Special Conditions: Boeing Model 747-8/-8F Airplanes, Interaction of Systems and Structures, 30523-30529 [2011-13022]

Download as PDF Federal Register / Vol. 76, No. 102 / Thursday, May 26, 2011 / Rules and Regulations that this rule will not have a significant economic impact on a substantial number of small credit unions within the meaning of the Regulatory Flexibility Act, 5 U.S.C. 601–612. Small Business Regulatory Enforcement Fairness Act The Small Business Regulatory Enforcement Fairness Act (SBREFA) of 1996, Public Law 104–121, provides generally for congressional review of agency rules. A reporting requirement is triggered in instances where NCUA issues a final rule as defined by Section 551 of the Administrative Procedures Act. 5 U.S.C. 551. The Office of Information and Regulatory Affairs, an office within the Office of Management and Budget, has reviewed this rule and determined that, for purposes of SBREFA, this is not a major rule. Paperwork Reduction Act This rule does not contain a ‘‘collection of information’’ within the meaning of section 3502(3) of the Paperwork Reduction Act of 1995 (44 U.S.C. 3502(3)) and would not increase paperwork requirements under the Paperwork Reduction Act of 1995 or regulations of the Office of Management and Budget. srobinson on DSK4SPTVN1PROD with RULES Executive Order 13132 Executive Order 13132 encourages independent regulatory agencies to consider the impact of their actions on state and local interests. In adherence to fundamental federalism principles, NCUA, an independent regulatory agency as defined in 44 U.S.C. 3502(5), voluntarily complies with the executive order. This rule would not have substantial direct effect on the states, on the connection between the national government and the states, or on the distribution of power and responsibilities among the various levels of government. NCUA has determined that this rule does not constitute a policy that has federalism implications for purposes of the executive order. The Treasury and General Government Appropriations Act, 1999—Assessment of Federal Regulations and Policies on Families NCUA has determined that this rule would not affect family well-being within the meaning of section 654 of the Treasury and General Government Appropriations Act, 1999, Public Law 105–277, 112 Stat. 2681 (1998). List of Subjects in 12 CFR Part 740 Advertisements, Credit unions, Signs and symbols. VerDate Mar<15>2010 16:14 May 25, 2011 Jkt 223001 By the National Credit Union Administration Board on May 19, 2011. Mary F. Rupp, Secretary of the Board. For the reasons discussed above, the NCUA Board amends 12 CFR part 740 as follows: PART 740—ACCURACY OF ADVERTISING AND NOTICE OF INSURED STATUS 1. The authority citation for part 740 continues to read as follows: ■ 30523 intended to convey information to the consumer. * * * * * * * * (c) * * * (7) Advertisements by radio that are less than fifteen (15) seconds in time; (8) Advertisements by television, other than display advertisements, that are less than fifteen (15) seconds in time; * * * * * [FR Doc. 2011–12825 Filed 5–25–11; 8:45 am] BILLING CODE 7535–01–P Authority: 12 U.S.C. 1766, 1781, 1785, and 1789. 2. Amend § 740.1 by redesignating current paragraphs (b) and (c) as paragraphs (c) and (d), respectively, and by adding a new paragraph (b) to read as follows: DEPARTMENT OF TRANSPORTATION § 740.1 [Docket No. NM400; Special Conditions No. 25–388A–SC] ■ Definitions. * * * * * (b) Advertisement as used in this part means a commercial message, in any medium, that is designed to attract public attention or patronage to a product or business. * * * * * ■ 3. Amend § 740.5(a) as follows: ■ a. Revise paragraph (a). ■ b. Revise the third sentence of paragraph (b). ■ c. Remove paragraph (c)(1) and redesignate paragraphs (c)(2) through (c)(12) as paragraphs (c)(1) through (c)(11) respectively. ■ d. Revise redesignated paragraphs (c)(7) and (c)(8). The revisions read as follows: § 740.5 Requirements for the official advertising statement. (a) Each insured credit union must include the official advertising statement, prescribed in paragraph (b) of this section, in all of its advertisements including, but not limited to, annual reports and statements of condition required to be published by law, and on its main Internet page, except as provided in paragraph (c) of this section. For annual reports and statements of condition required to be published by law, an insured credit union must place the official advertising statement in a prominent position on the cover page of such documents or on the first page a reader sees if there is no cover page. * * * * * (b) * * * The official advertising statement must be in a size and print that is clearly legible and may be no smaller than the smallest font size used in other portions of the advertisement PO 00000 Frm 00015 Fmt 4700 Sfmt 4700 Federal Aviation Administration 14 CFR Part 25 Special Conditions: Boeing Model 747– 8/–8F Airplanes, Interaction of Systems and Structures Federal Aviation Administration (FAA), DOT. ACTION: Final special conditions. AGENCY: These special conditions amend Special Conditions No. 25–388– SC for the Boeing Model 747–8/–8F airplanes. These special conditions were previously issued July 29, 2009, and became effective September 10, 2009. These special conditions are being amended to include additional criteria addressing the Outboard Aileron Modal Suppression System. The 747–8/–8F will have novel or unusual design features when compared to the state of technology envisioned in the airworthiness standards for transport category airplanes. These design features include their effects on the structural performance. These special conditions contain the additional safety standards that the Administrator considers necessary to establish a level of safety equivalent to that established by the existing airworthiness standards. Additional special conditions will be issued for other novel or unusual design features of the 747–8/–8F airplanes. DATES: Effective Date: June 27, 2011. FOR FURTHER INFORMATION CONTACT: Carl Niedermeyer, FAA, Airframe & Cabin Safety Branch, ANM–115, Transport Airplane Directorate, Aircraft Certification Service, 1601 Lind Avenue SW., Renton, Washington 98057–3356; telephone (425) 227–2279; e-mail Carl.Niedermeyer@faa.gov. SUMMARY: SUPPLEMENTARY INFORMATION: E:\FR\FM\26MYR1.SGM 26MYR1 30524 Federal Register / Vol. 76, No. 102 / Thursday, May 26, 2011 / Rules and Regulations srobinson on DSK4SPTVN1PROD with RULES Background On November 4, 2005, The Boeing Company, P.O. Box 3707, Seattle, WA 98124, applied for an amendment to Type Certificate Number A20WE to include the new Model 747–8 passenger airplane and the new Model 747–8F freighter airplane. The Model 747–8 and the Model 747–8F are derivatives of the 747–400 and the 747–400F, respectively. Both the Model 747–8 and the Model 747–8F are four-engine jet transport airplanes that will have a maximum takeoff weight of 970,000 pounds and new General Electric GEnx –2B67 engines. The Model 747–8 will have two flight crew and the capacity to carry 605 passengers. The Model 747– 8F will have two flight crew and a zero passenger capacity, although Boeing has submitted a petition for exemption to allow the carriage of supernumeraries. These special conditions were originally issued July 29, 2009, and published in the Federal Register on August 12, 2009 (74 FR 40479). Type Certification Basis Under the provisions of Title 14, Code of Federal Regulations (14 CFR) 21.101, Boeing must show that Model 747–8 and 747–8F airplanes (hereafter referred as 747–8/–8F) meet the applicable provisions of part 25, as amended by Amendments 25–1 through 25–117, except for earlier amendments as agreed upon by the FAA. These regulations will be incorporated into Type Certificate No. A20WE after type certification approval of the 747–8/–8F. In addition, the certification basis includes other regulations, special conditions and exemptions that are not relevant to these proposed special conditions. Type Certificate No. A20WE will be updated to include a complete description of the certification basis for these model airplanes. If the Administrator finds that the applicable airworthiness regulations (i.e., 14 CFR part 25) do not contain adequate or appropriate safety standards for the 747–8/–8F because of a novel or unusual design feature, special conditions are prescribed under the provisions of § 21.16. Special conditions are initially applicable to the model for which they are issued. Should the type certificate for that model be amended later to include any other model that incorporates the same or similar novel or unusual design feature, or should any other model already included on the same type certificate be modified to incorporate the same or similar novel or unusual design feature, the special conditions would also apply to the other model under § 21.101. VerDate Mar<15>2010 16:14 May 25, 2011 Jkt 223001 In addition to the applicable airworthiness regulations and special conditions, the 747–8/–8F must comply with the fuel vent and exhaust emission requirements of 14 CFR part 34 and the noise certification requirements of 14 CFR part 36. Special conditions, as defined in § 11.19, are issued under § 11.38, and become part of the type certification basis under § 21.101. Novel or Unusual Design Features The Boeing Model 747–8/8F is equipped with systems that affect the airplane’s structural performance, either directly or as a result of failure or malfunction. That is, the airplane’s systems affect how it responds in maneuver and gust conditions, and thereby affect its structural capability. These systems may also affect the aeroelastic stability of the airplane. Such systems represent a novel and unusual feature when compared to the technology envisioned in the current airworthiness standards. A special condition is needed to require consideration of the effects of systems on the structural capability and aeroelastic stability of the airplane, both in the normal and in the failed state. Discussion The Boeing 747–8F airplane exhibits an aeroelastic mode of oscillation that is self-excited and does not completely damp out after an external disturbance. The sustained oscillation (also known as a limit cycle oscillation or limit cycle flutter) is caused by an unstable aeroelastic mode that is prevented from becoming a divergent oscillation due to one or more nonlinearities that exist in the airplane. While the sustained oscillation is not divergent, the FAA considers it to be an aeroelastic instability. Boeing has proposed the addition of an Outboard Aileron Modal Suppression (OAMS) system to the fly-by-wire (FBW) flight control system to reduce, but not eliminate, the amplitude of the sustained oscillation and control the aeroelastic instability. Section 25.629 requires the airplane to be free of any aeroelastic instability, including flutter. It also requires the airplane to remain flutter free after certain failures. The regulations do not anticipate the use of systems that control flutter modes but do not completely suppress them. The use of the OAMS system is a novel and unusual design feature that the airworthiness standards do not adequately address. The FAA believes such systems can be used to ensure that limit cycle (non-divergent) flutter is PO 00000 Frm 00016 Fmt 4700 Sfmt 4700 kept to safe levels. Therefore, the FAA proposes a special condition that addresses this particular sustained oscillation characteristic and provides the necessary standards that permit the use of such active flutter control systems. Discussion of Comments Notice of proposed special conditions No. 25–11–09–SC for Boeing 747–8/–8F airplanes was published in the Federal Register on March 16, 2011 (76 FR 14341). The standards in Section A were modified to incorporate the reference to Section C and remove ‘‘flutter control systems’’ from the applicability. Section B was already adopted in Special Conditions 25–388–SC and was included for reference. Comments were invited on the amended Section A and the proposed text of Section C, Outboard Aileron Modal Suppression System. Several comments were received from one commenter. Concerns With the Philosophy of Controlling Aeroelastic Instability (Flutter) With an Active Control System The commenter, Leth and Associates, LLC, expressed reservations with the philosophy of controlling an aeroelastic instability (flutter) with an active control system under the current rules and regulations, specifically § 25.629. The commenter’s position is that a safety issue is being solved by introducing more risk with the addition of an active suppression system. The commenter also expressed concern that the acceptance of this remedy to a design problem will encourage manufacturers to use a similar approach in solving potentially more onerous design flaws in the future. The commenter acknowledged that active flutter suppression systems may be introduced in advanced designs of the future, and stated that it is incumbent on the regulatory authorities to introduce regulations that clearly address the safety requirement of such systems. The commenter recommended that until these amended rules are in place to address aeroservoelastic systems, it is ill advised to accept ad hoc solutions to safety issues by adding more risk. The commenter further recommended that until such time as the rules have been changed, flutter prevention should rely on true and tested methods, using passive means of stabilization. The commenter did not offer any specific changes to the special conditions. However, the commenter suggested issues that should be addressed during future rulemaking. Although the FAA agrees with many of the statements and sentiments E:\FR\FM\26MYR1.SGM 26MYR1 Federal Register / Vol. 76, No. 102 / Thursday, May 26, 2011 / Rules and Regulations expressed by the commenter, we believe that the special conditions and the agreed upon means of compliance between the FAA and the applicant, adequately address the commenter’s concerns. The FAA does not agree that the acceptance of the use of the OAMS system and the type of sustained oscillation exhibited by the Boeing Model 747–8F need wait until new general rulemaking is completed. Special conditions are a form of rulemaking and are issued when the existing applicable airworthiness standards do not contain adequate or appropriate safety standards because of novel or unusual design features of the product to be type certificated. The phrase ‘‘novel or unusual’’ applies to design features of the product to be certificated when compared to the applicable airworthiness standards. This allows the FAA to make adjustments for individual type certificate projects by developing special conditions where novel or unusual design features are present. The special conditions addressing the OAMS system and the existence of the limit cycle flutter mode were formulated based on the characteristics observed during flight testing of the 747–8F and Boeing’s proposed solution to the problem. The FAA is requiring that the type of sustained oscillation covered by the special conditions must not be a hazard to the airplane nor its occupants with the active system inoperative or failed. This is assured by compliance with the requirements in the special condition. The FAA is taking a conservative approach to the introduction of active flutter suppression systems on transport category airplanes. At this point in time, the FAA is not prepared to certify active flutter suppression systems that control divergent flutter modes, or limit cycle flutter modes that do not meet the requirements of Section C, paragraphs 2.(a) and 2.(c), of these special conditions with the active system inoperative or failed. No changes were made as a result of these comments and the special conditions are adopted as proposed. srobinson on DSK4SPTVN1PROD with RULES Applicability As discussed above, these special conditions are applicable to Boeing Model 747–8/–8F airplanes. Should Boeing apply at a later date for a change to the type certificate to include another model incorporating the same novel or unusual design features, these special conditions would apply to that model as well under the provisions of § 21.101. VerDate Mar<15>2010 16:14 May 25, 2011 Jkt 223001 Conclusion This action affects only certain novel or unusual design features of the Boeing Model 747–8/–8F airplanes. It is not a rule of general applicability. List of Subjects in 14 CFR Part 25 Aircraft, Aviation safety, Reporting and recordkeeping requirements. The authority citation for this Special Condition is as follows: Authority: 49 U.S.C. 106(g), 40113, 44701, 44702, 44704. The Special Conditions Accordingly, pursuant to the authority delegated to me by the Administrator, the following amendment to Special Conditions 25–388–SC is issued as part of the type certification basis for the 747–8/–8F airplanes. A. General The Boeing Model 747–8/8F airplanes are equipped with automatic control systems that affect the airplane’s structural performance, either directly or as a result of a failure or malfunction. The influence of these systems and their failure conditions must be taken into account when showing compliance with the requirements of Subparts C and D of part 25. Except as provided in Section C of these special conditions, the following criteria must be used for showing compliance with these special conditions for airplanes equipped with flight control systems, autopilots, stability augmentation systems, load alleviation systems, fuel management systems, and other systems that either directly or as a result of failure or malfunction affect structural performance. If these special conditions are used for other systems, it may be necessary to adapt the criteria to the specific system. 1. The criteria defined here only address the direct structural consequences of the system responses and performances and cannot be considered in isolation; however, they should be included in the overall safety evaluation of the airplane. These criteria may in some instances duplicate standards already established for this evaluation. These criteria are only applicable to structural elements whose failure could prevent continued safe flight and landing. Specific criteria that define acceptable limits on handling characteristics or stability requirements when operating in the system degraded or inoperative mode are not provided in these special conditions. 2. Depending on the specific characteristics of the airplane, PO 00000 Frm 00017 Fmt 4700 Sfmt 4700 30525 additional studies may be required that go beyond the criteria provided in these special conditions in order to demonstrate the capability of the airplane to meet other realistic conditions such as alternative gust or maneuver descriptions for an airplane equipped with a load alleviation system. 3. The following definitions are applicable to these special conditions. (a) Structural performance: Capability of the airplane to meet the structural requirements of part 25. (b) Flight limitations: Limitations that can be applied to the airplane flight conditions following an in-flight occurrence and that are included in the airplane flight manual (AFM) (e.g., speed limitations, avoidance of severe weather conditions). (c) Operational limitations: Limitations, including flight limitations that can be applied to the airplane operating conditions before dispatch (e.g., fuel, payload and Master Minimum Equipment List (MMEL) limitations). (d) Probabilistic terms: The probabilistic terms (probable, improbable, extremely improbable) used in these special conditions are the same as those used in § 25.1309. (e) Failure condition: The term failure condition is the same as that used in § 25.1309, however these special conditions apply only to system failure conditions that affect the structural performance of the airplane (e.g., system failure conditions that induce loads, change the response of the airplane to inputs such as gusts or pilot actions, or lower flutter margins). The system failure condition includes consequential or cascading effects resulting from the first failure. B. Effects of Systems on Structures 1. General. The following criteria will be used in determining the influence of a system and its failure conditions on the airplane structural elements. 2. System fully operative. With the system fully operative, the following apply: (a) Limit loads must be derived in all normal operating configurations of the system from all the limit conditions specified in subpart C (or used in lieu of those specified in subpart C), taking into account any special behavior of such a system or associated functions or any effect on the structural performance of the airplane that may occur up to the limit loads. In particular, any significant nonlinearity (rate of displacement of control surface, thresholds or any other system nonlinearities) must be accounted for in a realistic or E:\FR\FM\26MYR1.SGM 26MYR1 30526 Federal Register / Vol. 76, No. 102 / Thursday, May 26, 2011 / Rules and Regulations conditions need not be considered when it can be shown that the airplane has design features that will not allow it to exceed those limit conditions. (c) The airplane must meet the aeroelastic stability requirements of § 25.629. 3. System in the failure condition. For any system failure condition not shown to be extremely improbable, the following apply: (a) At the time of occurrence, starting from 1-g level flight conditions, a realistic scenario including pilot corrective actions, must be established to determine the loads occurring at the time of failure and immediately after failure. (1) For static strength substantiation, these loads multiplied by an appropriate factor of safety that is related to the probability of occurrence of the failure are ultimate loads to be considered for design. The factor of safety (F.S.) is defined in Figure 1. (2) For residual strength substantiation, the airplane must be able to withstand two thirds of the ultimate loads defined in subparagraph 3(a)(1). For pressurized cabins, these loads must be combined with the normal operating differential pressure. (3) Freedom from aeroelastic instability must be shown up to the speeds defined in § 25.629(b)(2). For failure conditions that result in speeds beyond VC/MC, freedom from aeroelastic instability must be shown to increased speeds, so that the margins intended by § 25.629(b)(2) are maintained. (4) Failures of the system that result in forced structural vibrations (oscillatory failures) must not produce loads that could result in detrimental deformation of the affected structural elements. (b) For continuation of flight, for an airplane in the system failed state and considering any appropriate reconfiguration and flight limitations, the following apply: (1) The loads derived from the following conditions (or used in lieu of the following conditions) at speeds up to VC/MC, or the speed limitation prescribed for the remainder of the flight, must be determined: (i) The limit symmetrical maneuvering conditions specified in § 25.331 and in § 25.345. (ii) The limit gust and turbulence conditions specified in § 25.341 and in § 25.345. (iii) The limit rolling conditions specified in § 25.349 and the limit unsymmetrical conditions specified in §§ 25.367 and 25.427(b) and (c). (iv) The limit yaw maneuvering conditions specified in § 25.351. (v) the limit ground loading conditions specified in §§ 25.473, 25.491 and 25.493. (2) For static strength substantiation, each part of the structure must be able to withstand the loads in paragraph (3)(b)(1) of the special condition multiplied by a factor of safety depending on the probability of being in this failure state. The factor of safety is defined in Figure 2. VerDate Mar<15>2010 16:14 May 25, 2011 Jkt 223001 PO 00000 Frm 00018 Fmt 4700 Sfmt 4700 E:\FR\FM\26MYR1.SGM 26MYR1 ER26MY11.004</GPH> srobinson on DSK4SPTVN1PROD with RULES conservative way when deriving limit loads from limit conditions. (b) The airplane must meet the strength requirements of part 25 (i.e., static strength, residual strength), using the specified factors to derive ultimate loads from the limit loads defined above. The effect of nonlinearities must be investigated beyond limit conditions to ensure the behavior of the system presents no anomaly compared to the behavior below limit conditions. However, conditions beyond limit Federal Register / Vol. 76, No. 102 / Thursday, May 26, 2011 / Rules and Regulations Where: Tj = Average time spent in failure condition j (in hours) Pj = Probability of occurrence of failure mode j (per hour) srobinson on DSK4SPTVN1PROD with RULES Note: If Pj is greater than 10¥3 per flight hour then a 1.5 factor of safety must be applied to all limit load conditions specified in Subpart C. VerDate Mar<15>2010 16:14 May 25, 2011 Jkt 223001 (3) For residual strength substantiation, the airplane must be able to withstand two thirds of the ultimate loads defined in paragraph (3)(b)(1) of the special condition. For pressurized cabins, these loads must be combined with the normal operating differential pressure. (4) If the loads induced by the failure condition have a significant effect on PO 00000 Frm 00019 Fmt 4700 Sfmt 4700 fatigue or damage tolerance then their effects must be taken into account. (5) Freedom from aeroelastic instability must be shown up to a speed determined from Figure 3. Flutter clearance speeds V′ and V″ may be based on the speed limitation specified for the remainder of the flight using the margins defined by § 25.629(b). E:\FR\FM\26MYR1.SGM 26MYR1 ER26MY11.005</GPH> Qj = (Tj)(Pj) 30527 Federal Register / Vol. 76, No. 102 / Thursday, May 26, 2011 / Rules and Regulations V′ = Clearance speed as defined by § 25.629(b)(2). V″ = Clearance speed as defined by § 25.629(b)(1). Qj = (Tj)(Pj) Where: Tj = Average time spent in failure condition j (in hours) Pj = Probability of occurrence of failure mode j (per hour) srobinson on DSK4SPTVN1PROD with RULES Note: If Pj is greater than 10¥3 per flight hour, then the flutter clearance speed must not be less than V″. (6) Freedom from aeroelastic instability must also be shown up to V′ in Figure 3 above, for any probable system failure condition combined with any damage required or selected for investigation by § 25.571(b). (c) Consideration of certain failure conditions may be required by other sections of part 25 regardless of calculated system reliability. Where analysis shows the probability of these failure conditions to be less than 10¥9, criteria other than those specified in this paragraph may be used for structural substantiation to show continued safe flight and landing. 4. Failure indications. For system failure detection and indication, the following apply: (a) The system must be checked for failure conditions, not extremely improbable, that degrade the structural capability below the level required by part 25 or significantly reduce the VerDate Mar<15>2010 16:14 May 25, 2011 Jkt 223001 reliability of the remaining system. As far as reasonably practicable, the flight crew must be made aware of these failures before flight. Certain elements of the control system, such as mechanical and hydraulic components, may use special periodic inspections, and electronic components may use daily checks, in lieu of detection and indication systems to achieve the objective of this requirement. These Certification Maintenance Requirements (CMRs) must be limited to components that are not readily detectable by normal detection and indication systems and where service history shows that inspections will provide an adequate level of safety. (b) The existence of any failure condition, not extremely improbable, during flight that could significantly affect the structural capability of the airplane and for which the associated reduction in airworthiness can be minimized by suitable flight limitations, must be signaled to the flight crew. For example, failure conditions that result in a factor of safety between the airplane strength and the loads of subpart C below 1.25, or flutter margins below V″, must be signaled to the crew during flight. 5. Dispatch with known failure conditions. If the airplane is to be dispatched in a known system failure condition that affects structural performance, or affects the reliability of PO 00000 Frm 00020 Fmt 4700 Sfmt 4700 the remaining system to maintain structural performance, then the provisions of these special conditions must be met, including the provisions of paragraph 2 for the dispatched condition, and paragraph 3 for subsequent failures. Expected operational limitations may be taken into account in establishing Pj as the probability of failure occurrence for determining the safety margin in Figure 1. Flight limitations and expected operational limitations may be taken into account in establishing Qj as the combined probability of being in the dispatched failure condition and the subsequent failure condition for the safety margins in Figures 2 and 3. These limitations must be such that the probability of being in this combined failure state and then subsequently encountering limit load conditions is extremely improbable. No reduction in these safety margins is allowed if the subsequent system failure rate is greater than 10¥3 per hour. C. Outboard Aileron Modal Suppression System 1. In general, these special conditions apply to fly-by-wire active flutter suppression systems that are intended to operate on a certain type of aeroelastic instability. This type of instability is characterized by a low frequency, self-excited, sustained oscillation of an aeroelastic vibration E:\FR\FM\26MYR1.SGM 26MYR1 ER26MY11.006</GPH> 30528 srobinson on DSK4SPTVN1PROD with RULES Federal Register / Vol. 76, No. 102 / Thursday, May 26, 2011 / Rules and Regulations mode that is shown to be a stable limit cycle oscillation (LCO), with the system operative and inoperative. (An LCO is considered ‘‘stable’’ if it maintains the same frequency and amplitude for a given excitation input and flight condition.) In addition, the type of sustained oscillation covered by these special conditions must not be a hazard to the airplane nor its occupants with the active system failed. These systems must be shown to reduce the amplitude of the sustained oscillation to acceptable levels and effectively control the aeroelastic instability. Specifically, the following criteria address the existence of such a sustained oscillation on the Boeing Model 747–8/–8F airplanes and the Outboard Aileron Modal Suppression (OAMS) system that will be used to control it. 2. In lieu of the requirements contained in § 25.629, the existence of a sustained, or limit cycle, oscillation that is controlled by an active flight control system is acceptable, provided that the following requirements are met: (a) OAMS System Inoperative: The sustained, or limit cycle, oscillation must be shown by test and analysis to be stable throughout the nominal aeroelastic stability envelope specified in § 25.629(b)(1) with the OAMS system inoperative. This should include the consideration of disturbances above the sustained amplitude of oscillation. (b) Nominal Conditions: (1) With the OAMS system operative it must be shown that the airplane remains safe, stable, and controllable throughout the nominal aeroelastic stability envelope specified in § 25.629(b)(1) by providing adequate suppression of the aeroelastic modes being controlled. All applicable airworthiness and environmental requirements should continue to be complied with. Additionally, loads imposed on the airplane due to any amplitude of oscillation must be shown to have a negligible impact on structure and systems, including wear, fatigue and damage tolerance. The OAMS system must function properly in all environments that may be encountered. (2) The applicant must establish by test and analysis that the OAMS system can be relied upon to control and limit the sustained amplitude of the oscillation to acceptable levels (per § 25.251) and control the stability of the aeroelastic mode. This should include the consideration of disturbances above the sustained amplitude of oscillation; maneuvering flight, icing conditions; manufacturing variations; Master Minimum Equipment List (MMEL) items; spare engine carriage; engine removed or inoperative ferry flights; and VerDate Mar<15>2010 16:14 May 25, 2011 Jkt 223001 wear, repairs, and modifications throughout the service life of the airplane by: (i) Analysis to the nominal aeroelastic stability envelope specified in § 25.629(b)(1), and (ii) Flight flutter test to the VDF/MDF boundary. These tests must demonstrate that the airplane has a proper margin of damping for disturbances above the sustained amplitude of oscillation at all speeds up to VDF/MDF, and that there is no large and rapid reduction in damping as VDF/MDF is approached. (iii) The structural modes must have adequate stability margins for any OAMS flight control system feedback loop at speeds up to the fail-safe aeroelastic stability envelope specified in § 25.629(b)(2). (c) Failures, Malfunctions, and Adverse Conditions: (1) For the OAMS system operative and failed, for any failure, or combination of failures not shown to be extremely improbable, and addressed by §§ 25.629(d), 25.571, 25.631, 25.671, 25.672, 25.901(c) or 25.1309 that results in LCO, it must be established by test or analysis up to the aeroelastic stability envelope specified in § 25.629(b)(2) that the LCO: (i) Is stable and decays to an acceptable limited amplitude once an external perturbing force is removed; (ii) Does not result in loads that would cause static, dynamic, or fatigue failure of structure during the expected exposure period; (iii) Does not result in repeated loads that would cause an additional failure due to wear during the expected exposure period that precludes safe flight and landing; (iv) Has, if necessary, sufficient indication of OAMS failure(s) and crew procedures to properly address the failure(s); (v) Does not result in a vibration condition on the flight deck that is severe enough to interfere with control of the airplane, ability of the crew to read the flight instruments, perform vital functions like reading and accomplishing checklist procedures, or to cause excessive fatigue to the crew; (vi) Does not result in adverse effects on the flight control system or on airplane stability, controllability, or handling characteristics (including airplane-pilot coupling (APC) per § 25.143) that would prevent safe flight and landing; and (vii) does not interfere with the flight crew’s ability to correctly distinguish vibration from buffeting associated with the recognition of stalls or high speed buffet. PO 00000 Frm 00021 Fmt 4700 Sfmt 4700 30529 (2) The applicant must show that particular risks such as engine failure, uncontained engine, or APU rotor burst, or other failures not shown to be extremely improbable, will not adversely or significantly change the aeroelastic stability characteristics of the airplane. (3) No MMEL dispatch is allowed with the OAMS system inoperative. Issued in Renton, Washington, on May 20, 2011. Ali Bahrami, Manager, Transport Airplane Directorate, Aircraft Certification Service. [FR Doc. 2011–13022 Filed 5–25–11; 8:45 am] BILLING CODE 4910–13–P DEPARTMENT OF TRANSPORTATION Federal Aviation Administration 14 CFR Part 39 [Docket No. FAA–2010–0994; Directorate Identifier 2009–NE–39–AD; Amendment 39– 16707; AD 2011–11–08] RIN 2120–AA64 Airworthiness Directives; Rolls-Royce plc (RR) RB211–535 Series Turbofan Engines Federal Aviation Administration (FAA), DOT. ACTION: Final rule. AGENCY: We are adopting a new airworthiness directive (AD) for the products listed above. This AD results from mandatory continuing airworthiness information (MCAI) issued by an aviation authority of another country to identify and correct an unsafe condition on an aviation product. The MCAI describes the unsafe condition as: SUMMARY: There have been several findings of cracking at the firtrees of LP Turbine discs. Fatigue crack initiation and subsequent crack propagation at the firtree may result in multiple LP Turbine blade release. The latter may potentially be beyond the containment capabilities of the engine casings. Thus, cracking at the firtrees of LP Turbine discs constitutes a potentially unsafe condition. We are issuing this AD to detect cracks in the low-pressure (LP) turbine stage 1, 2, and 3 discs, which could result in an uncontained release of LP turbine blades and damage to the airplane. DATES: This AD becomes effective June 30, 2011. ADDRESSES: The Docket Operations office is located at Docket Management Facility, U.S. Department of Transportation, 1200 New Jersey Avenue, SE., West Building Ground E:\FR\FM\26MYR1.SGM 26MYR1

Agencies

[Federal Register Volume 76, Number 102 (Thursday, May 26, 2011)]
[Rules and Regulations]
[Pages 30523-30529]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 2011-13022]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 25

[Docket No. NM400; Special Conditions No. 25-388A-SC]


Special Conditions: Boeing Model 747-8/-8F Airplanes, Interaction 
of Systems and Structures

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Final special conditions.

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SUMMARY: These special conditions amend Special Conditions No. 25-388-
SC for the Boeing Model 747-8/-8F airplanes. These special conditions 
were previously issued July 29, 2009, and became effective September 
10, 2009. These special conditions are being amended to include 
additional criteria addressing the Outboard Aileron Modal Suppression 
System. The 747-8/-8F will have novel or unusual design features when 
compared to the state of technology envisioned in the airworthiness 
standards for transport category airplanes. These design features 
include their effects on the structural performance. These special 
conditions contain the additional safety standards that the 
Administrator considers necessary to establish a level of safety 
equivalent to that established by the existing airworthiness standards. 
Additional special conditions will be issued for other novel or unusual 
design features of the 747-8/-8F airplanes.

DATES: Effective Date: June 27, 2011.

FOR FURTHER INFORMATION CONTACT: Carl Niedermeyer, FAA, Airframe & 
Cabin Safety Branch, ANM-115, Transport Airplane Directorate, Aircraft 
Certification Service, 1601 Lind Avenue SW., Renton, Washington 98057-
3356; telephone (425) 227-2279; e-mail Carl.Niedermeyer@faa.gov.

SUPPLEMENTARY INFORMATION:

[[Page 30524]]

Background

    On November 4, 2005, The Boeing Company, P.O. Box 3707, Seattle, WA 
98124, applied for an amendment to Type Certificate Number A20WE to 
include the new Model 747-8 passenger airplane and the new Model 747-8F 
freighter airplane. The Model 747-8 and the Model 747-8F are 
derivatives of the 747-400 and the 747-400F, respectively. Both the 
Model 747-8 and the Model 747-8F are four-engine jet transport 
airplanes that will have a maximum takeoff weight of 970,000 pounds and 
new General Electric GEnx -2B67 engines. The Model 747-8 will have two 
flight crew and the capacity to carry 605 passengers. The Model 747-8F 
will have two flight crew and a zero passenger capacity, although 
Boeing has submitted a petition for exemption to allow the carriage of 
supernumeraries.
    These special conditions were originally issued July 29, 2009, and 
published in the Federal Register on August 12, 2009 (74 FR 40479).

Type Certification Basis

    Under the provisions of Title 14, Code of Federal Regulations (14 
CFR) 21.101, Boeing must show that Model 747-8 and 747-8F airplanes 
(hereafter referred as 747-8/-8F) meet the applicable provisions of 
part 25, as amended by Amendments 25-1 through 25-117, except for 
earlier amendments as agreed upon by the FAA. These regulations will be 
incorporated into Type Certificate No. A20WE after type certification 
approval of the 747-8/-8F.
    In addition, the certification basis includes other regulations, 
special conditions and exemptions that are not relevant to these 
proposed special conditions. Type Certificate No. A20WE will be updated 
to include a complete description of the certification basis for these 
model airplanes.
    If the Administrator finds that the applicable airworthiness 
regulations (i.e., 14 CFR part 25) do not contain adequate or 
appropriate safety standards for the 747-8/-8F because of a novel or 
unusual design feature, special conditions are prescribed under the 
provisions of Sec.  21.16.
    Special conditions are initially applicable to the model for which 
they are issued. Should the type certificate for that model be amended 
later to include any other model that incorporates the same or similar 
novel or unusual design feature, or should any other model already 
included on the same type certificate be modified to incorporate the 
same or similar novel or unusual design feature, the special conditions 
would also apply to the other model under Sec.  21.101.
    In addition to the applicable airworthiness regulations and special 
conditions, the 747-8/-8F must comply with the fuel vent and exhaust 
emission requirements of 14 CFR part 34 and the noise certification 
requirements of 14 CFR part 36.
    Special conditions, as defined in Sec.  11.19, are issued under 
Sec.  11.38, and become part of the type certification basis under 
Sec.  21.101.

Novel or Unusual Design Features

    The Boeing Model 747-8/8F is equipped with systems that affect the 
airplane's structural performance, either directly or as a result of 
failure or malfunction. That is, the airplane's systems affect how it 
responds in maneuver and gust conditions, and thereby affect its 
structural capability. These systems may also affect the aeroelastic 
stability of the airplane. Such systems represent a novel and unusual 
feature when compared to the technology envisioned in the current 
airworthiness standards. A special condition is needed to require 
consideration of the effects of systems on the structural capability 
and aeroelastic stability of the airplane, both in the normal and in 
the failed state.

Discussion

    The Boeing 747-8F airplane exhibits an aeroelastic mode of 
oscillation that is self-excited and does not completely damp out after 
an external disturbance. The sustained oscillation (also known as a 
limit cycle oscillation or limit cycle flutter) is caused by an 
unstable aeroelastic mode that is prevented from becoming a divergent 
oscillation due to one or more nonlinearities that exist in the 
airplane.
    While the sustained oscillation is not divergent, the FAA considers 
it to be an aeroelastic instability. Boeing has proposed the addition 
of an Outboard Aileron Modal Suppression (OAMS) system to the fly-by-
wire (FBW) flight control system to reduce, but not eliminate, the 
amplitude of the sustained oscillation and control the aeroelastic 
instability.
    Section 25.629 requires the airplane to be free of any aeroelastic 
instability, including flutter. It also requires the airplane to remain 
flutter free after certain failures. The regulations do not anticipate 
the use of systems that control flutter modes but do not completely 
suppress them. The use of the OAMS system is a novel and unusual design 
feature that the airworthiness standards do not adequately address. The 
FAA believes such systems can be used to ensure that limit cycle (non-
divergent) flutter is kept to safe levels. Therefore, the FAA proposes 
a special condition that addresses this particular sustained 
oscillation characteristic and provides the necessary standards that 
permit the use of such active flutter control systems.

Discussion of Comments

    Notice of proposed special conditions No. 25-11-09-SC for Boeing 
747-8/-8F airplanes was published in the Federal Register on March 16, 
2011 (76 FR 14341). The standards in Section A were modified to 
incorporate the reference to Section C and remove ``flutter control 
systems'' from the applicability. Section B was already adopted in 
Special Conditions 25-388-SC and was included for reference. Comments 
were invited on the amended Section A and the proposed text of Section 
C, Outboard Aileron Modal Suppression System. Several comments were 
received from one commenter.

Concerns With the Philosophy of Controlling Aeroelastic Instability 
(Flutter) With an Active Control System

    The commenter, Leth and Associates, LLC, expressed reservations 
with the philosophy of controlling an aeroelastic instability (flutter) 
with an active control system under the current rules and regulations, 
specifically Sec.  25.629. The commenter's position is that a safety 
issue is being solved by introducing more risk with the addition of an 
active suppression system. The commenter also expressed concern that 
the acceptance of this remedy to a design problem will encourage 
manufacturers to use a similar approach in solving potentially more 
onerous design flaws in the future. The commenter acknowledged that 
active flutter suppression systems may be introduced in advanced 
designs of the future, and stated that it is incumbent on the 
regulatory authorities to introduce regulations that clearly address 
the safety requirement of such systems. The commenter recommended that 
until these amended rules are in place to address aeroservoelastic 
systems, it is ill advised to accept ad hoc solutions to safety issues 
by adding more risk. The commenter further recommended that until such 
time as the rules have been changed, flutter prevention should rely on 
true and tested methods, using passive means of stabilization. The 
commenter did not offer any specific changes to the special conditions. 
However, the commenter suggested issues that should be addressed during 
future rulemaking.
    Although the FAA agrees with many of the statements and sentiments

[[Page 30525]]

expressed by the commenter, we believe that the special conditions and 
the agreed upon means of compliance between the FAA and the applicant, 
adequately address the commenter's concerns. The FAA does not agree 
that the acceptance of the use of the OAMS system and the type of 
sustained oscillation exhibited by the Boeing Model 747-8F need wait 
until new general rulemaking is completed. Special conditions are a 
form of rulemaking and are issued when the existing applicable 
airworthiness standards do not contain adequate or appropriate safety 
standards because of novel or unusual design features of the product to 
be type certificated. The phrase ``novel or unusual'' applies to design 
features of the product to be certificated when compared to the 
applicable airworthiness standards. This allows the FAA to make 
adjustments for individual type certificate projects by developing 
special conditions where novel or unusual design features are present.
    The special conditions addressing the OAMS system and the existence 
of the limit cycle flutter mode were formulated based on the 
characteristics observed during flight testing of the 747-8F and 
Boeing's proposed solution to the problem. The FAA is requiring that 
the type of sustained oscillation covered by the special conditions 
must not be a hazard to the airplane nor its occupants with the active 
system inoperative or failed. This is assured by compliance with the 
requirements in the special condition.
    The FAA is taking a conservative approach to the introduction of 
active flutter suppression systems on transport category airplanes. At 
this point in time, the FAA is not prepared to certify active flutter 
suppression systems that control divergent flutter modes, or limit 
cycle flutter modes that do not meet the requirements of Section C, 
paragraphs 2.(a) and 2.(c), of these special conditions with the active 
system inoperative or failed.
    No changes were made as a result of these comments and the special 
conditions are adopted as proposed.

Applicability

    As discussed above, these special conditions are applicable to 
Boeing Model 747-8/-8F airplanes. Should Boeing apply at a later date 
for a change to the type certificate to include another model 
incorporating the same novel or unusual design features, these special 
conditions would apply to that model as well under the provisions of 
Sec.  21.101.

Conclusion

    This action affects only certain novel or unusual design features 
of the Boeing Model 747-8/-8F airplanes. It is not a rule of general 
applicability.

List of Subjects in 14 CFR Part 25

    Aircraft, Aviation safety, Reporting and recordkeeping 
requirements.

    The authority citation for this Special Condition is as follows:

    Authority: 49 U.S.C. 106(g), 40113, 44701, 44702, 44704.

The Special Conditions

    Accordingly, pursuant to the authority delegated to me by the 
Administrator, the following amendment to Special Conditions 25-388-SC 
is issued as part of the type certification basis for the 747-8/-8F 
airplanes.

A. General

    The Boeing Model 747-8/8F airplanes are equipped with automatic 
control systems that affect the airplane's structural performance, 
either directly or as a result of a failure or malfunction. The 
influence of these systems and their failure conditions must be taken 
into account when showing compliance with the requirements of Subparts 
C and D of part 25. Except as provided in Section C of these special 
conditions, the following criteria must be used for showing compliance 
with these special conditions for airplanes equipped with flight 
control systems, autopilots, stability augmentation systems, load 
alleviation systems, fuel management systems, and other systems that 
either directly or as a result of failure or malfunction affect 
structural performance. If these special conditions are used for other 
systems, it may be necessary to adapt the criteria to the specific 
system.
    1. The criteria defined here only address the direct structural 
consequences of the system responses and performances and cannot be 
considered in isolation; however, they should be included in the 
overall safety evaluation of the airplane. These criteria may in some 
instances duplicate standards already established for this evaluation. 
These criteria are only applicable to structural elements whose failure 
could prevent continued safe flight and landing. Specific criteria that 
define acceptable limits on handling characteristics or stability 
requirements when operating in the system degraded or inoperative mode 
are not provided in these special conditions.
    2. Depending on the specific characteristics of the airplane, 
additional studies may be required that go beyond the criteria provided 
in these special conditions in order to demonstrate the capability of 
the airplane to meet other realistic conditions such as alternative 
gust or maneuver descriptions for an airplane equipped with a load 
alleviation system.
    3. The following definitions are applicable to these special 
conditions.
    (a) Structural performance: Capability of the airplane to meet the 
structural requirements of part 25.
    (b) Flight limitations: Limitations that can be applied to the 
airplane flight conditions following an in-flight occurrence and that 
are included in the airplane flight manual (AFM) (e.g., speed 
limitations, avoidance of severe weather conditions).
    (c) Operational limitations: Limitations, including flight 
limitations that can be applied to the airplane operating conditions 
before dispatch (e.g., fuel, payload and Master Minimum Equipment List 
(MMEL) limitations).
    (d) Probabilistic terms: The probabilistic terms (probable, 
improbable, extremely improbable) used in these special conditions are 
the same as those used in Sec.  25.1309.
    (e) Failure condition: The term failure condition is the same as 
that used in Sec.  25.1309, however these special conditions apply only 
to system failure conditions that affect the structural performance of 
the airplane (e.g., system failure conditions that induce loads, change 
the response of the airplane to inputs such as gusts or pilot actions, 
or lower flutter margins). The system failure condition includes 
consequential or cascading effects resulting from the first failure.

B. Effects of Systems on Structures

    1. General. The following criteria will be used in determining the 
influence of a system and its failure conditions on the airplane 
structural elements.
    2. System fully operative. With the system fully operative, the 
following apply:
    (a) Limit loads must be derived in all normal operating 
configurations of the system from all the limit conditions specified in 
subpart C (or used in lieu of those specified in subpart C), taking 
into account any special behavior of such a system or associated 
functions or any effect on the structural performance of the airplane 
that may occur up to the limit loads. In particular, any significant 
nonlinearity (rate of displacement of control surface, thresholds or 
any other system nonlinearities) must be accounted for in a realistic 
or

[[Page 30526]]

conservative way when deriving limit loads from limit conditions.
    (b) The airplane must meet the strength requirements of part 25 
(i.e., static strength, residual strength), using the specified factors 
to derive ultimate loads from the limit loads defined above. The effect 
of nonlinearities must be investigated beyond limit conditions to 
ensure the behavior of the system presents no anomaly compared to the 
behavior below limit conditions. However, conditions beyond limit 
conditions need not be considered when it can be shown that the 
airplane has design features that will not allow it to exceed those 
limit conditions.
    (c) The airplane must meet the aeroelastic stability requirements 
of Sec.  25.629.
    3. System in the failure condition. For any system failure 
condition not shown to be extremely improbable, the following apply:
    (a) At the time of occurrence, starting from 1-g level flight 
conditions, a realistic scenario including pilot corrective actions, 
must be established to determine the loads occurring at the time of 
failure and immediately after failure.
    (1) For static strength substantiation, these loads multiplied by 
an appropriate factor of safety that is related to the probability of 
occurrence of the failure are ultimate loads to be considered for 
design. The factor of safety (F.S.) is defined in Figure 1.
[GRAPHIC] [TIFF OMITTED] TR26MY11.004

    (2) For residual strength substantiation, the airplane must be able 
to withstand two thirds of the ultimate loads defined in subparagraph 
3(a)(1). For pressurized cabins, these loads must be combined with the 
normal operating differential pressure.
    (3) Freedom from aeroelastic instability must be shown up to the 
speeds defined in Sec.  25.629(b)(2). For failure conditions that 
result in speeds beyond VC/MC, freedom from 
aeroelastic instability must be shown to increased speeds, so that the 
margins intended by Sec.  25.629(b)(2) are maintained.
    (4) Failures of the system that result in forced structural 
vibrations (oscillatory failures) must not produce loads that could 
result in detrimental deformation of the affected structural elements.
    (b) For continuation of flight, for an airplane in the system 
failed state and considering any appropriate reconfiguration and flight 
limitations, the following apply:
    (1) The loads derived from the following conditions (or used in 
lieu of the following conditions) at speeds up to VC/
MC, or the speed limitation prescribed for the remainder of 
the flight, must be determined:
    (i) The limit symmetrical maneuvering conditions specified in Sec.  
25.331 and in Sec.  25.345.
    (ii) The limit gust and turbulence conditions specified in Sec.  
25.341 and in Sec.  25.345.
    (iii) The limit rolling conditions specified in Sec.  25.349 and 
the limit unsymmetrical conditions specified in Sec. Sec.  25.367 and 
25.427(b) and (c).
    (iv) The limit yaw maneuvering conditions specified in Sec.  
25.351.
    (v) the limit ground loading conditions specified in Sec. Sec.  
25.473, 25.491 and 25.493.
    (2) For static strength substantiation, each part of the structure 
must be able to withstand the loads in paragraph (3)(b)(1) of the 
special condition multiplied by a factor of safety depending on the 
probability of being in this failure state. The factor of safety is 
defined in Figure 2.

[[Page 30527]]

[GRAPHIC] [TIFF OMITTED] TR26MY11.005

Qj = (Tj)(Pj)

Where:

Tj = Average time spent in failure condition j (in hours)
Pj = Probability of occurrence of failure mode j (per 
hour)

    Note: If Pj is greater than 10-3 per 
flight hour then a 1.5 factor of safety must be applied to all limit 
load conditions specified in Subpart C.

    (3) For residual strength substantiation, the airplane must be able 
to withstand two thirds of the ultimate loads defined in paragraph 
(3)(b)(1) of the special condition. For pressurized cabins, these loads 
must be combined with the normal operating differential pressure.
    (4) If the loads induced by the failure condition have a 
significant effect on fatigue or damage tolerance then their effects 
must be taken into account.
    (5) Freedom from aeroelastic instability must be shown up to a 
speed determined from Figure 3. Flutter clearance speeds V' and V'' may 
be based on the speed limitation specified for the remainder of the 
flight using the margins defined by Sec.  25.629(b).

[[Page 30528]]

[GRAPHIC] [TIFF OMITTED] TR26MY11.006

V' = Clearance speed as defined by Sec.  25.629(b)(2).
V'' = Clearance speed as defined by Sec.  25.629(b)(1).
Qj = (Tj)(Pj)

Where:

Tj = Average time spent in failure condition j (in hours)
Pj = Probability of occurrence of failure mode j (per 
hour)

    Note: If Pj is greater than 10-3 per 
flight hour, then the flutter clearance speed must not be less than 
V''.

    (6) Freedom from aeroelastic instability must also be shown up to 
V' in Figure 3 above, for any probable system failure condition 
combined with any damage required or selected for investigation by 
Sec.  25.571(b).
    (c) Consideration of certain failure conditions may be required by 
other sections of part 25 regardless of calculated system reliability. 
Where analysis shows the probability of these failure conditions to be 
less than 10-9, criteria other than those specified in this 
paragraph may be used for structural substantiation to show continued 
safe flight and landing.
    4. Failure indications. For system failure detection and 
indication, the following apply:
    (a) The system must be checked for failure conditions, not 
extremely improbable, that degrade the structural capability below the 
level required by part 25 or significantly reduce the reliability of 
the remaining system. As far as reasonably practicable, the flight crew 
must be made aware of these failures before flight. Certain elements of 
the control system, such as mechanical and hydraulic components, may 
use special periodic inspections, and electronic components may use 
daily checks, in lieu of detection and indication systems to achieve 
the objective of this requirement. These Certification Maintenance 
Requirements (CMRs) must be limited to components that are not readily 
detectable by normal detection and indication systems and where service 
history shows that inspections will provide an adequate level of 
safety.
    (b) The existence of any failure condition, not extremely 
improbable, during flight that could significantly affect the 
structural capability of the airplane and for which the associated 
reduction in airworthiness can be minimized by suitable flight 
limitations, must be signaled to the flight crew. For example, failure 
conditions that result in a factor of safety between the airplane 
strength and the loads of subpart C below 1.25, or flutter margins 
below V'', must be signaled to the crew during flight.
    5. Dispatch with known failure conditions. If the airplane is to be 
dispatched in a known system failure condition that affects structural 
performance, or affects the reliability of the remaining system to 
maintain structural performance, then the provisions of these special 
conditions must be met, including the provisions of paragraph 2 for the 
dispatched condition, and paragraph 3 for subsequent failures. Expected 
operational limitations may be taken into account in establishing Pj as 
the probability of failure occurrence for determining the safety margin 
in Figure 1. Flight limitations and expected operational limitations 
may be taken into account in establishing Qj as the combined 
probability of being in the dispatched failure condition and the 
subsequent failure condition for the safety margins in Figures 2 and 3. 
These limitations must be such that the probability of being in this 
combined failure state and then subsequently encountering limit load 
conditions is extremely improbable. No reduction in these safety 
margins is allowed if the subsequent system failure rate is greater 
than 10-3 per hour.

C. Outboard Aileron Modal Suppression System

    1. In general, these special conditions apply to fly-by-wire active 
flutter suppression systems that are intended to operate on a certain 
type of aeroelastic instability. This type of instability is 
characterized by a low frequency, self-excited, sustained oscillation 
of an aeroelastic vibration

[[Page 30529]]

mode that is shown to be a stable limit cycle oscillation (LCO), with 
the system operative and inoperative. (An LCO is considered ``stable'' 
if it maintains the same frequency and amplitude for a given excitation 
input and flight condition.) In addition, the type of sustained 
oscillation covered by these special conditions must not be a hazard to 
the airplane nor its occupants with the active system failed. These 
systems must be shown to reduce the amplitude of the sustained 
oscillation to acceptable levels and effectively control the 
aeroelastic instability. Specifically, the following criteria address 
the existence of such a sustained oscillation on the Boeing Model 747-
8/-8F airplanes and the Outboard Aileron Modal Suppression (OAMS) 
system that will be used to control it.
    2. In lieu of the requirements contained in Sec.  25.629, the 
existence of a sustained, or limit cycle, oscillation that is 
controlled by an active flight control system is acceptable, provided 
that the following requirements are met:
    (a) OAMS System Inoperative: The sustained, or limit cycle, 
oscillation must be shown by test and analysis to be stable throughout 
the nominal aeroelastic stability envelope specified in Sec.  
25.629(b)(1) with the OAMS system inoperative. This should include the 
consideration of disturbances above the sustained amplitude of 
oscillation.
    (b) Nominal Conditions:
    (1) With the OAMS system operative it must be shown that the 
airplane remains safe, stable, and controllable throughout the nominal 
aeroelastic stability envelope specified in Sec.  25.629(b)(1) by 
providing adequate suppression of the aeroelastic modes being 
controlled. All applicable airworthiness and environmental requirements 
should continue to be complied with. Additionally, loads imposed on the 
airplane due to any amplitude of oscillation must be shown to have a 
negligible impact on structure and systems, including wear, fatigue and 
damage tolerance. The OAMS system must function properly in all 
environments that may be encountered.
    (2) The applicant must establish by test and analysis that the OAMS 
system can be relied upon to control and limit the sustained amplitude 
of the oscillation to acceptable levels (per Sec.  25.251) and control 
the stability of the aeroelastic mode. This should include the 
consideration of disturbances above the sustained amplitude of 
oscillation; maneuvering flight, icing conditions; manufacturing 
variations; Master Minimum Equipment List (MMEL) items; spare engine 
carriage; engine removed or inoperative ferry flights; and wear, 
repairs, and modifications throughout the service life of the airplane 
by:
    (i) Analysis to the nominal aeroelastic stability envelope 
specified in Sec.  25.629(b)(1), and
    (ii) Flight flutter test to the VDF/MDF 
boundary. These tests must demonstrate that the airplane has a proper 
margin of damping for disturbances above the sustained amplitude of 
oscillation at all speeds up to VDF/MDF, and that 
there is no large and rapid reduction in damping as VDF/
MDF is approached.
    (iii) The structural modes must have adequate stability margins for 
any OAMS flight control system feedback loop at speeds up to the fail-
safe aeroelastic stability envelope specified in Sec.  25.629(b)(2).
    (c) Failures, Malfunctions, and Adverse Conditions:
    (1) For the OAMS system operative and failed, for any failure, or 
combination of failures not shown to be extremely improbable, and 
addressed by Sec. Sec.  25.629(d), 25.571, 25.631, 25.671, 25.672, 
25.901(c) or 25.1309 that results in LCO, it must be established by 
test or analysis up to the aeroelastic stability envelope specified in 
Sec.  25.629(b)(2) that the LCO:
    (i) Is stable and decays to an acceptable limited amplitude once an 
external perturbing force is removed;
    (ii) Does not result in loads that would cause static, dynamic, or 
fatigue failure of structure during the expected exposure period;
    (iii) Does not result in repeated loads that would cause an 
additional failure due to wear during the expected exposure period that 
precludes safe flight and landing;
    (iv) Has, if necessary, sufficient indication of OAMS failure(s) 
and crew procedures to properly address the failure(s);
    (v) Does not result in a vibration condition on the flight deck 
that is severe enough to interfere with control of the airplane, 
ability of the crew to read the flight instruments, perform vital 
functions like reading and accomplishing checklist procedures, or to 
cause excessive fatigue to the crew;
    (vi) Does not result in adverse effects on the flight control 
system or on airplane stability, controllability, or handling 
characteristics (including airplane-pilot coupling (APC) per Sec.  
25.143) that would prevent safe flight and landing; and
    (vii) does not interfere with the flight crew's ability to 
correctly distinguish vibration from buffeting associated with the 
recognition of stalls or high speed buffet.
    (2) The applicant must show that particular risks such as engine 
failure, uncontained engine, or APU rotor burst, or other failures not 
shown to be extremely improbable, will not adversely or significantly 
change the aeroelastic stability characteristics of the airplane.
    (3) No MMEL dispatch is allowed with the OAMS system inoperative.

    Issued in Renton, Washington, on May 20, 2011.
Ali Bahrami,
Manager, Transport Airplane Directorate, Aircraft Certification 
Service.
[FR Doc. 2011-13022 Filed 5-25-11; 8:45 am]
BILLING CODE 4910-13-P