time for both 
actions, to ease the demand on hangar space and to spread the cost out 
over two fiscal years instead of one. In addition, one commenter is 
concerned that service instructions are not yet available.
    Two of the commenters, including the airplane manufacturer, 
recommend a longer compliance period for modification of fuel system 
wiring. One commenter recommends 36 months because of the lack of 
immediate safety concern associated with the existing wiring 
configuration and because of logistical considerations for 
accomplishing the modification. In addition, this commenter notes that 
the fuel system modification for Model 737-100 through -500 series 
airplanes required by this AD, as well as the modification for Model 
747 series airplanes required by AD 98-20-40, amendment 39-10808 (63 FR 
52147, September 30, 1998), will affect up to 3,500 airplanes, and the 
requirements for manpower and hangar space will require that the work 
be spread out over several years. The other commenter recommends that 
the compliance time for the fuel system wiring modification be extended 
to 72 months, adding that such an extension would accommodate a flow 
time of 12 months to develop service instructions and 36 months to 
fabricate the required parts, as well as a projected incorporation rate 
that allows operators to complete the modification during a normal 
``D'' check interval.
    Two of the commenters state that the proposed compliance period for 
installation of vent system flame arrestors is too short, based on 
anticipated parts availability. The airplane manufacturer recommends a 
3-year compliance period for that action, based on anticipated 
availability of parts and service instructions.
    The FAA concurs with the request to extend the compliance period 
for accomplishment of the actions required by this AD. Generally, the 
commenters recommend that the compliance period for the wiring 
modification be different from that for the flame arrestor 
installation. The FAA concurs with this approach and has revised the 
final rule to extend the compliance period from 12 months to 48 months 
for modification of the fuel system wiring, and from 12 months to 36 
months for installation of fuel vent system flame arrestors and 
pressure relief valves. These extensions are intended to allow 
sufficient time for the fabrication of required parts and subsequent 
modification of most of the affected airplanes during scheduled 
maintenance visits. The FAA has determined that these extensions will 
not have a significant adverse effect on the safety of the fleet of 
Model 737-100 through -500 series airplanes.
    The FAA also agrees that, as these modifications are spread out 
over several years, the cost per year is reduced and the demand for 
hangar space and manpower is reduced. The FAA finds that both 
compliance periods allow ample time for development of service 
instructions and the fabrication of parts. The FAA has taken into 
account the size of the fleet in determining appropriate compliance 
times. The airplane manufacturer recommends a 72-month compliance time 
to accomplish fuel system wiring modifications. However, the FAA has 
determined that this activity may be completed in 48 months. This

[[Page 4962]]

determination was made by accepting the maximum compliance period 
requested from commenters (other than the manufacturer) and allowing 12 
months for development of service instructions and retrofit kits. The 
manufacturer indicates that service information will be available 
within 12 months, and sufficient parts to support all U.S.-registered 
airplanes will be available within 24 months. In addition, the 
manufacturer predicts an incorporation rate of 50 airplanes per month. 
In light of these numbers (all of which the FAA considers to be 
conservative), wiring modifications on the U.S.-registered fleet can be 
accomplished in a total of 36 months. Recognizing that non-U.S.-
registered airplanes will also be requiring parts, which will delay 
incorporation on U.S.-registered airplanes, the FAA believes it is 
sufficient to extend the compliance period for an additional 12 months 
for a total of 48 months.

Request To Delay Issuance of the AD Pending Release of Service 
Information

    Two commenters, comprising an association of airlines operating in 
the United States and an operator of U.S.-registered airplanes, note 
that detailed compliance methods for the fuel system wiring 
modification and flame arrestor installation must be developed before 
the AD is released. The commenters indicate that, without such detailed 
instructions, the operators will have to be reactive instead of 
proactive; therefore, design and implementation errors may be 
introduced. One of the commenters stresses that the compliance methods 
must be based on results from EMI tests conducted on Model 737 FQIS's 
and that caution should be taken because wiring modifications may cause 
damage to existing wiring. The other commenter stresses that, because 
of the fleet size and the relatively short proposed compliance times, 
the rule should not be released until compliance methods are available.
    The FAA concurs partially. The FAA does not concur that delaying 
this action until after the release of the manufacturer's planned 
service instructions is warranted, because sufficient technology 
currently exists to devise and install the required features within 
reasonable compliance times. However, as discussed previously, the 
final rule has been modified to allow 36 months to install fuel vent 
system flame arrestors and 48 months to modify fuel system wiring.
    The FAA has taken into account the size of the fleet in determining 
appropriate compliance times and has adopted the recommendation of the 
airplane manufacturer relative to the compliance period for the 
installation of fuel vent system flame arrestors. The selection of a 
48-month compliance time for fuel system wiring modification also has 
taken into account the fleet size (explained in detail under the 
heading ``Request to Extend Compliance Time,'' above).
    The FAA does not concur with the request to delay release of the 
rule to complete further EMI testing on additional Model 737 series 
airplanes. The airplane manufacturer has completed testing on one Model 
737 series airplane to date. The FAA has determined that the test 
procedures used during the EMI testing are not representative of the 
many possible conditions on an airplane in operation. Specifically, no 
attempt was made to represent any system failure conditions or 
compromise shielding/grounding provisions on the systems that were 
powered and switched. Also, because of the way airplane wire bundles 
are manufactured and installed, significant variation in levels of 
coupling between systems has been seen in the past and would be 
expected on Model 737 series airplanes.
    Moreover, the FAA's determination of the existence of an unsafe 
condition is not wholly dependent on the results of the EMI testing. In 
the Model 747 fuel system wiring safety analysis and airplane 
inspections performed by the NTSB during the investigation of the 1996 
accident, several tank ignition scenarios were identified involving a 
combination of a latent failure or aging condition inside the fuel tank 
and a subsequent failure or electromagnetic coupling outside the tank. 
Various FAA and NTSB activities identified actual examples of the 
specific potential for each of those types of contributing conditions 
on Model 747 series airplanes. In addition, the FAA has determined that 
these same types of scenarios are applicable to Model 737-100 through -
500 series airplanes.
    The FAA shares the commenters' concern that modification of fuel 
system wiring may damage existing wiring, and the airplane manufacturer 
has carefully considered this concern as well. To minimize possible 
damage, the manufacturer's service instructions will not specify 
removal of any of the existing wiring; instead, this wiring will be 
terminated properly and retained in the airplane. In addition, newly 
installed shielded wiring will be spatially separated from all other 
airplane wiring.

Preference for a Specific Design Solution

    Two commenters discuss application of transient suppression devices 
as they relate to the proposed AD. Responses to these comments have not 
been included in this AD because the optional requirement for 
installation of transient suppression devices has been removed from the 
final rule.
    Based on comments from the airplane manufacturer, and on its own 
further analysis, the FAA has determined that installation of transient 
suppression devices alone would not meet the intent of the rule. The 
FAA has concerns that transient suppression devices may have latent 
failure modes that would render the transient suppression function 
inoperative, or may have failure modes that would cause introduction of 
high voltage signals into the fuel tank that otherwise would not have 
occurred. Therefore, paragraph (a) of the final rule has been revised 
to eliminate the general requirement for transient suppression 
components and to delete the reference to ``install components.'' 
Operators that have specific design changes other than those required 
by the AD that may provide an acceptable level of safety may request 
approval of an alternative method of compliance in accordance with 
paragraph (c) of this AD.

Request To Separate the Proposed Rule Into Two AD's

    One commenter, an operator of U.S.-registered airplanes, requests 
that the AD be divided into two AD's. The commenter points out that the 
corrective actions cannot be done in one maintenance visit.
    The FAA does not concur with the request to separate the rule. 
Although both required actions most likely will not be accomplished 
during the same shop visit, the FAA notes that more than one shop visit 
to accomplish the actions required by an AD is not uncommon. The 
manufacturer plans to issue service information for each modification 
separately, which will allow the actions to be readily performed at 
different maintenance visits. No change to the AD in this regard is 
required.

Request To Revise Cost Estimate for Wiring Modification

    Two commenters, an operator of U.S.-registered airplanes and the 
airplane manufacturer, discuss work hour and cost estimates regarding 
modification of fuel system wiring. One commenter questions how the FAA 
determined the work hour and cost estimates for wiring changes in the 
proposed rule. The other

[[Page 4963]]

commenter provides its own specific work hour and cost estimates for 
wiring modifications.
    The FAA infers that the commenters request a revision of the work 
hour and cost estimates for the wiring modification. The FAA concurs. 
In the absence of specific instructions addressing wiring 
modifications, the FAA based its original work hour estimate (40 work 
hours) and cost estimate ($12,400 per airplane) on similar 
modifications accomplished on other airplane models. The cost impact 
information, below, has been revised in this regard, based on the 
information provided by the manufacturer.

Request To Revise Cost Estimate for Installation of Flame Arrestor

    Three commenters, comprising an operator of U.S.-registered 
airplanes, an association of airlines operating in the U.S., and the 
airplane manufacturer, discuss work hour and cost estimates regarding 
installation of fuel vent system flame arrestors and pressure relief 
valves. One commenter suggests that the FAA's determination of 48 work 
hours to install flame arrestors is underestimated. Another commenter 
questions the method the FAA used to estimate the work hours and parts 
necessary to install the flame arrestors. A third commenter provides 
its own specific work hour and cost estimates.
    The FAA infers that the commenters request a revision of the cost 
estimate for this installation. The FAA concurs partially. The FAA 
considers the cost estimates provided in the proposed rule to be 
generally representative of the actual costs associated with this 
modification. The FAA's estimated work hours and costs are based on 
previously released service instructions from the airplane manufacturer 
that detailed installation of fuel vent system flame arrestors and 
pressure relief valves on Model 737-200 series airplanes. The airplane 
manufacturer's labor cost estimate is comparable to the FAA's estimate 
and its parts cost estimate is actually lower than that of the FAA. The 
cost impact information, below, has been revised in this regard, based 
on the information provided by the manufacturer.

Request To Maintain Minimum FQIS Performance Requirements

    One commenter, a manufacturer of fuel system components, requests 
that the minimum performance requirements for FQIS's regarding maximum 
allowable energy into the fuel tank not be changed as a result of this 
AD. The commenter states that a change to the minimum performance 
requirements implies the currently certified FQIS is not safe.
    The FAA concurs with the request and finds that the changes that 
result from this AD do not directly affect the minimum performance 
requirements for fuel system wiring and components in the future. 
Though the AD does not specifically address the performance 
requirements, the FAA notes studies are in progress that may address 
the currently accepted maximum allowable energy levels in fuel tanks. 
If, as part of this study activity, it is determined that the currently 
recognized levels need to be adjusted, then the FAA may consider 
further rulemaking to address that. As stated previously, the fact that 
two unexplained center fuel tank explosions have occurred in the last 
eight years on Boeing airplanes leads the FAA to conclude that 
modifications to the fuel system wiring are necessary. The FAA has 
determined that wire separation and shielding is the appropriate action 
to take at this time. These modifications do not directly affect the 
minimum performance requirements for fuel system wiring. Therefore, no 
change to the AD in this regard is required.

Concerns Regarding Flame Arrestor Qualification Tests

    One commenter expresses concern that flame arrestor qualification 
tests are not sufficiently defined and that the installation of fuel 
vent system flame arrestors would not have prevented the 1990 center 
fuel tank explosion on a Model 737-300 series airplane.
    The FAA recognizes there are credible explanations for the accident 
that do not involve an external flame front traveling through the vent 
system into the center fuel tank. Regardless of the role a fuel vent 
system flame arrestor may have played in that specific accident, the 
FAA has determined that the lack of fuel vent system flame arrestors in 
Model 737-100 through -500 series airplanes creates an unacceptable 
risk of fuel tank explosion and constitutes an unsafe condition. Based 
on comments received on the NPRM, this opinion appears to be held by a 
number of commenters (including the airplane manufacturer) as well. The 
sufficiency of qualification testing for flame arrestors does not have 
a specific bearing on this AD.
    However, the FAA is interested in obtaining more information 
regarding this commenter's concerns. The FAA has asked the commenter to 
submit additional detailed information on this concern to the Seattle 
Aircraft Certification Office for consideration.

Concerns Regarding Detection of Wire Chafing

    One commenter, a manufacturer of electronic test equipment, states 
it believes that electrical coupling between adjacent wires is not 
plausible as a cause for either accident referred to previously. The 
commenter notes these wires have been adjacent to other wires for years 
with no apparent problems. In addition, the commenter suggests the test 
equipment utilized by industry is not sophisticated enough to detect 
the types of wire damage that may be present in the fuel system wiring. 
The commenter also details the benefits of utilizing more advanced test 
equipment for detection of wire damage. The commenter further indicates 
that it manufactures this advanced equipment.
    The FAA does not agree with the commenter's opinion that electrical 
coupling between adjacent wires could not be a factor in either the 
737-300 or the 747-100 fuel tank explosion. As noted in the proposed 
rule, the FAA participated in testing of fuel system wiring in which 
electrical coupling was induced in combination with an aging condition 
or a latent failure of the FQIS probes, which resulted in energy in 
excess of that required to ignite fuel vapor. The fact that the wires 
had been adjacent for years with no apparent problems prior to the tank 
ignition may only indicate that neither the aging condition nor the 
latent failure inside the tank was present during that time to allow 
the induced voltage to cause an ignition source inside the fuel tank.
    Regarding the advanced test equipment discussed by the commenter, 
the FAA cannot dictate the types of electrical equipment that industry 
utilizes in conducting airplane wiring tests. This AD is based on the 
determination that separation and shielding of the fuel system wiring 
is currently the only practical method to ensure that induced 
transients or wire-to-wire hot shorts do not cause an ignition source 
inside the fuel tank. No change to the AD in this regard is required.

Clarification of Systems Affected

    Since the issuance of the NPRM, the FAA recognized the proposed AD 
may be unclear with respect to which electrical circuits were intended 
to be affected by the proposed AD. The NPRM proposed, and the final 
rule requires, providing shielding and separation of the fuel system 
wiring (that is routed to the fuel tanks) from adjacent wiring. The FAA 
considers ``fuel system wiring'' to include all electrical circuits 
associated with the control or indication

[[Page 4964]]

of the fuel quantity on the airplane. This would include, but not be 
limited to, the FQIS tank probe circuits, the volumetric shutoff 
compensator circuits, densitometer circuits, and float switch circuits. 
The term ``circuits'' is considered by the FAA to include airplane 
wiring as well as wiring within electrical equipment.

Conclusion

    After careful review of the available data, including the comments 
noted above, the FAA has determined that air safety and the public 
interest require the adoption of the rule with the changes described 
previously. The FAA has determined that these changes will neither 
increase the economic burden on any operator nor increase the scope of 
the AD.

Cost Impact

    There are approximately 2,780 airplanes of the affected design in 
the worldwide fleet. The FAA estimates that 1,140 airplanes of U.S. 
registry will be affected by this AD.
    It will take approximately 278 work hours per airplane to 
accomplish the required installation of shielding/separation 
components, at an average labor rate of $60 per work hour. Required 
parts will cost approximately $4,500 per airplane. Based on these 
figures, the cost impact of this action on U.S. operators is estimated 
to be $24,145,200, or $21,180 per airplane.
    It will take approximately 48 work hours per airplane to accomplish 
the required installation of flame arrestors, at an average labor rate 
of $60 per work hour. Required parts will cost approximately $17,100 
per airplane. Based on these figures, the cost impact of this action on 
U.S. operators is estimated to be $22,777,200, or $19,980 per airplane.
    The cost impact figures discussed above are based on assumptions 
that no operator has yet accomplished any of the requirements of this 
AD action, and that no operator would accomplish those actions in the 
future if this AD were not adopted.

Regulatory Impact

    The regulations adopted herein will not have substantial direct 
effects on the States, on the relationship between the national 
government and the States, or on the distribution of power and 
responsibilities among the various levels of government. Therefore, in 
accordance with Executive Order 12612, it is determined that this final 
rule does not have sufficient federalism implications to warrant the 
preparation of a Federalism Assessment.
    For the reasons discussed above, I certify that this action (1) is 
not a ``significant regulatory action'' under Executive Order 12866; 
(2) is not a ``significant rule'' under DOT Regulatory Policies and 
Procedures (44 FR 11034, February 26, 1979); and (3) will not have a 
significant economic impact, positive or negative, on a substantial 
number of small entities under the criteria of the Regulatory 
Flexibility Act. A final evaluation has been prepared for this action 
and it is contained in the Rules Docket. A copy of it may be obtained 
from the Rules Docket at the location provided under the caption 
ADDRESSES.

List of Subjects in 14 CFR Part 39

    Air transportation, Aircraft, Aviation safety, 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:

99-03-04 Boeing: Amendment 39-11018. Docket 98-NM-50-AD.

    Applicability: All Model 737-100, -200, -300, -400, and -500 
series airplanes; 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 (c) 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 possible ignition of fuel vapors in the fuel tanks, 
and external ignition of fuel vapor exiting the fuel vent system and 
consequent propagation of a flame front into the fuel tanks, 
accomplish the following:
    (a) Within 48 months after the effective date of this AD, 
provide shielding and separation of the fuel system wiring (that is 
routed to the fuel tanks) from adjacent wiring, in accordance with a 
method approved by the Manager, Seattle Aircraft Certification 
Office (ACO), FAA, Transport Airplane Directorate.
    (b) Within 36 months after the effective date of this AD, 
install flame arrestors and pressure relief valves in the fuel vent 
system, in accordance with a method approved by the Manager, Seattle 
ACO.
    (c) 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 2: Information concerning the existence of approved 
alternative methods of compliance with this AD, if any, may be 
obtained from the Seattle ACO.

    (d) 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.
    (e) This amendment becomes effective on March 9, 1999.

    Issued in Renton, Washington, on January 26, 1999.
Darrell M. Pederson,
Acting Manager, Transport Airplane Directorate, Aircraft Certification 
Service.
[FR Doc. 99-2272 Filed 2-1-99; 8:45 am]
BILLING CODE 4910-13-U