SUPERCOOLED CLOUDS GROUND LEVEL TO 10,000 FEET
2.0 F LIQUID WATER CONTENT (gm3) 1.5 6 0 to - 15°
1.0 66 20
0.521 50 -20 to 20 - 15 TO - 20 ° C DURATION (NMI) .04 -25 °C
0.0 10 20 - 30 40 50 MEDIAN VOLUME DIAMETER (um) FIGURE AC 29.877-5 Figures AC: 29 877-1 through 4 represent one approach to a 10,000-foot altitude limit and Figure AC 29.877-5 represents another. See Paragraph 386b(5)(ii) for discussion of the individual application of the two approaches.
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CHAPTER 2. PART 29
AIRWORTHINESS STANDARDS
TRANSPORT CATEGORY ROTORCRAFT
SUBPART E - POWERPLANT
POWERPLANT - GENERAL AC 29.901. § 29.901 (Amendment 29-17) INSTALLATION.
a. Section 29.901(a):
(1) Explanation. Paragraph (a) provides a definition of areas of rotorcraft for which safety requirements are set forth under the general title, SUBPART E - POWERPLANT. This subpart includes not only major propulsive elements and power transmissive components but also powerplant controls and instruments, safety devices, including fire protection and other devices to protect personnel, and critical flight structure in event of fires.
(2) Procedures. To ensure that no certification aspect is overlooked in establishing compliance, certification engineers should make at least an informal breakdown of all components of the rotorcraft, assigning responsibility to powerplant certification engineers of all items within the above definition. While this procedure is usually straightforward, the following items of FAA/AUTHORITY powerplant responsibility are listed to minimize questions regarding authority and responsibility.
(i) Drive system components. All parts of the transmission, clutches, shafting, including the driveshafts (masts) of main and auxiliary rotors, powerplant cooling components, and powerplant instrumentation requirements under §§ 29.1305,
29.1337, 29.1543, 29.1549, 29.1551, 29.1553, 29.1555, and 29.1583.
NOTE: The division of responsibility between FAA/AUTHORITY airframe engineers and FAA/AUTHORITY powerplant engineers (in accordance with FAA/AUTHORITY practice) regarding the driveshaft is at the flange or spline interface between the driveshaft and the rotor hub. Rotor hubs, controls, blades, and associated components are the airframe engineers’ responsibility. (Industry practice may not agree with this concept.)
(ii) Engines, except for mount structure.
(iii) Auxiliary power units, except for mount structure.
(iv) Combustion heaters, except for downstream ventilation air ducting, mixing, and distribution systems and for electrical aspects of controls and safety devices.
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(v) Water/alcohol or other fluid power augmentation systems.
(vi) Engine induction systems including induction icing and snow ingestion, and exhaust systems, including exhaust shrouds and drains.
(vii) All fuel systems, including those serving engines, auxiliary power units, combustion heaters, power augmentation systems, etc., and vents and drains for those systems.
(viii) Oil systems for engines, auxiliary power units, rotor drive transmissions, and gearboxes, including grease lubricated gears and bearings of the drive system.
(ix) Cooling aspects of engines, rotordrive transmissions and gearboxes, and auxiliary power units (APU). Electrical generating equipment and hydraulic component cooling may be the responsibility of the systems and equipment engineer provided agreement is established among responsible personnel.
(x) Rotor brakes, except hydraulic, electrical, and structural aspects of nonrotating brake components.
(xi) Fire protection, including firewalls, fire extinguisher systems, fire detector systems, flammable fluid lines, fittings, and shutoff valves. The powerplant engineer has responsibility for evaluating compliance with §§ 29.861 and 29.863 as they pertain to fuel and oil systems.
(xii) Engine and transmission cowling and covering, including latches.
(xiii) Powerplant flexible controls (reference § 29.1141(c)).
(xiv) Powerplant accessories.
(xv) Pneumatic systems (engine bleed air) within the engine or APU compartments, including shut-off valves and engine isolation features of bleed systems.
(xvi) Powerplant aspects of instrument markings and powerplant aspects of flight manuals, including limitations, normal and emergency procedures, engine performance; powerplant aspects of maintenance manuals, with emphasis on the limitations section of the manual and verification of the limitations established under § 29.1521.
b. Section 29.901(b):
(1) Explanation. Paragraph (b) requires compliance with the engine manufacturers’ approved installation instructions and any applicable provisions of this subpart that the powerplant installation must be installed in a manner to ensure Page E - 2
continued safe operation, that accessibility for inspection and maintenance is provided, that appropriate electrical connections (ground connections) are provided, and that allowance is provided for thermal expansion of turbine engines.
(2) Procedures.
(i) Engine Installation. Compliance with most of the detail requirements in the engine installation manual can be established by test or by design features and arrangements negotiated between the rotorcraft manufacturer and the FAA/AUTHORITY powerplant engineer. Some aspects, usually involving inlet and/or exhaust distortion limitations, vibration limitations, and aircraft/engine interface items may require direct assistance and information from the engine manufacturer to determine that compliance with the installation manual exists. Fuel control/engine/rotor system torsional matching is usually a developmental problem to be worked out before presentation of the rotorcraft to the FAA/AUTHORITY; however, final flight tests for surge or stall, torsional stability, and acceleration/deceleration schedules may require direct coordination among FAA/AUTHORITY installation engineers, engine manufacturers’ representatives, and the FAA/AUTHORITY engine certification engineers. These items are addressed specifically under § 29.939. Reciprocating, carburetor-equipped engines usually require a particular carburetor configuration to achieve adequate engine cooling. This configuration, identified as a “carburetor parts list,” must be approved for the engine under Part 33 and should be listed with the engine on the type data sheet for the rotorcraft.
(ii) Arrangement and Construction. Each item of the powerplant area of responsibility should be shown to be suitable for its intended purpose and installed to operate satisfactorily and safely between normal inspections and overhauls.
Accessories mounted on engine or transmission drive pads should be determined to be compatible with the pad limits including fit and speed range, overhang moment loads, running torque, and static torque. This latter term pertains to protection of the engine or transmission, which drives the accessory, from damage to be expected from malfunction of the accessory. This protection is usually supplied by providing a shear section in the accessory drive shaft designed to fail before exceeding the static torque limit of the engine or transmission driving component. Note that when evaluating the strength of the mechanical shear section, material allowables quoted in materials handbooks should not be used since these are minimum strength values. Shear sections should consider maximum strength values to be expected which are on the order of 130 percent of the minimum strength values. Also, it should be verified that design data for shear sections are dimensioned to limit the maximum diameter as well as the minimum diameter. Installation of starter -generators may also require verification that horsepower extraction limits are not exceeded. Special flightcrew instructions in the flight manual to monitor generator load or to disconnect electrically loaded items to protect accessory or engine-transmission pad limits should be avoided.
Environmental qualification requires consideration or protection against adverse effects of heat, sand or dust, humidity and rain, salt-laden atmosphere, and extremes of cold weather. Accessories such as generators, pumps, etc., are subjected to many of these Page E - 3
aspects during the individual qualification tests; however, satisfactory overall integrated system performance under these adverse conditions should be verified. Cold weather testing should include verification that lubricating oils and greases function properly and that engine starting procedures are safe and do not impose excessive loads on accessories, engines, or drive system components. Powerplant engineers should coordinate compliance efforts in this area with the system engineer’s investigations of compliance with §§ 29.1301 and 29.1309. Full -scale rotorcraft operations in cold weather should be required. Performance tests are required at the minimum temperature to be certified. Propulsion systems may usually be evaluated at this time.
Cold soak or overnight exposure to cold weather is appropriate followed by starting and pretakeoff procedures in accordance with the flight manual. Attention should be given to the practicality of important mandatory inspection procedures as affected by cold weather.
(iii) Accessibility. Accessibility for maintenance should be reviewed.
Typically, some maintenance activities must involve disassembly or removal of adjacent components. This should be avoided if repetitive activity can jeopardize the performance of critical or safety-related equipment. Verify that easy access exists to items such as oil system sight gauges or dip sticks, filler ports and drain valves for engines, auxiliary propulsion units, transmissions, fuel tanks and filters, etc.
(iv) Electrical (Grounding). Electrical interconnections to prevent difference of potential should be provided in the form of grounding straps or wires sized to carry the currents to be expected. Verify that the attachments for these grounding devices are not compromised by paint or zinc chromate which will tend to electrically insulate the engine or component. Note that engine mount structure should not be accepted as a grounding device since electrical current will cause corrosion at attachment points.
(v) Thermal Expansion. Axial and radial expansion of turbine engines is usually not a problem unless redundant mount arrangements are used. Special expansion provisions are usually required if engine components other than mounting points are attached to bulkheads, firewalls, other engines, or drive system components.
Engine output shaft axial or bending loads due to thermal expansion and to deflection of supports under ground or flight loads should be checked. Other components of concern are compressor inlet flanges, exhaust ducts, and rigid fluid or air lines between aircraft structure and the engine. The engine installation data will provide limit loads to be considered for parts of the engine which normally are attached to airframe components.
c. Section 29.901(c):
(1) Explanation. Paragraph (c) requires, with notable exceptions, a detailed failure modes and effects analysis (FMEA) of the various powerplant systems and components to establish that anticipated failures will not jeopardize the safe operation of the rotorcraft. Alternative methods such as top- down analysis may also be used.
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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