Events Leading to the Issuance of This AD \n\tA proposal to amend part 39 of the Federal Aviation Regulations (14 CFR part 39) to include an AD that would apply to certain Cessna 300 and 400 series airplanes was published in the Federal Register as a notice of proposed rulemaking (NPRM) on July 6, 1999 (64 FR 36307). The NPRM proposed to supersede AD 75-23-08 R5, Amendment 39-5451, with a new AD. AD 75-23-08 R5 currently requires repetitively inspecting, using visual methods, the exhaust system on certain Cessna 300 and 400 series airplanes; and repairing or replacing any unserviceable parts. The actions specified in the NPRM proposed to replace the inspections and replacements that are required by AD 75-23-08 R5 with inspections and replacements containing new simplified procedures for all 300 and 400 series airplanes (models affected by the current AD plus additional models). The NPRM also proposed to revise the inspection intervals and proposed to require replacing certain unserviceable parts and removing the exhaust system for a detailed inspection. Other provisions included in the NPRM, as currently written, are: \n\n\t- Prohibiting patch-type repairs; and \n\n\t- Removing the exhaust system and sending it to a designated facility for metallic identification, airworthiness determinations, and repair or replacement of any unserviceable parts. \n\n\tThe NPRM was the result of numerous incidents and accidents relating to the exhaust systems on Cessna 300 and 400 series airplanes dating from the middle 1970's to the present, including six incidents since issuance of AD 75-23-08 R5 where exhaust problems were cited. \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. Introduction to the Comment Disposition \n\n\tThe FAA received over 350 comments on the NPRM. Many of the comments indicate that some kind of action needs tobe taken regarding the ongoing problems with the exhaust systems on Cessna 300 and 400 series airplanes. Many commenters present detailed suggestions for alternatives to the proposed actions included in the NPRM. The FAA believes that, for the most part, these suggestions and alternatives have merit, and the final rule reflects many of these suggestions and alternatives. \n\n\tThe FAA will continue to make available information that relates to the exhaust system problems on the Cessna 300 and 400 series airplanes. However, the FAA does not believe that this advisory information alone will alleviate and eliminate the unsafe condition of the exhaust system problems on the Cessna 300 and 400 series airplanes. The FAA also does not believe that continuing to only mandate the actions of AD 75-23-08 R5 will provide the safety level that is necessary for the affected airplanes. \n\n\tThe NPRM proposed to require an inspection to determine the type of material (Inconel or stainless steel) and the condition of the exhaust system. Of note is that the minimum wall thickness criteria was established as an attempt to remove from service those systems that were over 30 years old. However, the FAA did not account for those unused or recently installed exhaust systems that were manufactured over 30 years ago and either are currently held as or until recently were held as spares. The final rule accounts for this by requiring an inspection of the tailpipes 5 years after installation of an unused or overhauled exhaust system or within 100 hours time-in-service (TIS) after the effective date of the AD (the prevalent one being that which occurs later). \n\n\tIn addition, the FAA has found that Cessna has not manufactured any exhaust assemblies that are 100-percent Inconel material. Much of the confusion raised on and in opposition to the proposal stems from sending the exhaust system to a facility to get a determination on whether the system was a stainless steel or Inconel exhaust system. The different compliance times for the different systems adds to the confusion and opposition. The FAA has revised the proposal to include the same compliance times for all airplanes regardless of the exhaust system material and to remove the proposed requirement of sending the exhaust system to a specific facility for a material determination. \n\n\tThe final rule reflects other changes made based on the FAA's analysis of the comments received and all other information related to the exhaust systems on the Cessna 300 and 400 series airplanes. All changes, like the ones referenced above, will alleviate the burden upon the public as proposed in the NPRM while still providing the necessary safety level intended by this AD. \n\n\tThe following paragraphs present the comments received with the FAA's response and changes to the AD, as applicable:\n\nComment Issue No. 1: Include Alternative Proposals \n\tNumerous commenters recommend that the FAA incorporate the provisions of proposals that the Cessna Pilot's Association and Twin Cessna Flyer submitted. The commenters state that there is a need for the AD, and that these proposals provide a viable safety alternative. \n\n\tThe FAA evaluated both of these proposals, determined that many of these comments have merit, and has made changes to the final rule. Among the items in the proposals that the FAA incorporated into the final rule include: \n\n\t- eliminating the check of the system for wall thickness; \n\n\t- having the same compliance schedule for all airplanes regardless of whether the exhaust systems are made of Inconel or stainless steel; and \n\n\t- eliminating the proposed requirement of removing the exhaust system and sending it to a specific facility for a material determination. \n\nComment Issue No. 2: The Existing AD is Sufficient \n\tMany commenters state that the current actions of AD 75-23-08 R5 are sufficient to meet the necessary safety level intended by this AD for the exhaust systems of the Cessna 300 and 400 series airplanes. Several commenters state that, if AD 75-23-08 R5 was complied with in a correct and timely matter, the incidents referenced in the NPRM may not have happened. Some commenters believe that changing the inspection requirements from that already required by AD 75-23-08 R5 will cause confusion and add unnecessary costs to the inspections. One other commenter suggests that the FAA issue a Special Airworthiness Information Bulletin (SAIB) to address the requirements of the AD. The FAA does not concur that AD 75-23-08 R5 is sufficient. Analysis of the incidents and accidents pertaining to the exhaust systems on the Cessna 300 and 400 series airplanes that have occurred since the issuance of AD 75-23-08 R5 reveals the need to require different inspection requirements to meet the conditions known today. The FAA believes that the changes made to the final rule will also make the inspections easier to accomplish and will allow them to be accomplished to coincide with regularly scheduled maintenance. \n\n\tThe FAA does not concur that an SAIB should be issued instead of an AD. An SAIB is an "information only" document and has no regulatory requirement; therefore, it is not mandatory. The only vehicle the FAA has of assuring that certain actions are complied with is through the issuance of an AD. No changes have been made to this AD as a result of this comment. \n\nComment Issue No. 3: Cost Impact \n\tMany commenters state that the FAA's estimate of the cost impact upon U.S. owners/operators of the affected airplanes is incorrect. Some also believe that the FAA should have completed the Regulatory Flexibility Analysis before issuing the NPRM. Among the specific cost issues that were identified is the FAA's failure to account for the revenue lost due to airplane downtime and the fact that the cost of the proposed AD would affect the airplanes' value and make them unaffordable. \n\n\tThe FAA does not concur that the estimate of the costimpact upon U.S. owners/operators of the affected airplanes is incorrect. The FAA has no way of determining the number or extent of repairs and replacements that would be necessary based on the inspections proposed in the NPRM. Therefore, the FAA can only account for the costs of the inspections. The FAA believes it is the owners'/operators' responsibility to repair or replace parts when found damaged, regardless of whether the action is required by AD. \n\n\tThe FAA does not concur that it was necessary to complete the Regulatory Flexibility Analysis before issuing the NPRM. Having this analysis completed prior to issuing the NPRM is preferred; however, the FAA did not believe it could wait to initiate rulemaking on this subject. The FAA has until 180 days after issuance of the final rule AD action to have the completed Regulatory Flexibility Analysis in the docket file. \n\n\tThe FAA concurs that airplane downtime is not accounted for in the estimate of the cost impact. The FAAhas no way of determining the operational characteristics of each owner/operator of the affected airplanes. Therefore, estimating the lost expenses due to the affected airplanes being out of service is not possible. Even if this were possible, the safety aspects of the proposed rule would outweigh the potential lost revenue due to airplane downtime. \n\nComment Issue No. 4: V-Band Clamp Replacements \n\tThe FAA's intent was to maintain the V-band replacements from AD 75-23-08 R5. Based on this and after evaluating all the comments and information on this subject, the FAA has revised the proposal to only require replacement of the multi-band V-clamps at 500-hour TIS intervals. Inspection of the other V-band clamps is part of the exhaust system inspections required by this AD. \n\nComment Issue No. 5: Concerns With the Slip Joint Requirement \n\tMany commenters express concerns regarding the requirements of the slip joints, specifically either require (1) replacement of the old stylejoints; (2) lubrication of the slip joints; or (3) a change to the compliance time of the slip joint removal and inspection requirements. The majority of these commenters state that removing the slip joints would cause more damage than would be caused during normal usage. \n\n\tThe FAA concurs that removing the slip joints too frequently could cause damage. The FAA has determined that the necessary safety level intended by this AD will be reached by requiring the slip joints to be annually inspected for freedom of movement without removing the slip joints from the nacelle. The slip joints will be removed for inspection at each 2,500-hour TIS inspection. The FAA believes that the inspections will reveal deterioration of the older style joints and require replacement. \n\nComment Issue No. 6: Stainless Steel versus Inconel \n\tMany commenters state that the different compliance times for stainless steel exhaust systems and Inconel exhaust systems need clarification. These commentersrequest that the FAA define an "all Inconel system" since all exhaust systems consist of some stainless steel parts. Several commenters state that having different compliance times for different exhaust systems is confusing, and request that all exhaust systems be treated equally. \n\n\tThe FAA concurs that no exhaust system is made exclusively of Inconel alloy and that the current compliance times could cause confusion among those airplane owners/operators and mechanics trying to accomplish the AD. The FAA has revised the AD to provide compliance times that are applicable to all exhaust systems. This eliminates the need to send the exhaust system to an authorized facility for material determination. The FAA has revised the compliance times to coincide with regularly scheduled maintenance. \n\nComment Issue No. 7: Facilities and Personnel \n\tNumerous commenters express concern about the FAA's requirement of the qualifications of the personnel to accomplish the work and what facilities must be used to accomplish portions of this AD. These concerns include: \n\n\t- The three approved facilities would not be able to accomplish the parts evaluations and inspections on all of the affected airplanes in a timely manner; \n\n\t- Foreign airworthiness authorities that adopt an FAA AD verbatim for their countries would then require all airplanes certificated for operation in those countries to have the parts evaluations and inspections accomplished at one of the three U.S. facilities; and \n\n\t- Maintenance personnel in foreign countries with equivalent ratings to those specified in the proposed AD would not be able to accomplish the work under the current wording of this AD. \n\n\tThe FAA has evaluated these concerns and has changed this AD to include: \n\n\t- clarifying who can accomplish what actions in this AD, including a clause of "or for non U.S.-registered airplanes: the state of registry's equivalent facility in accordance with their applicable procedure"; \n\n\t- consolidating the actions of all airplanes into one compliance program so the need to send to one of the three facilities to determine the material used for the exhaust system and the condition is no longer necessary; and \n\n\t- changing the facilities required to do the repair work to any FAA-approved exhaust repair facility. \n\nComment Issue No. 8: Compliance Times \n\tMany commenters request changes to the proposed compliance times. The main reason for these proposed changes is to time the actions specified in the NPRM to coincide with regular maintenance intervals, i.e., engine overhaul and annual inspections. Several commenters also request a 10-percent adjustment on inspection compliance times. \n\n\tThe FAA has re-evaluated the compliance times and has changed the final rule to add provisions that would make the actions coincide with regularly scheduled maintenance activities. Having one compliance time for all airplanes, regardless of the exhaust system type (Inconel or stainless steel) allowed this to be accomplished. The FAA is also allowing the 10-percent adjustment allowance to allow the actions to be accomplished with other scheduled maintenance. All of these adjustments actually reflect a reduction in the burden upon U.S. operators over that proposed in the NPRM. \n\nComment Issue No. 9: Cessna Service Bulletins \n\tA few commenters suggest that the FAA issue an AD that mandates the Cessna service bulletins that relate to this subject instead of what is proposed in the NPRM. These commenters state that the actions specified in the service bulletins are adequate to address the unsafe condition. \n\n\tThe FAA does not concur. The Cessna service bulletins were not available at the time of issuance of the NPRM. Cessna has issued the following service bulletins since the NPRM:\n\n\t- Service Bulletin (SB) MEB99-8, SB MEB99-11, SB MEB99-14, and SB MEB99-15, all dated August 2, 1999. These service bulletins specify and include procedures for replacing thecrossfeed fuel lines with stainless steel cross feed lines. Each service bulletin applies to various Cessna airplane models. \n\n\t- SB MEB99-6, SB MEB99-9, and SB MEB99-12, all dated August 2, 1999. These service bulletins specify and include procedures for installing access panels to help with exhaust system inspections. Each service bulletin applies to various Cessna airplane models. \n\n\t- SB MEB99-7, SB MEB99-10, and SB MEB99-13, all dated August 2, 1999. These service bulletins specify and include procedures for installing stainless steel engine beam covers and inspecting the engine beams. Each service bulletin applies to various Cessna airplane models. \n\n\tThe FAA has determined that the best course of action is accomplishing that specified in the final rule (the actions of the NPRM as modified based on the comments received) instead of incorporating the Cessna service bulletins. Reasons include: \n\n\t- The service bulletins focus more on the protection of the affected airplanes once the exhaust system has failed; and \n\n\t- The service bulletins do not address the turbocharger installation on the firewall, including the engine exhaust pipes and the tail pipe. The leakage of exhaust gases in this area is considered the unsafe condition. \n\n\tThe FAA does believe that installing the access panels as specified in the Cessna service bulletins will aid in the repetitive exhaust system inspections. The FAA has added a note to the AD to include this access panel information. No other changes to the final rule have been made as a result of these comments. \n\nComment Issue No. 10: Supplemental Type Certificates and Parts Manufacturer Approvals \n\tTwo commenters suggest that airplanes that have been modified through the incorporation of Riley Aviation supplemental type certificates (STC's) not be subject to this AD, or that the FAA wait for the Riley Aviation solution to the unsafe condition for those affected airplanes. In addition, two commenters request explanation related to installation requirements of STC and parts manufacturer approval (PMA) parts as they relate to the exhaust systems on Cessna 300 and 400 series airplanes. \n\n\tThe FAA does not concur. The Riley Aviation modification through STC's utilizes design parts that are equivalent to the original type design. The FAA has determined that exhaust systems that have been modified through Riley Aviation STC's are subject to the unsafe condition addressed by this AD. Although Riley Aviation may indeed develop actions to address this unsafe condition, the FAA cannot delay AD action waiting for actions that have yet to be developed or approved. However, any owner/operator of the affected airplanes can present data to show that their exhaust systems utilize design parts that should not be subject to this AD by submitting an alternative method of compliance request in accordance with the procedures specified in this AD. The FAA will evaluate the merits of each request and either grant or deny the alternative method of compliance. No changes have been made to this AD as a result of these comments. \n\nComment Issue No. 11: Maintenance and Pilot Training \n\tNumerous commenters state that part of the safety problem comes from inadequate maintenance and the need for pilot training. These commenters suggest that additional pilot training and mandated preflight checks could alleviate the unsafe condition. Many commenters feel that the FAA is arbitrarily punishing the majority of owners/operators of the affected airplanes because of the inadequate maintenance practices of a few operators. These commenters state that the existing maintenance requirements are adequate to provide the necessary safety level intended by this AD, and that if the FAA enforced the existing rules there would not be any problems. \n\n\tThe FAA concurs that pilot training and preflight checks could reduce the potential for the unsafe condition from occurring. However, the FAA has determined that the unsafe condition is in part the result of maintenance practices that are not adequate to provide the necessary safety level intended by this AD. The FAA has determined that the condition should be addressed through inspections and exhaust system repair and parts replacement. No changes to this AD have been made as a result of these comments. \n\nComment Issue No. 12: Part 135 Operations \n\tFive commenters suggest that the FAA exempt those airplanes that are regulated by a maintenance program such as that required for airplanes operating in accordance with part 135 of the Federal Aviation Regulations (14 CFR part 135). The commenters state that such maintenance programs already require the actions specified in the NPRM. \n\n\tThe FAA partially concurs. The FAA agrees that certain actions may already be accomplished by maintenance programs required under 14 CFR part 135. A note has been added to this AD that specifies that the owners/operators of those airplanes operating under 14 CFR part 135 may have already had the actions of this AD incorporated, and appropriate "unless already accomplished" credit could be taken for the applicable portion of this AD. The FAA cannot exempt these airplanes from this AD because operators are not obligated to fly predominately in part 135 operations and could operate under part 91 of the Federal Aviation Regulations (14 CFR part 91).\n\nComment Issue No. 13: Leak Testing for Cracks \n\tSeveral commenters suggest that the FAA allow a leak test to detect cracked exhaust system parts. The FAA presumes that these commenters would prefer the leak test over the currently proposed pressure tests. \n\n\tThe FAA has determined that the pressure checks required in this AD will detect cracks, pinholes, or other damage, and that leak testing is not required. Owners/operators of the affected airplanes can submit an alternative method of compliance to the FAA that contains appropriate data and information to show that an equivalent level of safety to this AD would be obtained through leak testing. No changes to this AD have been made as a result of these comments. \n\nComment Issue No. 14: Firewall, Bulkhead, Engine Beams, and Fuel Lines \n\tMany commenters request modification or explanation concerning the need to inspect the firewall, bulkheads, engine beams, and fuel lines. The commenters suggest that the FAA only require inspection of the fuel lines and areas behind the firewall if damage has occurred or work has been done in the firewall area. These commenters also request the FAA define the acceptable limits of corrosion in the engine beams and associated structure. \n\n\tThe FAA maintains that the firewalls, canted bulkheads, and engine beams should be inspected and has written the compliance time of these inspections to allow them to be accomplished during the regular maintenance schedule that coincides with other inspections or repairs. The FAA concurs that the fuel lines should only be inspected upon condition, and this AD has been changed to only require the inspections if there is evidence of past damage to the firewalls, canted bulkheads, and engine beams. The fuel lines will be replaced if damage is found.\n\nComment No. 15: Wall Thickness \n\t Numerous commenters state that the wall thickness inspection is unworkable due to the thickness limit of .025 inches. Some of these commenters are concerned that some new parts would not pass the thickness requirement. The commenters recommend specific thickness of .049 inches for the "wye" and .035 inches for the tailpipe. \n\n\tAfter further analysis of the wall thickness inspection requirement, the FAA has determined that overly thin parts will be detected and corrected in the general airworthiness inspections required on the "wye" and tailpipe. Therefore, the FAA has deleted this requirement from this AD. \n\nComment Issue No. 16: Install an Insulation Blanket \n\tFive commenters suggest installing an insulation blanket (such as Kevlar) as an alternative to the actions specified in the NPRM. Another commenter states that installing this insulation blanket would complicate inspections. \n\n\tThe FAA concurs that the addition of an insulation blanket could relieve some of the potential difficulties, although it would only alleviate the condition and would not provide the necessary safety level intended by this AD. Also, the FAA concurs that installing an insulation blanket could make already required inspections difficult to accomplish. Based on this, the FAA has determined that the installation of an insulation blanket will not meet the necessary safety level intended by this AD and the FAA has not incorporated this suggestion. No changes have been made to this AD as a result of these comments. \n\nComment Issue No. 17: Inadequate Maintenance Practices \n\tSeveral commenters state that the NPRM lacks test and inspection procedures. These commenters suggest specific changes or additions to these inspection methods, including: \n\n\t1. make a video tape of the inspection process; \n\n\t2. require an inspection for exhaust stains; \n\n\t3. specify wear rates and leakage rates on the pressure tests; \n\n\t4. include information about the confusion concerning the various types of slip joints utilized on the affected airplanes; \n\n\t5. clarify what is meant by an exhaust repair station; \n\n\t6. require only visual inspections; \n\n\t7. clarify the pressure check requirements because this check is too judgmental, and that an unacceptable leak is not identified; \n\n\t8. add a "tap test" to check parts; and \n\n\t9. clarify and mandate assembly and torquing sequence requirements. \n\n\tThe FAA concurs with some of the recommendations, as follows: \n\n\t1. The FAA believes a video could be a great visual aid in illustrating the inspection, but the FAA has determined that it could only be an informational aid and cannot be mandated by AD action. No changes have been made to this AD as a result of this comment; \n\n\t2. The FAA does not consider the exhaust stains to be a reliable indication of whether exhaust problems exist. Stains could be a sign to look further, but not a true indicator. No changes have been made to this AD as a result of this comment; \n\n\t3. As specified in Comment Issue No. 13, the FAA has determined that the pressure checks required in this AD will detect cracks, pinholes, or other damage, and that leak testing is not required. Owners/operators of the affected airplanes can submit an alternative method of compliance to the FAA that contains appropriate data and information to show that an equivalent level of safety to this AD would be obtained with this method. No changes have been made to this AD as a result of this comment; \n\n\t4. The FAA has revised this AD to only require removal of the slip joints during the 2,500-hour TIS engine overhaul inspection; \n\n\t5. The FAA has revised the AD to specify an FAA-approved exhaust system repair facility. The means a facility that has FAA approval to work on exhaust systems; \n\n\t6. Due to the extent and location of the damage found on the Cessna 300 and 400 series airplanes, the FAA has determined that visual inspections will not provide the necessary safety level intended by this AD. No changes have been made to this AD as a result of this comment; \n\n\t7. The pressure check is intended to identify leakage that is considered to be excessive or in locations where it will help identify crack, pinholes, or damage. Any application of the pressure test will be judgmental; however, many owners/operators have already accomplished this test on the affected airplanes with success so the FAA has determined that authorized or appropriate maintenance personnel can accomplish the procedure repeatedly with acceptable results. No changes have been made to this AD as a result of this comment; \n\n\t8. The FAA concurs that a "tap test" may be helpful in identifying damaged parts, however, the FAA has determined that this procedure is not definitive and any suspect part should be further investigated. No changes have been made to this AD as a result of this comment; and \n\n\t9. After re-examining the procedures and information in the maintenance manuals and service information for the affected airplanes, the FAA has determined that the assembly and torquing techniques are acceptable to meet the necessary safety level intended by this AD. No changes have been made to this AD as a result of this comment. \n\nComment Issue No. 18: Incorporate a Design Change \n\tMany commenters recommend that the FAA incorporate a design change to the exhaust systems rather than requiring repetitive inspections and testing. One commenter states that various failure modes of the system should be analyzed and that various system changes should be implemented to prevent failure. Five commenters suggest that adding provisions to isolate the crossfeed lines or adding crossfeed valves couldbe a proposed solution to the problem. Each of the other commenters recommend at least one of the following: \n\n\t- installing a fire detector system; \n\n\t- incorporating a "tell tale" patch that changes color with heat exposure, or using paint that changes color when exposed to heat; \n\n\t- incorporating heat shields to protect the fuel lines that are behind the firewall from the effects of the exhaust heat; and \n\n\t- adding heat shields to the firewall. \n\n\tThe FAA concurs that adding a design change would be a more desirable solution to the exhaust system problems on the Cessna 300 and 400 series airplanes rather than relying on repetitive inspections and testing to detect any problems. The FAA reviewed many of the design ideas presented above, and found that they are designed to mitigate the effects of an exhaust system failure, but none prevent failure of the exhaust system. The FAA currently knows of no such design changes that would provide the same safety level as those actions in this AD. The FAA will look at any design changes on an individual basis if they are submitted as an alternative method of compliance in accordance with the procedures specified in this AD. No changes have been made to this AD as a result of this comment.\n\nComment Issue No. 19: V-Band Clamps \n\tOne commenter recommends that the FAA change the word V-band clamps in paragraph (g) of the NPRM to multi-segment V-band clamps. This commenter states that this was an oversight by the FAA. \n\n\tThe FAA concurs and has revised this AD accordingly. \n\nComment Issue No. 20: All Airplanes Should Not Be Affected \n\tFive commenters suggest that there are design differences in the affected airplanes and believe that this AD should not apply to all airplanes. One commenter states that less demand is placed on the exhaust system of unpressurized airplanes and this AD should only apply to pressurized airplanes. \n\n\tThe FAA's analysis and interpretation of the service history on the exhaust systems of the Cessna 300 and 400 series airplanes does not indicate that certain designs are more/less susceptible to the exhaust system problems than others. No changes have been made to this AD as a result of this comment. \n\nComment Issue No. 21: Lesser Requirements for Newer Exhaust Systems \n\tSeveral commenters believe that less stringent initial inspection requirements should exist for airplanes with newer exhaust systems installed. The commenters do not feel that the potential for damage exists for airplanes with exhaust systems that have not been in service for very long. \n\n\tThe FAA sees merit in this comment and has re-evaluated the compliance time of the initial inspection for cracks, corrosion, holes, or distortion, which is the inspection that requires removal of the tailpipes. The FAA has determined that the initial inspection compliance time should read "upon the accumulation of 5 years since installing a new or overhauled exhaust system or within the next 100 hours time-in-service (TIS) after the effective date of this AD, whichever occurs later." \n\n\tThe FAA has revised the AD accordingly. \n\nComment Issue No. 22: Certification Process of Exhaust Systems \n\tOne commenter believes that the FAA is changing the certification process of exhaust systems because the requirements of this AD were not required at the time the airplanes were type certificated.\n \n\tThe FAA does not concur. The exhaust systems that were certificated with the airplane met all design criteria at the time of certification. are not available to the field or the current maintenance procedures are AD's are the vehicle that the FAA uses to mandate modifications, inspections, etc. to correct an unsafe condition that is caused by airplane usage (fatigue), quality control, or maintenance problems (where the procedures to accomplish such maintenance not meeting the necessary safety level). The FAA has determined that the current maintenance procedures for the exhaust systems of the Cessna 300 and 400 series airplanes, including those required by AD 75-23-08 R5, are not adequate to eliminate the unsafe condition. No changes have been made to this AD as a result of this comment. \n\nComment Issue No. 23: Welds and Weld Repairs \n\tSeveral commenters express opinions concerning welds and the use of weld repairs in the NPRM. The comments vary and include the following: \n\n\t- patch welds should be banned; \n\n\t- patch welds should be retained; \n\n\t- inlay weld repairs should be allowed; \n\n\t- multi-seam welds should be defined; \n\n\t- butt welds are a better type of weld; \n\n\t- no welds should be allowed; and \n\n\t- patch or multi-seam weld repairs should not be left in service for 500 hours TIS and should be removed after 100 hours TIS. \n\n\tThe FAA has further examined the subject of welds on the exhaust systems as a method of repair and has incorporated the following into this AD: \n\n\t- overlay patch-type and parallel multi-seam weld repairs will not be permitted; \n\n\t- inlay patch repairs and multi-seam welds at the joints that are similar to the original construction are acceptable;\n\n\t- inspection schedules have been adjusted; and \n\n\t- removal of patch and multi-seam welds will not be required at 100 hours TIS, and will be inspected on condition until removed with the rest of the exhaust system.\n\nComment Issue No. 24: Exhaust System Removal Requirement\n\tOne commenter recommends that the FAA remove paragraph (i) from the NPRM. This paragraph specifies removal of the exhaust system from the slip joints and specifies the system be sent to an exhaust repair facility to be inspected for serviceable condition with accomplishment of necessary repairs. The FAA infers that the commenter believes that these requirements are not necessary. \n\n\tThe FAA does not concur. Based on its analysis of all information related to this subject, the FAA has determined that the removal, inspection, and possible repair requirements arenecessary to reach the necessary safety level intended by this AD. The FAA has revised the compliance time to coincide with engine overhauls, when the system is removed for other reasons, thereby reducing the downtime of the airplane. \n\nComment Issue No. 25: No Compelling Safety Issues \n\tFive commenters state that there are no compelling safety issues driving this AD action. These commenters further explain that this is evidenced through the AD process delays and the amount of time it took the FAA to issue the NPRM. The FAA infers that the commenters would like the NPRM withdrawn. \n\n\tThe FAA does not concur. The FAA has determined that an unsafe condition exists and this condition must be corrected. No changes have been made to this AD as a result of this comment. \n\nComment Issue No. 26: No Guarantee That the AD Will Work \n\tOne commenter states that there is no guarantee that the actions specified in the NPRM will eliminate the unsafe condition on the affected airplanes. The FAA infers that the commenter wants the NPRM withdrawn. \n\n\tThe FAA believes that, based on its analysis and evaluation of all available information related to this subject, the actions in this AD address items that have directly contributed to exhaust system incidents and accidents on the Cessna 300 and 400 series airplanes. The FAA also believes that the final rule AD (with the changes made to the NPRM) will be easier to comply with than AD 75-23-08 R5.\n\nComment Issue No. 27: Impossible to Comply With the AD \n\tOne commenter states that Environmental Protection Agency (EPA) requirements make it impossible to comply with the NPRM. The commenter expresses that this is due to the requirement to use certain solvents that the EPA has banned. \n\n\tNo banned substances are required to accomplish this AD. No changes have been made to this AD as a result of this comment.\n\nComment Issue No. 28: Extend the Comment Period \n\tOne commenter requests an extension to the comment period to allow persons to comment. The commenter states that this is necessary because the existence of the NPRM was not widely known. \n\n\tThe FAA does not concur. Based on the fact that over 350 comments were received, the FAA believes that it was widely known that the NPRM was issued and available. The FAA is aware that several owner associations sent their members individual letters advising them of the content and availability of the NPRM, and encouraging the owners to comment. In addition, the FAA is aware of several news articles that publicized the proposed action. The FAA has determined that there was adequate time to comment on the NPRM. No changes have been made to this AD as a result of this comment. \n\nComment Issue No. 29: More Information on the Accident Airplanes \n\tOne commenter requests more information on the accidents referenced in the NPRM. The FAA infers that the commenter does not believe the action is justified based on the information provided in the NPRM. The commenter is requesting information such as the age of the airplanes, the maintenance of the airplanes, the frequency in which the airplanes were flown, the States where the accidents occurred, any temperature swings that were involved, and the provider of the failed parts. \n\n\tThe FAA did a thorough investigation and examination of all the information available on the exhaust system failures of the Cessna 300 and 400 series airplanes, and has determined that the explanation presented in the NPRM adequately explained the situation. No changes have been made to this AD as a result of this comment. \n\nComment Issue No. 30: Exhaust System Time Is Not Always Recorded \n\tOne commenter states that, although required by FAA regulations, exhaust system component time is not always recorded or recorded correctly. The commenter states that improper maintenance and recordkeeping can negate any mandated action. The commenter makes no suggestion as to modifying or eliminating this AD action. \n\n\tNo changes have been made to this AD as a result of this comment. \n\nComment Issue No. 31: Exhaust Systems Have a Limited Life \n\tOne commenter states that exhaust system components have a limited life. This commenter believes that the FAA should require replacement of the exhaust system at a certain time of hours TIS. \n\n\tThe FAA concurs that exhaust systems have a limited life. However, the utilization differences between operators and the environment where the airplanes are operated contribute to the condition. For these reasons, a definite life limit on the exhaust systems could not be established and the FAA is requiring repetitive inspections and tests to assure that the condition of the systems is adequate. No changes have been made to this AD as a result of this comment.\n\nComment Issue No. 32: Apply a Corrosion Standard \n\tSeveral commenters suggest that the FAA should incorporate a 10-percent corrosion standard for the corrosion inspection of the engine beams and bulkhead. These commenters state that the proposed AD will require structural repair if any corrosion is found on the engine beams, canted bulkhead, or firewalls. \n\n\tThe FAA concurs that a reasonable standard should be applied. Revisions have been incorporated that require further investigation if corrosion or damage is found during the inspections. This includes holes or defects in the structural components. A 10-percent material thickness requirement for engine beam damage has been included in the AD. \n\nComment Issue No. 33: Visual Examination and Pressure Tests Are Adequate \n\tMany commenters believe that visual examination and pressure tests of the exhaust systems are adequate to meet the necessary safety level intended by this AD. These commenters state that they have found defects by visual and pressure checking. \n\n\tThe FAA does not concur. Although visual examination and pressure tests will reveal defects, many defects may go undetected if only these tests are utilized. No changes have been made to this AD as a result of this comment. \n\nThe FAA's Determination \n\tAfter careful review of all available information related to the subject presented above, the FAA has determined that air safety and the public interest require the adoption of the rule as proposed except for the changes discussed above in the comment disposition and minor editorial corrections. The FAA has determined that these changes and minor corrections will not change the meaning of this AD and will not add any additional burden upon the public than was already proposed. In fact, the changes made based on the comments received will actually reduce the burden that was originally proposed in the NPRM. \n\nComments Invited \n\tAlthough this action is in the form of a final rule that was preceded by notice and opportunity for comment, public comments are again invited on this rule. The FAA has determined that because of the large number of comments received on the proposed rule and the controversial nature of the situation, the public should be provided an opportunity to comment on the changes being made in this final rule. In addition, the FAA is in the process of completing a regulatory flexibility analysis for this action. The FAA anticipates completion of the analysis well within 180 days after issuance of this AD and will accept comments on the analysis at any time, even after the comment closing date for comments on this final rule. The FAA is particularly interested in receiving factual information on alternative means of compliance with the AD as well as the regulatory flexibility analysis. Interested persons are invited to comment on this rule by submitting such written data, views, or arguments as they may desire. Communications should identify the Rules Docket number and be submitted in triplicate to the address specified above. 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\tComments are specifically invited on the overall regulatory, economic, environmental, and energy aspects of the rule that might suggest a need to modify 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 No. 97-CE-67-AD." The postcard will be date stamped and returned tothe commenter. \n\nCost Impact \n\tThe FAA estimates that 6,500 airplanes in the U.S. registry will be affected by this AD. The cost of the inspections will be as follows at an average labor rate of approximately $60 per hour. The cost of any necessary repair depends on the extent of the rework and replacement needed based on the results of the inspections. \n\n\t- The repetitive visual inspections of the exhaust system will take approximately 3 workhours to accomplish, with a labor cost of $180 per airplane for each inspection; \n\n\t- The repetitive visual inspections of the removed tailpipes will take approximately 1 workhour per tailpipe to accomplish, with a labor cost of $120 per airplane for each inspection; \n\n\t- The inspection of the engine beams and canted bulkheads, as a result of damage to the tailpipes, will take approximately 3 workhours to accomplish, with a labor cost of $180 per airplane; \n\n\t- The inspection of the fuel tubing behind the firewall, as a result of damage to the tailpipes, engine beams, and canted bulkheads, will take approximately 16 workhours to accomplish, with a labor cost of $960 per airplane; \n\n\t- The replacement of the fuel tubing, if necessary, will take approximately 30 workhours to accomplish, with a labor cost of $1,800 per airplane; \n\n\t- The requirement of removing exhaust system prior to shipping to an approved facility will take approximately 8 workhours, with a labor cost of $480 per airplane. The cost of shipping the exhaust system to the facility and the inspections by the facility is estimated at $500 per airplane; \n\n\t- The repetitive pressure test is estimated to take 1 workhour, with a labor cost of $60 per airplane; and \n\n\t- The multi-band V-clamp replacement is estimated to take 1 workhour, with a labor cost of $60 per airplane. \n\n\tThe total cost impact on the U.S. operators for the initial inspections is estimated to be $28,210,000, or $4,340 per airplane. The maximum expense for full exhaust partsreplacement is estimated to be approximately $60,000 per airplane. These figures do not take into account the costs of any repetitive inspections or repairs or replacements that may be necessary. The FAA has no way of determining the number of repetitive inspections an owner/operator will incur over the life of the airplane, or the extent of the repairs and replacements that may be necessary for any affected airplane. \n\nCompliance Time of This AD \n\tCertain repetitive inspections of this AD are presented in both calendar time and hours time-in-service (TIS). The unsafe condition specified in this AD is a result of the stress cracking and/or corrosion that results over time. Stress corrosion starts as a result of high local stress incurred through operation of the affected part (the exhaust systems). Corrosion can then develop regardless of whether the airplane is in operation. The cracks may not be noticed initially as a result of the stress loads, but could then progress as aresult of corrosion. The stress incurred during flight operations (while in-flight) or temperature changes (either while in-flight or on the ground) could then cause rapid crack growth. In order to assure that these stress corrosion cracks do not go undetected, a compliance time of specific hours TIS and calendar time (whichever occurs first) is utilized. \n\nRegulatory Flexibility Analysis \n\tThe FAA believes that this regulation may have a significant economic impact on a substantial number of small businesses. Due to the urgent nature of the safety issues addressed, the FAA was not able to complete a regulatory flexibility analysis prior to issuing the NPRM. As stated in the NPRM, the FAA will complete the final regulatory flexibility analysis within 180 days after issuance of this AD. Copies of this analysis may be obtained at that time at the Central Region, Office of the Regional Counsel, Attention: Rules Docket No. 97-CE-67-AD, 901 Locust, Room 506, Kansas City, Missouri 64106. \n\nRegulatory Impact \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 various levels 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) may have a significant economic impact, positive or negative, on a substantial number of small entities under the criteria of the Regulatory Flexibility Act. \n\nList of Subjects in 14 CFR Part 39 \n\tAir transportation, Aircraft, Aviation safety, Safety.\n\n Adoption of the Amendment \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\t1. The authority citation for part 39 continues to read as follows: \n\tAuthority: 49 U.S.C. 106(g), 40113, 44701.\n\n§ 39.13 (Amended) \n\t2. Section 39.13 is amended by removing Airworthiness Directive (AD) 75-23-08 R5, Amendment 39-5451, and by adding a new AD to read as follows: