Discussion \n\n\tWe issued a notice of proposed rulemaking (NPRM) to amend 14 CFR part 39 to include an airworthiness directive (AD) that would apply to certain Boeing Model 737-300 and -400 series airplanes. That NPRM was published in the Federal Register on January 10, 2008 (73 FR 1846). That NPRM proposed to require testing and inspecting a certain web panel of the main wheel well pressure deck to determine the material type and thickness; and related investigative and corrective actions if necessary. \n\nComments \n\n\tWe gave the public the opportunity to participate in developing this AD. We considered the comments received. \n\nRequest To Change the Description of the Unsafe Condition \n\n\tBoeing asks that the unsafe condition (end level effect) specified in the applicable sections of the NPRM be changed from "rapid decompression'' to "controlled decompression.'' Boeing states that the most probable result of the cracking would be pressure loss or controlled depressurization. Boeing has received reports of cracks ranging from 4.5 to 8 inches in the web panel of the main wheel well pressure deck; the reports included the following data: \n\tCabin crews reported a loud hissing noise coming from the area below seats 14A, B, and C. No depressurization was reported. \n\tThe crew reported a loud hissing noise from the cabin lining on the left-hand side at row 15. The cabin windows along the left-hand side progressively frosted up until, after about 2 hours, all the windows were frosted up between rows 11 through 17. \n\tIt was reported that it was not possible to pressurize another airplane. \n\tWe partially agree with Boeing. We agree to change the end level effect of the unsafe condition by removing the word "rapid,'' since Boeing has provided data verifying that the decompression does not happen quickly. However, we do not agree that the decompression is "controlled'' because of the safety implications and consequences associated with cracking on a web panel of the main wheel well pressure deck. We have changed the applicable sections in this AD accordingly. \n\nRequest To Clarify Certain Compliance Times \n\n\tBoeing asks that we clarify the different compliance times for replacing discrepant web panels, depending on the thickness, as specified in Table 1 of Part 1.E., Compliance, of Boeing Special Attention Service Bulletin 737-57-1289, dated June 13, 2007. (The service bulletin was referenced in the NPRM as the appropriate source of service information for accomplishing the specified actions.) Boeing states that, as written, paragraphs (g)(1) and (g)(2) of the NPRM would allow 30 months or 6,000 flight cycles, whichever is later, to replace discrepant web panels. Boeing notes that this is acceptable for discrepant web panels with a material thickness that is found to be greater than or equal to 0.037 inch, and less than 0.047 inch; however, for web panels with a material thickness of less than 0.037 inch the specified compliance time is before further flight. Boeing adds that web panels with a material thickness of less than 0.037 inch do not meet the ultimate regulatory load requirements. Boeing also asks that the related investigative and corrective actions be clarified. \n\tWe agree with Boeing that some clarification is necessary. Paragraph (g)(1) of the AD requires accomplishing all applicable related investigative and corrective actions before further flight (which includes replacing any discrepant web panels) by doing all the actions specified in the Accomplishment Instructions of the service bulletin. The Accomplishment Instructions do not clearly identify web panels with a material thickness of less than 0.037 inch; however, the web panels are clearly identified in paragraph 1.E. of the service bulletin. Paragraph 1.E. specifies replacing web panels with a material thickness of less than 0.037 inch, as specified in paragraph 3.B.7. of the Accomplishment Instructions of the service bulletin. Therefore, we have clarified paragraph (g)(1) of this AD to add that the corrective actions include replacing any web panel with a material thickness of less than 0.037 inch before further flight. We have also changed paragraph (g)(2) of this AD to clarify that the compliance time in that paragraph is separate from the compliance time specified in paragraph (g)(1). The related investigative and corrective actions are defined in the service information section of the NPRM; therefore, no change is necessary in this regard. \n\nRequest To Clarify That Additional Action Is Necessary \n\n\tBoeing also asks that we change paragraph (e) of the NPRM to clarify that additional action is necessary for operators that inspected web panels using instructions that were contained in Boeing Communication messages that were sent out on January 17, 2006, prior to the release of the referenced service bulletin. Boeing states that the messages were sent to airlines that were operating airplanes that could have a discrepant web panel. Boeing adds that following release of those messages, as part of the information being developed for release in the referenced service bulletin, it was determined that additional details were necessary to accurately define the instructions to inspect for discrepant web panels. Boeing notes that the additional details, which affect both the chemical spot test and the ultrasonic thickness inspections, have been included in the service bulletin referenced in the NPRM as the source of service information for doing the specified actions; therefore, inspections accomplished without these additional steps could result in incorrect identification of discrepant web panels. Boeing also suggests that this language be added to paragraph (e) of the AD. \n\tWe acknowledge Boeing's concerns; however, paragraph 1.E, "Compliance,'' of the referenced service bulletin specifies that the inspection instructions contained in the subject Boeing messages sent out on January 17,2006, did not include certain steps. That section specifies that the chemical spot test and ultrasonic thickness inspections must be done again by following the procedures in the referenced service bulletin. In addition, paragraph (e) of this AD states that if the actions required by this AD have been done previously, they do not need to be done again. Therefore, we have made no change to the AD in this regard. \n\nConclusion \n\n\tWe reviewed the relevant data, considered the comments received, and determined that air safety and the public interest require adopting the AD with the changes described previously. We also determined that these changes will not increase the economic burden on any operator or increase the scope of the AD. \n\nCosts of Compliance \n\n\tThere are about 31 airplanes of the affected design in the worldwide fleet. This AD affects 1 airplane of U.S. registry. The required tests and inspections take about 3 work hours per airplane, at an average labor rate of $80 per work hour. Based on these figures, the estimated cost of the AD for this U.S. operator is $240. \n\nAuthority for This Rulemaking \n\n\tTitle 49 of the United States Code specifies the FAA's authority to issue rules on aviation safety. Subtitle I, section 106, describes the authority of the FAA Administrator. "Subtitle VII: Aviation Programs,'' describes in more detail the scope of the Agency's authority. \n\tWe are issuing this rulemaking under the authority described in "Subtitle VII, Part A, Subpart III, Section 44701: General requirements.'' Under that section, Congress charges the FAA with promoting safe flight of civil aircraft in air commerce by prescribing regulations for practices, methods, and procedures the Administrator finds necessary for safety in air commerce. This regulation is within the scope of that authority because it addresses an unsafe condition that is likely to exist or develop on products identified in this rulemaking action. \n\nRegulatory Findings \n\n\tThisAD will not have federalism implications under Executive Order 13132. This AD 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. \n\tFor the reasons discussed above, I certify that this AD: \n\t(1) Is not a "significant regulatory action'' under Executive Order 12866, \n\t(2) Is not a "significant rule'' under DOT Regulatory Policies and Procedures (44 FR 11034, February 26, 1979), and \n\t(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. \n\tYou can find our regulatory evaluation and the estimated costs of compliance in the AD Docket. \n\nList of Subjects in 14 CFR Part 39 \n\n\tAir transportation, Aircraft, Aviation safety, Incorporation by reference, Safety. \n\nAdoption of the Amendment \n\nAccordingly, under the authority delegated to me by the Administrator, the FAA amends 14 CFR part 39 as follows: \n\nPART 39--AIRWORTHINESS DIRECTIVES \n\n 1. 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\n2. The FAA amends Sec. 39.13 by adding the following new AD: