thus, a means to safely contain, vent, or drain hazardous fluids or gases should be employed.
(vi) Nickel cadmium (Ni-CAD) batteries have also been used for many years in aircraft. These batteries have a wide application from the main starting battery to smaller batteries integrated into individual equipment.
(A) Twenty cell Ni-CAD batteries have been used to prevent thermal runaway; however, aircraft bus level voltage may not be an adequate source to properly charge a 20 cell Ni-CAD battery. The nominal voltage of a Ni-CAD cell is 1.5 volts and if the number of battery cells is 20 then; 1.5 Volts/Cell X 20 Cells = 30 volts required to even match the nominal voltage of the installed battery. (This is actually inadequate to charge the Ni-CAD cell, as a few tenth volts above that of the nominal voltage is required.) The maximum rotorcraft bus voltage available is typically 28.5 volts, which is deficient by 1.5 volts to just meet the nominal battery voltage. A compensating factor for this battery charging design is to require frequent battery maintenance activities. Future battery installation designs should not depend on the use of frequent battery maintenance as a compensating factor for designs that are inherently deficient in battery charging capability. Battery charging from the rotorcraft bus voltage is the prevalent method used to charge batteries, since no separate battery charger is needed. However, for the 20 cell NI-CAD example, using the rotorcraft bus voltage as the battery-charging source can introduce battery capacity limitations. Several options are available to address these concerns. One option would be to use a 19-cell battery.
Since Ni-CAD batteries were first introduced, much has been learned about their care and use, plus design improvements have been made that lessen the possibility of thermal runaways. Another approach could be to oversize the battery capacity, recognizing that the battery will be in a "less than" nominal capacity condition. This philosophy would also carry with it a Certification Maintenance Requirement.
Compliance could also be shown by using a dedicated battery charger that can profile the charging based on state of charge. This can be much more efficient and make it possible to have a better charged battery with less frequent maintenance and increased battery life.
(B) If NI-CAD batteries are used for engine starts and compliance with a requirement is achieved through the use of a temperature monitoring system, 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.
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(vii) For battery replacement with any battery other than the one that was installed and accepted as part of the original type design, consideration of several aspects of battery design and installation is important to ensure that the type design is not negatively impacted.
(A) The replacement battery should provide at least the minimum usable capacity as required by the current load analysis. This requirement of minimum usable capacity should be valid over the entire range of environmental conditions. Different types of batteries have different inherent characteristics. Ni-CAD batteries have a voltage discharge curve that allows most of the amp-hour capacity to be usable from a delivered voltage standpoint, where only about 66% of the amp-hour capacity for lead acid batteries is usable due to the delivered voltage becoming too low to use. It a replacement battery has a similar discharge rate, and the capacity is similar throughout the original battery's environmental range, then no further testing should be needed.
(B) An analysis for a replacement or system modification may be needed to show that the electrical system still complies to the original operation under IFR and Category A certification requirements for rotorcraft that depend on fault clearing features. Subsequent analysis validation by testing may be needed in those cases where the system that provides fault clearing is difficult to evaluate by analysis alone. In these cases, the effects of a change in battery internal impedance on the fault clearing teatures is uncertain and may not be adequately determined by analysis. There are high rate discharge types of batteries and slow-rate discharge batteries that have different internal impedances and their behavior in a fault clearing circuit is also different. The regulatory basis for analysis and testing for fault clearing is contained in Appendix B to part 27 and appropriate Category A requirements.
(viii) There are widespread applications of batteries utilizing various lithium chemistries in aircraft today. These batteries are used as the main start battery and can be found integrated into various equipment installations. They furnish independent back up power to required equipment and many non-required applications such as entertainment systems. There are several different lithium battery chemistries available with a range of stability. Some of these lithium chemistries have been shown to be relatively unstable with failure modes that can result in explosive thermal runaway under certain conditions. A hazard analysis and system safety analysis should be accomplished, and design mitigation features should be tailored to mitigate hazards associated with the particular application.
(A) An acceptable means to show compliance with the § 27.1353 airworthiness requirements for installed rechargeable lithium battery and battery systems on rotorcraft is found in AC 20-184, Guidance on Testing and Installation of Rechargeable Lithium Batteries and Battery Systems on Aircraft. AC 20-184 provides Page F - 62.7
6/23/23 AC 27-1B, Chg 9 guidance for obtaining installation approval for installed rechargeable lithium battery and battery systems on rotorcraft.
(B) Installation requirements for installed non-rechargeable lithium batteries have some common attributes with rechargeable lithium batteries. These batteries usually exhibit less capacity and a hazard risk proportional to the capacity, but may also supply a critical and required function.
1. Each non-rechargeable lithium battery installation should address the following ten objectives, which are critical for safe installation:
Objective 1: Maintain safe cell temperatures and pressures under all foreseeable operating conditions to prevent fire and explosion.
Objective 2: Prevent the occurrence of self-sustaining, uncontrollable increases in temperature or pressure.
Objective 3: Prevent any emission of explosive or toxic gases that may accumulate in hazardous quantities, either during normal operation or as a result of a failure.
Objective 4: Meet the flammable fluid fire protection requirements of § 27.863.
Objective 5: Prevent corrosive fluids or gases that may escape from damaging surrounding structure or adjacent systems, equipment, or electrical wiring.
Objective 6: Prevent any hazardous effect on rotorcraft structure or systems caused by the maximum amount of heat generated from any battery or individual cell failure.
Objective 7: Automatically control the discharge rate of each cell to prevent cell imbalance, back charging, overheating, and uncontrollable temperature and pressure.
Objective 8: Automatically disconnect the battery in the event of an overtemperature condition, cell failure, or battery failure.
Objective 9: Alert the flight crew if an equipment state or failure affects safe operation of the rotorcraft.
Objective 10: Have a means for the flight crew to determine the battery charge state (for required battery systems).
Note: A battery system consists of the battery and any protective, monitoring, and alerting circuitry or hardware inside or outside of the battery. It also includes vents (where necessary) and packaging.
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2. The following means of compliance (MOC) have been accepted to ensure that the battery meets the objectives discussed above. Some failures may still occur due to various factors beyond the control of the designer; these MOCs are also intended to protect the rotorcraft if such failures occur.
Note: Some of the methods below reference RTCA DO-347, Certification Test Guidance for Small and Medium Size Rechargeable Lithium Batteries and Battery Systems, dated December 18, 2013. Although DO-347 provides standards for small and medium sized rechargeable lithium batteries, the FAA considers these tests appropriate as guidance for similar sized non-rechargeable lithium batteries. Procedures that are only applicable to rechargeable batteries (such as connecting input power) need not be performed. When a test method specifies charging the battery before the test, a new fully charged battery should be used.
MOC for Objectives 1, 2, and 7:
(a) Meets the requirements in paragraph 3 of TSO-C142a, Nonrechargeable Lithium Cells and Batteries;
(b) Meets the criteria to pass in Appendix 1, Table 2 of TSO-C142a;
and
(c) Passes the following tests in RTCA Document DO-347:
1. Section 2.3.7 Short-circuit Test of a Cell
2. Section 2.3.8 Short-circuit Test with Protection Enabled
3. Section 2.3.9 Short-circuit Test with Protection Disabled
4. Section 2.3.10 Insulation Resistance Test
5. Section 2.3.15 Thermal Runaway Containment Test: Perform step e. of test method 2.3.15.1 in lieu of steps c. and d.
(d) Objective 1 requires that each individual cell within a battery be designed to maintain safe temperatures and pressures. These same issues are also critical at the battery level. Objective 2 requires the battery to be designed to prevent propagation of a thermal event (i.e., self-sustained, uncontrollable increases in temperature or pressure) from one cell to adjacent cells.
їЇ. MOC for Objective 3: RTCA Document DO-347, section 3.1.3 discusses potential gas emissions from batteries and their effects. Acceptable means of Page F- 62.9
complying with Objective 3 include using the tests required by the MOC for Objectives 1, 2 and 7 to demonstrate that all emitted gasses are contained or vented overboard through designed ports. Objective 3 does allow explosive and toxic gases to be uncontained and not vented overboard if they do not accumulate in hazardous quantities within the rotorcraft. If you choose to use the test data to demonstrate compliance by this means, explain how you will demonstrate that hazardous quantities of gases will not accumulate within the rotorcraft. If your design will have ports to vent gases overboard, ensure there is a means to protect ground personnel from exposure to these gases.
iii. MOC for Objective 4: Section AC 27.863 of this guidance, subparagraph (c), provides an acceptable method of compliance for flammable fluids.
iv. MOC for Objective 5:
(a) Show that if fluid escapes the battery, it is not corrosive or it is managed in a way (e.g., contained, isolated or vented overboard) that protects the surrounding structure and adjacent systems, equipment, and electrical wiring.
(b) Any failure of the battery includes, but is not limited to, internal and external short circuit. See the tests listed in the MOC for Objectives 1, 2, and 7 for Failure Conditions that are evaluated.
V. MOC for Objective 6:
(a) Force a cell in the battery into thermal runaway by conducting the RTCA Document DO-347, section 2.3.15, Thermal Runaway Containment Test.
Perform step e. of test method 2.3.15.1 in lieu of steps c. and d. when conducting this test.
(b) Show that the effects of the heat and any related effects do not constitute a hazard to the structure or systems of the rotorcraft. If the effects of the heat or any related effects constitute a hazard to the structure or systems of the rotorcraft, design mitigation at the aircraft level may be applied to bring the design into compliance with Objective 6.
vi. MOC for Objective 8: A well designed disconnection system will function during the tests required by the MOC for Objectives 1, 2, and 7, except for the test in RTCA Document DO-347, section 2.3.9, where the protection is already disabled.
This is one of the means of preventing the Failure Conditions discussed in the Evaluation Criteria sections of RTCA Document DO-347 for these tests. Confirm the battery automatically disconnects from its discharging circuit during these tests.
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vil. MOC for Objective 9: Objective 9 specifies flight crew alerting if the failure of a battery installation, in itself or in relation to a system that performs an aircraft-level function, could result in "unsafe system operating conditions." The alert should comply with the applicable color requirements in § 27.1322. The applicant should refer to Section AC 27.1322 for an acceptable method of compliance.
vili. MOC for Objective 10: The following are examples of "means" for flight crew or maintenance personnel to determine the battery charge state:
(a) A manually activated system (e.g., push button) that displays the available charge of each battery unit.
(b) An automatic system that records the available charge of each battery unit on an appropriate interval and makes it available to maintenance personnel before the next flight.
(c) Physical access that allows a maintenance person to measure the battery charge state.
3. The MOCs for Objectives 1 through 10 are only acceptable for required functions if ICAs include a check of the battery charge state within an interval that will ensure sufficient charge for each flight considering the worst case scenario and replacement of the battery when the charge is not sufficient.
4. MOC for Very Small Non-Rechargeable Lithium Batteries:
An acceptable MOC for Objectives 1 through 8 for non-rechargeable lithium battery installations that have less than 2 watt-hours of energy is showing these batteries meet Underwriters Laboratories (UL) 1642 or UL 2054.
(ix) Part 27 Compliance. Non-rechargeable lithium battery installations should satisfy each of the MOCs identified in this guidance to comply with all applicable part 27 requirements. The following provides means of compliance and other information on some part 27 requirements:
(A) System Safety Assessment. Although these hazard conditions require specific functionalities and capabilities and address certain critical failure modes of nonrechargeable lithium batteries and their installations, the applicant must also meet the requirements of § § 27.1301 and 27.1309. To date, in-service experience has shown that non-rechargeable lithium battery thermal/pressure runaway conditions are not extremely improbable. Assume such failures could occur sometime during the life of the battery installation when demonstrating compliance with § 27.1309. Application of Page F - 62.11
6/23/23 AC 27-1B, Chg 9 § 27.1309(b) and (c) may result in required periodic maintenance actions or flight crew alerting features. For example, an over-temperature warning system may be necessary to allow the flight crew to manage potentially unsafe system operating conditions.
Provide rationale for alerting requirements (or for why an alert is not needed) in the system safety assessment of the battery installations to demonstrate compliance with § 27.1309(c). Such alerts, if provided, must comply with § 27.1322.
(B) The ICA should include the following provisions to comply with § 27.1529:
1. Maintenance requirements to replace each non-rechargeable lithium battery within an interval that will ensure there is sufficient charge to power equipment.
2. A requirement to only replace non-rechargeable lithium batteries with batteries from the same manufacturer with the same part number or to obtain a new FAA approval for installing a different battery. Refer to the battery Original Equipment Manufacturer's maintenance manual.
3. Procedures to ensure that each non-rechargeable lithium battery has not:
i. Experienced degraded charge retention ability or other damage during storage.
ii. Been damaged from environmental or physical impacts such as mechanical shock, vibration, heat and possible abuses encountered during storage, transportation prior to their installation, or maintenance activities on or around them.
ill. Precautions to prevent mishandling of replacement nonrechargeable lithium batteries prior to their installation that could result in short-circuit or other unintentional damage.
Note: Acceptable procedures for paragraphs b(2) (ix)(B)3.i. and b(2)(ix)(B)3.ii. may include a quality control process for packaging, storing, maintaining, and transporting non-rechargeable lithium batteries, including reporting of dropped or damaged batteries.
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AC 27-1B, Chg 4 3/31/2014 AC 27.1357. § 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.
b. Procedures.
(1) The circuit protective devices for systems essential to safety in flight should not be tripped by faults in other circuits.
(i) Systems that are essential to safety in flight 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.
(ii) An electrical utilization system is a system of electrical equipment, devices, and connected wiring using electrical energy to perform a specific aircraft function.
(ili) 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 that 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 Page F - 62.13
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 section requires protective devices for circuits essential to safety in flight to be accessible to the flight crew in the cockpit. These devices should be also readily accessible to the flight crew in the event it becomes necessary to manage smoke caused by an electrical failure. Their accessibility should also permit the flight crew to easily determine if they are in the "tripped" position prior to flight. 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 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 acceptable compliance with the "readily reset" provision of this guidance:
(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.
(ii) 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) A switch is a device intended for regular use to open or close a circuit. A circuit breaker is a device that protects a circuit by opening automatically at a predetermined current overload. Systems should be designed such that the primary means to remove or reset the power supply in normal operations is by means of a switch. The use of a circuit breaker for such normal operations is unacceptable, as it is not being used for its intended function.
Note: A switch-rated circuit breaker may be used for this function if it can be shown to be suitable for the number of switch cycles expected to be performed during the service life of the system.
(5) The spare fuse requirement applies only to fuses protecting systems required to show compliance with the regulations. Spare provisions need not be made for non-required convenience type installations, although it is encouraged. 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.
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AC 27-1B, Chg 4 3/31/2014
(6) 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 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.
AC 27.1361. § 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.
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AC 27.1365. § 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 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.
(ii) 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.
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AC 27-1B, Chg 4 3/31/2014
(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.
AC 27.1367. § 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.
(i) 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 turther 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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