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 all Boeing Model 737-600, -700, -700C, -800, and -900 series airplanes. That NPRM was published in the Federal Register on December 3, 2007 (72 FR 67864). That NPRM proposed to require repetitive inspections for any cracking of or damage to the left side and right side flight deck No. 2, No. 4, and No. 5 windows, as necessary, and corrective actions if necessary. \n\nChanges Made to This AD \n\n\tWe have revised the applicability of this AD to clearly identify that Boeing Model 737-900ER series airplanes are also subject to the requirements of this AD. These airplanes were not previously identified in the applicability of the NPRM because they were type certificated after issuance of Boeing Alert Service Bulletin 737-56A1022, dated July 18, 2007, which we referenced for the applicability of the NPRM. Although these airplanes are not explicitly identified in the effectivity of the service bulletin, they are included as Group 2 airplanes in the service bulletin. \n\tWe have deleted paragraph (h)(4) of the NPRM and added a new paragraph (h) to this AD specifying that installation of metallic window blanks at cockpit eyebrow windows No. 4 and No. 5 in accordance with Supplemental Type Certificate (STC) ST01630SE terminates the initial and repetitive inspections for the flight deck No. 4 and No. 5 windows required by paragraph (f) of this AD. Incorporation of STC ST01630SE is considered a terminating action, not an alternative method of compliance (AMOC), since an AMOC can be issued only after an AD has been issued. We also have reidentified the AMOC paragraph of the NPRM as paragraph (j) in this AD. \n\nComments \n\n\tWe gave the public the opportunity to participate in developing this AD. We considered the comments received from the three commenters. \n\nSupport for the NPRM \n\n\tBoeing and AirTran Airways agree with the NPRM. \n\nRequest To Add Terminating Action \n\n\tAirTran Airways and Continental Airlines (CAL) request that we revise the NPRM to specify that installing structural plugs at cockpit eyebrow windows No. 4 and No. 5 in accordance with Boeing Service Bulletin 737-56-1017, dated May 17, 2006; or Revision 1, dated February 15, 2007; terminates the initial and repetitive inspections for the flight deck No. 4 and No. 5 windows. CAL notes that a similar statement is found in Tables 2 and 3 of Boeing Alert Service Bulletin 737- 56A1022, dated July 18, 2007. (We referred to Boeing Alert Service Bulletin 737-56A1022 as the appropriate source of service information for accomplishing the proposed requirements of the NPRM.) \n\tWe agree with the commenters and have added a new paragraph (i) to this AD accordingly. \n\nRequest To Revise the Applicability \n\n\tCAL requests that we limit the applicability of the NPRM to airplanes delivered before line number 2589. As justification, CAL states that new production airplanes do not include the flight deck No. 4 and No. 5 windows, and that Boeing is in the final stages of approving a new, improved flight deck No. 2 window, part numbers 5- 89355-87 and -88. According to CAL, the new, improved No. 2 window, which is manufactured by PPG Aerospace, is designed specifically to address the unsafe condition of the NPRM. CAL also states that Boeing plans to install the new, improved No. 2 windows on new production airplanes this year. \n\tWe do not agree to exclude any airplanes from the applicability of this AD. The affected flight deck No. 2 windows are interchangeable with the new, improved windows manufactured by PPG Aerospace; therefore, the unsafe condition could be introduced on a new production airplane if an affected No. 2 window is installed after airplane delivery. We have not changed this AD in this regard. \n\nRequest To Track Compliance Time by Flight Cycles \n\n\tAirTran Airways requests that we add anoption to this AD to allow operators to track the inspections by airplane flight cycles instead of window flight hours, provided that any used window is inspected before it is installed. AirTran Airways states that tracking compliance by a component rather than by airplane is more difficult due to the extra work and documentation generated for the removal and installation of a component. According to AirTran Airways, tracking compliance by component also increases the opportunity for human factor errors. AirTran Airways also states that tracking inspections by airplane flight cycles will accomplish the inspections within the same timeframe as proposed in the NPRM and be less of a burden. \n\tWe disagree with allowing operators to track compliance by airplane flight cycles because the unsafe condition is primarily related to window flight hours. The utilization of airplanes within the fleet varies from short to long flight hours per flight cycle. Therefore, we have not changed this AD in this regard. \n\nRequest To Revise the Compliance Time for the No. 2 Window \n\n\tCAL requests that we revise the compliance time for the initial inspection of the flight deck No. 2 window to within 36 months or 7,500 flight hours, whichever occurs first, after the window installation; or to within 24 months, for windows installed more than 36 months ago or for windows where the number of flight hours is unknown. CAL states that the NPRM, which proposes to require the initial inspection within 24 months after the effective date of this AD regardless of the age or flight time of the window, unnecessarily penalizes operators who proactively inspect and replace the No. 2 window before the AD is issued. CAL also states that, according to the wording in the NPRM, a window replaced one day before the effective date of the AD would need to be re-inspected within 24 months, but a window inspected and replaced one day after the effective date of the AD would not need to be re-inspected until 36 months or 7,500 window flight hours. \n\tWe do not agree to revise the compliance time for the initial inspection of the flight deck No. 2 window. According to paragraph (e) of this AD, an operator is responsible for having the actions required by this AD performed within the compliance times specified, unless the actions have already been done. If the initial inspection of the No. 2 window was done before the effective date of this AD in accordance with Boeing Alert Service Bulletin 737-56A1022, dated July 18, 2007, then the initial inspection does not need to be accomplished again; only the repetitive inspections would need to be accomplished in accordance with the service bulletin at the applicable interval specified in the service bulletin. \n\nIf the initial and repetitive inspections of the No. 2 window are done before the effective date of this AD, but are not done in accordance with the service bulletin, then those inspections are not acceptable for compliance with this AD unless an AMOC is issued for those prior inspections. Under the provisions of paragraph (j) of this AD, we will consider requests for approval of an AMOC if sufficient data are submitted to substantiate that prior inspections incorporate similar criteria to what is provided for in the service bulletin. Therefore, no change to this AD is necessary in this regard. \n\nRequest for an AMOC for a Parts Manufacturer Approval (PMA) Equivalent Part \n\n\tCAL states that the FAA has approved a new, improved flight deck No. 2 window designed by GKN Aerospace Transparency Systems, under PMA Holder No. PQ1250NM, Supplement 10, dated September 17, 2007. CAL also states that the new, improved No. 2 window was designed to prevent the premature failure of the window, and that the new, improved window addresses the unsafe condition of the NPRM. CAL, therefore, requests that we add a new AMOC paragraph to this AD, which would exempt the new, improved No. 2 window from the required inspections. \n\tWe do notagree to allow the PMA equivalent No. 2 window as an AMOC to the required inspections. Although the window has been approved as a PMA equivalent part, the commenter has not provided data showing that the PMA equivalent window is not susceptible to the same vinyl interlayer cracking. However, under the provisions of paragraph (j) of this AD, we will consider requests for approval of an AMOC if sufficient data are submitted to substantiate that the design change would provide an acceptable level of safety. No change to this AD is necessary 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 2,127 airplanes of the affected design in the worldwide fleet. ThisAD affects about 737 airplanes of U.S. registry. The required actions take about 2 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 U.S. operators is $117,920, or $160 per airplane, per inspection cycle. \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 \nrulemaking action. \n\nRegulatory Findings \n\n\tThis AD 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\n Accordingly, under the authority delegated to me by the Administrator, the FAA amends 14 CFR part 39 as follows: \n\nPART 39--AIRWORTHINESS DIRECTIVES \n\n1. 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: