Appendix 3
(6) Critical control systems, such as primary and secondary flight controls, electrical power cables, systems and wiring, hydraulic systems, engines control systems, flammable fluid shut-off valves, and the associated actuation wiring or cables.
(7) Engine and APU fire extinguisher systems including electrical wiring and fire extinguishing agent plumbing to engine and APU compartments.
(8) Instrumentation necessary for continued safe flight and landing.
(9) Transmission and rotor drive shafts.
b. Location of Critical Systems and Components. The following design practices have been used to minimize hazards to critical components:
(1) Locate, if possible, critical components or systems outside the likely debris impact areas.
(2) Duplicate and separate critical components or systems if located in debris impact areas or provide suitable protection.
(3) Protection of critical systems and components can be provided by using airframe structure where shown to be suitable.
(4) Locate fluid shutoffs so that flammable fluids can be isolated in the event of damage to the system. Design and locate the shut-off actuation means in protected areas or outside debris impact areas.
(5) Minimize the flammable fluid spillage which could contact an ignition source.
(6) For airframe structural elements, provide redundant designs or crack stoppers to limit the subsequent tearing which could be caused by uncontained rotor fragments.
(7) Consider the likely damage caused by multiple fragments.
(8) Fuel tanks should not be located in impact areas. However, if necessitated by the basic configuration requirements of the rotorcraft type to locate fuel tanks in impact areas, then the engine rotorburst hazard should be minimized by use of design features such as minimization of hazardous fuel spillage (that could contact an ignition source by drainage or migration); by drainage of leaked fuel quickly and safely into the airstream; by proper ventilation of potential spillage areas; by use of shielding; by use of explosion suppression devices (i.e., explosion resistant foam or inert gases); and by minimization of potential fuel ignition sources or by other methods to reduce the hazard.
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Appendix 3
(9) The rotor integrity or containment capability demonstrated during APU evaluation to TSO-C77a, or JAR-APU should be considered for installation certification.
(10) The flight data recorder, cockpit voice recorder, and emergency locator transmitter, if required, should be located outside the impact zone when practical.
(11) Items such as human factors, pilot reaction time, and correct critical system status indication in the pilot compartment after an uncontained engine failure has occurred should be considered in design to permit continued safe flight and landing.
c. Rotorcraft Modifications. Modifications made to rotorcraft certified to this rule should be assessed with the considerations of this AC. These modifications include but are not limited to re-engining installations (including conversion from reciprocating to turbine powered), APU installations, fuselage stretch, and auxiliary fuel tank installations. Auxiliary fuel tank(s) should be located as much as practical so as to minimize the risk that this tank(s) will be hit by rotor failure fragments. The need to remain within the approved CG limits of the aircraft will of necessity limit the degree to which the risk may be minimized.
7. PROTECTIVE MEASURES. The following list is provided for consideration as some measures which may be used to minimize effects of a rotorburst:
a. Powerplant Containment.
(1) 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.
(2) 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.
Page A3-8
Appendix 3
b. 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 thicknesses 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.
c. Isolation or Redundancy.
(1) 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.
(2) 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 unacceptable isolation or shielding or other means of reducing the risk should be incorporated.
d. 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 788 of this AC, "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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Appendix 3 -1/3RD AND LARGER BLADED MASS (SEE FIGURE 2) ‡ 3° + 5°.
1/30 BLADED MASS (INTERMEDIATE) (SEE FIGURE 3) * 15°.
SMALL FRAGMENTS (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 АPХ3-1. ESTIMATED PATH OF FRAGMENTS Page A3-10
Appendix 3
MAXIMUM DIMENSION CG 1/3 b 120° Where R = disc radius b = blade length The CG is taken to lie on the maximum dimension as shown.
FIGURE APX3-2. SINGLE ONE-THIRD DISC FRAGMENT Page A3- 11
Appendix 3 1/3 (R + b) 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 АPХ3-3. INTERMEDIATE AND SMALL PIECES OF DEBRIS Page А3-12
7/30/97
GEOMETRIC CG 1/3 X WHERE X = AIRFOIL LENGTH (LESS BLADE ROOT & PLATFORM) FIGURE АPХ3-4 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 Page A3- 13 BLADE FRAGMENT DEFINITION Appendix 3 AC 29-2B
Appendix 3
FRAGMENT
ROTATIONAL KE = ½1∞2
TRANSLATIONAL KE = ½ mv 100 TOTAL
KINETIC ENERGY (PERCENT) 60 120 180 240 300 360
ROTATIONAL TRANSLATIONAL
FRAGMENT SIZE C FIGURE APХ3-5. DISTRIBUTION OF TRANSLATIONAL AND ROTATIONAL
KINETIC ENERGY OF ROTOR-COMPONENT FRAGMENTS AS A FUNCTION
OF FRAGMENT SIZE Page A3-14
Appendix 3 CG of Fragment Becomes Center of Rotation of Fragment For Separation Distance Calculations:
1/3 Rotor with 1/3 Blade Height FIGURE APX3-6. CROSS SECTION THROUGH AIRCRAFT AT PLANE
OF ROTATION OF THE ENGINE DISK FRAGMENT Page A3- 15