АС 29-2B 7/30/97
(4) Heater zone (or region). A geometric zone defined by the heater type, heater size, the location of heater system components, and the maximum safe distance determined under (3) above. The heater system components may affect the heater zone's size if they are closely located to the heat source. For example a heater fuel tank would not be part of the heater zone if it were located far away from the zone boundary; however, if it were adjacent or close to the boundary, it would be included in the heater zone.
(5) Fireproof. Fireproof is defined in § 1.1, "General Definitions."
(6) Severe Fire. The following thermodynamic definitions are based on AC 20-135, "Powerplant Installation and Propulsion System Component Fire Protection Test Methods, Standards and Criteria" and on the definitions in § 1.1 for fire resistant and fireproof materials. These definitions are provided for analytical purposes. A severe fire, when used with respect to fireproof materials, is one which reaches a steady state temperature of 2,000 150° F for at least 15 minutes. A severe fire, when used with respect to fire resistant materials, is one which reaches a steady state temperature of 2,000 ‡150° F for at least 5 minutes.
(7) Hazardous accumulation of water or ice. An accumulation of water or ice that causes a device to not perform its intended function in either normal operation or a survivable emergency situation.
c. Procedures. When suitable data is available, the heating system design should be thoroughly reviewed to determine which system components and arrangements must comply with each subsection of § 29.859. The method-of-compliance relative to each subsection of § 29.859 should then be determined. Acceptable, but not the only, methods of compliance are discussed on a section-by-section basis as follows.
(1) For compliance with § 29.859(a), combustion heater designs, their installations and their heater zones must be identified and thoroughly evaluated. The most direct method of compliance for the heater, itself, is to procure units that already have internal design features that meet the relevant requirements of this section;
otherwise, design features should be provided and evaluated during certification that meet these same requirements. Several combustion heaters are approved under TSO-C20. TSO-C20 provides the procurement sources and the detailed approval standards for these combustion heaters. Each heater, its installation and its heater zone should be reviewed against the criteria of §§ 29.1181 through 29.1191 and §§ 29.1195 through 29.1203 (reference Paragraphs 584 through 589 and 592 through 596 of this AC) to ensure compliance. Next, the fire detector installation drawings and specifications should be reviewed for each heater region. The review should consider all reasonable hazards and failure modes of the heater and the detection system, itself.
If not previously TSO approved, the detectors themselves should be evaluated and 532 Par 360
approved during the overall system certification effort. Then, the drainage and venting system for each heater installation should be reviewed to ensure that areas of fuel or fuel vapor collection are properly drained or vented. The capacity of each drain or vent should be determined and, unless impracticable, the flow capacity should be a minimum of 3-to-1 over the worst-case leakage anticipated (including the adverse effects of surface tension). Phased inspections to eliminate clogging should be considered. Finally, the drainage and ventilation systems should be reviewed to ensure that discharges do not create external hazards by entering or contacting external ignition sources such as engine inlets and hot exhausts. If an accurate determination cannot be made by a design review, ground and/or flight test work with dyed, inert fluids or vapors should be conducted to accurately display discharge patterns.
(2) For compliance with § 29.859(f), the ventilating air duct design should be reviewed to determine what ducts are routed through heater zones. Once this has been determined, each duct section running through the heater zone should be made fireproof by either using a fireproof shroud around the existing duct or by using fireproof material for the duct wall. A primary purpose of these certification measures is to eliminate any system leakage that would allow carbon monoxide (a poisonous gas) to enter occupied areas, incapacitate the crew or passengers, and cause a crash.
Regardless of the method-of-compliance chosen, periodic checks should be performed during certification using carbon monoxide detection equipment to certify the leak-free integrity of the system. Several such checks should be done during flight test, especially after rigorous maneuvers, to ensure no leakage. It is also recommended that periodic checks using a carbon monoxide detector be conducted in conjunction with phased visual inspections (typically at a less frequent interval than each visual inspection) to ensure continued airworthiness. Carbon monoxide tests are reliable and quickly accomplished without any system disassembly. Continued airworthiness considerations are very important since carbon monoxide is a colorless, odoriess, tasteless, poisonous gas that incapacitates an occupant without warning. Carbon monoxide's ability to incapacitate increases with altitude, and has long been suspected as a probable cause for many aircraft accidents. It is the subject of General Aviation Airworthiness Alert No. 137, dated December 1983.
(3) For compliance with § 29.859(c), any design using combustion air ducts should be reviewed to ensure that the ducts are either made from fireproof material or shrouded with a fireproof shroud over a safe distance (see definition). The safe distance should be determined analytically, by test, or a combination, if the analytical results are not conclusive. The design should be reviewed to ensure that combustion air ducts are not connected to the ventilating airstream, except when an informal equivalent safety finding can be made that shows backfires or reverse burning cannot induce flames or fumes into the ventilating airstream under any failure condition or malfunction of the heater or its associated components. Such a finding should require analysis, testing, or a combination for a proper determination. A hazard FMEA should be conducted to ensure that no flames or fumes can be induced under any failure mode.
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(4) For compliance with § 29.859(d), the design and installation of all standard heater control components, control tubing and safety controls should be reviewed to determine the probable points of water or ice accumulation (e.g., sumps, rough surfaces, joints, etc.). If a design review cannot accurately determine these accumulation points, then bench tests and flight tests should be conducted for proper determination. Once these points are identified, the ability of the effected part (or parts) to perform its intended function when water or ice has fully accumulated must be determined for both normal and survivable emergency operations. If the part (or parts) either has not lost its ability to function; has lost part of its ability to function; or has lost all of its ability to function; and the entire system's function is not impaired, then nothing further should be required. However, if the overall system's function is hazardously impaired or lost, as a result of water or ice accumulation on a part (or parts), then rectifying design improvements should be made prior to final approval. These improvements should either alter the part's environment (e.g., relocation, enclosure, insulation, etc.) or eliminate the hazardous accumulation of water or ice (e.g., provide drainage, better sealing, better location, different surface finish, etc.).
(5) For compliance with § 29.859(e), combustion heaters, if used, must have separate, independent safety controls from their standard controls (e.g., air temperature, air flow, fuel flow, etc.) which are remotely located in case of a heater fire, are operable by the crew and automatically shut off the ignition and fuel supply when a hazardous condition exists, (as defined by § 29.859(g)). These separate safety controls must comply with § 29.859(g) (1), must keep the heater off until restarted by the crew or ground maintenance, and must warn the crew when an essential heater is automatically shut down. The safety control system design should be thoroughly reviewed and tested to ensure that it complies and that no hazardous failure modes exist. An FMEA should be conducted to ensure proper compliance.
(6) For compliance with § 29.859(f), each combustion and ventilating air intake's location should be identified, reviewed, and tested to ensure that no flammable fluids or vapors can enter the heater system, ignite and create a fire. If a combustion or ventilating air intake's location is critical or questionable, it should be relocated, shielded, drained, or other equivalent means provided to eliminate the potential fire hazard. If engineering analysis and evaluation are not adequate to make an acceptable safety finding, testing using dyed, inert, leaked fluids or vapors should be conducted.
(7) For compliance with § 29.859(g), each heater exhaust system design should be reviewed, tested, or a combination to ensure proper compliance with § 29.1121 and § 29.1123 (reference AC Paragraphs 548 and 549, respectively). Each exhaust shroud should be sealed to ensure that leaked flammable fluids or vapors do not contact the hot exhaust and cause a fire. The seal design should be reviewed to ensure that the sealing material is fireproof, is chemically compatible with the relevant fuels and vapors, is durable and is functionally adequate. If the design review is not conclusive for compliance purposes, then the seal system should be bench tested 534 Par 360
7/30/97/ AC 29-2B under pressure while undergoing critical service loads and motions to ensure no leakage occurs. Phased seal inspections should be considered to ensure continued airworthiness. An analysis should be conducted to determine the structural effects on the exhaust system of the worse case restricted backfire (typically a shock wave analysis can be used to determine the peak internal pressure and the resultant load on the exhaust system.) If structural failure would occur, based on the analysis, either the backfire restriction should be reduced or the exhaust design should be structurally improved to eliminate the failure.
(8) For compliance with § 29.859(h), each heater's fuel system design must be reviewed to ensure compliance with the powerplant fuel system requirements of Part 29 that are necessary for safe operation to be achieved. An equivalent safety finding should be made if an application is received that requests partial compliance or non-compliance with the powerplant fuel system requirements of Part 29. The finding should ensure that the safety intent of § 29.859() is achieved. Analysis, engineering evaluation, testing, or a combination should be used to substantiate the heater fuel system design. Heater fuel system components that, by leakage or other failures, can induce flammable fluids or vapors into the ventilating air stream should be shrouded by drainable, fireproof shrouds.
(9) For compliance with § 29.859(1), the drain system design should be reviewed to identify parts that may be subjected to high temperature and parts that may be subjected to hazardous ice accumulation in service. The high temperature parts should be evaluated using the methods of compliance for heater exhausts (reference Paragraph c(7), above and Paragraph 549 of this AC). Drains that would be stopped up from ice accumulation should be protected by relocation, size, shields, heating, or a combination to ensure hazardous fluids and vapors are properly drained away.
361. § 29.861 FIRE PROTECTION OF STRUCTURE, CONTROLS, AND OTHER PARTS.
a. Explanation.
(1) As stated in the rule, a Category B rotorcraft must be controllable until landed and a Category A rotorcraft must be controllable and continue its flight after a powerplant fire. For Category B rotorcraft designs with Category A powerplant isolation or Category A rotorcraft, a powerplant fire in one engine compartment must not adversely affect the remaining engine or engines. (Refer to § 29.903(b)). A policy statement on powerplant fire protection provisions was contained in the following note that appeared after Civil Air Regulation (CAR), Part 7, § 7.480, Designated Fire Zones.
NOTE: For Category B rotorcraft, the powerplant fire protection provisions are intended to ensure that the main and auxiliary rotors and controls remain operable, that the essential rotorcraft structure remains intact, and that the Par 360 535
passengers and crew are otherwise protected for a period of at least 5 minutes after the start of an engine fire to permit a controlled autorotative landing.
(2) To achieve the objectives of the rule, each part of the rotorcraft, as stated in the rule, must be isolated from a powerplant fire by a firewall (§ 29.1191), or for
(i) Category A, must be fireproof and must also comply with § 29.903(b).
(Il) Category B, must be protected so that they can perform their essential tunctions for at least 5 minutes under any foreseeable powerplant fire condition.
Review Case No. 26 pertains to CAR, Part 6, §§ 6.384 and 6.483. These rules were replaced by §§ 27.861 and 27.1191 respectively. Even though these rules pertain to normal category rotorcraft requirements, the objective statements contained in the review case pertain to the interpretation of the time interval specified by CAR, Part 7, § 7.384(b) and the note under CAR, Part 7, § 7.480 for Category B rotorcraft. These rules have been replaced by § 29.861(b) and § 29.1191, respectively. In the review case, the FAA stated, in part, that the firewall must be fireproof, support appropriate flight and landing condition loads, and prevent flame penetration when subjected to a flame of 2000° F for 15 minutes. Essential structure and controls must be protected for the duration of time appropriate to the rotorcraft operation and be able to carry loads and resist any failure that could cause hazardous loss of control when subjected to the temperature resulting from any foreseeable powerplant fire. Insufficient protection to provide enough time for a controlled landing would represent an unsafe feature or characteristic for the rotorcraft design.
(3) In addition, Paragraph 590 of § 29.1193(c) pertains to allowable opening in engine cowls and to fireproof skins in specified cases.
b. Procedures.
(1) If each part described in the rule is isolated completely by firewalls, compliance is obtained for Category A or B.
(2) If each part described by the rule is made of fireproof material such as steel, compliance is obtained for Category A or B.
(3) For some Category A rotorcraft, § 29.903(b) also imposes additional considerations where structure, controls, and other parts are common to the engine installation. For example, an interconnected engine mount must be fireproof and also perform its function and not affect the remaining engine in case of a powerplant fire. An evaluation should involve propulsion and airframe disciplines.
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(4) For Category B certification, if each part described by the rule does not comply as stated in (1) or (2), it must be proven that it will perform its function under the prescribed conditions. Compliance for Category B may be demonstrated by the following criteria:
(i) The parts must have a positive margin of safety for the appropriate flight and landing condition, including appropriate engine power conditions, under any foreseeable powerplant fire condition. The time interval under consideration here is the time necessary to complete an emergency descent (as described in the flight manual) and landing from the maximum operating altitude for which certification is requested. In no case is the total time interval to be less than 5 minutes.
(ii) The factors affecting the time interval should include the maximum height above the terrain, the maximum operating altitude, the flight manual recommendations for rate of descent, and a reasonable time for recognizing a powerplant fire.
(iii) The factors affecting the change in physical characteristics (strength primarily) of the parts are the temperature of the part, time interval at the elevated temperature, size, heat absorption or rejection.
(iv) The factors affecting the temperature of the part are location and distance from fire and flames, and temperature of the flames (2,000° F +50° F should be used unless proven otherwise).
(V) The rule requires substantiations for any foreseeable powerplant fire condition. Each rotorcraft design is unique and an evaluation of each design is necessary to establish the fire and flight conditions under consideration.
(vi) A very brief and simple example of compliance noted here may be helpful. This example pertains to a single engine Category B rotorcraft with the engine mounted on top at the fuselage center line. The engine is supported by all steel tubular mounts. The fuselage panel serves as a work deck as well as a firewall. A 15-minute duration is appropriate for this design. A representative panel of the firewall (deck) skin may be subjected to the autorotation flight loads and the landing load. A flame from an appropriate size burner, measuring 2,000° 150° F at the skin surface, should impinge on the loaded panel for 15 minutes. The panel may deform but must remain intact and sustain the appropriate load. The flame must not penetrate the panel skin.
(vii) Other rotorcraft designs may have engines located on top of the fuselage and under the main rotor. If cowls or firewalls do not isolate the rotors and essential controls, it must be determined by a rational analysis or by temperature measurement that the rotor and essential controls will perform their functions. Air flow through the rotor and factors noted in Paragraphs b(4)(ii), (ili), and (iv) are important to Par 361 537
an analysis. Compliance with § 29.1193(e)(3), fireproof skins will involve airframe and propulsion disciplines for rotorcraft designs that do not have cowls.
361A. § 29.861 (Amendment 29-30) FIRE PROTECTION OF STRUCTURE.
CONTROLS, AND OTHER PARTS.
a. Explanation.
(1) Amendment 29-30 revised the standard for Category B rotorcraft to allow use of parts made from standard fireproof materials of known acceptable dimensions in areas affected by powerplant fires without further proof of qualification. Previously the standard imposed a performance criterion for Category B applications regardless of the materials and part dimensions used.
(2) Fireproof and fire resistant are defined in FAR Part 1, § 1.1.
b. Procedures.
(1) A part with acceptable geometry that is made of steel, or another fireproof material, may be used to comply with the standard.
(2) A material system, panel, or assembly would be equivalent to steel provided it successfully completes the flammability tests described in Paragraph 361b4(vi) of this AC for the appropriate time period that includes fire recognition.
362. § 29.863 (Amendment 29-17) FLAMMABLE FLUID FIRE PROTECTION.
a. Background.
(1) The development of current § 29.863 can be traced through CAR 7.483, § 29.863 (1968), NPRM 68-18 (1968), and NPRM 75-26 (Airworthiness Review Notice November 7, 1975) and subsequent Amendment 29-17.
(2) Investigation of two accidents disclosed evidence of in-flight fires caused by leakage of flammable fluids to ignition sources. The revisions to § 29.863 adopted by Amendment 29-17 require significantly more attention to overall fire protection and prevention.
b. Explanation.
(1) Prior to Amendment 29-17, this rule only required either a means to prevent ignition of flammable fluids or vapors or a means to control any resulting fire. Isolation of flammable fluids and vapors from ignition sources by shrouding or sealing was the normal method of compliance. The revised rule further requires the assumption that 538 Par 361
these means fail or are ineffective and a fire does actually occur. Means to minimize the consequence of these fires must be provided. Specifically identified considerations must include the flammability of any combustible or absorbing materials, electrical faults, malfunction of protective devices, etc.
(2) The rule does not go so far as to require the entire rotorcraft to be a "designated fire zone."
c. Methods of Compliance.
(1) To minimize the probability of ignition of fluids and vapors after single failure of a component or systems, the following methods may be used:
(I) Shroud and drain flammable fluid systems (including steel fluid lines), fittings, etc. and/or provide fuel and vapor seals with respect to ignition sources (electrical wiring and equipment, hot bleed air lines, etc.).
(ii) Provide other effective separation, ventilation, or overheat shutdown devices, etc., to preclude ignition.
(iii) Assure that electrical equipment in the areas subject to flammable fluids and vapors is either hermetically sealed or has been tested and shown to be free of ignition capability. Paragraph 621b(1) (iv) of this advisory circular describes an acceptable standard for such laboratory testing.
(iv) Place a restricting orifice in fluid pressure lines routed to instruments and transducers.
(v) Assure fluid lines are not located so as to be subject to abrasion during normal operations. Cargo compartments should be evaluated for potential line damage due to cargo movement.
(2) To minimize the hazards if ignition occurs:
(i) Provide fireproof designs, fire wall isolation, or equivalent means for critical structure, equipment and personnel areas.
Consider fire detection, extinguishment, shutoff valves, fire suppression systems, etc.
(3) In considering compliance the actual protective measures may be related to the situation, considering the quantity and flammability characteristics of the fluid, the fire damage tolerance of the area, and the means available to the crew to minimize hazards from the fire. If action by the crew is necessary, quick-acting means (not necessarily fire detectors) must be provided to alert the crew in the event of a fire.
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Details of any action required by the crew should be included in the Rotorcraft Flight Manual.
(4) Compliance with § 29.863(d) requires as a minimum, type design data defining each area where flammable fluids or vapors might escape.
363.-372. RESERVED.
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SECTION 22. EXTERNAL LOAD ATTACHING MEANS.
373. § 29.865 (Amendment 29-12) EXTERNAL LOAD ATTACHING MEANS.
a. Background. The external load attaching means standards for transport and normal category rotorcraft were originally contained in Subpart D, "Airworthiness Requirements of FAR Part 133, Rotorcraft External-Load Operations."
Amendment 29-12, in 1977, added a new § 29.865, which moved these standards from Part 133 to Part 29. An identical transfer occurred in 1977 for Part 27. Transport Category A and B rotorcraft were initially used under Part 133 operations and, after Amendment 133-6, restricted category rotorcraft were also included under Part 133 operations. The use of restricted category first came about when an operator, exempt from Part 133, transferred harbor pilots to and from ships by a hoist and sling. The exemption was granted to study the feasibility of passenger transfer outside of the cabin. Subsequently, Amendment 133-9, adopted in January 1987, established a new Class D rotorcraft load combination for transporting passengers external to the rotorcraft. Amendment 133-9 also provided for the limitations and conditions for external passenger transportation and the necessary, associated safety requirements.
Part 29 rules have not yet been changed to reflect the Class D requirements.
b. Explanation. While the regulation only addresses external load attaching means, this advisory material also includes guidance for certification of external load carrying devices for rotorcraft to be used in conjunction with Part 133, "Rotorcraft External Load Operationsi Subpart D of Part 133 contains supplemental airworthiness requirements. Part 1 defines four classes of rotorcraft load combinations which are operationally approvable under the Part 133 operating rules and, thus, are eligible for certification under § 29.865. Parts 1 and 133 (through Amendment 133-9) contain a new rotorcraft load combination, Class D, that addresses personnel carried externally.
The four classes of rotorcraft load combinations are summarized in Table 373-1 and are discussed in detail in Paragraph c. For further information, AC 133-1A, "Rotorcraft External-Load Operations in Accordance with FAR Part 133," October 16, 1979, may be reviewed. Also, Paragraph 43 of this AC (reference § 29.25) concerns, in part, jettisonable external cargo.
c. Procedures.
(1) The applicant should clearly identify the Parts 1 and 133 rotorcraft load combination classes (A, B, C, or D) that are being applied for. The loads and operating envelopes for each class should be determined and used to formulate the flight manual supplement and basic loads report. The applicant should show by analysis, test, or both, that the rotorcraft structure, the external load attachment means, and (for Class D operations) the personnel carrying device meets the requirements of §§ 29.865(a),
133.41, 133.43, and 133.45(e)(3) for the proposed operating envelope.
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(2) For rotorcraft load combination classes A, B, and C, § 29.865 requires use of 2.5 g vertical limit load factor (Nzw) at the maximum substantiable cargo load (which is typical for cargo hauling configurations). This 2.5 g limit load factor is based on an engineering evaluation and a rationalization of § 29.337 for high gross weight applications. However, for lower gross weight configurations (which are more typical of a Class D application; i.e., personnel transport or evacuation), a higher limit load factor is recommended to ensure that limit load is never exceeded in service. For example, a Class D external load carrying device which is certified to a limit vertical load factor of
2.5 g and is installed in a minimum gross weight configuration rotorcraft capable of generating a vertical limit load factor of 3.2 g's could experience ((3.2/2.5 X 1.5) X 100) - 85 percent of ultimate load under emergency conditions with new external hardware. However, if factors such as wear and corrosion have effected the structural integrity of the external hardware ultimate load could be exceeded in emergency service. In any case, FAA/AUTHORITY policy is to not exceed limit load in service. The higher load factor for Class D cases should be the analytically derived maximum vertical limit load factor for the restricted operating envelope being applied for; or, as a conservative option, a vertical limit load factor of 3.5 g's (reference § 29.337). Unless a more rational proposal is received, for Class D cases where maximum operating gross weight for external load is between design maximum weight and design minimum weight, linear interpolation can be used between Nzw MIN and Nw MAX versus gross weight for design limit load factor determination.
(3) For applications that employ winches (or hoists) to raise or lower an external load from a hover (or another phase of flight), limit load must be properly determined based on the characteristics of the winch system and its installation such as mechanical advantage, static strength of the winch, static strength of its installation and the payload for any operating scenario being applied for. One acceptable method of determining limit load is by the following procedure:
(i) Determine the basic loads that fail and unspool the winch or its installation, respectively (Note: This determination should be based primarily on static strength; however, any dynamic load magnification factors that are significant should be accounted for).
(ii) Select the lower of the two values from (i) as the ultimate load of the winch system installation.
(ili) Divide the selected ultimate load by 1.5 to determine the limit load of the system.
(iv) Compare the system's derived limit load to the applied for one "g" payload multiplied by the maximum downward vertical load factor (Nzw MAx) from Paragraph (2) to determine the critical payload's limit value.
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(V) If the critical limit payload is equal to or less than the system's derived limit load the installation is structurally approvable as presented.
(vi) If the critical limit payload exceeds the system's derived limit load then one of the following options should be considered:
(A) Disapproval.
(B) Application for exemption.
(C) Reduction of the applied for critical limit payload to less than or equal to the system's derived limit load.
(D) Redesign of the winch system (and installation) to increase its derived limit load to equal to or greater than the critical payload.
A combination of options (C) and (D).
(F) Approvable operating restrictions to reduce Nzw MAx and, the corresponding critical limit payload to less than or equal to the system's derived limit load.
(4) In all approved cases, appropriate winch system placards and flight manual restrictions should be provided. Also, for Class D load combinations, the winch or hoist should have a demonstrated, acceptable level of reliability (for the phases of flight in which it is operable and in which the Class D load is carried externally). The winch should be disabled (or utilize an overriding mechanical safety device such as a flagged removable shear pin) to prevent inadvertent load unspooling or release during the phases of flight that the load is carried externally and operation is not intended. The maximum allowable winch cable angle should be determined and approved. This is primarily a structural requirement but should also be reviewed from an interference and flight handling criteria standpoint.
(5) It is recommended that winch or hoist systems be demonstrated as follows:
At least 1/3 of the demonstration cycles should include the maximum aft angular displacement of the load from the drum applied for under § 29.865(a).
(ii) The load versus speed combinations of the winch should be demonstrated by showing repeatability of the no load-speed combination, the 50 percent load-speed combination, the 75 percent load-speed combination and the system limit load-speed combination.
(ill) A minimum of six consecutive, complete operation cycles should be conducted at the system's critical limit load speed combination.
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