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 McDonnell Douglas Model DC-9-10, DC-9-20, DC-9-30, DC-9-40, and DC-9-50 series airplanes, equipped with certain tail cone evacuation slide containers. That NPRM was published in the Federal Register on August 6, 2007 (72 FR 43578). That NPRM proposed to require modifying the tail cone slide. That NPRM also proposed to require additional tail cone drops and slide deployments, and repair if necessary. \n\nComments \n\n\tWe gave the public the opportunity to participate in developing this AD. We considered the comments received from the one commenter. \n\nRequest To Clarify Paragraph (g) of the NPRM \n\n\tNorthwest Airlines (NWA) requests that we clarify whether the 150- flight-cycle compliance time specified in paragraph (g) of the NPRM starts after the first airplane is modified or after the last airplane ismodified. Unless the FAA intends to have operators perform slide deployments while the fleet is still being modified, NWA recommends that the 150-flight-cycle clock start after the modification, or within 24 months after the effective date of the AD, whichever occurs first. NWA also states that it assumes the 150-day compliance clock in that same paragraph is intended for those operators who have already complied with the intent of the AD or who will comply very quickly after issuance of the AD. NWA states that it would be helpful if this was stated. \n\tWe agree with NWA's request for clarification. Our intent was for the operator to start and complete the tail cone modification and fly a minimum of 150 flight cycles before the additional tail cone deployment test, accomplished within 24 months after the effective date of the AD. When there are multiple airplanes, the 150 flight cycles apply to each individual airplane, and start after the modification is done to each airplane individually. \n\tScenario: An operator completes the modification on the first airplane, and then completes the minimum 150 flight cycles two months after the modification. After the operator successfully performs the tail cone slide deployment test on the first airplane, the second airplane is modified a week later. The second airplane will also be required to fly a minimum of 150 flight cycles before the deployment test of the tail cone slide. If the operator has 100 airplanes, then the operator must demonstrate a successful deployment test on 10 percent (ten) of the modified airplanes as terminating action for the AD. \n\tWe agree that the proposed 150-day compliance time specified in paragraph (g) of the NPRM needs not only to be clarified, but also revised. The 150-day requirement could impose a schedule hardship for some operators who might need more time to complete the modification. Our intent was to allow the operator time to modify the tail cone slide cover and to perform the deployment test after a minimum of 150 flight cycles after modification, and no later than 24 months after the effective date of this AD. \n\tFor all these reasons, we have revised paragraph (g) and added a new paragraph (h) to the AD to clarify the compliance time. The new paragraph (g) begins as follows: "* * * no earlier than 150 flight cycles after doing the modification required by paragraph (f) of this AD, and no later than 24 months after the effective date of this AD. * * *'' The new paragraph (h) states that operators should contact the\nManager, Los Angeles Aircraft Certification Office (ACO), FAA, if the repeat deployment cannot be performed as required by paragraph (g) of this AD. \n\nRequest for Exemption From Proposed Requirements \n\n\tNWA also requests exemption from the proposed requirements of paragraph (g) of the NPRM. NWA states that while performing testing to obtain Supplemental Type Certificate (STC) ST01967CH, it successfully performed 5 tail cone drops and slide deployments with the new design slide installation. NWA believes that the requirement to perform additional slide deployments is arbitrary, and that the 5 tail cone drops performed as part of the STC approval are sufficient to prove the design reliability of its airplanes. \n\tWe disagree with the request for exemption. The 5 tail cone drops and slide deployments that NWA did during the STC approval process did not represent the severity of the actual operating environment for the tail cone, including temperature and high takeoff and landing loads, nor did they represent repeated flight cycles with various types of contamination such as dirt and fuel. Tail cone slides must be overhauled, repacked and re-rigged every 3 years and have no other maintenance requirements in order to verify successful deployment. We have not changed the AD in this regard. \n\nExplanation of Change to Paragraph (f) and Removal of Note 1 of the NPRM \n\n\tWe have revised paragraph (f) of this AD, and have removed Note 1of this AD, to remove reference to Northwest Airlines STC ST01967CH and Northwest Airlines Drawing 9B25-41477, Revision B, dated September 14, 2006; and Northwest Airlines Drawing 9B25-90399, Revision D, dated December 21, 2006. However, we have approved Northwest Airlines STC ST01967CH as a method for modifying the tail cone slide. Operators may contact the Manager, Los Angeles ACO, for information regarding Northwest Airlines STC ST01967CH for modifying the tail cone slide, as required by paragraph (f) of this AD. \n\nConclusion \n\n\tWe reviewed the relevant data, considered the comment 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 400 airplanes of the affected design in the worldwide fleet. This AD affects about 300 airplanes of U.S.registry. The tail cone drops/slide deployments take about 16 work hours per airplane, at an average labor rate of $80 per work hour. Required parts cost about $1,300 per airplane. Based on these figures, the estimated cost of the AD for U.S. operators is $774,000, or $2,580 per airplane. \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\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 theAD Docket. \n\nList of Subjects in 14 CFR Part 39 \n\n\tAir transportation, Aircraft, Aviation safety, 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: