AC 27-1B, Chg 1 2/12/03 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 helicopter 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 helicopter 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 go to 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 thani nominal capacity condition. This philosophy would also carry with it a Certification Maintenance Requirement as a limitation on the helicopter approval. Compliance could 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.
(C) One of the significant design issues associated with batteries is the internal impedance. Low internal impedance usually characterizes batteries capable of high discharge rates. Starting and fault clearing requirements are typically best satisfied by batteries with low internal impedance. The other extreme is batteries with high internal impedances that are generally more economical and perform low rate, long-time interval discharges well. This type of batteries can provide the emergency power for equipment efficiently. If both starting/fault clearing and emergency power requirements are to be provided using a single battery, a workable compromise on the internal impedance must be accomplished.
(D) The battery must be qualified to the helicopter's defined environmental specifications associated with the battery installation location, as a minimum. High level environmental qualification may preclude the installation location dependency for the battery. In addition to the environmental qualification, the design/installation design should consider the risk of explosion presented by the natural production of gases. An explosive resistant case or container is an acceptable way of showing compliance to the rule requiring that the structure and essential systems do not suffer hazardous effects.
There may be other ways to show compliance, but use of the explosion-resistant case is the predominant way at this time.
(її) The part of the battery installation approval associated with installation into the helicopter should consider the adequacy of the associated battery interface components and the physical aspects of installing the battery.
(A) The adequacy of the associated components that are necessary to interface with the battery to provide functionality must be shown to perform their intended function for the installation environment and operations. These components should be qualified to meet the most severe environment of their installation and Page F - 60
operate properly within all specified limits. Operational specified limits may be set by battery charging in-rush currents or possible fault currents.
(B) 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 batteries and associated bus on an over-temperature condition, provisions should be provided in the cockpit to warn of a disconnect.
(C) 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.
(D) Battery failure sensing and warning systems have also been used to show compliance with this rule.
(3) Battery replacement with any battery other than the one that was installed and accepted as part of the original type design must consider several aspects of battery design and installation to assure that the type design is not negatively impacted.
(i) 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. Nickel cadmium 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. If 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.
Replacement batteries must be compatible with type design requirements of the helicopter (reference paragraph b(2)(1)(A) above). For FR and CAT A certified helicopters that depend on fault clearing features, an analysis may be Page F - 61
AC 27-1B, Chg 4 3/31/2014 needed to show that the electrical system still complies to the original certification requirements. 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 features 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 Appendices B and C.
(ili) Replacement batteries must show compliance to the requirements of FAR 27 for the same areas of concern identified in paragraph b(2)(i)(B).
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AC 27.1353А. § 27.1353 (AMENDMENT 27-51) ENERGY STORAGE SYSTEMS.
a. Explanation. Amendment 27-51 revised § 27.1353 to generalize the regulation to apply to any energy storage system. Additionally, the amendment changed the section title from "Storage battery design and installation" to "Energy storage systems." This AC material is updated to address lithium batteries. The guidance pertaining to lead-acid and nickel-cadmium batteries has not changed.
(1) Electrical equipment, controls, and wiring must be installed so that operation of any one unit or system of units will not adversely affect the simultaneous operation of any other electrical unit or system essential to safe operation to meet the requirements of § 27.1351. Additionally, electrical equipment, controls and wiring installation design should be documented sufficiently to maintain configuration control for manufacturing and to ensure that characteristics such as electromagnetic and thermal attributes remain the same as the certification sample.
(2) The applicant should submit the results of qualification testing. Environmental qualification of electrical equipment may be accomplished using the appropriate revision of RTCA DO-160. Electrical equipment should be tested as installed to show there are no adverse environmental effects on previously installed equipment and that newly installed equipment is not adversely affected by previously installed equipment.
Potential adverse effects include, but are not limited to, electromagnetic interference or overheating.
(3) Installed energy storage systems serve many required and non-required rotorcraft functions. Energy storage systems vary in size, capacity, chemistry, discharge rates, recharge rates, function, function criticality, and reliability. They supply power for the primary electrical system architecture, backup power for required functions, and backup power for non-required functions. Throughout the life of the aircraft, these systems and their components will be maintained, replaced, and modified. Replacement systems or components should be identical or all differences assessed and justified.
Replacement or system modification data should show that any changes to rotorcraft systems and wiring remain in compliance and that functionality and reliability meet or exceed current requirements. Capacity requirements should result from a careful and accurate system's load analysis. System management requirements such as electrical load shedding should safely integrate into the overall cockpit management scheme and not adversely affect pilot workload for any normal or emergency operation.
(4) Energy storage systems may present hazards when installed on rotorcraft.
Energy storage may be achieved by various methods and each may present different hazards that must be addressed for installation. Hazards associated with installation of any energy storage system should be identified and evaluated to determine any effects on continued safe flight and landing. If required, methods should be developed to mitigate those effects.
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6/23/23 AC 27-1B, Chg 9
(5) Energy storage systems that have the potential to emit corrosive fluids, explosive gasses, or toxic gasses will require a design scheme to contain, drain, or vent these fluids or gasses, to meet the requirements of § 27.1353(c). This design scheme must not expose structure or adjacent equipment to corrosive fluids, as required by § 27.1353(c), or allow explosive or toxic gasses to accumulate to hazardous levels, as required by § 27.1353(b).
(6) Energy storage systems that are susceptible to thermal runaway should include a sense and control capability to prevent such an event or a design to safely contain or vent heat and pressure. Designs for energy storage systems with potential for thermal runaway must prevent an uncontrolled cascading failure that results in a hazardous condition, to meet the requirements of § 27.1353(d).
b. Procedures.
(1) General. Chapter 11 of AC 43.13-1B, Acceptable Methods, Techniques, and Practices-Aircraft Inspection and Repair, includes considerable guidance regarding the installation of electrical systems (routing, separation, typing, clamping, j-box installations, etc.). The following areas are overlooked in many cases, and special emphasis should be placed on them during the compliance inspection of the rotorcraft:
(i) Feeder wires from the rotorcraft's generators and energy storage systems should be routed separately from utilization system wiring.
(ii) Generator field wiring should be routed separately from generator output wiring. This should begin at the generator and continue to the voltage regulator.
(2) Energy Storage System Installation.
(i) Installation approval of an energy storage system consists of two parts.
One part is the approval of the energy storage system performance specifications that meet the rotorcraft requirements and the other part is the actual installation of the energy storage system into the rotorcraft. The following methods of showing compliance can be demonstrated by a combination of tests and analysis.
(ii) Approval of the energy storage system for installation is dependent on rotorcraft requirements for the installed equipment and operations.
Page F - 62.2
6//23/23 AC 27-1B, Chg 9 (A) The energy storage system capacity must be sufficient to supply the required current at a voltage level that allows proper operation of the equipment or functions dependent on the energy storage system. This capacity must be shown to be sufficient at the proper voltage levels for all operational and environmental conditions, as required by § 27.1351(a) (1). Any energy storage system required for continued safe flight and landing of the rotorcraft must have monitoring features and the means to indicate to the pilot the status of all critical system parameters (see § 27.1353(e)).
Accordingly, warning, caution, or advisory indication must be provided appropriate to any hazard condition. Section 91.167 requires Rotorcrafts approved for IFR operations have sufficient back-up power to operate a minimum set of required equipment for a time long enough to find a suitable place to land and effect a safe landing if primary power generation fails. The minimum time for IFR is 30 minutes. Additionally, for rotorcraft that employ electrical power for starting, to meet the requirements in § 27.1351, the energy storage system must have sufficient capacity to provide adequate engine starting on the ground, and also to provide power for the air start function, when required. The power requirement for air start could have high-level safety considerations depending on the rotorcraft power distribution system and the number of engines.
(B) The energy storage system must be qualified for the specific location and rotorcraft environment. Acceptable environmental test procedures may be found in RTCA DO-160, which provides test procedures to meet a wide range of environmental conditions that may exist on a wide variety of aircraft. Environmental tests should meet or exceed the expected installation environment. In addition to the environmental qualification, the equipment and installation design should consider the risk of explosion presented by the production of gases that occur during normal operation or as a result of any failed condition. An explosive resistant case or safe venting may be an acceptable way of showing compliance if it is shown that structure and essential systems do not suffer hazardous effects that jeopardize continued safe flight and landing.
(C) Energy storage systems shall have protective features to prevent unsafe conditions during operation. The protective features shall automatically control all critical operational parameters of the system in order to prevent any unsafe condition. The term "automatically" is intended to preclude pilot action as a mitigating feature and is not intended to require a specific mitigating design feature. Development assurance levels (DAL) for hardware and software that implement monitoring and control functions should be commensurate with the effect associated with system failure. An acceptable certification process for airborne software is found in AC 20-115 and, for complex hardware, in AC 20-152.
(D) The occurrence of self-sustaining, uncontrolled increases in temperature or pressure must be prevented for normal energy storage system operation or as a result of a failure of any part of the system installation (see § 27.1353(d)).
System architecture should ensure that a component failure does not propagate to adjacent components. A safe design may require a compartmentalized structure to prevent cascading failures. Any required monitoring and control functions implemented in hardware or software should have a DAL commensurate with the hazard associated Page F - 62.3
with the loss of function. Installation design features to mitigate explosive increases in temperature or pressure may be required if energy storage system DALs are not suitable for the inherent hazard.
(E) Fluids or gases that may escape from an energy storage system or its installation may not damage surrounding structure or any adjacent systems, equipment, or electrical wiring of the rotorcraft (see § 27.1353(c)). Installation design data should include features to contain, vent, or drain these fluids or gases away from the rotorcraft. Flight tests may be required to evaluate the effectiveness of venting or draining throughout the normal operating envelope.
(F) Explosive or toxic gases that may be emitted by an energy storage system must not accumulate in hazardous quantities within the rotorcraft (see § 27.1353(b)). Installation design data should include features to contain or vent these gases away from the rotorcraft. Flight tests may be required to evaluate the effectiveness of venting or draining throughout the normal operating envelope.
(G) The energy storage system and its installation must prevent any hazardous effects on the rotorcraft structure or other installed systems caused by the maximum amount of heat generated during normal operation or by a failure of any part of the energy storage system or its installation (see § 27.1353(d)). Shielding, cooling, or venting provisions may be required to ensure that temperatures do not exceed allowed temperatures of any adjacent equipment, electrical wiring, hydraulic lines, or structure.
(H) Energy storage system temperatures and pressures must be maintained within a safe operating range during all foreseeable charging and discharging conditions, and during any failure of the control or monitoring functions (see § 27.1353(d)). Energy storage system installation design features must mitigate all hazards associated with monitoring and control failures (see § 27.1353(e)). The monitoring and control function implemented in hardware or software should have a DAL commensurate with the hazard associated with the loss of function.
(I) The instructions for continued airworthiness (ICA) must include maintenance requirements to ensure that required energy storage system maintenance functions are performed at appropriate intervals specified for each energy storage system component, to meet the requirements in Appendix A to Part 27. This is necessary to ensure system performance will not degrade below specified energy levels sufficient to power Rotorcraft systems for intended applications. Similarly, the ICA must also include procedures for the maintenance of energy storage system components in spares storage to prevent the replacement of components with stored components that have experienced degraded functional capability or other damage due to prolonged storage. Replacement energy storage system components must be of the same manufacturer and part number as initially approved, unless additional installation approval is obtained (see § 21.9(a)). Precautions should be included in the ICA maintenance instructions to prevent mishandling of the energy storage system components that could result in unintentional impact damage caused by dropping or other destructive means that could result in personal injury or property damage.
Page F - 62.4
(J) The energy storage system installation approval should consider the adequacy of associated interface components that may affect system performance.
Components necessary to interface with the energy storage system to provide functionality should be shown to perform their intended function for the installation environment and operations. These components should be qualified to meet the most severe environment of the installation and operate properly within all specified limits.
Operational limits should be set for all energy storage system parameters identified as critical for safely performing the intended function. The intended function of energy storage system and knowledge of the charging function supplied by the rotorcraft are major design considerations. The type of charging function may be a major factor in determining the required design assurance level. If the energy storage system is not adequately charged, the capacity may not be available, when required, to meet emergency power demands. Overcharging has been shown to result in a thermal runaway with certain battery chemistries. Monitoring and control functions may be required to prevent safety critical parameters from exceeding specified limits. Such critical parameters may include charging currents, fault currents, temperature, or pressure.
(ili) Batteries are widely used as energy storage devices and utilize different chemistries that may affect their performance. For the purpose of this guidance, a battery or battery system is referred to as a battery.
Battery chemistries in widespread use today are lead acid, nickel cadmium, and lithium.
Each type exhibits different failure modes. These batteries also come in a wide range of sizes and energy density. Large batteries are used for engine starting and as systems' backup power in the event all electrical generation capacity is lost. Smaller batteries are used in many electrical components independent of the aircraft power generation and distribution systems. Some of these batteries are non-rechargeable and some are recharged by an electrical generation system. The hazard analysis should take into account the failure modes, the intended function, and the supporting rotorcraft interface.
(iv) A significant design issue associated with batteries is internal impedance.
Low internal impedance usually characterizes batteries capable of nigh discnarge rates.
Starting and fault clearing requirements are typically best satisfied by batteries with low internal impedance. The other extreme is batteries with high internal impedances that are generally more economical and perform low rate, long-time interval discharges well.
These types of batteries can provide the emergency power for equipment efficiently. If both starting/fault clearing and emergency power requirements are to be provided using a single battery, a workable compromise on the internal impedance should be accomplished.
(v) Lead acid batteries have been used in aircraft for many years. As such, the failure modes are well documented. These batteries contain corrosive materials, and consideration should be given to protect rotorcraft structures and systems from inadvertent leakage. Hazardous gases may also be released under certain conditions;
Page F - 62.5
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.
Page F - 62.6
(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.
Page F - 62.8
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