Exceptions include engine rotor discs and structural elements for which the probability of failure can be shown to be “extremely remote.” Items in this latter case would include all components of the rotor drive system evaluated under § 29.571 provided that the reliability of any item or system exempted under § 29.901(c)(1) is not jeopardized by the failure of other systems/components which themselves may be less reliable than “extremely remote.” Items of consideration here would include, but not be limited to, powerplant cooling systems, probable maintenance errors, deterioration/failure of seals and other time/temperature/weather sensitive nonmetallics, high energy fragment impact damage of nearby dynamic components, etc. Some items in these categories are addressed by specific rules in this subpart which override consideration under § 29.901(c). For example, § 29.927 sets forth specific tests to demonstrate acceptable safety levels in event of overtorque, overspeed, and transmission lubrication system failures. Further consideration of failures in these areas (under § 29.901(c)(1)) probably would be inappropriate. It would not, however, be appropriate to assume that an engine certified under Part 33, an auxiliary power unit qualified under TSO C-77, or other components qualified under various TSO’s or military specifications would not be subject to failure. As a general rule, any component or system whose failure is “probable” and the failure, in conjunction with probable combinations of failures, significantly degrades safe operation and/or impairs the capability of the crew to operate the rotorcraft safely constitutes an apparent noncompliance unless it is compensated for by alternate components, systems, or if appropriate, special operating procedures which essentially restore a safe level of operation of the rotorcraft. Normally, safe “continued” flight is intended; however, for the special case of the single-engine rotorcraft, safe entry into autorotation after engine failure is an acceptable means of compliance provided that other coincidental or associated failures or malfunctions do not jeopardize this maneuver.
(2) Procedures.
(i) The general techniques of AC 25.1309-1, System Design Analysis, present an acceptable means of evaluating the powerplant systems/components for compliance. However, the quantitative assessments of the probability classifications in AC 25.1309-1 have not been universally adopted for powerplant systems and components. Other procedural techniques in AC 25.1309- 1 may be impractical for powerplant systems. This does not preclude using a similar but simplified methodology in conjunction with conservative engineering judgment to arrive at a determination of compliance or identification of noncompliance aspects, using the following as a guide (extracted from AC 25.1309-1). Develop a matrix of all applicable powerplant components/systems which includes:
(A) Possible modes of failure, including malfunctions and damage from external sources.
(B) The probability of multiple failures and undetected failures.
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(C) The resulting effects of the rotorcraft and occupants, considering the stage of flight and operating conditions, and (D) The crew warning cues, corrective action required, and the capability of detecting faults.
(ii) Prepare an item-by-item, system-by-system FMEA. The analysis to identify failure conditions should be qualitative. An assessment of the probability of a failure condition can be qualitative or quantitative. An analysis may range from a simple report which interprets test results or presents a comparison between two similar systems to a fault/failure analysis which may (or may not) include numerical probability data. An analysis may make use of previous service experience from comparable installations in other aircraft.
(iii) Powerplant engineers normally find that believable statistical failure data on powerplant components are not readily available. Therefore, the simpler form of analysis involving assumption of failure with either benign results or dependence on alternate or redundant systems/components becomes the most feasible method of finding compliance. Repetitive inspections and preflight checks are a significant part of this finding, particularly if the backup system/component is used or checked routinely in the operation of the rotorcraft.
d. Section 29.901(d):
(1) Explanation. This paragraph provides a generalized basis for requiring compliance with any rules in this Part applicable to safe installation and operation of auxiliary power units (APU’s). The wording of the rule is generalized to permit (and require) a detailed review of this Part to identify any existing rule related to this type of equipment. Generally, any rule related to engines and their installation, support systems, and fire protection should be considered to be applicable to APU’s. This review may result in a designation of “nonapplicable” to certain engine-related rules if limitations such as “ground-use-only” are applied or if the APU serves only nonessential services. Any questionable aspects or interpretation/policy involved in establishing the applicable rules should be coordinated with the FAA Aircraft Certification Office.
Notwithstanding the generalization discussed above, a number of specific rules in subparts E and F include reference to APU’s in their applicability. The presence of these references should not be interpreted as excluding applicability of other appropriate rules as discussed above. In addition, the APU itself must be shown to be safe and reliable. Normally, this aspect is satisfied by showing that the APU model is included in the qualified parts list of TSO-C77a. This TSO also requires establishment (by the APU manufacturer) of limitations and installation data peculiar to the model APU. A showing of compliance with these data for the APU installed in the rotorcraft will be expected.
(2) Procedures.
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(i) Verify that the Model APU is listed as qualified to TSO-C77(a) or other suitable specifications. Note that TSO qualification is not regulatory but simply defines an acceptable base qualification standard. Other standards may be acceptable or deviations from the TSO may be acceptable if evaluated and found not pertinent to the planned installation.
(ii) Review the installation data provided for the APU and determine that the installation is in compliance. Exceptions may be taken as discussed above. Note that the TSO provides different qualification standards for “essential” and “nonessential” service APU’s. However, it does not distinguish between “flight -use” and “ground-use-only” APU’s. Some deviations to the TSO may be authorized based on this aspect; i.e., operation during negative “g” conditions.
(iii) Review Part 29, especially subparts E and F for all rules related to engines, engine support/service systems, intakes, exhausts, instrumentation, fire protection, pneumatic systems, etc., for applicability to installation and operation of the APU. Develop and accomplish a compliance program for the rules identified by this review following policy and procedures used for engines with exceptions which may be justified as discussed above.
(iv) For reference, the following rules specifically refer to APU’s. Some comments regarding compliance are offered.
(A) Section 29.1041, Cooling. APU installation data should define limits to be substantiated.
(B) Section 29.1091, Air Induction. Note the requirements of paragraph (f).
(C) Section 29.1103, Induction System Ducts. Note the special requirements of paragraphs (a), (e), and (f).
(D) Section 29.1121, Exhaust Systems.
(E) Section 29.1142, Controls.
(F) Section 29.1181, Designated Fire Zones.
(G) Section 29.1191, Firewalls. Firewall construction should be provided to completely separate the APU from other parts of the rotorcraft.
(H) Section 29.1195, Fire Extinguishers. Note that only one adequate discharge is required.
(I) Section 29.1203, Fire Detector Systems. Detectors are required for each fire zone which would include APU installations.
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AC 29-2C, Chg 3 9/30/2008 (J) Section 29.1305, Powerplant Instruments. TSO -C77(a) specifies provisions for measuring gas temperature, rotor RPM, and any other parameter necessary for safe operation of the APU.
(K) Section 29.1337, Powerplant Instruments.
(v) Additional comments. APU fuel sources which tap into engine fuel systems should be carefully designed and arranged to minimize the probability that an APU fuel line failure will jeopardize continued normal engine operation. If the APU provides essential services, it should be provided with an independent fuel system.
Also, engine fuel systems which operate at negative pressures should not be tapped for APU fuel source since air leaks back through the APU fuel control or small leaks in the APU fuel system likely will fail the engine.
AC 29.901A. § 29.901 (Amendment 29-26) INSTALLATION.
a. Explanation. Amendment 29-26 changes § 29.901(b)(2) to require a satisfactory determination that the rotorcraft can operate safely throughout adverse environmental conditions such as high altitude and temperature extremes. This amendment was needed to provide consistent application of the environmental qualification aspects of the installation. This amendment also added a new paragraph § 29.901(b)(6) to require design precautions to minimize the potential for incorrect assembly of components and equipment essential to safe operation.
b. Procedures. All of the policy material pertaining to this section remains in effect with the addition of design precautions. Design precautions should be taken to minimize the possibility of improper assembly of the components essential to the safe operation of the rotorcraft. Fluid lines, electrical connectors, control linkages, etc., should be designed so that they cannot be incorrectly assembled. This can be achieved by incorporating different sizes, lengths, and types of connectors, wires, fluid lines, and mounting methods. The applicant should perform a detailed maintenance assessment to clearly define the maintenance requirements, reliability, and serviceability of the drive system design. The applicant should consider all design qualification tests and service history data, if available. A review of accident data supports the importance of this assessment. Some applicants have utilized drive system vibration monitoring to verify continuing safe operation of their drive system.
AC 29.901B. § 29.901 (Amendment 29-36) INSTALLATION.
a. Explanation. Prior to Amendment 29-36, paragraph (c) exempted engine rotor disc failures (engine rotorburst) from consideration as a failure that could jeopardize the safe operation of the rotorcraft. Amendment 29- 36 removes this exclusion. Therefore, engine rotor disc failures should be considered as a failure that would jeopardize the safe operation of the rotorcraft.
b. Procedures. The method of compliance for this section is unchanged.
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AC 29.903. § 29.903 (Amendment 29-12) ENGINES.
a. Explanation. While paragraph (a) of this section requires engines to be type certificated under Part 33 of this chapter, engines certificated under other approved certification rules (CAR Part 13 and § 21.29 for imported engines) are also eligible. The fact that a component, system, or arrangement for which Part 29 standards exist is approved as a part of a certificated engine should not, except when specifically stated in Part 29, relieve an applicant of the necessity for compliance with Part 29. Even if the component, system, or arrangement supplied as a part of a certificated engine does meet the Part 29 standard, the possibility that subsequent changes to these components, systems, or arrangements by the engine manufacturer could negate compliance with Part 29 must be considered. For example, an engine may initially be equipped by the engine manufacturer with an oil tank filler cap that meets the Category A requirements of § 29.1013(c)(2) but is subsequently changed to a simpler and less expensive cap complying with § 33.71(c)(4). Continued monitoring of the engine configuration by the rotorcraft certification team would be needed to preclude an occurrence of noncompliance.
b. Procedures.
(1) Category A; Engine Isolation. This rule is one of the most significant safety rules in Subpart E of Part 29. Compliance involves a very extensive and rigorous evaluation not only of essentially all systems of the rotorcraft, but of the controls, both flight and powerplant, instruments, cockpit arrangement, cockpit switches, and operating procedures. A complete failure modes and effects analysis is involved.
Section 29.903(b)(1) should be rigorously applied to rotorcraft engine control arrangements which utilize governors responding to main rotor speed to modulate power rather than power levers preset to produce equal or less than limit power.
Section 29.903(b)(2) precludes “immediate action by any crewmember for continued safe operation.” This should be interpreted as requiring all powerplant systems to operate safely and continuously without crew attention (except to maintain flight using primary flight controls) in event of an engine failure from any cause, including fire. The collective is considered a primary flight control and not a powerplant control even though collective movement affects engine operation. No adjustment to powerplant controls or configuration can be allowed for certification purposes for performance credit or for safety. The time increment associated with “immediate” action may vary among different designs; however, it must not be less than that required to established engine-out flight profiles and climb rates associated with Category A performance.
During critical takeoff flight regimes, flight translation to at least published takeoff safety speed is needed before crew attention can be mandated to modulate powerplant controls or change aircraft configuration (i.e., landing gear, power lever or rotorspeed governor setting, etc.) to achieve published flight performance. This does not mean crew action is prohibited--only that no credit for crew action can be allowed for any resulting improved performance in the performance section of the flight manual.
(2) Category A; Control of Engine Rotation.
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(i) Means for stopping any engine in flight is to be considered unless it is shown that after critical failure of the engine, or components/accessories driven by the engine (not including rotor drive system components), no hazard results from rotation during the coast-down period. If continued rotation occurs, no hazard should result due to rotation during the period that the rotation is expected to continue. (Consider unbalanced rotors, bearing failures, accessory failures, lack of lubrication to other engine rotors, etc.) Note that after emergency engine shutdown, coast -down and continued rotation speed can be influenced by ram air flow into the compressor and, for multiengine rotorcraft, drag through the freewheeling unit.
(ii) A requirement exists for Category A rotorcraft to incorporate a means for restarting any engine individually in flight. Compliance is usually obtained during official flight tests and/or applicant tests in accordance with an approved test plan by requiring actual engine air-start demonstrations to define an acceptable restart envelope. These air -starts should be conducted at various altitudes, ambient temperatures, and fuel temperatures using the fuel type most critical, unless the applicant can show that this parameter is not pertinent. Other concerns involve the pilot station arrangement for flight controls and engine starting controls; i.e., verify that the engine start can be accomplished without jeopardizing continued safe operation of the rotorcraft, considering the pilot workload for the preexisting one-engine-inoperative situation, the location of the restart system controls, availability of a second pilot, etc.
Also, verify that the emergency/malfunction instruction sections of the RFM present a detailed definition of the approved restart envelope and detailed instructions for the restart, including eligible ambient atmospheric conditions, prestart arrangement of fuel, electrical and pneumatic systems (as applicable), delay time between start attempts (to allow for waste fuel drainage), starter duty cycle (if different from ground start duty cycle), and prestart situation analysis (i.e., Should a restart be attempted in view of the cause for initial shutdown? Is inlet system ice ingestion a possibility? Is reignition of fuel in the engine nacelle a possibility? Is sufficient restart time available? Is power available and is altitude sufficient to maintain terrain clearance?). Although restart capability from an all-engines-out flight condition is not required, special instructions for restarting from this situation should also be included commensurate with the system capability to accomplish the starts.
(3) Although restart capability is required for only Category A rotorcraft, the applicant should be encouraged to provide air start instructions in accordance with the above criteria for both single and multiengine Category B rotorcraft, including all-engine-out instructions if reasonable and practicable.
c. Turbine Engine Installation.
(1) Explanation. The certification of turbine engines and particularly the qualification of turbine rotors assume that the limitations established during these certifications will be accurately and rigorously observed during ground and flight operations in an aircraft. This paragraph is intended to promote this concept.
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(2) Procedures. Primary engine li mitations in the form of time, gas temperature, torque, and rotational speed and their corresponding allowable transient values are defined in the approved engine installation manual. The rotorcraft manufacturer must provide reliable, accurate means to assure that these limitations are not exceeded. These means may be in the form of automatic limiters or by crew monitoring of appropriately marked instruments. The FAA/AUTHORITY powerplant certification engineer and the rotorcraft manufacturer’s staff should verify these aspects by:
(i) Evaluating all applicable instrument, indicator, or warning devices, including transmitters, and limiting devices, if any, for system tolerances.
(ii) Closely reviewing the component qualification reports of items in c(2)(i) above to verify that these devices are properly qualified and that any deviations are acceptable.
(iii) Assuring that maintenance data are provided for functional checks and calibration of instruments and devices which are used to monitor or protect critical turbine rotor limitations. Preflight checks for automatic limiter devices may be appropriate.
(iv) Verifying that instrument markings are clear and relatively simple, that corresponding flight manual instructions and descriptions are straightforward and complete, and that instruments are located and orientated to minimize the probability of reading error.
AC 29.903A. §29.903 (Ame ndment 29-26) ENGINES.
a. Explanation. Amendment 29-26 adds § 29.903(a) that requires reciprocating engines used in rotorcraft to be certified in accordance with the rotorcraft engine testing requirements in § 33.49(d). This change is incorporated to ensure that certification requirements are not overlooked when reciprocating engines are installed in rotorcraft to be certified under Part 29 requirements. Section 29.903(b)(2) was revised to identify and clarify crew action; i.e., normal pilot action allowable with primary flight controls, in determining if adequate powerplant systems isolation is provided. This change 0eliminates any possible confusion that may exist regarding the acceptability of modifying optimum flight control manipulation to protect engine parameters.
Section 29.903(c)(3) was added and requires engine restart capability to be available throughout the flight envelope appropriate to the rotorcraft. This will avoid the concept that an in-flight engine restart envelope constitutes acceptable compliance with this rule.
b. Procedures.
(1) Engine type certification. All eng ines installed in rotorcraft should have a type certificate. The specific certification requirements for installation of reciprocating engines in rotorcraft are found in Part 33. Engines certificated under other approved Page E - 11
AC 29-2C, Chg 4 5/1/2014 certification rules (CAR Part 13 and FAR § 21.29, for imported engines) are also eligible. If a component, sys tem, or arrangement is certified under Part 33 or other requirement, the applicant is not relieved of the necessity to comply with the requirements of Part 29. If the component, system, or arrangement, supplied as a part of a certificated engine, meets the Part 33 and Part 29 requirements, subsequent changes to these components, systems, or arrangements could negate compliance with Part 29. For example, an engine may init ially be equipped by the engine manufacturer with an oil tank filler cap that complies with the Category A requirements of § 29.1013(c)(2) but is subsequently changed to a simpler and less expensive cap that complies with § 33.71(c)(4). The airfra me manufacturer should ensure that the requirements of § 29.1013(c)(2) are maintained.
(2) Category A: control of engine rotation. Section 29.903(c)(3) requires an engine restart capability which is appropriate to the rotorcraft. T he minimum envelope for the restart capability should be equal to or better than the rotorcraft takeoff/landing maximum altitude and temperature limits. Compliance is usually shown by conducting actual in-flight restarts during flight tests and/or other tests in accordance with an approved test plan. Restarts should be conducted at various altitudes, ambient temperatures, and fuel temperatures using the fuel type most critical, unless the applicant can show that this parameter is not pertinent. Other concerns involve the pilot station arrangement for flight controls and engine starting controls. It should be verified that the engine start can be accomplished without jeopardizing continued safe operation of the rotorcraft. Pilot wo rkload for a preexisting one-engine-inoperative situation, the location of the restart system controls, and the availability of a second pilot should be considered. The emergency/malfunction instruction sections of the rotorcraft flight manual (RFM) should present a detailed definition of the approved restart envelope and detailed instructions for the restart. Eligible ambient atmo spheric conditions, prestart requirements (to allow for waste fuel drainage), starter duty cycle (if different from the ground start duty cycle), and prestart situation analysis should be included. The prestart situation analysis should consider the following questions:
Should a restart be attempted in view of the cause for initial shutdown?
Is inlet system ice ingestion a possibility?
Is reignition of fuel in the engine nacelle a possibility?
Is sufficient restart time available?
Is power available?
Is altitude sufficient to maintain terrain clearance?
Although restart capability from an all-engines-out flight condition is not required, special instructions for restarting from this situation should also be included commensurate with the system capability to accomplish the starts.
AC 29.903B. § 29.903 (Amendment 29-31) ENGINES.
a. Explanation. Amendment 29-31 clarif ied the requirements for control of engine rotation and in-flight restart of engines. Section 29.903(c)(1) was changed by adding Page E - 12
the word “or” at the end of the paragraph, which provided an option on how to protect the engine stopping system from fire.
b. Procedures. All of the policy material per taining to this section remains in effect.
Additionally, the new § 29.903(e) requires that any engine should have a restart capability that has been demonstrated throughout a flight envelope to be certificated for the rotorcraft. However, § 29.903(e)(2) does not require in-flight demonstration of restart capability for single-engine rotorcraft or for all-engine shutdown of multi-engine rotorcraft. In the past, engine relight c apability for single engine rotorcraft has been demonstrated on the ground taking into account altitude effects, warm engine characteristics, depleted battery, etc. The minimum restart envelope for category A rotorcraft is discussed in section 29.903A of this AC. The restart capability can consider windmilling of the engine as part of this restart capability; however, most rotorcraft airspeeds and the locations of the engines do not support engine windmilling up to start speeds. Only electrical power requirements were considered for restarting; however, other factors that may affect this capability are permitted to be considered. Engine restart capability following an in-flight shutdown of the engine in single-engine rotorcraft, or all engines in a multi-engine rotorcraft, is the primary requirement, and the means of providing this capability is left to the applicant. To minimize any potential height loss following the failure of one or more engines, engine restart should be available at the earliest opportunity. T he engine certification should be checked to ensure that the flight manual instructions for in-flight restart are consistent with any specific engine restart requirements.
AC 29.903C. § 29.903 (Amendment 29-36) TURBINE ENGINE INSTALLATION.
a. Explanation. Amendment 29-36 revises § 29.903(d) to require that design precautions should be taken to minimize hazards to the rotorcraft in the event of an engine failure. This advisory material sets forth a method of compliance with the requirements of §§ 29.901, 29.903(b)(1), and 29.903(d)(1) of the Federal Aviation Regulations (FAR) pertaining to design precautions taken to minimize the hazards to rotorcraft in the event of uncontained engine rotor (compressor and turbine) failure. It is for guidance and to provide a method of compliance that has been found acceptable.
As with all AC material, it is not mandatory and does not constitute a regulation.
b. Procedures. Although turbine engine manufacturers are making efforts to reduce the probability of uncontained rotor failures, service experience shows that such failures continue to occur. Failures have re sulted in high velocity fragment penetration of fuel tanks, adjacent structures, fuselage, system components and other engines of the rotorcraft. Since it is unlikely that uncontained rotor failures can be completely eliminated, rotorcraft design precautions should be taken to minimize the hazard from such events. These design precautions should recognize rotorcraft design features that may differ significantly from that of an airplane, particularly regarding an engine location and its proximity to another engine or to other systems and components.
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AC 29-2C, Chg 4 5/1/2014
(1) Uncontained gas turbine engine rotor failure statistics for rotorcraft are presented in the Society of Automotive Engineers (SAE) Report No.’s AIR 4003 (period 1976-83) and AIR 4770 (period 1984-89).
(2) The statistics in the SAE studies indicate the existence of some failure modes not readily apparent or predictable by failure analysis methods. Because of the variety of uncontained rotor failures, it is difficult to analyze all possible failure modes and to provide protection to all areas. Howeve r, design considerations outlined in this AC provide guidelines for achieving the desired objective of minimizing the hazard to rotorcraft from uncontained rotor failures. These guidelines, therefore, assume a rotor failure will occur and that analysis of the effects or evaluation of this failure is necessary.
These guidelines are based on service experience and tests but are not necessarily the only means available to the designer.
c. Definitions.
(1) Minimize. Reduce to the least possible amount by means that can be shown to be both technically feasible and economically justifiable.
(2) Separation. Positioning of redundant critical structure, systems, or system components within the impact area such that the distance between the components minimizes the potential impact hazard. Redundant critical components should be separated within the spread angles of a rotor by a distance at least equal to either a ½ unbladed disk (hub, impeller) sector, or a 1/3 bladed disk (hub, impeller) sector with 1/3 blade height, with each rotating about its center of gravity (CG), whichever is greater (see figure AC 29.903C-6).
(3) Isolation. A means to limit system damage so as to maintain partial or full system function after the system has been damaged by fragments. Limiting the loss of hydraulic fluid by the use of check valves to retain the capability to operate flight controls is an example of “isolation.” S ystem damage is confined allowing the retention of critical system functions.
(4) Rotor.
(i) Rotor means the rotating components of the engine and APU that analysis, test results, and/or experience has shown can be released during uncontained failure with sufficient energy to hazard the rotorcraft.
(ii) The engine or APU manufacturer should define those components that constitute the rotor for each engine and APU type design. Typical rotors have included, as a minimum, disks, hubs, drums, seals, impellers, and spacers.
(5) Uncontained Engine or APU Failure (or Rotorburst). For the purposes of rotorcraft evaluations in accordance with this AC, uncontained failure of a turbine engine is any failure which results in the escape of rotor fragments from the engine or Page E - 14
could create a hazard to the rotorcraft. Rotor failures of concern are those in which released fragments have sufficient energy to create a hazard to the rotorcraft.
Uncontained failures of APU’s which are “ground operable only” are not considered hazardous to the rotorcraft.
(6) Critical Component (System). A critical component is any component or system whose failure or malfunction would contribute to or cause a failure condition that would prevent the continued safe flight and landing of the rotorcraft. These components (systems) should be considered on an individual basis and in relation to other components (systems) that could be degraded or rendered inoperative by the same fragment or by other fragments during any uncontained failure event.
(7) Fragment Spread Angle. The fragment spread angle is the angle measured, fore and aft, from the center of the plane of rotation of the disk (hub, impeller) or other rotor component initiating at the engine or APU shaft centerline or axis of rotation (see figure AC 29.903C-1). The width of the fragment should be considered in defining the path of the fragment envelope’s maximum dimension.
(8) Ignition Source. Any component that could precipitate a fire or explosion.
This includes existing ignition sources and potential ignition sources due to damage or fault from an uncontained rotor failure. Potential ignition sources include hot fragments, damage or faults that produce sparking, arcing, or overheating above the auto-ignition temperature of the fuel. Existing ignition sources include items such as unprotected engine or APU surfaces with temperature greater than the auto-ignition temperature of the fuel or any other flammable fluid.
d. Safety Assessment.
(1) Procedure. Assess the potential hazard to the rotorcraft using the following procedure:
(i) Minimizing Rotorburst Hazard. The rotorburst hazard should be reduced to the lowest level that can be shown to be both technically feasible and economically justifiable. The extent of minimization that is possible will vary from new or amended certification projects and from design to design. Thus the effort to minimize must be determined uniquely for each certification project. Design precautions and techniques such as location, separation, isolation, redundancy, shielding, containment and/or other appropriate considerations should be employed, documented, agreed to by the certifying authority, and placed in the type data file. A discussion of these methods and techniques follows.
(ii) Geometric Layout and Safety Analysis. The applicant should prepare a preliminary geometric layout and safety analysis for a minimum rotorburst hazard configuration determination early in the design process and present the results to the certification authority no later than when the initial design is complete. Early contact and coordination with the certifying authority will minimize the need for design Page E - 15
modification later in the certification process. The hazard analysis should follow the guidelines indicated in paragraphs AC 29.901c(2) and AC 29.903Cd(6). Geometric layouts and analysis should be used to evaluate and identify engine rotorburst hazards to critical systems, powerplants, and structural components from uncontained rotor fragments, and to determine any actions which may be necessary to further minimize the hazard. Calculated geometric risk quantities may be used in accordance with paragraph d(4) following, to define the rotorcraft configuration with the minimum physical rotorburst hazard.
(2) Engine and APU Failure Model. The safety analysis should be made using the following engine and APU failure model, unless for the particular engine/APU type concerned, relevant service experience, design data, test results or other evidence justify the use of a different model. In particular, a suitable failure model may be provided by the engine/APU manufacturer. This may show that one or more of the considerations below do not need to be addressed.
(i) Single One-Third Disc Fragment. It should be assumed that the one-third disc fragment has the maximum dimension corresponding to one-third of the disc with one-third blade height and a fragment spread angle of ±3°. Where energy 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 AC 29.903C-2).
(ii) 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 ±5°. 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 AC 29.903C-3).
(iii) Alternative Engine Failure Model. For the purpose of the analysis, as an alternative to the engine failure model of paragraphs d(2)(i) and d(2)(ii) above, the use of a single one-third piece of disc having a fragment spread angle of ±5° would be acceptable, provided that the objectives of the analysis are satisfied.
(iv) 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 ±15°. 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.
(v) 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 Page E - 16