CFR Final Rule
Federal Register Information
[Federal Register: June 29, 1992 (Volume 57, Number 125)]
DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
14 CFR Part 25
[Docket No. 26007; Amendment No. 25-77]
Vibration, Buffet and Aeroelastic Stability Requirements for Transport Category Airplanes
AGENCY: Federal Aviation Administration, DOT
ACTION: Final Rule
SUMMARY: This amendment revises the airworthiness standards of the Federal Aviation Regulations (FAR) for type certification of transport category airplanes concerning vibration, buffet, flutter and divergence. It clarifies the requirement to consider flutter and divergence when treating certain damage and failure conditions required by other sections of the FAR and adjusts the safety margins related to aeroelastic stability to make them more appropriate for the conditions to which they apply. These changes are made to provide consistency with other sections of the FAR and to take into account advances in technology and the evolution of the design of transport airplanes.
EFFECTIVE DATE: This rule becomes effective July 29, 1992.
FOR FURTHER INFORMATION CONTACT: James Haynes, Airframe and Propulsion Branch (ANM-112), Transport Airplane Directorate, Aircraft Certification Service, FAA, 1601 Lind Avenue SW., Renton, Washington 98055-4056, telephone (206) 227-2131.
The term "aeroelastic" is applied to an important class of phenomena which involves the mutual interaction between the inertial, aerodynamic, and elastic forces in a structure. These forces can interact to give rise to a variety of aeroelastic phenomena ranging from transient or dynamic responses as a result of external forces (vibration or buffeting) to aeroelastic instabilities (flutter or divergence). The importance distinction between response and instability phenomena is that instabilities are self-excited, that is, they can exist even in smooth air in the absence of any external forces. A slight perturbation of the structure at or above the critical airspeed is all that is needed to initiate the unstable condition which then may be maintained or grow to destructive proportions in the absence of any external forces.
Few aeroelastic phenomena fit neatly into classifications where exact definitions can be considered to apply without qualification. The following definitions should be considered to apply to classical aeroelastic phenomena and used with a certain amount of judgment since not even the experts in the field would agree completely on any set of definitions.
1. Vibration. An oscillation of the structure or a control surface resulting from an independent external excitation.
2. Buffeting. A random oscillation of the structure resulting from unsteady aerodynamic forces, usually associated with separated airflow.
3. Flutter. The unstable self-excited structural oscillation at a definite frequency where energy is extracted from the airstream by the motion of the structure. The deformation and motion of the structure result in forces on the structure that tend to maintain or augment the motion. The displacement modes associated with flutter instabilities are sometimes called "flutter modes."
4. Whirl Flutter. Flutter in which the aerodynamic and gyroscopic forces associated with rotations and displacements in the plane of a propeller or large turbofan play an important role. The displacement modes associated with whirl flutter are sometimes called "whirl modes."
5. Divergence. A static instability at a speed where the aerodynamic forces resulting from the deformation of the structure exceed the elastic restoring forces resulting from the same deformation.
6. Control Reversal. A condition generally occurring at higher speeds in which the intended effects of displacing a given component of the control system are completely overcome by the aeroelastic effects of structural deformation, resulting in reversed command.
7. Deformation Instability. The loss of airplane stability and control as a result of the aeroelastic effects of structural deformation.
Many of the above terms have been used in the airworthiness regulations and associated advisory material for many years and there is no intent to redefine these phenomena or require consideration of new phenomena by this amendment.
This amendment is based on Notice of Proposed Rulemaking (NPRM) No. 89-24 which was published in the Federal Register on September 12, 1989, (54 FR 37768). The notice proposed to revise and update the requirements concerning vibration, buffet, and aeroelastic stability to make these requirements more consistent with modern transport airplane designs. It was proposed to augment the list of failures, malfunctions and adverse conditions by including additional damage and failure conditions that have been added to other sections of the FAR. In addition, the FAA proposed in the NPRM to revise the safety margins for aeroelastic stability to make them more appropriate for the conditions to which they applied and more consistent with advances in technology of transport airplane design. Additional proposals were to reorganize certain requirements so that structural load requirements, flight requirements, and aeroelastic stability requirements would be set forth in the proper sections and subparts of part 25.
In the 1940's, when the first transport airplane flutter and divergence requirements were introduced, a safety margin was established by requiring that the airplane be designed to be free from flutter and divergence at an airspeed 20 percent greater than the maximum design dive speed. Flutter analyses, using the available theoretical methods of that time, were used to show compliance. The 20 percent margin was intended to account for the inaccuracy in the analytical prediction of the flutter speed, as established by those early methods, and to provide for production and service variations. The ability of the industry to substantiate freedom from flutter and other aeroelastic instability phenomena has been continually improving. Current analytical methods employ finite element solutions with advanced unsteady aerodynamic theories and can accommodate airplanes of complex configurations. In addition, model testing, ground vibration testing and flight flutter testing techniques have all undergone significant improvements. Complete airplane experimental modal analyses are now commonplace. Furthermore, the cost of these analytical methods and testing techniques has been kept reasonable by the advances in computer technology. Because of these improvements, the FAA proposed in Notice 89-24 to reduce the 20 percent margin to 15 percent.
Part 25 has been continually upgraded with failure and damage requirements in other sections. Among these requirements are the criteria for complete loss of all engines in Sec. 25.671, the empennage bird strike criteria of Sec. 25.631, and the discrete source damage criteria of Sec. 25.571(e). These sections generally require "no catastrophic failure" or "safe flight and landing" or similar provisions in the event of specified failure conditions. These regulations have been interpreted to require flutter substantiation if the failure or damage event could have a significant effect on the flutter modes. In Notice 89-24 the FAA proposed to amend Sec. 25.829 to directly reference many of these requirements to make it clear that freedom from aeroelastic instability is required to be demonstrated for these additional failure and damage conditions.
The design margin for the fail-safe design conditions has been the margin between design cruise speed, and design dive speed, . This margin originally was 25 percent, but has since been reduced by the incorporation of an upset criterion to establish (Sec. 25.385(b)). This criterion generally results in a margin of between 15 and 20 percent on modern conventional transport airplanes at altitudes where is not limited by Mach number. One recent airplane design incorporating a speed protection system would have resulted in even lower margins had the FAA not issued a special condition requiring that this margin be at least 15 percent. In Notice 69-24 the FAA proposed that the fail-safe margin not be allowed to be lower than 15 percent for the fail-safe design conditions. However, further adjustments in the margin were proposed for altitudes where design speeds are limited by Mach number.
Discussion of Comments
Comments were received from foreign and domestic airplane manufacturers, foreign airworthiness authorities, airplane operator and manufacturer trade groups, pilots associations and private individuals. The majority of commenters express support for the proposals, especially in regard to the attempt to modernize the requirements and adjust the safety margins so that they are more appropriate for modern transport airplane designs and take into consideration modern technology. As a result of the comments, several changes were made to the proposals to improve their organization and clarity.
One commenter suggests that the references to Sec. 25.1309 and the use of the phrase "extremely improbable" in the proposed rule be accompanied with a numerical probability value. The phrase "extremely improbable" was contained in the previous rule and was not a new proposal in the NPRM. Acceptable methods of compliance are described in FAA Advisory Circular 1309-1A, System Design and Analysis. However, the FAA appreciate the commenter's design for specific compliance criteria and is currently assessing the need for additional advisory material to treat failure analyses as they relate to flutter. If additional guidance is found necessary, it will be included in the appropriate advisory circular.
The same commenter suggests that the requirement concerning oscillatory failures in the proposed Sec. 25.305(f) was more restrictive than the current requirement. The commenter believes that the requirement for the resulting loads to be considered as limit load conditions is an increase in the current requirements and not consistent with conditions related to failures which should be treated as ultimate conditions.
The FAA disagrees. Limit loads (the maximum loads to be expected in service) are required to be sustained without permanent deformation of the structure. Ultimate loads are loads that are required to be sustained without failure, although permanent deformation is allowed. Section 25.301(a) states that all loads prescribed in the FAR are limit loads unless otherwise specified. Only loads from certain failure conditions, as specified by the regulations, are allowed to be treated as ultimate load conditions. These are generally load conditions that are independent of the failure event and not likely to be achieved during the time the failure exists. However, the oscillatory load condition concerns loads that result directly from the failure itself and involve a repetition of these loads at a rapid frequency. These loads have historically been treated as limit loads, and this amendment merely clarifies the requirement that this failure condition is to be treated as a limit load condition.
Several commenters object to the provisions relating to damage tolerance contained in paragraphs Sec. 25.629(d)(2) (i) and (ii) of the NPRM, which were intended to provide a means of establishing the necessity for considering single failures of engine structures, engine mounts, and supports for external bodies, propellers or rotating machinery. The commenters believe that it is inappropriate to establish damage tolerance criteria in Sec. 25.629 that are different and could be more restrictive than Sec. 25.571 which specifically covers damage tolerance evaluation. The FAA agrees, and the paragraphs have been revised to provide relief from the single failure requirement from these structures if an analysis under Sec. 25.571(b) and 25.571(e) indicate that consideration of a single failure is unnecessary for meeting those requirements. For the purposes of organizational clarity, this revised requirement is consolidated with Sec. 25.629(d)(3)(ix) of the proposal, which also referred to Sec. 25.571, and set forth in Sec. 25.629(d)(8) of this amendment. Further consolidation of the proposed Secs. 25.629(d)(3)(viii) and 25.629(d)(3)(ix) resulted in Sec. 25.629(d)(9) of this amendment.
Several commenters suggest that a specific minimum damping value be provided in the rule to define a proper margin of damping for aeroelastic modes; however, no suggestions for specific criteria were provided. The current Advisory Circular (AC) 25.629-1, Flutter Substantiation of Transport Airplanes, provides guidance relative to establishing a proper margin of damping which depends on the analytical methodology and on the general character of the aeroelastic mode. It is not practicable to establish a regulatory requirement for a specific damping margin that would be appropriate in all cases.
The majority of commenters express support for the change in the flutter substantiation speed margin from 1.2 to 1.15 . However, several commenters are concerned that the modern analytical methods, which they believed to be the basis for making this reduction, are not mandated by regulation nor necessarily practiced by all manufacturers. As discussed previously, the reduction was not proposed as a result of improvements in analytical methodology alone; but is also attributable to improved testing methods and improvements in other related requirements. Furthermore, an analytical speed margin alone does not in itself provide a guarantee of freedom from flutter regardless of its actual value. This is because many modes can become critical well within the flight envelope by only small changes in other parameters. An extensive parametric investigation to establish sensitivities and to develop a proper margin with respect to all important parameters (altitude, air forces, rigidity, mass balance, etc.) is an essential part of any aeroelastic investigation. This is a required certification practice for transport airplanes with respect to flutter substantiation as explained in AC 25.629-1.
Furthermore, the analytical speed margins required by the previous regulation were inconsistent with the accuracy associated with predicting flutter for the various conditions. For modern transport category airplanes, the 20 percent margin was required for the nominal (unfailed) airplane at the lower altitudes and these are the most reliable conditions to analyze. However, the analytical speed margins for the nominal airplane at altitudes where operating speed is limited by Mach number, and for failure cases at any altitude, were permitted to be much less than 20 percent even though aeroelastic instabilities for these conditions are less reliably predicted. This amendment establishes a more consistent speed margin for all conditions including failure cases.
Another commenter suggests that the change in the speed margin should not be allowed as long as the FAA accepts the traditional "strip theory" method of flutter analysis and does not mandate the more recently developed "doublet-lattice" method which the commenter asserts to be more reliable. Since all analytical methods have deficiencies with respect to certain configurations, the FAA prefers not to mandate specific theoretical methods by regulation. In many cases, more than one analytical method may be necessary in order to overcome deficiencies that a particular method might have with specific configurations. It is necessary that any analytical methodology used for flutter substantiation be validated for the specific application and be shown to reliably predict the aeroelastic characteristics of the airplane. This validation is normally based on correlation with actual test data such as wind tunnel data, ground vibration test data, and flight test results. Guidance pertaining to validation of analytical methodology is contained in AC 25.629-1.
One commenter states that the requirement to consider mismanagement of fuel conditions is considerably beyond the normal design practices. The FAA disagrees since consideration of fuel mismanagement conditions has been a standard practice for many years, and, in fact, although not explicitly listed, has been considered necessary in showing compliance with Sec. 25.629. The new rule makes this condition explicit by adding it to the list of failure and adverse conditions so that it cannot be overlooked.
Another commenter suggests that the requirement for the treatment of whirl flutter should include a specific requirement to consider the influence of a non-uniform airstream on propellers installed in a pusher configuration. The general objective language, as proposed, is sufficient for requiring these considerations. These analytical details will be considered for inclusion in the appropriate advisory circular.
The same commenter also points out that, in addition to pitch and yaw rigidity, the translational rigidity of propeller axes can also be important for certain configurations. The FAA agrees and paragraph (d)(5) has been revised to delete the words "pitch and yaw" so that it addresses "rigidity" in general.
One commenter suggests that the consideration of single failures in flutter damper systems should not be required if they can be shown to be extremely improbable. The FAA disagrees; this single failure requirement already existed in the previous regulation and was intended to provide a single failure requirement for passive flutter dampers, equivalent to that already provided in Sec. 25.671(c)(1) for flight control systems. Although flutter dampers are typically mechanical components, similar in design and criticality to mechanical control system components, they may not necessarily be considered part of the flight control system. Therefore, it is necessary to provide a separate single failure requirement for them in Sec. 25.629(d).
One additional change was to delete a statement in the proposal that provided for substantiation of the failure and damage events by showing that losses in rigidity or changes in frequency, mode shape, or damping are within the parameter investigations shown to be satisfactory in the flutter and divergence investigations. While there is no intent to eliminate this approach as an acceptable means of compliance, the FAA considers it unnecessary to prescribe it in the regulations. This method of compliance is specifically provided for in AC 25.629-1.
This section summarizes the full regulatory evaluation prepared by the FAA that provides more detailed estimates of the economic consequences of this regulatory action. This summary and the full evaluation quantify, to the extent practicable, estimated costs to the private sector, consumers, Federal, State and local governments, as well as anticipated benefits.
Executive Order 12291, dated February 17, 1981, directs Federal agencies to promulgate new regulations or modify existing regulations only if potential benefits to society for each regulatory change outweigh potential costs. The order also requires the preparation of a Regulatory Impact Analysis of all "major" rules except those responding to emergency situations or other narrowly defined exigencies. A "major" rule is one that is likely to result in an annual increase in consumer costs, a significant adverse effect on the economy of $100 million or more, a major increase in consumer costs, a significant adverse effect on competition, or is highly controversial.
The FAA has determined that this rule is not "major" as defined in the executive order, therefore a full regulatory analysis, that includes the identification and evaluation of cost reducing alternatives to this rule, has not been prepared. Instead, the agency has prepared a more concise document termed a regulatory evaluation that analyzes only this rule without identifying alternatives. In addition to a summary of the regulatory evaluation, this section also contains a regulatory flexibility determination required by the Regulatory Flexibility Act of 1989 (Pub. L. 96-354) and an international trade impact assessment. If more detailed economic information is desired than is contained in this summary, the reader is referred to the full regulatory evaluation in the docket.
This rule applies to manufacturers of airplanes built to part 25 standards. It will have no impact, positive or negative, on the level of safety associated with the operation of transport category airplanes. It will provide a limited, but undetermined, amount of cost savings to manufacturers by reducing the design margin for airspeed. Another benefit of the rule is that it will update, reorganize and clarify the intent of various sections within part 25 concerning vibration, buffet, flutter and divergence. Since no increase in cost is associated with this rule, and since there are benefits of the rule, associated with cost reduction to transport airplane manufacturers, and improved organization, consistency, and clarity within part 25, this rule is cost-effective.
The following table summarizes each of the changes and briefly assesses their economic impact.
International Trade Impact Assessment
This rule will have little or no impact on the trade opportunities for both U.S. firms doing business in foreign countries and foreign firms doing business in the United States. If foreign nations do not adopt U.S. standards, their manufacturers may be at a disadvantage in the U.S. market. However, the impact is expected to be slight. If foreign manufacturers do adopt U.S. standards, U.S. manufacturers selling abroad could continue to design to foreign standards which would also meet U.S. standards.
Regulatory Flexibility Determination
Under the criteria of the Regulatory Flexibility Act of 1980 and FAA Order 2100.14A, (Regulatory Flexibility Criteria and Guidance), the FAA has determined that the rule will not have a significant economic impact on a substantial number of small entities. Only U.S. manufacturers of transport category airplanes will be affected, and none of the transport category airplane manufacturers in the United States meets the criteria of a small entity.
The regulations adopted herein do 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 such a regulation does not have sufficient federalism implications to warrant the preparation of a Federalism Assessment.
Because the requirement to consider flutter and divergence when testing certain damage and failure conditions required by the FAR is not expected to result in a substantial cost, the FAA has determined that this final rule is not major as defined in Executive Order 12291. This final rule is considered to be significant as defined in Department of Transportation Regulatory Policies and Procedures (44 FR 11034, February 26, 1979). In addition, since there are no small entities affected by this rulemaking, it is certified, under the criteria of the Regulatory Flexibility Act, that this final rule, at promulgation, will not have a significant economic impact, positive or negative, on a substantial number of small entities. A copy of the final regulatory evaluation prepared for this project may be examined in the public docket or obtained from the person identified under the caption "For Further Information Contact."
List of Subjects in 14 CFR Part 25
Air transportation, Aircraft, Aviation safety, Safety.
Accordingly, 14 CFR part 25 of the Federal Aviation Regulations (FAR) is amended as follows:
Part 25-Airworthiness Standards: Transport Category Airplanes
1. The authority citation for part 25 continues to read as follows:
Authority: 49 U.S.C. 1344, 1354(a), 1355, 1421, 1423, 1424, 1425, 1428, 1429, 1430, 49 U.S.C. 106(g) and 49 CFR 1.47(a).
2. By amending Sec. 25.251 by revising paragraphs (a) and (b) to read as follows:
Sec. 25.251 Vibration and buffeting.
(a) The airplane must be demonstrated in flight to be free from any vibration and buffeting that would prevent continued safe flight in any likely operating condition.
(b) Each part of the airplanes must be demonstrated in flight to be free from excessive vibration under any appropriate speed and power conditions up to . The maximum speeds shown must be used in establishing the operating limitations of the airplane in accordance with Sec. 25.1505.
* * * * *
3. By amending Sec. 25.305 by adding new paragraphs (e) and (f) to read as follows:
Sec. 25.305 Strength and deformation.
* * * * *
(e) The airplane must be designed to withstand any vibration and buffeting that might occur in any likely operating condition up to , including stall and probable inadvertent excursions beyond the boundaries of the buffet onset envelope. This must be shown by analysis, flight tests, or other tests found necessary by the Administrator.
(f) Unless shown to be extremely improbable, the airplane must be designed to withstand any forced structural vibration resulting from any failure, malfunction or adverse condition in the flight control system. These must be considered limit loads and must be investigated at airspeeds up to .
4. By revising Sec. 25.629 to read as follows:
Sec. 25.629 Aeroelastic stability requirements.
(a) General. The aeroelastic stability evaluations required under this section include flutter, divergence, control reversal and any undue loss of stability and control as a result of structural deformation. The aeroelastic evaluation must include whirl modes associated with any propeller or rotating device that contributes significant dynamic forces. Compliance with this section must be shown by analyses, wind tunnel tests, ground vibration tests, flight tests, or other means found necessary by the Administrator.
(b) Aeroelastic stability envelopes. The airplane must be designed to be free from aeroelastic instability for all configurations and design conditions within the aeroelastic stability envelopes as follows:
(1) For normal conditions without failures, malfunctions, or adverse conditions, all combinations of altitudes and speeds encompassed by the versus altitude envelope enlarged at all points by an increase of 15 percent in equivalent airspeed at both constant Mach number and constant altitude. In addition, a proper margin of stability must exist at all speeds up to and, there must be no large and rapid reduction in stability as is approached. The enlarged envelope may be limited to Mach 1.0 when is less than 1.0 at all design altitudes, and
(2) For the conditions described in Sec. 25.629(d) below, for all approved altitudes, any airspeed up to the greater airspeed defined by;
(i) The envelope determined by Sec. 25.335(b); or,
(ii) An altitude-airspeed envelope defined by a 15 percent increase in equivalent airspeed above at constant altitude, from sea level to the altitude of the intersection of 1.15 with the extension of the constant cruise Mach number line, then a linear variation in equivalent airspeed to at the altitude of the lowest intersection; then, at higher altitudes, up to the maximum flight altitude, the boundary defined by a .05 Mach increase in at constant altitude.
(c) Balance weights. If concentrated balance weights are used, their effectiveness and strength, including supporting structure, must be substantiated.
(d) Failures, malfunctions, and adverse conditions. The failures, malfunctions, and adverse conditions which must be considered in showing compliance with this section are:
(1) Any critical fuel loading conditions, not shown to be extremely improbable, which may result from management of fuel.
(2) Any single failure in any flutter damper system.
(3) For airplanes not approved for operation in icing conditions, the maximum likely ice accumulation expected as a result of an inadvertent encounter.
(4) Failure of any single element of the structure supporting any engine, independently mounted propeller shaft, large auxiliary power unit, or large externally mounted aerodynamic body (such as an external fuel tank).
(5) For airplanes with engines that have propellers or large rotating devices capable of significant dynamic forces, any single failure of the engine structure that would reduce the rigidity of the rotational axis.
(6) The absence of aerodynamic or gyroscopic forces resulting from the most adverse combination of feathered propellers or other rotating devices capable of significant dynamic forces. In addition, the effect of a single feathered propeller or rotating device must be coupled with the failures of paragraphs (d)(4) and (d)(5) of this section.
(7) Any single propeller or rotating device capable of significant dynamic forces rotating at the highest likely overspeed.
(8) Any damage or failure condition, required or selected for investigation by Sec. 25.571. The single structural failures described in paragraphs (d)(4) and (d)(5) of this section need not be considered in showing compliance with this section if;
(i) The structural element could not fail due to discrete source damage resulting from the conditions described in Sec. 25.571(e), and
(ii) A damage tolerance investigation in accordance with Sec. 25.571(b) shows that the maximum extent of damage assumed for the purpose of residual strength evaluation does not involve complete failure of the structural element.
(9) Any damage, failure, or malfunction considered under Secs. 25.631, 25.671, 25.672, and 25.1309.
(10) Any other combination of failures, malfunctions, or adverse conditions not shown to be extremely improbable.
(e) Flight flutter testing. Full scale flight flutter tests at speeds up to must be conducted for new type designs and for modifications to a type design unless the modifications have been shown to have an insignificant effect on the aeroelastic stability. These tests must demonstrate that the airplane has a proper margin of damping at all speeds up to , and that there is no large and rapid reduction in damping as , is approached. If a failure, malfunction, or adverse condition is simulated during flight test in showing compliance with paragraph (d) of this section, the maximum speed investigated new not exceed if it is shown, by correlation of the flight test data with other test data or analyses, that the airplane is free from any aeroelastic instability at all speeds within the altitude-airspeed envelope described in paragraph (b)(2) of this section.
Issued in Washington, DC, on June 22, 1992.
Barry Lambert Harris,
[FR Doc. 92-15130 Filed 6-26-92; 8:45 am]
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