The FAA has received a report that severe corrosion was found on a keel beam splice on two Boeing Model 737-700 series airplanes. At the time the severe corrosion was found, the airplanes had been in service for approximately 22 months since date of manufacture. This corrosion has been attributed to the material of the keel beam splice plates that were installed during production of certain Boeing Model 737-600, -700, and -800 series airplanes. The material, 7150-T6511, is known to be highly susceptible to corrosion. Such corrosion, if not found and fixed, could cause cracking of the keel beam splices, which in turn could lead to rapid degradation of the strength of the keel beam splices, and result in failure of the keel beam and consequent failure of the forward fuselage of the airplane. \n\n\tThis unsafe condition may exist or develop on Model 737-600 and -700 series airplanes up to and including line number 908; and on Model 737-800 series airplanes up toand including line number 455. The keel beam splices on airplanes after those line numbers are made of a more corrosion-resistant material. \n\nOther Relevant Rulemaking \n\n\tOn November 5, 1990, we issued AD 90-25-01, amendment 39-6789 (55 FR 49263, November 27, 1990). That AD applies to all Boeing Model 737 series airplanes and requires implementation of a corrosion prevention and control program (CPCP) specified in Boeing Document Number D6-38528 "Aging Airplane CPCP, Model 737," Revision A, dated July 28, 1989. \n\n\tThe airplanes subject to this new AD are also subject to AD 90-25-01. However, we have previously approved an alternative method of compliance (AMOC) to paragraph (a) of that AD for Boeing Model 737 "Next Generation" airplanes (which includes Model 737-600, -700, and -800 series airplanes). This AMOC allows certain inspection thresholds and repetitive intervals listed in Section 8 ("Structural Maintenance Program") of Boeing Document Number D626A001, dated June 2000 (the "Maintenance Planning Document" (MPD) for the Boeing 737 Next Generation airplanes), to be used as an alternative to the thresholds and intervals listed in Boeing Document Number D6-38528, Revision A. \n\nFAA's Determination \n\n\tWe have determined that existing inspections of the keel beam splices included in the CPCP required by the existing AD and in Boeing Document Number D626A001 are not sufficient to ensure that the splices are inspected for corrosion and cracking in a timely manner. This determination is based on the following information: \n\nTask Number 53-210-00 of Boeing Document Number D626A001, Section 8, dated June 2001, includes repetitive general visual inspections for any discrepancy of the keel beam under the wing-to-body fairing, including the keel beam splice (among other areas). We find that the procedures involved in this inspection are sufficient to ensure that corrosion and cracking of the keel beam splices are found. However, the compliance time for this inspection is 12 years since the airplane's date of manufacture or 36,000 total flight cycles, whichever occurs first, and the repetitive interval is 8 years or 24,000 flight cycles, whichever occurs first. We have determined that the compliance threshold is not early enough and the repetitive interval is too long to ensure that corrosion and cracking of the keel beam splices is found and fixed in a timely manner. (As stated above, severe corrosion of the keel beam splice plates has been found on two Model 737-700 series airplanes within 22 months after the date of manufacture of those airplanes.) \n\nTask 53-828-00 of Boeing Document Number D626A001, Section 7 ("Zonal Inspection Program"), dated June 2001, contains instructions for an optional general visual inspection for discrepancies in a specific area aft of the keel beam at a suggested repetitive interval of 18 months. However, the procedures do not specifically state that the keel beam splices should be inspected. \n\nDeterminationof Compliance Time \n\n\tAlthough this action is in the form of a final rule that involves requirements affecting flight safety and, thus, was not preceded by notice and an opportunity for public comment, the FAA and representatives of the airplane manufacturer met on July 23 and 25, 2001. (Records of these meetings are available in the Rules Docket for examination by interested persons.) The purpose of these meetings was to allow the manufacturer to provide revised engineering data that could potentially affect the compliance time for the actions required by this AD. Although the manufacturer concurs with our determination that the corrosion addressed by this AD represents a safety concern, it asserts that the compliance times could be increased over the times we planned to require for the actions in this AD. \n\n\tThe revised data presented primarily consisted of a finite-element analysis (FEA) of the wing and fuselage of the Boeing Model 737 "Next Generation" airplanes. The FEA included results of a "splice plates intact" model, and an analysis of the model with the keel beam chords rendered ineffective (due to corroded or cracked splice plates) at the body station (BS) 540 location. The manufacturer asserted that the FEA was necessary to properly analyze this area because this area, the lower wing-to-fuselage connection, is highly complex and redundant. \n\n\tThe manufacturer asserted that the buttock line 41 fuel beam and the under-wing longeron would be adequate to react limit load in the case of failure of the keel beam chord splices at BS 540. (Limit load is defined as the highest application of load that is expected to occur in service.) Based on these data, the manufacturer suggested that an 18-month repetitive inspection interval, similar to the MPD inspection of an adjacent area which was described previously, would provide an adequate level of safety. \n\n\tWe have reviewed the revised data provided by the manufacturer and concur that the area is structurally very complex and difficult to analyze, due to the structural interactions of the fuselage and wing. We accept that the alternate load paths shown by the manufacturer's analysis are adequate to react limit load in the event of failure of the keel beam splices at BS 540. However, the fatigue life of the alternate load paths is unknown and is expected to be reduced due to the significant increase in loads. \n\n\tGiven the level of risk, we conclude that urgent airworthiness action continues to be necessary and requires the immediate adoption of this AD without notice and opportunity for prior public comment. However, we have determined that the manufacturer's analysis allows for an increase in the initial inspection threshold and repetitive inspection interval over what we planned to require, as well as an increase in the planned grace period (for airplanes over the initial inspection threshold). \n\n\tThe initial reports of severe corrosion were received in July 2000, on airplanes withline numbers 73 and 90. As described previously, at that time, these airplanes had been in service for approximately 22 months. It was not until May 2001, that we determined the actual extent of the corrosion of the splice plate. As a result, it is possible that there are approximately 400 airplanes at present that are at two years or more since date of manufacture, with some airplanes being as old as 4 years since date of manufacture. Inspection of these airplanes may reveal corrosion considerably in excess of the severe corrosion observed on line numbers 73 and 90. \n\n\tWe originally intended to set a compliance threshold of 12 months since date of manufacture for the initial inspection, with a repetitive inspection interval of 12 months. We intended to allow a grace period of 30 days after the effective date of this AD for airplanes older than 12 months since date of manufacture. As discussed above, due to the revised data provided by the manufacturer, we have determined that the following changes to the compliance times for this AD will provide an acceptable level of safety: \n\nFor airplanes at less than 18 months since date of manufacture as of the effective date of this AD, extension of the initial inspection threshold to the later of 18 months since date of manufacture or 90 days after the effective date of this AD. \n\nFor airplanes at 18 months or more since date of manufacture as of the effective date of this AD, extension of the initial inspection threshold to the later of 24 months since date of manufacture or 30 days after the effective date of this AD. \n\nFor all airplanes, extension of the repetitive inspection interval to 18 months. \n\nExplanation of the Requirements of the Rule \n\n\tSince an unsafe condition has been identified that is likely to exist or develop on other airplanes of the same type design, this AD is being issued to find and fix corrosion or cracking of the keel beam splices, which could lead to rapid degradation of the strength of the keel beam splices, and result in failure of the keel beam and consequent failure of the forward fuselage of the airplane. This AD requires repetitive detailed visual inspections for corrosion or cracking of the keel beam splices, and repair or replacement of splice plates and bolts with new, improved parts, if necessary. This action also provides an optional terminating action for the repetitive inspections. \n\nInterim Action \n\n\tThis is considered to be interim action. We are currently considering requiring the replacement of existing splice plates and bolts with new, improved parts, which is included in this AD as an optional terminating action that terminates the repetitive inspections required by this AD action. However, the planned compliance time for the replacement is sufficiently long so that notice and opportunity for prior public comment will be practicable. \n\nDifference Between This AD and Service Document \n\n\tTask Number 53-210-00 of Boeing Document Number D626A001 describes a general visual inspection for discrepancies of the keel beam under the wing-to-body fairing, including the keel beam splice. However, we have determined that it is necessary for this AD to require a detailed visual inspection for corrosion or cracking of the keel beam splice only. \n\nDetermination of Rule's Effective Date \n\n\tSince a situation exists that requires the immediate adoption of this regulation, it is found that notice and opportunity for prior public comment hereon are impracticable, and that good cause exists for making this amendment effective in less than 30 days. \n\nComments Invited \n\n\tAlthough this action is in the form of a final rule that involves requirements affecting flight safety and, thus, was not preceded by notice and an opportunity for public comment, comments are invited on this rule. Interested persons are invited to comment on this rule by submitting such written data, views, or arguments as they may desire. Communications shall identify the Rules Docket number and be submitted in triplicate to the address specified under the caption ADDRESSES. All communications received on or before the closing date for comments will be considered, and this rule may be amended in light of the comments received. Factual information that supports the commenter's ideas and suggestions is extremely helpful in evaluating the effectiveness of the AD action and determining whether additional rulemaking action would be needed. \n\n\tSubmit comments using the following format: \n\nOrganize comments issue-by-issue. For example, discuss a request to change the compliance time and a request to change the service bulletin reference as two separate issues. \n\nFor each issue, state what specific change to the AD is being requested. \n\nInclude justification (e.g., reasons or data) for each request. \n\n\tComments are specifically invited on the overall regulatory, economic, environmental, and energy aspects of the rule that might suggest a need tomodify the rule. All comments submitted will be available, both before and after the closing date for comments, in the Rules Docket for examination by interested persons. A report that summarizes each FAA-public contact concerned with the substance of this AD will be filed in the Rules Docket. \n\n\tCommenters wishing the FAA to acknowledge receipt of their comments submitted in response to this rule must submit a self-addressed, stamped postcard on which the following statement is made: "Comments to Docket Number 2001-NM-236-AD." The postcard will be date-stamped and returned to the commenter. \n\nRegulatory Impact \n\n\tThe regulations adopted herein will not have a substantial direct effect on the States, on the relationship between the national Government and the States, or on the distribution of power and responsibilities among the various levels of government. Therefore, it is determined that this final rule does not have federalism implications under Executive Order 13132. \n\n\tThe FAA has determined that this regulation is an emergency regulation that must be issued immediately to correct an unsafe condition in aircraft, and that it is not a "significant regulatory action" under Executive Order 12866. It has been determined further that this action involves an emergency regulation under DOT Regulatory Policies and Procedures (44 FR 11034, February 26, 1979). If it is determined that this emergency regulation otherwise would be significant under DOT Regulatory Policies and Procedures, a final regulatory evaluation will be prepared and placed in the Rules Docket. A copy of it, if filed, may be obtained from the Rules Docket at the location provided under the caption ADDRESSES. \n\nList of Subjects in 14 CFR Part 39 \n\n\tAir transportation, Aircraft, Aviation safety, Safety. \n\nAdoption of the Amendment \n\n\tAccordingly, 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\nPART 39--AIRWORTHINESS DIRECTIVES \n\n\t1. The authority citation for part 39 continues to read as follows: \n\n\tAuthority: 49 U.S.C. 106(g), 40113, 44701. \n\nSec. 39.13 (Amended) \n\n\t2. Section 39.13 is amended by adding the following new airworthiness directive: