SECTION 36. ELECTRICAL SYSTEMS AND EQUIPMENT
652. § 27.1351 (Amendment 27-13) GENERAL.
a. Explanation. With the advent of more sophisticated rotorcraft and operations under more critical conditions, such as IFR and icing, it is essential that the electrical system be very carefully analyzed and evaluated to assure proper operation under any foreseeable operating condition and that hazards do not result from any malfunctions or tailures.
b. Procedures.
(1) Electrical System Capacity. Rotorcraft electrical systems have grown in capacity, complexity, and impact on safety. This paragraph requires adequate electrical system capacity for safe operation of load circuits essential for safe operation at continuous rated power. If this capacity can be shown by electrical measurements, an electrical load analysis is not required.
Load circuits (systems) that are essential for safe operation are those systems necessary to maintain controlled flight and land safely and are generally those systems required to show compliance with the certification regulations. This includes most electrical utilization systems.
An electrical utilization system is a system of electrical equipment, devices, and connected wiring using electric energy to perform a specific aircraft function.
The specific utilization systems, which are necessary to maintain controlled flight and land safely, will vary with the type of rotorcraft and with the nature of operations. Examples of systems which may be essential are basic flight instruments, minimum navigation equipment, minimum radio communications, and flight control systems.
(2) Function.
(1) Electrical equipment, controls, and wiring must be installed so that failure of a source will not interrupt supply of power by other sources to any systems that are essential for safe operation. Generating systems should be analyzed, inspected, or tested to ensure that no probable malfunction in the generating system or in the generator drive system may cause permanent loss of service to systems essential for safe operation. A probable malfunction is any single electrical or mechanical malfunction or failure which is considered probable on the basis of past service experience with similar components in rotorcraft applications. The experience with similar components in other aircraft may be used, if applicable, when there is 866 Par 652
insufficient rotorcraft experience. This definition should be extended to multiple malfunctions when:
(A) The first malfunction would not be detected during normal operation of the system, including periodic checks established at intervals which are consistent with the degree of hazard involved; or (B) The first malfunction would inevitably lead to other malfunctions.
(I)) The generator drive system includes the prime movers (propulsion engines or other) and coupling devices such as gearboxes or constant speed drives.
(ill) Where crew corrective action is necessary:
(A) Adequate warning should be provided for any malfunction or failure requiring such corrective action;
(B) Controls should be located to permit such corrective action during any probable flight situation;
(C) If corrective action must be taken within a specified time to continue safe operation of the generating system, it should be demonstrated that such corrective action can be accomplished within the specified time during any probable flight situation; and (D) The procedure to be followed by the crew should be detailed in the Rotorcraft Flight Manual.
(iv) Chapter 11 of Advisory Circular 43.13-1A, "Acceptable Methods, Techniques, and Practices; Aircraft Inspection and Repair," includes guidance on installation of electrical systems (routing, separation, tying, clamping, j-box installations, etc.). Special emphasis should be placed on wire routing during the rotorcraft compliance inspection. Control wires to the rotorcraft's generators should be routed separately from generator output wiring. This should begin at the generator and continue to the voltage regulator.
(3) Generating System. When electrical power is needed for essential equipment, this paragraph requires at least one generator with adequate capacity for safe operation. Complete electrical failures have been caused by loss of voltage control in the voltage regulator. Overvoltage conditions can destroy electronic equipment. An acceptable method of overvoltage protection is the use of a separate overvoltage sensing relay to trip the generator off the line when overvoltage is detected.
Another acceptable method is use of a voltage regulator with built-in overvoltage protection.
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(4) Instruments. Voltage and current supplied by each generator are parameters which define system operation. Some systems are provided with voltmeters and ammeters to display these parameters to the crew. These instruments may be multifunctional with switches to select the functions displayed. Some designs have annunciated sate operation of each generator with lights and have no voltmeter and ammeter. If additional limitations, such as maximum loading of portions of the systems, are necessary to account for fault condition, that information should be made avallable to appropriate personnel (crew, owner, modifier, etc.) to ensure the limits are not exceeded.
(5) An external power source supplying reverse polarity or reverse phase sequence to the rotorcraft electrical system could seriously damage the system. This paragraph requires a means to prevent such an occurrence. This can be accomplished by use of a standard polarized receptacle and protective relays.
653. § 27.1353 (Amendment 27-14) STORAGE BATTERY DESIGN AND INSTALLATION.
a. Explanation. Batteries must not be designed and installed to create a hazard to the rotorcraft under any operating conditions.
b. Procedures.
(1) As part of the electrical system evaluation, the battery installation should be reviewed to ensure the battery is vented and drained. If there is some doubt regarding the ability of the drain to satisfactorily dispose of corrosive fluids, TIA tests should be conducted to resolve the issue. Normally this is done by expelling a dye solution through the drain system during different phases of flight to ensure that fluids are drained clear of the rotorcraft. Some aircraft rely on the installation of a sump jar to dispose of corrosive fluids.
(2) If nickel cadmium batteries are used for engine starts, compliance with § 27.1353(g) may be achieved through:
A battery charge control system may be used that automatically controls the battery charge to prevent battery overheating. Unless otherwise specified by the battery manufacturer, temperatures above 140° F are considered overheat for NI-CAD batteries. The system is acceptable if the charge rate is automatically adjusted by controlling the charging current as a function of battery temperature, and in an over-temperature condition, the charge current is automatically reduced to a safe value.
Zero to 10 amperes has been considered safe for batteries rated at less than 34 amp-hours, and zero to 15 amperes has been considered safe for batteries rated at 34 amp-hours or more. The actual number chosen should be substantiated. Means and/or procedures should be provided for the crew to monitor the charger performance or the battery condition. If there is an automatic disconnect of the charger from the 868 Par 652
batteries and associated bus on an over-temperature condition, provisions should be provided in the cockpit to warn of a disconnect.
(il) If a temperature monitoring system is used, the temperature sensor should be located in a position that will most accurately reflect the internal battery temperature without causing adverse effects to the sensor. The location normally used is near the center of the battery. If the sensor is placed between two cells, the indication should be very close to the actual temperature within the cell. If the sensor is placed in a cell strap, there will normally be a period of time just after a heavy current drain (e.g. engine start) when the sensor shows a temperature that is hotter than the actual cell temperature.
(ill) Battery failure sensing and warning systems have also been used to show compliance with this rule.
(3) Other aspects of the battery installation can be resolved by reviewing AC 43.13-1A, "Acceptable Methods, Techniques, and Practices: Aircraft Inspection and Repairs" and AC 43.13-2A, "Acceptable Methods, Techniques, and Practices: Aircraft Alterations."
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655. § 27.1357 (Amendment 27-13) CIRCUIT PROTECTIVE DEVICES.
a. Explanation. Circuit protective devices are normally installed to limit the hazardous consequences of overloaded or faulted electrical circuits. These devices are resettable (circuit breakers) or replaceable (fuses) to permit the crew to restore service when nuisance trips occur or when the abnormal circuit condition can be corrected in flight. Chapter 11 of Advisory Circular 43.13-1A, "Acceptable Methods, Techniques, and Practices: Aircraft Inspection and Repair," includes guidance on selection of circuit protective devices.
b. Procedures.
(1) The circuit protective devices for systems essential to flight safety should not be tripped by faults in other circuits.
Systems that are "essential to flight safety" are generally those systems that are required to show compliance with the regulations. These essential systems include the basic electrical system, the distribution system, and many electrical utilization systems.
An electrical utilization system is a system of electrical equipment, devices, and connected wiring using electrical energy to perform a specific aircraft function.
(ill) The specific utilization systems, which are necessary to maintain controlled flight and land safely, will vary with the type of rotorcraft and with the nature of operations. Examples of systems which may be essential are basic flight instruments, minimum navigation equipment, minimum radio communications, and flight control systems.
(2) Automatic reset circuit breakers, which automatically reset themselves, should not be used as circuit protective devices. If an abnormal circuit condition cannot be corrected in flight, the decision to restore power to the circuit should result from a careful analysis by the flightcrew and cannot be performed by automatic reset circuit breakers.
To ensure crew supervision over the reset operation, circuit protective devices should be designed to require a manual operation to restore service after tripping. Circuit breakers must be designed such that the tripping mechanism cannot be overridden by the operating control. These are known as the "trip free" type.
(3) This paragraph requires protective devices for circuits essential to safety in flight to be accessible to the crew in the cockpit. Again, this generally applies to systems required for compliance as discussed above. If continued safe flight to the destination is sufficiently assured, certain required circuits have been excepted from Par 655 871
АС 27-1A 7/30/97 this accessibility. Voltmeter and ammeter circuit protective devices are examples of ones that have been excepted. Some utilization systems, although not specifically required by Part 27, may be required for the particular design to be certified. Circuit protective devices for these systems should be accessible. The following are considered to be acceptable compliance with the "readily reset" provision of this paragraph:
(i) For operation by a single pilot with seat belt and shoulder harness normally adjusted, the pilot should be able to identify and reset or replace the opened circuit protector while flying the rotorcraft. Circuit protection should not be located aft of a vertical plane passing left to right (laterally) through the pilot's body.
For a crew of two, it is satisfactory for one crewmember to move his seat and loosen his shoulder harness to identify and reset or replace the circuit protective device. It is not satisfactory for one of the crewmembers to leave his seat to reset or replace the circuit protective device.
(4) The spare fuse requirement applies only to fuses protecting systems required to show compliance with the regulations. Spare provisions are encouraged but not required for nonrequired convenience type installations. The spare fuses should be stored in a location readily accessible to the crew. For spare fuses not directly visible to the crew, location information should be provided. One acceptable location is on the fuse panel in a holder without wire terminations. The spare fuse should be identified "spare" with the fuse rating.
(5) Passive circuit protection has been utilized to a limited degree in some designs. To accommodate special installation problems, unprotected wire runs of up to 2 feet have been accepted in a few instances when associated with detailed specific installation data and regular periodic inspections. Specific installation data would normally include information such as routing requirements, clamp locations, requirement for conduit, etc. Electrical master junction boxes usually rely to some degree on passive circuit protection for protection against short circuits on distribution bars. This reliance is normally supported by considerations such as careful layout to minimize the possibility of shorts from loose objects, extensive use of nonconductive materials, terminal covers for relays, etc. Periodic inspections are also normally required. It is desirable to install junction boxes so loose objects will tend to fall away from internal circuitry. Also, careful consideration should be given to flammability characteristics when selecting a nonconductive material.
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657. RESERVED.
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658. § 27.1361 MASTER SWITCH.
a. Explanation. This paragraph provides for a master switch to allow for a quick disconnect of electric power sources. This provision was intended to minimize the probability of electrical power providing an ignition source during a crash.
b. Procedures.
(1) It has been determined that bypassing the master switch with small load circuits may not significantly increase the probability of electrical ignition of fuel.
Therefore, it is permissible to allow live circuits as described in Paragraph (b) of this section.
(2) The pilot should be able to readily identify and operate the master switch from his normal crew position with seat belt and shoulder harness normally adjusted.
The master switch and switch positions should be labeled. The labels should be readily recognized under all certificated flight conditions.
(3) Designs that include multiple power sources may include a "master switch arrangement" instead of a "master switch." This is done to minimize the possibility of a single failure resulting in a total loss of electrical power.
(4) In addition to carefully evaluating the functional aspects of an installation, the malfunction aspects must also be considered as required by § 27.1309. Normally, the installation is protected against inadvertent actuation of the function.
659. § 27.1365 ELECTRIC CABLES.
a. Explanation. The FAA does not have a wire standard and, in general, relies on military specifications. Where a military specification does not exist, manufacturers' specifications, along with appropriate qualification test data, have been accepted.
b. Procedures.
(1) Chapter 11 of Advisory Circular 43.13-1A, "Acceptable Methods, Techniques and Practices: Aircraft Inspection and Repair," contains a listing of wiring that has been accepted for aircraft installations.
(2) In many instances, references to a basic specification are not adequate since several configurations may exist, and reference to a supplemental specification sheet will also be necessary.
(3) Where wire with thin wall insulation (thickness of at least 10.5 mils.) has been used, some problems can occur if special precautions are not taken when the Par 658 875
wire is stamped for identification. The areas of concern are temperature, pressure, and dwell time of the stamp.
(4) Some additional types included in Tables A-l and A-ll of MIL-W-5088H, Appendix A, have also been evaluated and accepted for civil applications. Use of a specific type of wiring selected from this listing should be coordinated with FAA engineering personnel.
(5) Wire insulated with KAPTON® polyimide film manufactured to MIL-W-81381A, has been used in aeronautical products with varying degrees of success. The U.S. Navy had such a bad service history with KAPTON® insulated interconnect wire in aircraft that in the mid-1980s the U.S. Navy no longer allows the use of KAPTON® insulated wire. U.S. Army policy also bans the use of KAPTON® wire in their rotorcraft. Although the FAA has taken no such action, the use of KAPTON® insulated wire requires very special handling. The following areas should be observed when utilizing KAPTON® insulated wire:
The instructions in the KAPTON® wire "Handling Manual" should be strictly followed. This manual may be obtained from E.I. Du Pont de Nemours and Company, Polymer Products Department, Industrial Film Division, Wilmington, Delaware 19898.
(ïi) Use in special wind and moisture problem (SWAMP) areas, such as wheel wells, usually requires additional protection for the cable bundles.
(ili) The wire should not be exposed to a combination of either high stress (U.V. or physical) in the presence of water, high humidity, or high PH factor liquids.
(iv) The stiffness and permanent set (memory) of KAPTON® may cause chafing in unrestrained bundles or where KAPTON® insulated wire is bundled with wires of other insulation types.
(v) Care should be exercised in the stripping, stamping, and terminating of KAPTON® insulated wires.
NOTE: KAPTON® is a registered trademark of E.I. Du Pont de Nemours and Company.
(6) Compliance to the flame resistance requirement should be shown by testing in accordance with FAR 25, Appendix F, Part 1, Paragraph (b)(7) for the sixty-degree burn test. FAR 25, Appendix F contains test procedures and pass-fail criteria for this requirement.
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660. § 27.1367 SWITCHES.
a. Explanation. Qualification data that are available from the switch manufacturer should provide information regarding contact ratings and environmental limitations.
b. Procedures.
(1) Contact ratings are normally provided by the switch manufacturer. If the particular application is not specifically addressed by the switch manufacturer, additional information is available in Chapter 11, Section 2 of Advisory Circular 43.13-1A, "Acceptable Methods, Techniques and Practices: Aircraft Inspection and Repair."
(2) The rule requires all switches to be accessible.
For operation by a single pilot with seat belt and shoulder harness normally adjusted, the pilot should be able to identify and operate essential switches while flying the rotorcraft. Essential system switches should be located forward of a vertical plane passing left to right (laterally) through the pilot's body.
(ii) For a crew of two, switches for essential systems can be further back and beyond the reach of the pilot if readily identifiable and accessible to the other pilot or crewmember.
(3) This paragraph requires labeling of all switches. Each switch should be labeled for the circuit controlled, and each switch position should also be labeled.
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