A proposal to amend part 39 of the Federal Aviation Regulations (14 CFR part 39) to include an airworthiness directive (AD) that is applicable to certain Boeing Model 747 series airplanes was published as a supplemental Notice of Proposed Rulemaking (NPRM) in the Federal Register on December 11, 1995 (60 FR 63465). That action proposed to require modification of the left and right inboard elevator servo assemblies and the hydraulic routing of the right inboard elevator power control package (PCP). \n\nComments \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.\n\n Requests to Extend the Compliance Time for Hydraulic Tubing Modification \n\n\tTwo commenters request that the compliance time for accomplishing the proposed modification of the hydraulic tubing of the right inboard elevator PCP be extended from the proposed 1 year to 18 months. Thecommenters state that such an extension will allow the modification to be accomplished during a regularly scheduled "C" or "D" check, and thereby eliminate any additional expenses that would be associated with special scheduling. \n\n\tOne of these commenters notes that it had submitted an identical request to the FAA during the comment period for the original NPRM, but the FAA denied this request, in part, because it "determined that a heavy maintenance visit is not required to accomplish the modification." The commenter states that the proposed modification requires draining and purging of the stabilizer fuel tank, which is not an activity suited for a line or field maintenance environment. Additionally, access to remove the elevator actuator and modify the hydraulic tubing on the stabilizer aft spar would be difficult to accomplish during line maintenance or at a field maintenance facility. \n\n\tThe FAA does not concur with the commenters' request to extend the compliance time. The FAA agrees that, due to the complexity of the modification, it should be performed at a facility where special equipment and trained maintenance personnel will be available, if necessary. However, the FAA points out that the compliance time of one year was developed in consideration of not only the degree of urgency associated with addressing the unsafe condition, but such factors as the manufacturer's recommendations, the availability of required parts, and the practical aspect of installing the required modification within an interval of time that parallels normal scheduled maintenance for the majority of affected operators. In consideration of all of these factors, and in consideration of the amount of time that has already elapsed since issuance of the original NPRM, the FAA has determined that further delay of this modification is not appropriate.\n\n\tHowever, under the provisions of paragraph (c) of the final rule, the FAA may approve requests for adjustments to the compliance time if sufficient data are submitted to substantiate that such an adjustment would provide an acceptable level of safety. \n\nRequests to Extend to the Compliance Time for Servo Assembly Modification \n\n\tSeveral commenters request that the compliance time for accomplishing the proposed modification of the left and right servo assemblies of the inboard elevator PCP be extended from the proposed 3 years to 5 years or, preferably, to 7 years. Some of these commenters contend that, due to the complexity and proprietary technology used to manufacture the servo valve, Parker (the original equipment manufacturer/supplier of the assemblies) is the only facility that is qualified to perform the rework and retrofit that would be required by the proposed modification. These commenters state that Parker's current facility would be unable to support modification of the affected fleet within the proposed 3-year compliance time; therefore, additional time will be necessary for compliance. Onecommenter contends that, even if other maintenance facilities are available to accomplish the modification, a sufficient inventory of parts to support multiple repair facilities does not currently exist.\n \n\tThe FAA does not concur. In developing an appropriate compliance time for this modification, the FAA specifically considered the availability of required parts as well as the process necessary for modifying the affected fleet in an orderly and timely manner. The FAA maintains that other facilities will be able to support maintenance of the PCP's, even if the spool valve units must be modified only by Parker. In consideration of all of these items (as well as the revision to the applicability of this requirement, as explained below), the FAA finds that a compliance time of 3 years for this modification is appropriate. However, under the provisions of paragraph (c) of the final rule, the FAA may approve requests for adjustment of the compliance time if sufficient data are submitted to substantiate that such an adjustment would provide an acceptable level of safety.\n\n Requests to Reconsider Limiting the Applicability of Servo Assembly Modification \n\n\tSeveral commenters request that the applicability of proposed paragraph (b) (which would require the modification of the left and right servo assemblies of the inboard elevator PCP) be limited to only Model 747-200 and -300 series airplanes having cumulative line number (C/L) 696 and subsequent. These commenters request that airplanes having C/L 001 through 695 (which are commonly referred to as "classic" models) and Model 747-400 series airplanes, be exempt from this requirement. These commenters bring up three reasons to justify this request. \n\n\t1. No history of service actuator assembly failures on "classic" models. Several commenters state that the servo assembly modification is unjustified for "classic" model airplanes because the entire Model 747 fleet has accumulated over 87 million flight hours,during which time, there have been no reports of uncommanded elevator movement on the "classic" model airplanes. These commenters point out that they submitted an identical request to the FAA during the comment period for the original NPRM, but, again, the FAA denied the request. As part of the reason for its denial, the FAA indicated that only a small percentage of airplanes are equipped with a flight data recorder that records the position of the elevator; therefore, if an operator elects to record only the control column position and an uncommanded elevator motion occurred, the incident may not be recorded, due to the flight crew's inability to confirm the anomaly. However, several of the commenters point out the flight crew can quickly ascertain the cause of a sudden pitching moment, since the elevator position is indicated on the flight deck for the flightcrew to see. Another commenter states that, even if operators have elected not to record the control surface position, it still does not negate the fact that there have been over 87 million flight hours of safe flight of the Model 747 fleet on which the proposed modification has not been installed. Therefore, the commenters request that, when determining whether the current servo configuration is safe, the FAA should also consider this long history of absence of reports of uncommanded elevator motion. \n\n\t2. Potential structural damage concerns on Model 747-100, -100B SUD, -200, -300, SR, and SP series airplanes up to and including C/L 695. One commenter points out that it submitted a similar request, along with a justifying probability analysis, to the original NPRM. That analysis was conducted to demonstrate that the aft fuselage structural limit load cannot be exceeded for the "classic" model airplanes for any valve jam in any portion of the flight envelope. As a result of that analysis, the commenter states that the probability of exceeding the structural limit of the aft fuselage on these airplanes is less than 1 x10-9. The FAA did not concur with that request, and indicated that the analysis was based "on a sampling that was much too small from which accurate statistical conclusions could be drawn that would be representative of the fleet." The commenter states that this FAA statement and others in the Discussion section of the preamble to the supplemental NPRM indicate that clarification is necessary. The commenter notes that, the structural and systems configurations of "classic" model airplanes are such that, the maximum capability of the hydraulic system cannot induce a valve pressure on that airplane that exceeds structural limit loading; this point does not rely on the statistical sampling presented for the net valve differential pressure. The commenter maintains that the key point of the analysis it submitted previously is that only Model 747-200 and -300 series airplanes having C/L 696 and subsequent require the retrofit for potential structural damage concerns.3. Hydraulic tube change eliminates the possibility of high pressure spike for Model 747-400 series airplanes. Several commenters request that Model 747-400 series airplanes be removed from the applicability of proposed paragraph (b) and not be subject to the servo modification requirement. One commenter states that a high pressure spike in hydraulic system 4 was identified as the source of the valve movement that caused the initial incident on which this AD action is based; the commenter maintains that the hydraulic tubing modification required by proposed paragraph (a) of the AD will remove the possibility of such high pressure spikes. Therefore, the servo assembly modification would be unnecessary. Two commenters present test results and analyses defending this position. \n\n\tCertain commenters submitted a similar request, along with analysis of data, to the original NPRM, but note that the FAA denied it. As reason for its denial, the FAA concluded that the data submitted "donot substantiate. . . that routing the hydraulic system 3 to the sensitive side of the servo valve would preclude uncommanded elevator deflection." One of the commenters believes that the FAA drew these conclusions based on hydraulic pressure data for only one Model 747-400 airplane; the commenter contends that such data are too small for the FAA to draw accurate conclusions. In light of this, the commenters request that the proposed modification of the servo assembly be deleted until the FAA conducts further studies of hydraulic systems 3 and 4 pressure fluctuations on affected airplanes. \n\n\tThe FAA has reconsidered its previous position and partially concurs with the commenters' request. However, a clarification of certain points associated with the data that were submitted by these commenters in support of their initial request must first be addressed. \n\n\tAs for Item 1, indicated above, the FAA has considered the absence of reports of uncommanded elevator motion, as broughtup by these commenters. The FAA finds that an absence of reports may be due to the fact that a flight data recorder is not installed on all airplanes that records the elevator surface movement. However, the FAA agrees with the commenters that the recording of the elevator time history is not the sole determination in the reporting of an event. The FAA also agrees with the commenters that the occurrence of uncommanded elevator movement would most likely be quickly ascertained by the flight crew. However, the FAA still maintains that the ability to confirm the anomaly may not exist. \n\n\tAs for Item 2, indicated above, the FAA has re-reviewed the probability analysis that Boeing submitted. Based on Boeing's clarification of the previously submitted data, the FAA has determined that the "classic" model airplanes should not be subject to the modification of the servo assembly. The FAA has determined that the "classic" model airplanes have sufficient structural strength to sustain a potential valve jam in the elevator PCP. Based on the configuration and loading of the "classic" model airplanes, the limit load cannot be exceeded with this type of valve jam. While the possibility of degradation in the controllability of these airplanes still exists, it is extremely improbable. \n\n\tIn addition, the FAA has determined that Model 747-200 and -300 series airplanes having C/L 696 and subsequent, should continue to be subject to the requirements of paragraph (b) of the AD. The FAA bases this determination on the fact that the limit load capability of the Model 747- 200 and -300 series airplanes having C/L 696 and subsequent, commonly referred to as "common tail classic" model airplanes, is similar to that of the "classic" model airplanes, but have a similar control configuration as that of Model 747-400 series airplanes. Due to the changes made in the configuration of the "common tail classic" model airplanes, uncommanded elevator motion could cause structural loadingon these airplanes, which could result in structural damage and reduced controllability of the airplane. \n\n\tAs for Item 3, indicated above, Boeing has conducted further testing, since issuance of the supplemental NPRM. The FAA has reviewed the results of this testing and has determined that the data presented does not adequately support the assertion that the high pressure spike in the hydraulic system 4 is the only potential cause of a secondary slide jam; or a pressure fluctuation will not occur in the hydraulic system 3, which could lead to a potential uncommanded valve movement. \n\n\tIn addition, the FAA has re-reviewed the data submitted by the commenters and maintains that: \n\n\t The analysis is based on a sampling size that is too small from which accurate statistical conclusions can be drawn that would be representative of the fleet. \n\t\n The variability in the data and the tests for normality indicate that the data depart from the mean. \n\t\n Due to the sampling size ofthe data, a strong argument supporting normality cannot be inferred. \n\t\n The flow rate and differential pressures used by the commenters in the analysis were not substantiated to be the worst case scenario. \n\n\t Uncommanded elevator motion may occur on all Model 747 series airplanes if the servo valve secondary slide moves to the valve's internal stop, regardless of hydraulic system routing. \n\tTherefore, the FAA finds that accomplishment of the hydraulic tubing modification alone, as required by paragraph (a) of the AD, does not adequately eliminate the possibility of crossflow in the servo valve on Model 747-400 series airplanes. The FAA acknowledges the possibility that uncommanded valve motion may be reduced following the installation of the hydraulic tubing modification on Model 747-400 series airplanes. However, the FAA finds that the data presented, as well as service history, do not adequately demonstrate that uncommanded motion is the only potential cause of a secondaryslide jam, or that a pressure fluctuation will not occur in hydraulic system 3 and lead to uncommanded movement of the valves.\n \n\tIn light of the all of factors discussed above and the review of all of the data submitted, the FAA has determined that the servo assembly modification is warranted only for: \n\n\t1. Model 747-200 and -300 series airplanes having C/L 696 and subsequent (the "common tail classic" model airplanes) and \n\n\t2. the Model 747-400 series airplanes. \n\n\tThe applicability of the final rule and of paragraph (b) have been revised accordingly. \n\n\tIn addition, the FAA has revised the cost impact information, below, to indicate that the number of airplanes affected by this final rule has been reduced. \n\nRequest to Require Only Certain Type of Testing \n\n\tOne commenter requests that paragraph (b) of the proposed AD be revised to require that the servo valve be tested only for the crossflow neutralization improvement feature in accordance with Parker Service Bulletins 93600-27-173 and 327400-27-171. The commenter states that certain specific tests that are called out in the service bulletins, such as the PCU dielectric test and eight- hour duty cycle test, are unrelated to the subject of the crossflow neutralization improvement. The commenter considers that revising the proposal as requested will eliminate redundant and unnecessary testing when reworking the PCP. The commenter contends that accomplishment of many of the tests in accordance with the subject service bulletins will not appreciably affect the flightworthy status of the PCP, as related to the crossflow enhancement, but will appreciably increase the cost of performing maintenance on the PCP. The commenter also requests that FAA allow operators to test the main manifold and assembled PCP's in accordance with existing approved procedures. \n\n\tThe FAA does not concur with the commenter's requests. The FAA has reviewed and approved all of the procedures specified in the Parker service bulletins that are referenced in paragraph (b) of the AD as the appropriate source of service information. The FAA has determined that accomplishment of the modification, including the various follow-on testing, in accordance with these service bulletins will prevent blockage of the hydraulic balance passageway by the spring guide, which can contribute to the uncommanded motion of the PCP. The FAA points out that some of the tests identified in the subject service bulletins are optional. \n\nRequests to Revise the Cost Impact Statement \n\n\tSeveral commenters question the FAA's cost estimate presented in the preamble to the supplemental NPRM. These commenters consider the cost estimate to be significantly understated. Some commenters point out that the cost of a serviceable elevator PCP will be far greater than the FAA's estimated cost of $5,910 per unit. One of these commenters states that a serviceable elevator PCP costs $23,633; another commenter states that the cost is closer to $45,500. \n\n\tAdditionally, some of the commenters note that, since many of the affected inboard elevator PCP's are older units, they may need additional rework or overhaul if they do not pass their respective acceptance tests. However, the FAA's cost estimate did not take these costs into consideration. In addition, one of these commenters states that the FAA's estimates failed to address the cost to modify spare units. \n\n\tThe FAA finds that clarification of the costs associated with the requirements of this AD is necessary. The FAA points out that the economic analysis of the AD is usually limited only to the cost of actions actually required by the rule. It does not consider the costs of "on condition" actions (e.g., "repair if a unit fails the functional test"), since those actions would be required to be accomplished, regardless of AD direction, in order to correct an unsafe condition identified in an airplane, and to ensure operation of that airplane in an airworthycondition, as required by the Federal Aviation Regulations. \n\n\tThe AD's cost estimate also does not consider the costs of "spare parts," since part 39 ("Airworthiness Directives") of the Federal Aviation Regulations (14 CFR part 39) does not permit AD's to address parts or components that are not installed on an airplane ("a product"). Therefore, the FAA cannot require via an AD that operators modify a "spare part;" an AD can only require that the part be modified before it is installed on an affected airplane.\n \n\tIn addition, the FAA points out that many operators have their own shop facilities in which to disassemble the PCP and accomplish the PCP overhaul/modification; for these operators, the costs associated with those actions may be far less than estimated by the FAA. \n\n\tAs for the exact cost of required parts, the FAA has verified with the manufacturer that the parts costs that was presented in the cost impact information in the preamble to the supplemental NPRM, and reiterated below, are valid.\n\n FAA's Conclusions \n\n\tAfter careful review of the available data, including the comments noted above, the FAA has determined that air safety and the public interest require the adoption of the rule with the changes previously described. The FAA has determined that these changes will neither increase the economic burden on any operator nor increase the scope of the AD. \n\nCost Impact \n\n\tThere are approximately 30 Model 747-200 and -300 series airplanes; and 332 Model 747-400 series airplanes; of the affected design in the worldwide fleet (362 airplanes total). \n\n\tThere currently are no Boeing Model 747-200 and -300 series airplanes on the U.S. Register that are subject to the requirements of this AD. All of the affected airplanes of those models that are included in the applicability of this rule currently are operated by non-U.S. operators under foreign registry; therefore, they are not directly affected by this AD action. However, the FAA considers that inclusion of these airplanes in the rule is necessary to ensure that the unsafe condition is addressed in the event that any of these airplanes are imported and placed on the U.S. Register in the future. \n\n\tShould any affected Model 747-200 and -300 series airplanes be imported and placed on the U.S. Register in the future, it would take approximately 73 work hours per airplane to accomplish the required modification, at an average labor rate of $60 per work hour. Required parts will cost approximately $7,440 per airplane. Based on these figures, the cost impact of the modification required by this AD on U.S. operators is estimated to be $11,820 per airplane. \n\n\tThe FAA estimates that 65 Model 747-400 series airplanes of U.S. registry will be affected by this AD, that it will take approximately 111 work hours per airplane to accomplish the required actions, and that the average labor rate is $60 per work hour. Required parts will cost approximately $12,269 per airplane.Based on these figures, the cost impact of the AD on U.S. operators of Model 747-400 series airplanes is estimated to be $1,230,385, or $18,929 per airplane. \n\n\tThe cost impact figures discussed above are based on assumptions that no operator has yet accomplished any of the requirements of this AD action, and that no operator would accomplish those actions in the future if this AD were not adopted. Regulatory 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) 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. A final evaluation has been prepared for this action and it is contained in the Rules Docket. A copy of it may be obtained from the Rules Docket at the location provided under the caption "ADDRESSES."\n\n List of Subjects in 14 CFR Part \n\n\tAir transportation, Aircraft, Aviation safety, Incorporation by reference, 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: Authority: 49 U.S.C. 106(g), 40113, 44701. § 39.13 - (Amended) \n\t2. Section 39.13 is amended by adding the following new airworthiness directive: