CFR NPRM

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[Federal Register: September 12, 1989 (Volume 54, Number 175)]
[Page 37768]

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DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
14 CFR Part 25
[Docket No. 26007; Notice No. 89-24]
RIN 2120-AD38
Vibration, Buffet and Aerolastic Stability Requirements for Transport Category Airplanes

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AGENCY: Federal Aviation Administration, DOT
ACTION: Notice of Proposed Rulemaking

SUMMARY: This notice proposes to revise the airworthiness standards of the Federal Aviation Regulations (FAR) for transport category airplanes concerning flutter, divergence, vibration, and buffet. It would clarify the requirement to consider flutter and divergence when treating certain damage and failure conditions required by other sections of the FAR. It would revise the required safety margins by slightly reducing the safety margin concerning airplane speed for normal configurations and providing a minimum safety margin concerning airplane speed for damage and failure configurations. These changes are intended to provide consistency with other sections of the FAR, to relieve a design burden which is now unnecessary as a result of advances in technology, and to improve certain safety margins as a result of the evolution in the design of transport airplanes.
DATES: Comments must be received on or before March 12, 1990.
ADDRESSES: Comments on this proposal may be mailed in duplicate to: Federal Aviation Administration, Office of the Chief Counsel, Attention: Rules Docket (AGC-10), Docket No. 26007, 800 Independence Avenue SW., Washington, DC 20591, or delivered in duplicate to: Room 915G, 800 Independence Avenue SW., Washington, DC 20591. Comments delivered must be marked: Docket No. 26007. Comments may be inspected in Room 915G weekdays, except Federal holidays, between 8:30 a.m. and 5:00 p.m. In addition, the FAA is maintaining an information docket of comments in the Office of the Regional Counsel (ANM-7), FAA, Northwest Mountain Region, 17900 Pacific Highway South, C-68966, Seattle, Washington 98168. Comments in the information docket may be inspected in the Office of the Regional Counsel weekdays, except Federal holidays, between 7:30 a.m. and 4:00 p.m.
FOR FURTHER INFORMATION CONTACT: James Haynes, Airframe and Propulsion Branch (ANM-112), Transport Airplane Directorate, Aircraft Certification Service, FAA, 17900 Pacific Highway South, C-68966, Seattle, Washington 98168; telephone (206) 431-2113.
SUPPLEMENTARY INFORMATION:

Comments Invited

Interested persons are invited to participate in the proposed rulemaking by submitting such written data, views, or arguments as they may desire. Comments relating to the environmental, energy, or economic impact that might result from adoption of proposals contained in this notice are also invited. Substantive comments should be accompanied by cost estimates. Commenters should identify the regulatory docket or notice number and submit comments, in duplicate, to the Rules Docket address specified above. All comments will be considered by the Administrator before taking action on the proposed rulemaking. The proposals contained in this notice may be changed in light of comments received. All comments will be available in the Rules Docket, both before and after the closing date for comments, for examination by interested persons. A report summarizing each substantive public contact with FAA personnel concerning this rulemaking will be filed in the docket. Commenters wishing the FAA to acknowledge receipt of their comments must submit with those comments a self-addressed, stamped postcard on which the following statement is made: "Comments to Docket No. 26007." The postcard will be date/time stamped and returned to the commenter.

Availability of NPRM

Any person may obtain a copy of this NPRM by submitting a request to the Federal Aviation Administration, Office of Public Affairs, Attention: Public Information Center, APA-430, 800 Independence Avenue SW., Washington, DC 20591; or by calling (202) 267-3484. Communications must identify the notice number of this NPRM. Persons interested in being placed on a mailing list for future NPRMs should also request a copy of Advisory Circular No. 11-2A, Notice of Proposed Rulemaking Distribution System, which describes the application procedures.

Background

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 important distinction between response and instability phenomena is that instabilities are self-excited, that is, they can exit 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. For the purpose of better understanding the proposals in this NPRM the following definitions are provided. They 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 of 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 flow.
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 potential flutter instabilities are often called "flutter modes" even though they may be well damped or do not become unstable within the flight envelope.
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 frequently 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 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 at higher speeds.
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 proposal.
Regulations dealing with flutter and divergence for transport category airplanes were first introduced in part 04 of the Civil Air Regulations (CAR) in the 1940's. 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. A 20 percent margin was chosen as the safety margin for civil airplanes after comparing analytical studies with the results of model testing conducted by the Army Air Corps. Based on the same studies and tests, a 15 percent margin was chosen by the Army Air Corps as the safety margin for the related military specification.
The flutter requirement of part 04 evolved into section 4b.306 of the CAR, where developing fail-safe philosophy continued to change the scope of the flutter and divergence substantiation requirements. Among the early fail-safe provisions were the requirements that control surface tabs and flutter damper systems be fail-safe. The most comprehensive fail-safe requirement was adopted in 1964 (29 FR 12609, September 5, 1964) and required compliance with the single failure criteria for the entire airplane, as well as compliance with special provisions for turbopropeller airplanes. The most recent substantive change in the fail-safe provisions was the addition of a requirement in Sec. 25.629(d) for freedom from flutter with any combination of failures not shown to be extremely improbable (Amendment 25-46, 43 FR 50578; October 30, 1978).
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). Those sections generally require "no catastrophic failure" or "safe flight and landing" or similar provisions. These regulations have been interpreted to require flutter substantiation if the failure or damage event could have a significant effect on the flutter modes.
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. 25.335(b)). This criterion generally results in a margin of between 15 and 20 percent of modern conventional transport airplanes at altitudes where is not limited by Mach number.
While the scope of the flutter requirements was being widened by additional fall-safe criteria, the ability of the industry to substantiate freedom from flutter and other aeroelastic instability phenomena was continually improving. At the time the 20 percent margin was established, the analytical capability was minimal and unreliable without a large speed margin. 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.
The requirements to withstand vibration and buffet, as contained in part 4b of the CAR at the time of recodification to part 25 of the FAR, were divided into two parts, (1) a flight demonstration requirement (section 4b.190) in subpart B "Flight" and (2) a design requirement (section 4b.308) in subpart C "Structure." As a result of the recodification, these requirements were combined into a single requirement in Sec. 25.251 of subpart B "Flight." In addition, since Sec. 4b.190 referred to both flutter and vibration, the fail-safe flutter requirements from section 4b.308 were also incorporated into Sec. 25.251 by reference to the new Sec. 25.629(d) which was the recodified section 4b.308(d). However, at the same time the reference to flutter was removed from Sec. 25.251 and it became only a vibration and buffet regulation. As a result of the placement in subpart B, there is a tendency to interpret the vibration ad buffet requirements as only flight requirements. Also, because of the reference to the fail-safe requirements of Sec. 25.629(d), Sec. 25.251 literally requires that freedom from excessive vibration be demonstrated with failure conditions. The rule also continued to reference the structural design dive speed rather than a flight speed such as .

Discussion

The scope of this proposal is to revise Sec. 25.629 "Flutter, deformation and fail-safe criteria" which includes several substantive changes, and to reorganize some requirements of Sec. 25.251 "Vibration and buffeting," without substantive changes.
The proposed changes to Sec. 25.251 involve the creation of a new Sec. 25.305(e) to incorporate the design requirements for buffet and vibration of Sec. 25.251(a) into Sec. 25.305, creating a new Sec. 25.305(e). This requirement is a structural design requirement and should be set forth in subpart C. Section 25.251(a) would be revised to require only a flight demonstration for freedom from vibration and buffet in any likely operating condition, including probable inadvertent excursions beyond the buffet boundaries. Furthermore, the third sentence of Sec. 25.251(b) would be deleted. This would provide relief from the requirement that freedom from excessive vibration be demonstrated in flight for the failure and damage conditions of Sec. 25.629(d). The speed referenced in Sec. 25.251 would be changed to the flight speed while the corresponding structural requirements that are moved to Sec. 25.305(e) would continue to reference the structural design dive speed .
It is also proposed to relocate the requirement for the evaluation of loads resulting from forced structural vibration after failures in the automatic flight control system. This requirement is currently located in Sec. 25.629(d)(4)(vi), although it is a structural loading condition for oscillatory failures rather than for aeroelastic instability. It is proposed that it be set forth in subpart C, specifically in Sec. 25.305, creating a new paragraph Sec. 25.305(f). Furthermore, it is proposed to clarify that the loads resulting from these forced structural vibrations are limit loading conditions.
Section 25.629 would be retitled "Aeroelastic stability requirements" to more accurately describe the objective of the revised section. Originally the regulation contained vibration, buffet requirements and oscillatory failure load requirements, as well as flutter, divergence, control reversal and deformation instability requirements. However, as a result of this proposal and previous amendments, the rule would only contain flutter (including whirl flutter), divergence, control reversal, and deformation instability requirements, all of which can be considered aeroelastic instabilities.
The references to propellers and turbopropellers in Sec. 25.629 would be replaced by "propeller or rotating device that contributes significant dynamic forces" to encompass all types of rotating machinery which could influence the basic aeroelastic modes or create new "whirl modes." The general growth of compressors to large bypass fans and now to unducted fans has obscured the differences between propellers and other rotating machinery. The proposed rule would impose the requirements for the consideration of gyroscopic inertial forces and whirl flutter analysis on a more objective basis.
It is proposed to reduce the design envelope in which freedom from aeroelastic instability is to be shown for the normal (undamaged) airplane. The requirement for a 20 percent increase in equivalent airspeed at both constant altitude and constant Mach number would be reduced to a 15 percent increase. The principal purpose of the 20 percent margin has been for substantiation reliability. When the 20 percent margin was first established, flutter and divergence substantiation was in its infancy, and a large margin was needed because of the unreliability of the techniques. In addition, there were no failure or damage conditions at that time and the 20 percent margin, by virtue of the added stiffness, provided some degree of protection against damage and failure conditions as well as production and service variations. The transport airplane aeroelastic stability requirements, as provided in this proposal, and advances in aeroelastic substantiation techniques are now sufficient to justify a reduction in the substantiation margin. These provisions now require a complete program validated with test data and full-scale flight flutter testing. Furthermore, previous amendments, as well as the provisions of this proposal, have significantly amplified the specific fail-safe and damage conditions which must be considered with a separate fail-safe aeroelastic stability envelope. Since many transport airplanes serve both civil and military purposes, an added benefit of this proposal would be similarity with the military specifications which have used a 15 percent margin for many years without adverse service experience.
A further proposal affecting the normal envelope would be the inclusion of a general statement concerning aeroelastic stability criteria within the design envelope. The statement would require that, for the normal airplane without failures, malfunctions, or adverse conditions, there must be a proper margin of damping up to and no large and rapid reduction in stability as is approached. These words are currently in the rule but are stated as a condition required in order to allow the limiting of the aeroelastic stability substantiation envelope to Mach 1.0 when is less than 1.0 This Mach 1.0 limitation would still be allowed, but the damping criteria would be a requirement in any case. Advisory Circular 25.629-1, Flutter Substantiation of Transport Category Airplanes, currently contains acceptable criteria for establishing a proper margin of damping.
It is also proposed that the fail-safe stability envelope be modified to provide a minimum speed margin. The margins between and have provided a sufficient margin in the past; however, with the advent of new types of propulsion systems, speed protection systems and unusual configuration, there is concern that this margin may be reduced to the point that it might not always serve as a sufficient margin for aeroelastic stability substantiation in the failed or damaged condition. Failures and damage conditions are typically substantiated by analyses or wind tunnel tests with very little flight test verification. The proposal would still require fail-safe aeroelastic stability substantiation within the structural design envelope, , however, a minimum margin would be provided to ensure protection against substantiation unreliability if the , envelope did not provide sufficient margin over . The minimum margin would be a 15 percent increase in equivalent airspeed over design cruise speed, at all altitudes from sea level up to the altitude of the intersection of the extension of the constant cruise Mach number line, with 1.15 . Then the minimum margin would be a linear variation is equivalent airspeed from that intersection to the point of intersection of the constant + .05 line with the altitude of the lowest intersection and a Mach increment of .05 over at higher altitudes. Figure 1. shows the minimum flutter substantiation envelope for fail-safe conditions.





Also proposed are additions to the specified failure, malfunction, damage and adverse conditions specified in Sec. 25.629(d). The list of conditions would be revised to add mismanagement of fuel not shown to be extremely improbable, the empennage bird strike requirement of Sec. 25.631, and the discrete source damage conditions of Sec. 25.571(e). Also included in a provision to consider inadvertent encounter with icing conditions, even though the airplane may not be approved for operation in icing conditions. Many of these are conditions that have generally required aeroelastic stability substantiation in order to show "safe flight and landing." This placement of the requirements in Sec. 25.629(d) would make the margins and substantiation criteria of Sec. 25.629 directly applicable to these aeroelastic stability substantiations.
The mismanagement of fuel condition is not specifically mentioned in part 25, although its consideration has been a practice for many years and has been required under general rules such as Sec. 25.629(d)(1)(ii). There is an increasing complexity in fuel loading configuration including empennage fuel and automatic fuel distribution systems and these can have a significant effect on aeroelastic stability. Therefore, it is proposed to specifically require consideration of fuel mismanagement conditions, not shown to be extremely improbable, in order to provide a probability basis consistent with other fail-safe flutter conditions and to assure that this condition is not overlooked.
The combinations of feathered propellers in Sec. 25.629(d) would also be revised to include any combination of feathered propellers (or rotating devices capable of significant dynamic forces) including all propellers feathered. The requirement in Sec. 25.671 for the airplane to be controllable with all engines inoperative has been made the current requirement inconsistent since the power failure requirement necessitates the feathering of all propellers.
The current Sec. 25.629(d) requires single failures to be considered in engine mounts, other attachments of external bodies and engine structure supporting propeller shafts. Relief from this requirement for structural elements of these attachments is provided if "conservative static strength margins" or "sufficient fatigue strength" are shown. This provision was adopted in 1964 (29 FR 12609, September 5, 1964) and was intended to require design integrity of mounts and engine structures sufficiently above the normal design load and fatigue requirements so that the probability of their failure could be considered "negligible." This has resulted in confusion and inconsistencies in the application of the regulation. It is proposed that the damage-tolerance requirements of Sec. 25.571(b) be used as a basis of evaluating these structures to determine if they should be treated under the single failure criteria of Sec. 25.629(d). However, in order to assure conservative margins above the normal requirements of Sec. 25.571, the damage-tolerance requirements would be applied with the specific loading conditions of Sec. 25.571(b) replaced by "all ground and flight load conditions specified in this part." The quoted phrase is taken from the current rule (Sec. 25.629(d)(3)(i)), and when combined with the damage-tolerance requirements of Sec. 25.571(b), should provide the conservatism necessary to warrant relief from the single failure requirement for the structural elements of these attachments. The proposal also provides a further alternative damage-tolerant method in case the inspection provisions of Sec. 25.571 are impracticable.
It is also proposed to revise the full-scale flight flutter test requirement to the extent that full-scale flight flutter tests would always be required for new designs. Currently, flight flutter tests are specifically required if is greater than .8. Indirectly, flight tests have always been necessary and required on transports, either as proof of freedom from flutter or as a means of validating the flutter analysis. The specific requirement for flight flutter testing on all new designs is considered necessary and consistent with the reduction of the normal flutter margins from 1.2 to 1.15 . It is also proposed to add a requirement that the flight test show a proper damping margin and that there be no large and rapid reduction in damping as is approached.

Regulatory Evaluation

This section reviews the economic impact of the proposed regulatory changes. Table 1 below shows the proposed changes and their associated economic impact. Additional detail is contained in the regulatory evaluation that has been placed in the docket.

Table 1. - Summary of Changes And Economic Impact

ChangeEconomic Impact
Changes to Sec. 25.251. Incorporate design requirements Sec. 25.251(a) into Sec. 25.305.Changes organization of regulations. No economic impact.
Provide for relief from requirement that freedom from vibration be demonstrated for failure conditions.Clarifies the intent of the rule. No economic impact.
Reorganize contents of Sec. 25.629 into a new section.Reorganization of rule. No economic impact.
Change title of Sec. 25.629....Editorial change. No economic impact.
Include requirements for rotating devices other than propellers.Clarifies the intent of the rule. No economic impact.
Reduce the design envelope from a 20 percent increase in equivalent airspeed to a 15 percent increase.Relieving, no derogation of safety. Reduction in costs.
Provide a minimum speed margin or floor for aeroelastic stability analysis.Ensures current practice as required by special conditions. No economic impact.
Provide additions to the failure, malfunction, damage and adverse conditions of Sec. 25.629(d).Consolidates existing requirements. No economic impact.
Require aeroelastic analysis of any combination of feathered propellers.Resolve inconsistencies in regulations. No economic impact.
Revise the damage tolerance requirements for aeroelastic analysis.Clarifies meaning of regulation. No economic impact.
Require full scale flight flutter tests for new designs.Codifies what is currently required in practice. No economic impact.

Table 1 shows that most of the changes have no cost implications and the change relating to reducing the design airspeed margin from 20 percent to 15 percent will reduce costs.

International Trade Impact Analysis

The proposals are likely to have little impact on trade for both U.S. firms doing business in foreign countries and foreign firms doing business in the U.S. The proposals which do have an economic impact are relieving in nature and will reduce costs. If foreign nations do not adopt U.S. standards, their manufacturers may be at a disadvantage in the U.S. market. U.S. manufacturers selling abroad could continue to design to foreign standards which would also meet U.S. standards. However the impact is expected to be slight.

Regulatory Flexibility Determination

Under the criteria of the Regulatory Flexibility Act of 1980, the FAA has determined that the proposed rule would not have a significant economic impact on a substantial number of small entities.
Since the Act applies to U.S. entities, only U.S. manufacturers of transport category airplanes would be affected. In the United States, there are two manufacturers that specialize in commercial transport category airplanes, the Boeing Company and the McDonnell Douglas Corporation. In addition, there are a number of general aviation (GA) entities that manufacturer other transport category airplanes such as large business jets, including Cessna Aircraft and Gates Lear Jet.
The FAA size threshold for a determination of a small entity for U.S. airplane manufacturers is 75 employees; any U.S. airplane manufacturer with more than 75 employees is considered not to be a small entity. None of the transport category airplane manufacturers is known to be a small entity. Thus, there would not be a significant economic impact on a substantial number of small entities as the result of the implementation of this proposal.

Federalism Implications

The regulations proposed herein would not have substantial direct effects on the states, on the relationship between the national government and the states, or on the distribution of power and responsibilities among the various levels of government. Therefore, in accordance with Executive Order 12612, it is determined that this proposal would not have sufficient federalism implications to warrant the preparation of a Federalism Assessment.

Conclusion

For the reasons given above, the FAA has determined that this proposed regulation is not considered to be major under Executive Order 12291, or significant under Department of Transportation Regulatory Policies and Procedures (44 FR 11034; February 26, 1979). In addition, the FAA certifies that this proposed rule, if promulgated, would not have a significant economic impact, positive or negative, on a substantial number of small entities under the criteria of the Regulatory flexibility Act, since none would be affected.

List of Subjects in 14 CFR Part 25

Air transportation, Aircraft, Aviation safety, Safety.

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The Proposed Amendment

Accordingly, the FAA proposes to amend part 25 of the Federal Aviation Regulations (FAR) 14 CFR part 25 as follows:

Part 25 -Airworthiness Standards: Transport Category Airplanes

1. The authority citation 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) (Revised Pub. L. 97-449, January 12, 1983), 49 CFR 1.47(a).

2. By amending section 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 airplane 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. 23.305 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 test, 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 exit 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 + .05 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 airplane must be shown to be free from any aeroelastic instability that would preclude continued safe flight and landing within the fail-safe envelope described in paragraph (b)(2 of this section for each of the failures described in paragraph (d)(3) of this section.
(1) Safety following the failures, malfunctions, or adverse conditions of paragraph (d)(3) may be substantiated by showing that losses in rigidity or changes in frequency, made shape, or damping are within the parameter investigations.
(2) The structural failures described in paragraphs (d)(3)(iv) and (d)(3)(v) of this section need not be considered in showing compliance with this paragraph if a damage tolerance investigation, using a conservative operating spectrum, shows that the structural element is designed with:
(i) Damage tolerance in accordance with Sec. 25.571(b) using all the flight and ground loading conditions specified in this part in lieu of the loading conditions specified in Sec. 25.571, or
(ii) If compliance with the inspection provisions of Sec. 25.571 (b) is shown to be impracticable, damage tolerance without inspections where analysis supported by tests shows that each critical structural element is capable of sustaining two lifetimes of slow crack growth starting with a 0.05 inch deep flaw and terminating with the maximum extent of damage at the limit load conditions specified in this part.
(3) The damage, failures, malfunctions, and adverse conditions which must be considered in showing compliance with this section are:
(i) Any critical fuel loading conditions, not shown to be extremely improbable, which may result from mismanagement of fuel.
(ii) Any single failure in any flutter damper system.
(iii) For airplanes not approved for operation in icing conditions, the maximum likely ice accumulation expected as a result of an inadvertent encounter.
(iv) 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).
(v) 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 pitch or yaw rigidity of the rotational axis.
(vi) 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)(3)(iv) and (d)(3)(v) of this section.
(vii) Any single propeller or rotating device capable of significant dynamic forces rotating at the highest likely overspeed.
(viii) Any failure or malfunction, or combinations thereof, in the flight control system considered under Secs. 25.671, 25.672, and 25.1309.
(ix) Any damage or failure condition, required or selected for investigation by Sec. 25.571 or Sec. 25.631, and
(x) 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 they have been shown to have an insignificant effect on the aeroelastic stability. These tests must demonstrate that the airplane has a proper margin or 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 need 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

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Issued in Washington, DC, on September 5, 1989.
Thomas E. McSweeny,
Acting Director, Aircraft Certification Service.
[FR Doc. 89-21395 Filed 9-11-89; 8:45 am]
BILLING CODE 4910-13-M


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Other Notice of Proposed Rulemaking Actions:
Not Applicable.

Final Rule Actions:
Final Rule. Docket No. 26007; Issued on 06/22/92.