АС 29-2B 7/30/97 Appendix 3 considerations are relevant, the mass should be assumed to be one-third of the bladed disc mass and its energy--the translational energy (i.e., neglecting rotational energy) of the sector (See Figure АPX3-2).
(2) Intermediate Fragments. It should be assumed that the intermediate fragment has a maximum dimension corresponding to one third or the disc radius with one-third blade height and a fragment spread angle of t5°. Where energy considerations are relevant, the mass should be assumed to be 1/30th of the bladed disc mass and its energy--the translational energy (neglecting rotational energy) of the piece traveling at rim speed (See Figure APX3-3).
(3) Alternative Engine Failure Model. For the purpose of the analysis, as an alternative to the engine failure model of sections (1) and (2) above, the use of a single one-third piece of disc having a fragment spread angle of t5° would be acceptable, provided that the objectives of the analysis are satisfied.
(4) Small Fragments. It should be assumed that small fragments have a maximum dimension corresponding to the tip half of the blade airfoil and a fragment spread angle of t15°. Where energy considerations are relevant, the mass should be assumed to be corresponding to the above fragment dimensions and the energy is the translational energy (neglecting rotational energy) of the fragment traveling at the speed of its CG location. The effects of multiple small fragments should be considered during this assessment.
(5) Critical Engine Speed. Where energy considerations are relevant, the uncontained rotor event should be assumed to occur at the engine shaft speed for the maximum rating appropriate to the flight phase (exclusive of OEI ratings), unless the most probable mode of failure would be expected to result in the engine rotor reaching a red line speed or a design burst speed. For APU's, use the maximum rating appropriate to the flight phase or the speed resulting from a failure of any one of the normal engine control systems.
(6) APU Failure Model. Service experience has shown that some APU rotor failures produced fragments having significant energy to have been expelled through the APU tailpipe. For the analysis, the applicable APU service history and test results should be considered in addition to the failure model as discussed in Paragraph 5(b) above for certification of APU installations near critical items. In addition, the APU installer needs to address the rotorcraft hazard associated with APU debris exiting the tailpipe. Applicable service history or test results provided by the APU manufacturer may be used to define the tailpipe debris size, mass, and energy. The uncontained APU rotor failure model is dependent upon the design/analysis, test results and service experience.
(A) For APU's in which rotor integrity and blade containment have been demonstrated in accordance with TSO-C77a/JAR APU, i.e., without specific Page A3-4
Appendix 3 containment testing, paragraphs 5(b)(1), 5(b)(2), and 5(b)(4) or Paragraph 5(b)(3) and 5(b)(4) apply. If shielding of critical airframe components is proposed, the energy level that should be considered is that of the tri-hub failure released at the critical speed as defined in Paragraph 5(b)(5). The shield and airframe mounting points) should be shown to be effective at containing both primary and secondary debris at angles specified by the failure model.
(B) For APU rotor stages qualified as contained in accordance with the TSO, 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. Historical data shows that in-service uncontained failures have occurred on APU rotor stages qualified as contained per the TSO. These failure modes have included bi-hub and overspeed failure resulting in some fragments missing the containment ring. In order to address these hazards, the installer should use the small fragment failure model, or substantiated in-service data supplied by the APU manufacturer. Analytical substantiation for the shielding system if proposed is acceptable for showing compliance.
c. Engine/APU Rotorburst Data. The engine or APU manufacturer should provide the required engine data to accomplish the evaluation and analysis necessary to minimize the rotorburst hazard such as:
(1) Engine failure model (range of fragment sizes, spread angles and energy).
(2) Engine rotorburst probability assessment.
(3) List of components constituting the rotors.
d. Fragment Impact Risks. FAA/AUTHORITY research and development studies have shown that, for rotorcraft conventional configurations (one main rotor and one tail rotor), the main and tail rotorblades have minimal risks from a rotorburst, and thus, they require no special protection. However, unique main and tail rotor blade configurations should be carefully reviewed. Certain zones of the tail rotor drive shaft and other critical parts which may be necessary for continued safe flight and landing may not have natural, minimal risk from uncontained rotor fragments.
e. Engine Service History/Design. For the purpose of a gross assessment of the vulnerability of the rotorcraft to an uncontained rotorburst, it must be taken that an uncontained engine rotor failure (burst) will occur. However, in determining the overall risk to the rotorcraft, engine service history and engine design features should be included in showing compliance with § 29.903 to minimize the hazard from uncontained rotor failures. This is extremely important since the engine design and/or the service history may provide valuable information in assessing the potential for a rotorburst occurring and this should be considered in the overall safety analysis.
Page A3- 5
Appendix 3 Information contained in the recent SAE studies (see Paragraph (3)(a) should be considered in this evaluation.
f. Certification Data File. A report, including all geometric layouts, that details all the aspects of minimizing the engine rotorburst hazards to the rotorcraft should be prepared by the applicant and submitted to the certification authority. Items which should be included in this report are the identification of all hazardous failures that could result from engine rotor failure strikes and their consequences (i.e., an FMEA or equivalent analysis) and the design precautions and features taken to minimize the identified hazards that could result from rotor failure fragment strikes. Thus an analysis that lists all the critical components; quantifies and ranks their associated rotorburst hazard; and clearly shows the minimization of that quantified, ranked hazard to the "maximum practicable extent" should be generated and agreed upon during certification. Critical components should all be identified and their rotorburst hazard quantified, ranked, and minimized where necessary. Design features in which the design precautions of this guidance material are not accomplished should be identified along with the alternate means used to minimize the hazard. To adequately address minimizing the hazards, all rotorcraft design disciplines should be involved in the applicant's compliance efforts and report preparation.
6. DESIGN CONSIDERATIONS. Practical design precautions should be used to minimize the damage that can be caused by uncontained engine and APU rotor debris.
The following design considerations are recommended:
a. Consider the location of the engine and APU rotors relative to critical components, or areas of the rotorcraft such as:
(1) Opposite Engine - Protection of the opposite engine from damage from 1/3 disc rotor fragments may not be feasible. Protection of the opposite engine from other fragments may be provided by locating critical components, such as engine accessories essential for proper engine operation (e.g. high pressure fuel lines, engine controls and wiring, etc.), in areas where inherent shielding is provided by the fuselage, engine, or other structure.
(2) Engine Controls - Controls for the remaining engine(s) that pass through the uncontained engine failure zone should be separated/protected to the maximum extent practicable.
(3) Primary Structure of the Fuselage.
(4) Flight Crew - The flight crew is considered a critical component.
(5) Fuel system components, piping and tanks, including fuel tank access panels (NOTE: Spilled fuel into the engine or APU compartments, on engine cases or on other critical components or areas could create a fire hazard.)
Page A3-6
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.
Page А3- 7
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.
Page A3- 9
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