(1) Powerplant Containment.
(i) Engine Rotor Fragment Containment. It should be clearly understood that containment of rotor fragments is not a requirement. However, it is one of many options which may be used to minimize the hazards of an engine rotorburst.
Containment structures (either around the engine, or APU, or on the rotorcraft) that have been demonstrated to provide containment should be accepted as minimizing the hazard defined by the rotor failure model for that particular rotor component. Contained rotor in-service failures may be used to augment any design or test data. Containment material stretch and geometric deformation should be considered in conjunction with fragment energies and trajectories in defining the hazards to adjacent critical components such as structures, system components, fluid lines, and control systems.
Data obtained during containment system testing along with analytical data and service experience should be used for this evaluation.
(ii) APU Containment. Rotor integrity or containment capability demonstrated during APU TSO evaluation should be considered for installation certification. If rotor containment option was shown by analysis or rig test an objective review of the APU location should be made to ensure the hazard is minimized in the event of an uncontained APU rotor failure.
(2) Shields and Deflectors. When shields, deflection devices, or intervening rotorcraft structure are used to protect critical systems or components, the adequacy of the protection should be shown by testing or analysis supported by test data, using the impact area, fragment mass, and fragment energies based on the definitions stated herein. Analytical methods used to compute protective armor or shielding t hicknesses and energy absorption requirements should reflect established methods, acceptable to the certifying authority, that are supported by adequate test evidence. Protective armor, shielding, or deflectors that stop, slow down, or redirect uncontained fragments redistribute absorbed energy into the airframe. The resulting loads are significant for large fragments and should be considered as basic load cases for structural analysis purposes (reference § 29.301). These structural loads should be defined and approved as ultimate loads acting alone. The protective devices and their supporting airframe structures should be able to absorb or deflect the fragment energies defined herein and still continue safe flight and landing. If hazardous, the deflected fragment trajectories and residual energies should also be considered.
(3) Isolation or Redundancy.
(i) Other Engines - Although other engines may be considered critical, engine isolation from rotorburst on multi-engine rotorcraft is not mandatory. Other methods of minimizing the risk to the engine(s) may be acceptable.
(ii) Other Critical Components - Isolation or redundancy of other critical components, the failure of which would not allow continued safe flight and landing should be evaluated relative to the risk of occurrence and where the risk is deemed Page E - 21
unacceptable isolation or shielding or other means of reducing the risk should be incorporated.
(4) Composite Materials. If containment devices, shields, or deflectors are chosen by the applicant to be wholly or partially made from composites; they should comply with the structural requirements of AC 20-107A, “Composite Aircraft Structure,” and paragraph AC 29 MG 8, “Substantiation of Composite Rotorcraft Structure,” (which includes glass transition temperature considerations). Glass transition temperature considerations are critical for proper certification of composite or composite hybrid structures used in temperature zones that reach or exceed 200° to 250°F (93° to 121°C) for significant time periods. Hot fragment containment is typically accommodated in such protective devices by use of metal-composite hybrid designs that use the metal component’s properties to absorb the fragment heat load after the entire hybrid structure has absorbed the fragment’s impact load. These devices should comply with §§ 29.609 and 29.1529 to ensure continued airworthiness.
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/ 1/GRD AND LARGER BLADED MASS (SEE FIGURE 2) +3°4 + 5 ° 1/30 BLADED MASS (INTERMEDIATE) (SEE FIGURE 3) + 15 %
SMALL FRAGMENT S (SEE PARAGRAPH 5B(4))
FRAGMENT SPREAD ANGLE IS THE ANGLE
MEASURED FORE AND AFT, FROM THE
CENTER OF THE PLANE OF ROTATION
INITIATING AT THE ENGINE OR APU SHAFT
CENTERLINE NOTE: 1) THE POSSIBILITY OF TURBINE MOVEMENT SHOULD BE CONSIDERED.
2) ALL ROTORS ARE CONSIDERED TO BE FULLY BLADED FOR CALCULATING MASS.
3) FAILURE OF EACH ROTOR STAGE SHOULD BE CONSIDERED.
FIGURE AC 29.903C-1 ESTIMATED PATH OF FRAGMENTS Page E - 23
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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