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(4) 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.
(5) Engine Service History/Design .
(i) For the purpose of a gross a ssessment 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.
(ii) Information contained in the recent SAE studies should be considered in this evaluation (see paragraph b(1) above).
(6) 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.
e. 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:
(1) Consider the location of the engi ne and APU rotors relative to critical components, or areas of the rotorcraft such as:
Page E - 19
(i) 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.
(ii) 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.
(iii) Primary Stru cture of the Fuselage.
(iv) Flight Crew - The flight crew is considered a critical component.
(v) 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.)
(vi) 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.
(vii) Engine and APU fire extinguis her systems including electrical wiring and fire extinguishing agent plumbing to engine and APU compartments.
(viii) Instrumentation necessary for continued safe flight and landing.
(ix) Transmission and rotor drive shafts.
(2) Location of Crit ical Systems and Components. The following design practices have been used to minimize hazards to critical components:
(i) Locate, if possible, critic al components or systems outside the likely debris impact areas.
(ii) Duplicate and separate critic al components or systems if located in debris impact areas or provide suitable protection.
(iii) Protection of critical systems and components can be provided by using airframe structure where shown to be suitable.
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(iv) 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.
(v) Minimize the flammable fluid spillage which could contact an ignition source.
(vi) For airframe structural elem ents, provide redundant designs or crack stoppers to limit the subsequent tearing which could be caused by uncontained rotor fragments.
(vii) Consider the likely dam age caused by multiple fragments.
(viii) Fuel tanks should not be lo cated 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.
(ix) The rotor integrity or c ontainment capability demonstrated during APU evaluation to TSO-C77a, or JAR-APU should be considered for installation certification.
(x) The flight data recorder, cockpit voice recorder, and emergency locator transmitter, if required, should be located outside the impact zone when practical.
(xi) 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.
(3) 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.
f. Protective Measures . The following list is provided for consideration as some measures which may be used to minimize effects of a rotorburst:
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(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 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.
(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 - 22 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.
• 1/3RD AND LARGER BLADED MASS (SEE FIGURE 2) 13°+ ‡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, FROMTHE
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 1r3 b CG 120° Where RE= 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
[и3(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
WHERE X = AIRFOIL LENGTH (LESS BLADE ROOT & PLATFORM) CG IS TAKEN TO LIE AT THE CENTERLINE OF THE 1/3 FRAGMENT FRAGMENT VELOCITY TAKEN AT GEOMETRIC OG FRAGMENT MASS ASSUMED TO BE 1/3 OF THE AIRFOIL MASS GEOMETRIC OG × BLADE FRAGMENT DEFINITION FIGURE AC 29.903C-4 Page E - 26
FRAGMENT
ROTATIONAL KE = ½1@?
TRANSLATIONAL KE = ½ mv2 100 90 TOTAL
KINETIC ENERGY (PERCENT) 60 120 180 240 300 360
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:
13 Rotor with 1/3 Blade Height FIGURE 29.903С-6 CROSS SECTION THROUGH AIRCRAFT AT PLANE
OF ROTATION OF THE ENGINE DISK FRAGMENT Page E - 28
Page E - 29 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 supplements § 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