AD 99-23-20 ACTIVE

This amendment adopts a new airworthiness directive (AD), applicable to certain Boeing Model 737-100, -200, -300, -400, and -500 series airplanes. For certain airplanes, this AD requires installation of a transient suppression diode in the wiring circuit of the refueling valve-to-float switch of each fuel tank. For certain other airplanes, this AD requires replacement of the existing transient suppression diode with an improved diode. This AD also requires a functional test to verify proper installation of each diode, and corrective action, if necessary. This amendment is prompted by incidents of electrical fire during fueling of the airplane, due to a short circuit and overheating of a transient suppression diode. The actions specified by this AD are intended to prevent such conditions, which could result in electrical arcing and ignition of fuel vapors at the refueling receptacle for the fuel tanks, and consequent fire during airplane fueling.

Final rule

99-23-20 BOEING: Amendment 39-11416. Docket 99-NM-47-AD. Issued November 4, 1999. \n\n\tApplicability: Model 737-100, -200, -300, -400, and -500 series airplanes; line numbers 1 through 3016 inclusive; certificated in any category. \n\n\tNOTE 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. \n\n\tCompliance: Required as indicated, unless accomplished previously. \n\n\tTo prevent a short circuit and overheating of the transient suppression diode, which could result in electrical arcing and ignition of fuel vapors at the fueling receptacle for the fuel tanks, and consequent fire during airplane fueling, accomplish the following: \n\nCorrective Action \n\n\t(a) For Group 1 airplanes, as identified in Boeing Service Bulletin 737-28-1115, dated March 4, 1999: Within 12 months after the effective date of this AD, install a transient suppression diode, part number (P/N) 69-58806-4, in the wire bundle (W264) of the refueling valve-to-float switch of each fuel tank, in accordance with the service bulletin. \n\n\t(b)\tFor Groups 2, 3, and 4 airplanes, as identified in Boeing Service Bulletin 737-28-1115, dated March 4, 1999: Within 12 months after the effective date of this AD, replace the existing transient suppression diode, P/N 69-58806-1 or 69-58806-3, installed in the wire bundle (W264) of the refueling valve-to-float switch of each fuel tank, with an improved diode, P/N 69-58806-4, in accordance with the service bulletin. \n\n\t(c)\tPrior to further flight following accomplishment of the actions required by paragraph (a) or (b) of this AD, perform a functional test to verify proper installation of each diode in accordance with Boeing Service Bulletin 737-28-1115, dated March 4, 1999. If any discrepancy is detected during any functional test, prior to further flight, replace the discrepant diode and repeat the functional test, in accordance with the service bulletin. \n\nSpares Paragraph \n\n\t(d)\tAs of the effective date of this AD, no person shall install a transient suppression diode having P/N 69-58806-1 or 69-58806-3 on any airplane. \n\nAlternative Methods of Compliance \n\n\t(e)\tAn alternative method of compliance or adjustment of the compliance time that provides an acceptable level of safety may be used if approved by the Manager, Seattle Aircraft Certification Office (ACO), FAA, Transport Airplane Directorate. Operators shall submit their requests through an appropriate FAA Principal Maintenance Inspector, who may add comments and then send it to the Manager, Seattle ACO. \n\n\tNOTE 2: Information concerning the existence of approved alternative methods of compliance with this AD, if any, may be obtained from the Seattle ACO. \n\nSpecial Flight Permits \n\n\t(f)\tSpecial 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. \n\nIncorporation by Reference \n\n\t(g)\tThe corrective actions shall be done in accordance with Boeing Service Bulletin 737-28-1115, dated March 4, 1999. This incorporation by reference was approved by the Director of the Federal Register in accordance with 5 U.S.C. 552(a) and 1 CFR part 51. Copies may be obtained from Boeing Commercial Airplane Group, P.O. Box3707, Seattle, Washington 98124-2207. Copies may be inspected at the FAA, Transport Airplane Directorate, 1601 Lind Avenue, SW., Renton, Washington; or at the Office of the Federal Register, 800 North Capitol Street, NW., suite 700, Washington, DC. \n\n\t(h)\tThis amendment becomes effective on December 27, 1999.

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 certain Boeing Model 737-100, -200, -300, -400, and -500 series airplanes was published in the Federal Register on June 14, 1999 (64 FR 31762). That action proposed to require, for certain airplanes, installation of a transient suppression diode in the wiring circuit of the refueling valve-to-float switch of each fuel tank. For certain other airplanes, the proposal would require replacement of the existing transient suppression diode with an improved diode. The proposal also would require a functional test to verify proper installation of each diode, and corrective action, if necessary. \n\nComments \n\n\tInterested persons have been afforded an opportunity to participate in the making of this amendment. Due consideration has been given to the comments received. \n\nRequest to Revise Cost Estimate \n\n\tTwo commenters request that the FAA revise the cost estimate and the number of hours required to complete the installation or replacement. One commenter states that the estimated material cost alone, based on Boeing's quoted price for the wire kit, is $800. The other commenter states that the kit price is $1,106. In addition, one commenter estimates that 12 work hours are required to modify an airplane while another commenter estimates that 16 work hours are required to complete the modification. One of the commenters indicates that additional time is required to gain access to the transient suppression diodes, close up the area, and perform functional testing. \n\n\tThe FAA partially concurs. The cost estimate for required parts has been increased to $800 per airplane from $50 per airplane, using the kit price that the commenter states is based upon Boeing's quoted price. The FAA work hour estimate has been increased to 12 work hours from 7 hours based upon information supplied by the commenters. However, the FAA is not increasing the work hour estimate to account for functional testing since this has already been accounted for in the work hour estimate in the manufacturer's service bulletin. The final rule has been revised to incorporate the above changes in the cost estimate. \n\nRequest to Extend Compliance Time \n\n\tThree commenters request that the compliance period be extended to 18 months from 12 months. Two commenters state that the circuit that includes the transient suppression diode is only powered on the ground during fueling and has no function in the air. One of the commenters also notes that the same circuit is affected by AD 99-05-12, which requires either deactivation of the circuit or installation of double teflon sleeving over the float switch wiring for the center fuel tank to prevent a possible short in the system. A third commenter notes that extending the compliance time to 18 months will allow for diode replacement at the same time as thereplacement of the float switch wiring for the center fuel tank (per AD 99-05-12). \n\n\tAnother commenter indicates that extending the compliance period to 18 months will allow for installation or replacement (as applicable) during the next "C" check. In addition, this commenter states that the compliance time should be extended to account for the airplane manufacturer's estimate of a 300-day lead time for kits listed in the service bulletin. \n\n\tThe FAA does not concur with the commenters' request to extend the compliance time. The FAA agrees that the circuit which includes the transient suppression diode is powered only on the ground during fueling and has no function in the air. However, this fact does not nullify the safety hazard posed by overheating of the transient suppression diode. During the comment period for the proposed AD, an overheated transient suppression diode caused another fire during fueling. Although the fire was extinguished before extensive damage occurred, the FAA finds that this condition is a significant safety hazard. With regard to the comment that installation of an improved transient suppression diode should be performed at the same time as modification of wiring for the center tank float switch in accordance with AD 99-05-12, the actions required by the two AD's are performed in different locations on the airplane and do not have a direct bearing on each other. Additionally, the compliance threshold for AD 99-05-12 is 30,000 flight hours. The FAA estimates that there are more than 2,000 airplanes that currently have fewer than 30,000 flight hours, and operators of those airplanes are not required to modify the wiring of the center tank float switch in accordance with AD 99-05-12 until the airplanes have accumulated 30,000 flight hours. The FAA finds that extending the compliance threshold for this AD to 30,000 flight hours, to allow for installation of an improved transient suppression diode at the same time as modification ofwiring for the center tank float switch, is inappropriate because it would not address the identified unsafe condition in a timely manner. \n\n\tThe FAA has determined that a 12-month compliance period, as proposed, is warranted. The manufacturer has advised that an ample number of required parts will be available for installation in the U.S. fleet within the compliance period. The manufacturer indicated that the 300-day-lead-time quote was a standard quote for this type of part. However, production schedules have been modified to support this AD. The improved transient suppression diodes are being produced at a rate of 1,500 per month to ensure availability within the 12-month compliance period. In developing an appropriate compliance time for this action, the FAA considered not only the degree of urgency associated the addressing the subject unsafe condition, but the availability of required parts and the practical aspect of installing the required modification within an interval of time that parallels normal scheduled maintenance for the majority of affected operators. No change to the final rule is necessary in this regard. \n\nRequest to Develop a New Transient Suppression Diode \n\n\tOne commenter requests that the FAA require the airplane manufacturer to develop a transient suppression diode with better mechanical protection from stresses to prevent possible overheating. The commenter states that the improved transient suppression diode is made of the same components as the existing diode, with essentially the same manufacturing process and the same mechanical protection (heat-shrunk plastic sleeving); only the arrangement of the wiring is different. The commenter states that the lack of significant changes to the design may result in more failures of the improved diodes (due to damage during installation) than if the existing diodes had been left in place. \n\n\tThe FAA does not concur. The manufacturer has made production changes to eliminate the stress conditions which occurred in the existing diode design. Based upon the production changes, the FAA does not anticipate that variation in installation will lead to failures of the improved diode as the commenter suggests. The improved diodes have been used on other Boeing model airplanes. A review of the service history on the improved diodes on other Boeing model airplanes confirms that they do not have a history of failure in service. No change to the final rule is necessary in this regard. \n\nConclusion \n\n\tAfter 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 previously described. The FAA has determined that these changes will neither increase the economic burden on any operator nor increase the scope of the AD. \n\nCost Impact \n\n\tThere are approximately 2,897 airplanes of the affected design in the worldwide fleet. The FAA estimatesthat 1,126 airplanes of U.S. registry will be affected by this AD. \n\n\tFor all airplanes, it would take approximately 12 work hours per airplane to accomplish the replacement or installation (as applicable) and the functional test to verify proper installation, at an average labor rate of $60 per work hour. Required parts would cost approximately $800 per airplane. Based on these figures, the cost impact of the AD on U.S. operators is estimated to be $1,711,520 or $1,520 per airplane. \n\n\tThe cost impact figure discussed above is based on assumptions that no operator has yet accomplished any of the proposed requirements of this AD action, and that no operator would accomplish those actions in the future if this AD were not adopted. \n\nRegulatory Impact \n\n\tThe 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 variouslevels 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. \n\n\tFor 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." \nList of Subjects in 14 CFR Part 39 \n\n\tAir transportation, Aircraft, Aviation safety, Incorporation by reference, Safety. \n\nAdoption of the AmendmentAccordingly, 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:\n\n PART 39 - AIRWORTHINESS DIRECTIVES \n\n\t1. The authority citation for part 39 continues to read as follows: Authority: 49 U.S.C. 106(g), 40113, 44701. § 39.13 (Amended) \n\t2. Section 39.13 is amended by adding the following new airworthiness directive:

The service information referenced in this AD may be obtained from Boeing Commercial Airplane Group, P.O. Box 3707, Seattle, Washington 98124-2207. 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.

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.