MAXIMUM DIMENSION _ 1/3 b CG 120° Where R = disc radius b = blade length The CG is taken to lie on the maximum dimension as shown.
FIGURE AC 29.903C-2. SINGLE ONE-THIRD DISC FRAGMENT Page E - 24
1/3 (R + b} CG Where R = disc radius b = blade length Maximum dimension = 1/3 (R + b) Mass assumed to be 1/ 30 th of bladed disc CG is taken to lie on the disc rim FIGURE AC 29.903C-3. INTERMEDIATE AND SMALL PIECES OF DEBRIS Page E - 25
Page E - 26 AC 29-2C
GEOMETRIC OG 1/3 WHERE X = AIRFOIL LENGTH FIGURE AC 29.903C-4 (LESS BLADE ROOT & PLATFORM)
CG IS TAKEN TO LIE AT THE CENTERLINE OF THE 1/3 FRAGMENT
FRAGMENT VELOCITY TAKEN AT
GEOMETRIC CG
FRAGMENT MASS ASSUMED TO BE 1/3 OF THE AIRFOIL MASS BLADE FRAGMENT DEFINITION 9/30/99
FRAGMENT
ROTATIONAL KE = lø2
TRANSLATIONAL KE = ½ mv?
100 90 TOTAL
KINETIC ENERGY (PERCENT) 6C 120 180 240 300 360
ROTATIONAL TRANSLATIONAL
FRAGMENT SIZE O FIGURE AC 29.903C-5 DISTRIBUTION OF TRANSLATIONAL AND
ROTATIONAL KINETIC ENERGY OF ROTOR-COMPONENT FRAGMENTS AS A
FUNCTION OF FRAGMENT SIZEO Page E - 27
CG of Fragment Becomes Center of Rotation of Fragment For Separation Distance Calculations:
1/3 Rotor with 1/3 Blade Height FIGURE 29.903C-6 CROSS SECTION THROUGH AIRCRAFT AT PLANE
OF ROTATION OF THE ENGINE DISK FRAGMENT Page E - 28
AC 29.907. § 29.907 ENGINE VIBRATION.
a. Explanation. This very generalized requirement is authority to require substantiation of the effects of vibration on any part of the engine or the rotorcraft. In normal certification practice, the vibration effects of concern to the powerplant engineer are the vibratory loads or stresses in the engine and in the rotor drive system. Vibration effects on the rotor drive system are of concern if the corresponding loads or stresses result in fatigue damage. This aspect, however, is adequately addressed in § 29.571.
Vibration effects on the engine are usually categorized as “installation vibration” and “torsional vibration.” Methods of evaluation and limitations of these vibrations are established by the engine manufacturer.
b. Procedures. Review Order 8110.9, Handbook on Vibration Substantiation and Fatigue Evaluation of Helicopter and other Power Transmission Systems. Note that the mechanical coupling of the engines to the rotor drive system creates, for torsional vibration considerations, one, rather complicated, drive system which responds to any forced or resonant frequency. Antinodes or nodes and frequencies may exist in the engine shaft which are absent when the engine is operated on a test stand; therefore, the vibration investigation conducted under Part 33 is not conclusive with respect to torsionals. As noted in Order 8110.9, the engine manufacturers’ assistance is necessary to find compliance. Section 29.571 was amended by Amendment 29-13 to include “rotor drive systems between the engines and the rotor hubs” as part of the flight structure. This rule supplemen ts § 29.907 and requires coordination with the structures certification engineer to avoid duplication of effort by the rotorcraft manufacturer.
Advisory Circular 27 MG-11, Fatigue Evaluation of Rotorcraft Structure, which provides acceptable methods of compliance with § 29.571, may also be used to find compliance with § 29.907. In addition to basic drive system components such as main and auxiliary rotor drive shafts, the vibratory evaluation should include couplings, gear teeth, gear cases and splines, and should consider, where appropriate, low cycle fatigue associated with ground-air-ground cycles.
AC 29.908. § 29.908 (Amendment 29-13) COOLING FANS.
a. Section 29.908(a):
(1) Explanation. This paragraph applies to Category A rotorcraft and is intended to require that powerplant area cooling fans be designed and installed to enable continued safe operation of the rotorcraft after failure of a cooling fan blade. The phrase “except that the loss of cooling need not be considered” at the end of this paragraph is intended to make clear that for the purposes of this section, the FAA/AUTHORITY is concerned only with the fragmentation effect of a fan blade failure (reference Preamble Item 3-64 of Amendment 29-12).
(2) Procedures. If a fan shroud is provided, the applicant may demonstrate that the shroud configuration and strength are adequate to contain a failed fan blade and any other fan blades, guide vanes, etc., which can be expected to fail sequentially to the Page E - 29
initial blade failure. The demonstration can be facilitated by making a saw slot at the root of a blade sufficiently deep to weaken the blade retention strength and create a failure while the fan is rotating at the maximum speed established for the test. If the fan is driven by the rotor drive system, the test speed should be equal to or above the maximum transient speed to be expected with the rotor system. If the fan is driven by other means; i.e., bleed air turbine, hydraulic motor, engine N1 turbine, etc., the rotational speed for the blade failure demonstration should be based on a critical analysis of speed regimes to be expected. Containment is not required if the fan is located so that blade failure (and any sequential fan component failure) will not jeopardize safety. This may be shown by test or analysis. Segment shielding would likely be involved.
b. Section 29.908(b):
(1) Explanation. This paragraph applies to Category B rotorcraft and is intended to provide safety to the rotorcraft in the event of an assumed cooling fan blade failure or to prescribe a test to show that the cooling fan blade retention means is sufficient that blade failure is not a consideration.
(2) Procedures.
(i) The applicant may select § 29.908(b)(1), (b)(2), or (b)(3) to show compliance with this section. If § 29.908(b)(1) is selected, follow the procedures outlined above for Category A rotorcraft.
(ii) Section 29.909(b)(2) may be selected; however, without containment, damage to any component or structure in the plane of the fan rotor or any other trajectory to be expected should not cause the loss of any function essential to a controlled landing.
(iii) If § 29.908(b)(3) is selected, a spin test at 122.5 percent of the maximum speed associated with either engine terminal speed or an overspeed limiting device would be acceptable to show compliance. No failure should occur, and distortion should not result in fan element contact with housings or other adjacent components.
(Note: 150 percent of the centrifugal force is achieved at 122.5 percent of the rotational speed.)
AC 29.908A. § 29.908 (Amendment 29-26) COOLING FANS.
a. Explanation. Amendment 29- 26 requires that cooling fans be designed and installed to enable continued safe flight and adequate cooling of the rotorcraft following a fan blade failure. Compliance with the previous requirements could have resulted in hazards to the rotorcraft with the loss of cooling air to critical powerplant components. A new section was also added to the rule for cooling fans, which are not part of the powerplant installation and therefore not subject to the fatigue evaluation under Page E - 30
§ 29.571. It should be determined that no cooling fan blade resonant conditions exist within the operating limits of the rotorcraft unless a fatigue evaluation is conducted.
b. Procedures. Neither mechanical damage nor loss of cooling air should prevent “continued safe flight.” The definition of “continued safe flight” is contained in Appendix 1 of AC 20-136 and is quoted as follows:
Continued safe flight and landing. This phrase means that the aircraft is capable of safely aborting or continuing a takeoff; continuing controlled flight and landing, possibly using emergency procedures but without requiring exceptional pilot skill or strength.
Some aircraft damage may occur as a result of the failure condition or upon landing.
For airplanes, the safe landing must be accomplished at a suitable airport. For rotorcraft, this means maintaining the ability of the rotorcraft to cope with adverse operating conditions and to land safely at a suitable site.
The FAA/AUTHORITY has determined that for Category A rotorcraft the phrase, “continued safe flight” means that the rotorcraft retains the capability to return and land safely at the point of departure or continue and land safely at the original intended destination or a suitable alternate site.
(1) This section is intended to ensure that a cooling fan blade failure will not jeopardize safety of the rotorcraft. Three ways to show compliance with this section are as follows:
(i) A demonstration should be conducted to show that at the maximum fan speed to be expected, a failed blade will be contained within a housing or shroud which is included in the proposed type design and is designated as the containment shield;
(ii) It should be shown that the installed cooling fan is located such that a blade failure will not jeopardize the safety of the rotorcraft or its ability to continue safe flight (Category A) or land safely (Category B); or,
(iii) It should be shown that the cooling fan blades can withstand an ultimate load 1.5 times the maximum centrifugal force that may be expected in service.
The maximum centrifugal forces will occur at the maximum cooling fan rotational speeds. The maximum fan rotational speeds may be related to an overspeed limiting device or to the maximum transient speed to be expected from analysis or test of the engine, system, or component which drives the fan. The maximum rotational speed will be as follows:
(A) For fans driven directly by the engine:
(1) The terminal engine rotational speed that will occur under uncontrolled conditions; such as output shaft disconnect; or Page E - 31
(2) The maximum engine rotational speed that would be controlled by a reliable, approved engine overspeed limiting device.
(B) For fans driven by the rotor drive system, the maximum rotor drive system rotational speed to be expected in service including transients. (Note:
Capability to withstand the ultimate load of 1.5 times the centrifugal force means that no failure would occur and distortion should not result in fan element contact with housings or other adjacent components during the 122.5 percent spin test which equates to 150 percent centrifugal force.)
(2) Fatigue. If the cooling f an is not included in the fatigue evaluation under § 29.571, it should be shown that the cooling fan blades are not operating at resonant conditions within the normal operating limits of the rotorcraft.
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SUBPART E - POWERPLANT
ROTOR DRIVE SYSTEM AC 29.917. § 29.917 (Amendment 29-12) DESIGN.
a. Section 29.917(a) General:
(1) Explanation. This paragraph sets forth a definition of the rotor drive system and its associated components. The intent of this paragraph is to clarify and/or establish the identification of components to be considered in other rules which are applicable to the rotor drive system.
(2) Procedures. Coordinate with other certification personnel to ensure that other rules pertaining to rotor drive systems are properly addressed.
b. Section 29.917(b) Arrangement:
(1) Explanation.
(i) Section 29.917(b)(1) pertains to multiengine rotorcraft and requires the drive system arrangement to be such that the rotors will continue to be driven by the remaining engines in order to ensure that lift and control to be expected from the rotors are available if an engine fails.
(ii) Section 29.917(b)(2) pertains to single-engine rotorcraft and is similar to the requirement of paragraph AC 29.917b(1)(i) except that it requires each rotor necessary for operation and control to be driven by the main rotor(s) after disengagement of the engine from the main and auxiliary rotors.
(iii) Section 29.917(b)(3) is intended to require a design which allows the rotor system to be protected from the torsional drag of an inoperative engine.
(iv) Section 29.917(b)(4) pertains to optional torque limiting means (shear sections or clutches) and prohibits these devices from being located in the cross-shafting system between rotors.
(v) Section 29.917(b)(5) is intended to ensure that the design prevents rotors from contacting each other if intermeshing is possible.
(vi) Section 29.917(b)(6) is intended to ensure that locking devices are installed to keep rotors in proper phase if dephasing is possible.
(2) Procedures.
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(i) Section 29.917(b)(1) is normally complied with by cross-shafting between rotors, usually via one or more transmissions or gear boxes, to optimize the mechanical simplicity and weight aspects. Individual engine input arrangements are required.
(ii) Section 29.917(b)(2) may be complied with by cross-shafting between rotors. Usually this involves driving the antitorque rotor via a drive shaft from the main transmission.
(iii) Section 29.917(b)(3) may be complied with by installing “free-wheel” or “one-way” clutches in the engine output shaft or transmission input quill. Note that the output section of “free power turbine” engines is not an acceptable method of compliance.
(iv) Section 29.917(b)(4). Any torque limiting devices in the rotor system should be located in the engine output or transmission input quill to ensure that any disconnect from overtorque does not preclude continued normal function and relation of the rotors.
(v) Section 29.917(b)(5). Phase control of intermeshing rotors should utilize positive mechanical drive components. Deflections in both shafting (torsional) and rotors (blade chordwise bending) should be considered in establishing compliance.
(vi) Section 29.917(b)(6). Reconnection of dephased rotors should employ positive mechanical locking pins with secure locking methods.
AC 29.917A. § 29.917 (Amendment 29-40) DESIGN.
a. Explanation. Amendment 29- 40 introduces a new § 29.917(b). The previous § 29.917(b) has been redesignated as § 29.917(c). Section 29.917(a) sets forth a definition of the rotor drive system and its associated components and § 29.917(b) requires a design assessment to be performed. The intent of this paragraph (b) is to identify the critical components and to establish and/or clarify their design integrity to show that the basic airworthiness requirements, which are applicable to the rotor drive system, will be met.
b. Procedures.
(1) Section 29.917(a) General. The method of compliance for this section is unchanged.
(2) Section 29.917(b) Design Assessment. A design assessment of the rotor drive system should be carried out in order to substantiate that the system is of a safe design and that compensating provisions are made available to prevent failures classified as hazardous and catastrophic in the sense specified in paragraph (c) below.
In carrying out the design assessment, the results of the certification ground and flight Page E - 34
testing (including any failures or degradation) should be taken into consideration.
Previous service experience with similar designs should also be taken into account (see also § 29.601(a)).
c. Definitions. For the purposes of this assessment, failure conditions may be classified according to the severity of their effects as follows:
(1) Minor. Failure conditions which would not significantly reduce rotorcraft safety, and which involve crew actions that are well within their capabilities. Minor failure conditions may include, for example, a slight reduction in safety margins or functional capabilities, a slight increase in crew workload, such as routine flight plan changes, or some inconvenience to occupants.
(2) Major. Failure conditions which would reduce the capability of the rotorcraft or the ability of the crew to cope with adverse operating conditions to the extent that there would be, for example, a significant reduction in safety margins or functional capabilities, a significant increase in crew workload or in conditions impairing crew efficiency, or discomfort to occupants, possibly including injuries.
(3) Hazardous. Failure conditions which would reduce the capability of the rotorcraft or the ability of the crew to cope with adverse operating conditions to the extent that there would be--
(i) A large reduction in safety margins or functional capabilities;
(ii) Physical distress or higher workload such that the flight crew cannot be relied upon to perform their tasks accurately or completely;
(iii) Serious or fatal injury to a relatively small number of the occupants;
(iv) Loss of ability to continue safe flight to a suitable landing site.
(4) Catastrophic. Failure conditions which would prevent a safe landing.
(5) Minimize. Reduce to the least possible amount by means that can be shown to be both technically feasible and economically justifiable.
(6) Health Monitoring. A Vibration Health Monitoring System (VHM) is used to acquire and process helicopter drive system vibration signals.
(i) The principal purpose of a VHM is to increase the likelihood of detection of incipient faults in the rotor drive system that could prevent continued safe flight and safe landing by providing timely warning of potential failures to the pilot and maintenance personnel.
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