(II) Low Pressure. Although lines may only be subjected to low pressures, if they are located behind upholstery or for any reason are not 100 percent visible during normal operation, they should be solid metal lines or high pressure flexible lines such as Aeroquip 300 series hose, or Stratoflex 124, or 170 series hose assemblies. The so called "green lines" should only be used in locations that are 100 percent visible during normal operation. This would restrict their use to the run between the mask and the bulkhead disconnect in the aircraft cabin. Synthetic lines such as plastic, nylon, or rubber cannot be recommended for applications that will be exposed to continuous pressure (l.e., as opposed to pressurized when needed). These materials can cold flow.
(2) Fittings.
(i) High Pressure. Intercylinder connections are made with regular flared or flareless tube fittings with stainless steel. Usually fittings are of the same material as the lines. Mild steel or aluminum alloy fittings with stainless steel lines are discouraged. Titanium fittings should never be used because of a possible chemical reaction and resulting fire. An example of a series of fittings that has been accepted is the "SS" series Swagelok tube fittings (flareless).
(i) Low Pressure. Fittings for metallic low pressure lines are flared or flareless, similar to high pressure lines. Line assemblies should be terminated with "B" nuts in a similar manner to a manufactured terminating connection. Universal adapters (AN 807) or friction nipples used in conjunction with hose clamps are not accepted for use in pressurized oxygen systems.
(4) Shut-off Valve. Each system should contain a shutoff valve that is located as close as practical to the high pressure cylinders), and it should be assessable to a flightcrew member. High pressure cylinders should use slow opening/closing system shut-off valves. Where the regulator is part of the cylinder, and low pressure oxygen is controlled, the emphasis in slow acting valves is not as significant, and use of a flow fuse may be possible. Use of a flow fuse must be supported by a system fault analysis and testing to show maximum normal flow will not result in nuisance trips, and reliable trips will be provided for malfunction conditions resulting in excess flow.
(5) Regulators. The regulator should be mounted as close as possible to the cylinders (reference b(9)(ii)(B)(2)(i)). If nonaviation qualified regulators are to be considered, their service history should be reviewed and careful consideration should be given to the manufacturer's environmental qualification. Radio Technical Commission for Aeronautics Document D0-160 is a recognized and accepted standard for environmental considerations. As a minimum, consideration should be given to operation during altitude, temperature, and vibration extremes.
1254 Par 786
(Q) Placards. Appropriate placards should be provided with the installed system. Emphasis should be placed on any precautions that are appropriate during filling of the system and so forth.
(Z) Filler Connections. When a filler connection is provided, it is recommended it be located outside the fuselage skin or isolated in a manner that would prevent leaking oxygen from entering the rotorcraft. Careful evaluation should also be made of any nearby sources of fuel, oil, or hydraulic fluid under normal or malfunction conditions. Each filler connection should be placarded. In addition, any valves (on aircraft or ground servicing equipment) associated with high pressure should be slow acting.
(C) "Provisions Only" Considerations. In some instances systems are approved that only include provisions for a supply system consisting of the high pressure cylinders, regulators, and their associated lines and fittings. In these instances, a placard should be provided that refers to a supply system that is considered satisfactory for the remainder of the installation. Other supply system configurations may very well be acceptable; however, they should be evaluated by the ACO that evaluated the original installation. An example of an acceptable placard for this situation is:
Oxygen Supply System must be in accordance with the requirements given in STC SH .. Deviations to the configuration specified must be evaluated and approved by the Manager (include reference to the appropriate FAA ACO).
(11) Medical Communication Equipment. This equipment is provided to allow for communication between the rotorcraft and ground medical personnel. It includes voice communication and may also include telemetry equipment for the transmission of graphic data. It should be demonstrated that this equipment functions, and the range at which this determination was made should be recorded in the project file. The functional demonstration should include a 360° turn (clockwise and counterclockwise) to assure no significant sections of signal blanking exists. The remainder of the emphasis on this equipment should be to assure that operation of this equipment does not interfere with normal operation of any avionic systems whose installation is required for safe operation of the rotorcraft.
(12) Cabin Lighting. EMS interiors normally include higher intensity cabin lighting than other interiors. This lighting capability should be carefully evaluated to ensure it does not interfere with operation of the rotorcraft. In some installations a special curtain is required to separate the cockpit from any interterence by the lighting.
The FAA/AUTHORITY project file should include a short discussion of how this evaluation was conducted. See Paragraph b(6) for other curtain considerations.
Par 786 1255
АС 29-2B 7/30/97
(13) Other EMS Equipment. These items of equipment that are installed for the EMS mission are considered to be optional equipment and should be operated to assure they function properly. This evaluation would normally be done by someone that is knowledgeable about the particular type of equipment since correct operation of the equipment is essential to a valid determination that the required rotorcraft systems are not being interfered with. This includes all removable pieces of medical equipment that are used for patient care. The primary purpose of the evaluation of this equipment is to emphasize the possibility of any adverse interference between operation of the EMS equipment and the systems whose installation is required for safe operation of the aircraft, the adequacy of the installation provisions, and assurance that failure modes will not result in a hazardous condition for the rotorcraft.
(14) Miscellaneous. The following areas are not peculiar to EMS installations;
however, their significance is enhanced by the complexity of an EMS installation.
Compatibility. Many EMS installations are a collection of several STC's and may also include some "FAA/AUTHORITY field approvals." For this situation it should be shown that the overall installation provides for safe operation of the aircraft. Operation of a search light, if included, should be emphasized since this system can be difficult to keep out of the cockpit.
(її) Electrical Load Analysis. An electrical load analysis should be conducted, and additional guidance is available in Paragraph 776 of this AC. If the analysis indicates the generator(s) can be overloaded, appropriate measures should be taken to account for the problem. In some instances a placard that specifies certain operating limitations may be satisfactory while in other instances an electrical interlock may be in order. In general, if the amount of overload is relatively small, the placard solution will probably be satisfactory; whereas if the amount of possible overload is significant, it is more likely that an interlock scheme will be necessary.
Aircraft Grounding. It should be emphasized in an appropriate place in the STC data (RFM, maintenance information, etc.) that any time the EMS systems are being operated or serviced (oxygen for example) on the ground, the rotorcraft itself must be grounded.
(iv) Electrical Outlets. All electrical outlets provided in the cabin should be the three-prong grounded type. When not in use, these outlets should be suitably protected against the entry of fluids.
(v) Placards. All medical outlets should be placarded (air, oxygen, vacuum, etc.). Electrical power outlets should be placarded for type of voltage and amperage capacity. A placard stating no smoking when oxygen is in use should be included. Other placards would include information appropriate to the oxygen system, operation of special controls, etc.
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(VI) Equipment in Cargo and Baggage Compartments. When components are added to the compartment, rovisions should be made to protect the system components due to shifting cargo. In addition, when oxygen components are installed, the compartment should be placarded against the storage of oil or hydrocarbons. A smoke detector is recommended for a compartment if oxygen cylinders are installed in a closed, nonaccessible compartment. Also, the compartment weight limitations placard should be changed. Paragraph 336 of this AC pertains to cargo and baggage compartments.
(15) Equipment Substitution. The EMS modification that is presented for approval will contain specific items of equipment, and the approval will make reference to this equipment. If other equipment (new model, manufacturer, etc.) is to be substituted, then an evaluation should be made to assure the substitute equipment is also satistactory. This evaluation would normally consist of comparing the attachment means, design features, failure modes, specifications, and operation of the two units.
The purpose of the evaluation is to assure there are not differences that have an adverse effect on the airworthiness of the installation. Other differences would not be considered significant. A specific seat and litter design is approved as a part of the EMS configuration. Substitutions may be approved in accordance with the standards.
c. Related FAR Sections and References.
(1) FAR Sections. §§ 29.337, 29.471, 29.561, 29.773, 29.783, 29.785, 29.803,
29.805, 29.809, 29.811, 29.813, 29.815, 29.831, 29.853, 29.1301, 29.1309, 29.1353,
29.1357, 29.1411, 29.1431, 29.1557(d), 29.1583(d), 29.1585, and 29.1589.
(2) Other References.
(i) Helicopter Association International, Emergency Medical Services Recommended Guidelines, 1987, First Revision, 2 pages.
(ii) National Highway Traffic Safety Administration, Air Ambulance Guidelines, dated 1981.
(ill) FAR Part 91, General Operating and Flight Rules.
(iv) FAR Part 135, Air Taxi Operators and Commercial Operators.
(v) AC 67-1, Medical Information for Air Ambulance Operators, dated March 4, 1974.
(vi) AC 23-2, Flammability Tests, dated August 20, 1984.
(vii) Oxygen Equipment for Aircraft, Society of Automotive Engineers Aerospace Information Report No. 825B, Rev. 9/86.
Par 786 1257
(viii) Acceptable Methods, Techniques, and Practices--Aircraft Alterations, AC 43.13-2A, dated June 9, 1977.
(ix) Design and Installation of Liquid Oxygen Systems in Aircraft, General Specification for Military Specification MIL-D-19326G, dated October 1, 1985.
1258 Par 786
Par 786 INTEGRAL ASSEMBLY (Most Cases) 7/30/97
OVERBOARD B
VENT LINE PRESSURE BUILDUPIVENT
RELIEF VALVE TO
GAUGING SYSTEM
PRESSURE RELIEF PRES.
CLOSING LIQUID VALVE Oz
TO FLOW CONTAINER
PATIENT CONTRO REGULATOR ALV EVAPO- PRES.
BUILDUP
RATING COIL FILL COIL CHECK ¡VALVE ASSEMBLY IVALVE_ -
EMERGENCY
SHUTOFF VALVE FIGURE 786-1 TYPICAL LIQUID OXYGEN SYSTEM AC 29-2B 1259
INTEGRAL ASSEMBLY AC 29-2B (Most Cases)
OVERBOARD
VENT LINE
PRESSURE
RELIEF FILL, BUILDUP
AND VENT VALVE TO
GAUGING SYSTEM PRESSURE PRES.
RELIEF CLOSING LIQUID
VALVE CONTINER FLOW
CONTROL REGULATOR VALVE PRES.
EVAPO- BUILDUP
RATING COIL
COIL CHECK ASSEMBLY | VALVE
EMERGENCY
SHUTOFF VALVE FIGURE 786-2 TYPICAL LIQUID OXYGEN SYSTEM - USING COMBINATION VALVE 7/30/97 Par 786
787. RESERVED.
Intentionally Left Blank Par 787 1261
788. (Amendment 29-35) SUBSTANTIATION OF COMPOSITE ROTORCRAFT STRUÇTURE.
a. Reference FAR Sections. §§ 29.305, .307, 571, .603, .605, .609, .610, .611, .613, 629, .923, .927, .931, .1529 and Appendix A
b. Purpose. These substantiation procedures provide a more specialized supplement to the general procedures outlined by AC 20-107A, "Composite Aircraft Structure. These procedures address substantiation requirements for composite material system constituents, composite material systems, and composite structures common to rotorcraft. A uniform approach to composite structural substantiation is desirable, but it is recognized that in a continually developing technical area which has diverse industrial roots, both in aerospace and in other industries, some variations and deviations from the procedures described herein will be both necessary and acceptable. Significant deviations from this material should be coordinated in advance with the Rotorcraft Directorate.
c. Special Considerations. Since rotorcraft structure is configured uniquely and is inherently subjected to severe cyclic stresses, special consideration is required for the substantiation of all rotorcraft structure, including composites. This special consideration is necessary to ensure that the level of safety intended by the current regulations is attained during the type certification process for all structure with special emphasis on composite structure because of its unique structural characteristics, manufacturing quality and operational considerations, and failure mechanisms.
d. Background
(1) Historically, rotorcraft have required unique, conservative structural substantiation because of unique configuration effects, unique loading considerations, severe fatigue spectrum effects, and the specialized comprehensive fatigue testing required by these effects. Rotorcraft structural static strength substantiation for both metal and composite structure is essentially identical to that for fixed wing structure once basic loads have been determined. However, rotorcraft structural fatigue substantiation for metals is significantly different from fixed wing fatigue substantiation.
Since AC 20-107A, as developed, applies to both fixed wing aircraft and rotorcraft; it, of necessity, was finalized in a broad generic form. Accordingly, a need to supplement AC 20-107A for rotorcraft was recognized during type certification programs. One significant difference in traditional rotorcraft fatigue substantiation programs and fixed wing fatigue programs is the use of multiple full-scale specimen fatigue tests for rotorcraft programs rather than just one full-scale specimen test. Also, constant amplitude, accelerated load tests are typically used rather than spectrum tests because of the high frequency loads common to rotorcraft operations. These rotorcraft fatigue tests have traditionally involved the generation of stress versus life or cycle (S-N) curves for each critical part (most of which are subjected to the cyclic loading of the main or tail rotor system) using a monotonic (sinusoidal) fatigue spectrum based on 1262 Par 788
maximum and minimum service stress values. Unless configuration differences or flight usage data dictate otherwise, the monotonic fatigue spectrum's period is typically based on six ground-air-ground (GAG) cycles for each flight hour of operation. The S-N curves for the substantiation of each detailed part are typically generated by plotting a curved line through three data points (reference Appendix 1 of this AC, "Fatigue Evaluation of Transport Category Rotorcraft Structure (Including Flaw Tolerance)").
The three data points selected are a short specimen life (low cycle fatigue), an intermediate specimen life and a long specimen life (high cycle fatigue). Each raw data point is generated by monotonically fatigue testing at least two full-scale specimens (parts) to failure or run out for each data point on the S-N curve. The raw data point values are then reduced by an acceptable statistical method to a single value for plotting to ensure proper reliability of the associated S-N curve. Order 8110.9, "Handbook on Vibration Substantiation and Fatigue Evaluation of Helicopter and Other Power Transmission Systems and Appendix 1 contain comprehensive discussions of the S-N curve generation process. The rotorcraft S-N curve process contrasts sharply with the fixed wing process of using a single full-scale fatigue article (usually an entire wing or airframe, which constitutes a single full-scale assembly data point), generic material or full-scale assembly S-N data (e.g., MIL-HDBK-5 for metals, MIL-HDBK-17B for composites, or AFS-120-73-2 for full-scale assemblies), a non-monotonic spectrum and relatively large scatter factors to verify or determine the design fatigue life of the full-scale airplane.
(2) Also, rotorcraft have employed and mass produced composite designs in primary structure (typically main and tail rotor blades) since the early 1950's. This was 10 or more years before composites were type certificated for primary fixed-wing structure in either military or civil aircraft applications (with some notable limited production exceptions, such as the Windecker fixed wing aircraft). In any case, the early 1950 period was well before a clear, detailed understanding of composite structural behavior (especially in the areas of macroscopic and microscopic failure mechanisms and modes) was relatively common and readily available in a usable format for the average engineer working in this field. It also predated the initial issuance of AC 20-107. Currently, much composite design information is proprietary, either to government, industry or both, and many data gathering methods have not been completely standardized. Consequently, a significant variation from laboratory to laboratory in material property value determination methods and results can exist. The early rotor blade designs (as well as current designs) are by nature relatively low strain, tension structure designs. Also, by nature, these designs are not damage or flaw critical. Thus by circumstance as much as design, early composite rotor blade and other composite rotorcraft designs incorporated an acceptable fatigue tolerance level of safety. In the 1980's, more test data, analytical knowledge, and analytical methodology became available to more completely substantiate a composite design. Current FAR's 27 and 29 contain many sections (reference Paragraph a.) to be considered in substantiating composite rotorcraft structure, but this advisory material is needed to supplement the general guidance of AC 20-107A by providing specific rotorcraft Par 788 1263
guidance for obtaining consistent compliance with FAR sections applicable to rotorcraft.
e. Definitions. The following basic definitions are provided as a convenient reading reference. MIL-HDBK-17, and other sources, contain more complete glossaries of definitions.
(1) AUTOCLAVE. A closed apparatus usually equipped with variable conditions of vacuum, pressure and temperature. Used for bonding, compressing or curing materials.
(2) ALLOWABLES. Both A- basis and B- basis values statistically derived and used for a particular composite design.
(3) BALANCED LAMINATE. A composite laminate in which all laminae at angles other than 0° occur only in + pairs (not necessarily adjacent).
(4) A-BASIS ALLOWABLE. The "A" mechanical property value is the value above which at least 99 percent of the population of values is expected to fall, with a confidence of 95 percent.
(5) B-BASIS ALLOWABLE. The "B" mechanical property value is the value above which at least 90 percent of the population of values is expected to fall, with a confidence of 95 percent.
(6) BOND. The adhesion of one surface to another, with or without the use of an adhesive as a bonding agent.
(7) COCURE. The process of curing several different materials in a single step. Examples include the curing of various compatible resin system pre-pregs, using the same cure cycle, to produce hybrid composite structure or the curing of compatible composite materials and structural adhesives, using the same cure cycle, to produce sandwich structure or skins with integrally molded fittings.
(8) CURE. To change the properties of a thermosetting resin irreversibly by chemical reaction; i.e., condensation, ring closure, or addition. Cure may be accomplished by addition of curing (crosslinking) agents, with or without catalyst, and with or without heat.
(9) DELAMINATION. The separation of the layers of material in a laminate.
(10) DISBOND. A lack of proper adhesion in a bonded joint. This may be local or may cover a majority of the bond area. It may occur at any time in the cure or subsequent life of the bond area and may arise from a wide variety of causes.
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(11) EIBER. A single homogeneous strand of material, essentially one--dimensional in the macro-behavior sense, used as a principal constituent in advanced composites because of its high axial strength and modulus.
(12) EIBER VOLUME. The volume of fiber present in the composite. This is usually expressed as a percentage volume fraction or weight fraction of the composite.
(13) FILL. The 90° yarns in a fabric, also called the woof or weft.
(14) GLASS TRANSITION. The reversible change in an amorphous polymer or in amorphous regions of a partially crystalline polymer from (or to) a viscous or rubbery condition to (or from) a hard and relatively brittle one.
(15) GLASS TRANSITION TEMPERATURE. The approximate midpoint of the temperature range over which the glass transition takes place.
(16) HYBRID. Any mixture of fiber types (i.e., graphite and glass).
(17) IMPREGNATE. An application of resin onto fibers or fabrics by several processes: hot melt, solution coat, or hand lay-up.
(18) LAMINA. A single ply or layer in a laminate in which all fibers have the same fiber orientation.
(19) LAMINATE. A product made by bonding together two or more layers or laminae of material or materials.
(20) LOW STRAIN LEVEL. As used herein, is defined as a principal, elastic axial gross strain level, that for a given composite structure provides for no flaw growth and thus provides damage tolerance of the maximum defects allowed during the certification process using the approved design fatigue spectrum.
(21) MATERIAL SYSTEM CONSTITUENT. A single constituent (ingredient) chosen for a material system (e.g., a fiber, a resin).
(22) MATERIAL SYSTEM. The combination of single constituents chosen
(e.g., fiber and resin).
(23) MATRIX. The essentially homogeneous material in which the fibers or filaments of a composite are embedded. The resins used in most aircraft structure are thermoset polymers.
(24) MAXIMUM STRUCTURAL TEMPERATURE. The temperature of a part, panel or structural element due to service parameters such as incident heat fluxes, temperature, and air flow at the time of occurrence of any critical load case, (l.e., each Par 788 1265