Fire resistance. Electrical equipment must be so designed and installed that if there is a fire in the engine compartment, during which the surface of the firewall adjacent to the fire is heated to 2000 degrees F for five minutes or to a lesser temperature confirmed by the applicant, the equipment essential to continued safe operation and located behind the firewall will function satisfactorily and will not create an additional fire hazard.
Amendment 23-17 and Subsequent
A proposed revision to NPRM 75-10 requires overvoltage control of generators with the following explanation: “Complete electrical system failures on both single and twin engine aircraft continue to occur. The electrical failure that causes this problem is often the loss of voltage control in the voltage regulator. This usually results in a bus voltage well above the capabilities of the electrical equipment connected to the bus. This overvoltage condition frequently destroys electronic equipment and boils the electrolyte in the battery. The proposal is only directed at this overvoltage problem and therefore differs from Sec. 23.1351(b). The proposal would not require a specific method of overvoltage control.”
Amendment 23-20 and Subsequent
A proposed revision to NPRM 75-23 added external power requirements with the following explanation: “If an external power supply with reverse polarity or reverse phase sequence were to supply power to the airplanes electrical system extensive damage to the system could result. This proposal would require a means to prevent such an occurrence.”
Amendment 23-34 and Subsequent
A proposed revision to NPRM 83-17 added commuter category requirements from “part 135, Appendix A, sections 61 and 63”.
Amendment 23-43 and Subsequent
A proposed revision to NPRM 90-23 explains this amendment as follows: “This proposal would allow a generator to be installed and operate below its continuous rating when it has a rating higher than necessary, allow methods other than reverse current cutouts for protecting against reverse current, and would require the airplane to operate safely for 5 minutes without normal electrical power.
Conference proposal 453 recommended revising paragraphs (c)(1) and (c)(3) essentially as proposed herein. The justification given is that the generator-rated output may be higher than required for the electrical loads of the airplane and, in such case, the electrical system (generator output) is limited by its regulation system.
The consensus at the conference supported the objective of this conference proposal.
The FAA agrees with the need for changing paragraph (c)(1) to clarify its intent and to revise paragraph (c)(3) to relieve the burden to install a specific type of reverse current control where more efficient and less costly controls are now available.
Conference proposal 456 recommended adding paragraph (g), essentially as proposed herein except the requirement would only be applicable for airplanes operated above 25,000 feet. The justification was that part 23 airplanes that operate at high altitudes above 25,000 feet depend upon electrical power for safe operation.
Emergencies involving loss of normal electrical power at or above this altitude typically result in the loss of other systems, such as electric fuel pumps, pressurization system, warning system, navigation, communications, and instrumentation. The FAA developed special conditions for part 25 that initiated the requirement in this proposal and it was later adopted into part 25 by amendment
25.41, in 1977. Conference proposal 456 was essentially developed from the part 25 requirements except for the 25,000 foot applicability. When offered for comment at the conference, there were no objections on conference proposal 456. After further review, FAA has concluded that the proposal should not be limited to airplanes that operate above 25,000 feet since emergencies resulting in the loss of normal electrical power are critical for all airplanes. Five minutes is considered adequate time to cope with such an emergency so that pilot can operate the airplane safely and assess the reason for the loss of normal electrical power.
Conference proposal 452 recommended changing the phrase "essential for safe operation" to "essential to flight safety" for consistency in the regulations. A word search of the regulations indicated that there were other phrases such as "essential to safety of flight," and "essential to continued safe operation." All of these phrases have been interpreted to have the same meaning. Since the affected regulations have been administered effectively without significant problems, the FAA does not consider the recommended changes to be beneficial.”
Final Rule, Docket 26324, has the following clarification: “It has been brought to the FAAs attention that many electrical generating devices that are used on part 23 are now referred to as "alternators" and that there is some confusion about such units acceptability because Sec. 23.1351(c) continues to address "generators." To provide clarifications, ten locations in Sec. 23.1351, paragraphs (c), (c)(1), (c)(2),
(c)(3), (c)(4), and (c)(5), are being revised by changing the word "generator" to "generator/alternator." This proposal is adopted with the aforementioned change.”
Amendment 23-49 and Subsequent
A proposed revision to NPRM 94-21 explains this amendment as follows: “The proposal would revise current Sec. 23.1351 by removing portions of paragraphs
(b)(2) and (b)(3) and by removing all of paragraph (b)(4). The removed requirements 267
are applicable to alternators that depend upon the battery for initial excitation or for stabilization. This revision responds to a Joint Aviation Authority recommendation to remove the provisions that allow a battery failure to result in the loss of the alternator. Information in this recommendation showed that self-excited alternators are now available for installation on newly certificated airplanes. The FAA has verified that self-excited alternators are now available; therefore, there is no longer a need for the regulations to address alternators that depend upon a battery for initial excitation and stabilization.
Revised Sec. 23.1351(c)(3) would require an automatic means for reverse current protection. Reverse current protection is accomplished by means that automatically detect changes in the current. The proposed revised wording would more accurately define this function and the equipment that would accomplish the protection.
Finally, Sec. 23.1351(f) would be revised by adding a requirement that would require the ground power receptacle to be located where its use will not result in a hazard to the airplane or to people on the ground using the receptacle.”
Final Rule, Docket 27806, has the following clarification: “The proposals are adopted as proposed, except that paragraph (c)(3) has been revised to clarify that protection for any generator/alternator and the airplane electrical system must be provided.”
EASA AMC 23.1351(a)(2) is acceptable for FAA certification.
EASA AMC 23.1351(b)(5)(iv) is acceptable for FAA certification to 14 CFR, part 23, § 23.1351(b)(4)(iv).
23.1353 Storage battery design and installation
The corresponding rule in CAR 3 is CAR 3.683.
The corresponding rule in the Airship Design Criteria, FAA-P-8110-2, Change 2, is section 6.18.
Original Issue and Subsequent
See AC 43.13-1B, Change 1, “Acceptable Methods, Techniques, and Practices⎯Aircraft Inspection and Repair,” Section 8, for battery installation guidance. Replacement batteries would require PMA, unless exempted by part 21, § 21.303(b), whether the replacements are lead-acid or nickel-cadmium. The airworthiness standards of 14 CFR, part 23, §§ 23.1301, 23.1309, 23.1351, and
23.1353 should be considered for replacement battery installations.
Amendment 23-20 and Subsequent
A proposed revision to NPRM 75-23 explains this amendment as follows:
“Proposed Sec. 23.1353(b)(1) would add "or power" to the battery recharging standard. The proposal by adding an alternative standard is intended to clarify the present rule to more appropriately include all battery charging means and parameters. In some battery charging techniques, voltage is not a controlled factor except by response of the battery. Also, power is a measure of heat input to the battery. A similar proposal is made for Secs. 25.1353(c)(1)(i), 27.1353(b)(1), and
29.1353(c)(1)(i).
Short circuits of nickel cadmium batteries (either internally or to the airplane structure through the battery caps) have occurred in service. The heat generated by such short circuits may in some circumstances damage nearby structure or essential systems. It is therefore proposed to require provisions to prevent any hazardous effect from this cause. See also proposed new Sec. 23.1353(f) in Airworthiness Review Notice No. 2 (Notice 75-10), which is aimed at reducing the occurrence of short circuits that are caused by excessive battery charging current.”
Amendment 23-21 and Subsequent
A proposed revision to NPRM 75-10 explains paragraph (g) as follows: “The proposal conforms to paragraph (b) of AD 72-19-4, which applies to all nickelcadmium batteries that are capable of being used to start an aircraft engine or auxiliary power unit.”
Amendment 23-49 and Subsequent
A proposed revision to NPRM 94-21 explains the 30-minute rule in this amendment as follows: “Proposed new Sec. 23.1353(h) would require that, the event of a complete loss of the primary electrical power generating system, airplane battery capacity must be sufficient to supply at least 30 minutes of electrical power to those loads essential to the continued safe flight and landing of the airplane.
This proposal is not limited to airplanes that are approved for any particular type of operation. Although the battery capacity needed for an airplane approved for day VFR operations would be much less than the capacity for an airplane approved for day/night IFR operations, the same level of safety should be provided for all airplanes. While this proposal would add an additional requirement to part 23 for normal, utility, acrobatic, and commuter category airplanes, in practice this requirement to provide a battery capacity sufficient to supply at least 30 minutes of electrical power is not new to many airplane manufacturers. Certain other countries in which part 23 airplanes have been certificated have requirements for such a 30- minute battery capacity. Manufacturers experience with these requirements has shown that the only design impact that results from complying with these requirements is the need to install a battery with greater capacity than might otherwise be installed. Experience has also shown that a load shedding procedure may be necessary for certain airplanes. No other airplane design changes would be needed.
Despite the above referenced experience record, this requirement would be new to some manufacturers and they may have questions on how it would be applied. For that reason, this notice discusses compliance considerations that have emerged from experience based on substantively equivalent requirements.
This compliance experience has shown that the rating of the battery selected for the airplane should be sufficient to cover the loss of capacity that would occur with battery age and the reduced capacity that results from a realistic state of charge, which may be less than a full charge. Using a design battery capacity that is only 75 percent of the battery nameplate rating would be an acceptable way of accounting for these losses.
In addition to determining the battery rating that would be needed, the manufacturer would also need to determine the functions that would be necessary for 30 minutes of safe flight and the landing of the airplane. Again, experience has identified several functions. For a day VFR approved airplane, no functions may require battery power; however, it may be necessary to supply power for certain communication capacities or, if the airplane has electrically powered retractable landing gear.
Providing a secondary means for lowering the gear would be an acceptable alternative to providing electrical power or battery power for this function.
For other types of operating approvals, providing power for the following functions and equipment should be considered:
1. Any required flight and navigation instruments. Air driven instruments that would function over the required period can also be accepted for this function.
2. Cockpit and instrument lighting.
3. For IFR and icing approvals, power for the heated pitot tube.
4. For radio communication, usually one VHF communication system with power for three to five minutes of transmission would be acceptable.
5. Functions needed for safe night flight and night landing for the airplane.
6. Electronic engine ignition systems.
7. Any functions that cannot be readily shed following the loss of generator power.
8. Engine inlet heat or deicing protection required for normal operation of the airplane.
Although power for the listed functions may provide for the safe operation and landing of most airplanes, individual airplane designs may require the consideration of additional functions.
In applying these rules it may be assumed that airframe and engine icing protection equipment would not be operating at the time of the generator system failure. Power for icing protection would not be required if the icing protection equipment is not required for the normal operation of the airplane.
This proposal would require additional battery capacity and would not alter or supersede any other requirements in this part for separate or dedicated emergency power supplies. When requirements such as those in current Sec. 23.1331(a) or in proposed Sec. 23.1311(a)(5) are applicable to the airplane design, these power supplies are required to provide a needed level of safety for that function; therefore, that power source must be supplied.”
The requirements of § 23.1353(h) are as follows:
“In the event of a complete loss of the primary electrical power generating system, the battery must be capable of providing at least 30 minutes of electrical power to those loads that are essential to continued safe flight and landing. The 30 minute time period includes the time needed for the pilots to recognize the loss of generated power and take appropriate load shedding action.”
Previous guidance on this issue states the airplanes primary electrical power includes the airplanes electrical generation system and the airplanes starter battery when only one battery is installed. The battery for the 30-minute criteria, therefore, should be an independent power source from the airplanes starter battery. If adequate monitoring and procedures are incorporated so the pilot knows the airplanes starter battery 271
meets the 30-minute criteria after an engine start and during all other operations, an ELOS finding may be an acceptable method for using the airplanes starter battery.
Please refer to the guidance in AC 23.1309-1C, “Equipment, Systems, and Installations in Part 23 Airplanes,” for determining the loads that are essential to continued safe flight and landing. Continued safe flight and landing is defined as follows:
“This phrase means that the airplane is capable of continued controlled flight and landing, possibly using emergency procedures, without requiring exceptional pilot skill or strength. Upon landing, some airplane damage may occur as a result of a failure condition.”
The 30-minute power bus should include all systems that could cause a catastrophic failure condition under the § 23.1309, Failure Hazard Assessment. Sometimes, it may not be practical to include all systems on the 30-minute power bus that could cause a catastrophic failure condition. For example, systems with large heating loads for ice protection may not be included on the 30-minute electrical power bus;
however, the possible hazards that could cause catastrophic failure conditions should be minimized.
To minimize the hazard either is to reduce, lessen, or diminish to the least practical amount with current technology and materials. The least practical amount is that point at which the effort to further reduce a hazard significantly exceeds any benefit in safety gained from that reduction. Additional efforts would not result in any significant improvements to safety and would inappropriately add to the cost of the product.
Assuming operations under IFR conditions for parts 91 or 135 operations, the following systems should be included on the 30-minute power bus:
• If needed to comply with § 23.1325, one airspeed indicator and altimeter with a heated pitot tube and heated static pressure source;
• The magnetic compass, and any display necessary for continued safe flight and landing, sufficiently illuminated for night operation;
• One navigation system installation appropriate to the ground facilities to be used;
• One communication installation system;
• One gyroscopic pitch and bank indicator;
• One clock;
• Any display for the powerplant parameter necessary for continued safe flight and landing; and 272
• Any electrical loads unique for the airplane characteristics and needed for continued safe flight and landing for the intended operations.
Tests and analyses should be considered for determining the rated operating capacity of the battery, the normal service life, and the continued airworthiness requirement of § 23.1529. For these tests and analyses, the following should be established as follows:
(1) For the operating capacity: the discharge rate, temperature, end-point voltage, etc.; and
(2) For the airworthiness requirement: the inspection schedule, useful battery life, end-of-life, etc.
23.1357 Circuit protective devices
The corresponding rules in CAR 3 are CAR 3.690, 3.691 and 3.692.
The corresponding rule in the Airship Design Criteria, FAA-P-8110-2, Change 2, is section 6.19.
Original Issue and Subsequent
This rule allows only one essential circuit on one circuit protective device. The intent of the rule is met for installations that integrate position and anticollision lights on one wire when no single failure can cause the loss of any of the following:
(a) More than all anticollision lights.
(b) More than all position lights.
(c) More than one position light and the anticollision light adjacent to it.
The requirement in § 23.1357(e) applies to fuses for all circuits, not just essential circuits. Although spare fuses for non-essential systems and equipment do not have to be resettable in flight.
The phrase “essential to safe operation,” as used in part 135, Appendix A, paragraph 64, and the phrase “essential to flight safety” in § 23.1357(b) have the same meaning as “essential to safety in flight” in § 23.1357(d) and “essential to flight safety” in § 23.1357(b). All of these phrases are descriptive of equipment installed in order to comply with the airworthiness or operational requirements. The FAA recognizes that some required circuit protection devices are associated with circuits that can have no significant impact on safety in flight. Therefore, the responsible ACO, with the applicant, should identify which circuits and circuit protection devices are essential to safety in flight. The identified circuits should comply with § 23.1357(d) on the pilot's ability to reset them in flight.
Fuel quantity indicators are required by regulation but are not essential to safety in flight. The loss of an indicator will increase the pilot's workload, but it will not, in itself, cause either a loss or a forced landing of the airplane. It is acceptable, therefore, to have a protected circuit that includes more than one required indicator, which includes a fuel quantity indicator. Good design practice would include placing multiple indicators of the same parameter on separate protected circuits, and we encourage the use of warnings for low fuel and high oil temperature, to mitigate the effects of loss of indication.
For part 23 applications, the definitions of a switch and a circuit breaker are as follows: 1) A switch is a device for opening and closing or for changing the connection of a circuit; 2) A circuit breaker is a device designed to open and close a circuit by non-automatic means and to open the circuit automatically at a predetermined overload of current, without injury to itself when properly applied within its rating. Thus, circuit breakers used for operational functions are not acceptable because they are not performing their intended function, which is protection against overloads. Circuit breakers, even those suitable for frequent operation, should not be used as a switch to perform procedural functions.
A combination switch/circuit breaker is a device which can perform both as a switch for opening and closing a circuit as well as a circuit breaker, automatically opening the circuit at a predetermined overload current.
Amendment 23-20 and Subsequent
A proposed revision to NPRM 75-23 explains § 23.1357(b) as follows: “Proposed paragraph (b) would clarify the present rule by specifically prohibiting the use of a single protective device to protect an essential and a nonessential circuit, or more than one essential circuit. A protective device protecting two circuits would trip in response to a fault in either one; thus a fault in the nonessential circuit would render the essential circuit inoperative.”
Amendment 23-43 and Subsequent
A proposed revision to NPRM 90-23 explains this amendment as follows: “The intent has historically been to protect the airplane from the hazards of all electrical faults. Paragraph (a)(1) exempted starting motor circuits because they did not have power applied, except during engine starting. It is proposed to clarify the intent of paragraph (a)(1). The existing rules of paragraph (e) require spare fuses for all electric circuits. This proposal would require spare fuses for fuses identified as replaceable in flight, which would be those required by paragraph (d) and any other fuses identified as replaceable in flight. This proposal would also require the fuses be readily accessible and available.”
23.1359 Electrical system fire protection
See guidance in section 23.853 of this AC.
There is no corresponding rule in CAR 3 or the Airship Design Criteria.
Amendment 23-49 and Subsequent
A proposed revision to NPRM 94-21 explains this rule as follows: “Proposed new Sec. 23.1359 would require smoke and fire protection for electrical system installations. The provisions of Sec. 23.1359(a) of this proposal state that electrical systems must meet the applicable requirements of Secs. 23.863 and 23.1182.
Proposed Sec. 23.1359(b) would require that the electrical systems components installed in designated fire zones and used during emergency procedures be fire resistant. This provision is needed to clarify the requirements for electrical system components that may be installed in the designated fire zones identified in Sec.
23.1181.
Finally, Sec. 23.1359(c) provides burn criteria for electrical wire and cables. A proposed revision to appendix F of part 23 that would add appropriate wire testing criteria is included in this notice.
This proposed burn criteria for wire is necessary because of the increased use of electrical systems in the design of part 23 airplanes and the resulting increase in the amount of electrical wire being installed. This increased use results in the need to ensure that wire insulating material does not become the source of an in-flight fire and/or that it does not propagate a fire from another source. The electrical wire burn requirements in this proposal, along with the testing identified in revised appendix F, would ensure that installed electrical wire has insulating material that reduces the possibility of hazardous in-flight fires.”
23.1361 Master switch arrangement
The corresponding rule in CAR 3 is CAR 3.688.
The corresponding rule in the Airship Design Criteria, FAA-P-8110-2, Change 2, is section 6.20.
Amendment 23-20 and Subsequent
A proposed revision to NPRM 75-23 explains this amendment as follows: “Circuits by-passing the master switch remain energized after the master switch is opened. If such circuits fail during a crash landing they might possibly ignite nearby flammable fluids or vapors. This proposal would reduce that possibility.”
Amendment 23-43 and Subsequent
A proposed revision to NPRM 90-23 explains this amendment as follows: “The proposal clarifies the master switch arrangement requirement and permits new generations of engines to operate with the master switch turned off, as is necessary to isolate hazardous electrical faults.
Subsequent to the receipt of the proposals submitted for the Part 23 Review, engine designs have been developed that depend on an electrical power source for normal ignition and/or fuel pressure. An electrical fault that makes it necessary to turn off the master switches must not cause unintentional disabling of such designed engines.
However, the pilot must retain the capability to isolate all sources of electrical energy that might ignite flammable fluids that are likely to escape during a survivable crash landing.
When conference proposal 461 was discussed, a commenter specifically noted that he had no objections to the multiple circuits restriction in the proposal and supported that position. The commenter did express a concern that the proposal, "Load, circuits, such as cabin entry lights whose functions are needed prior to entering the cockpit," may unnecessarily limit those circuits to those functions that are needed before entering the cockpit. It was noted that there are other continuously energized circuits that do meet the "needed prior to entering the cockpit" definition in the proposal. A circuit for an electrical clock was cited as an example.
The FAA has reviewed the proposal and this discussion and agrees that while the proposed wordage was only intended to provide an example of the type of circuit that was permitted by this section, it could be interpreted as more restrictive than intended; therefore, this example language has not been included in this proposal.
This action by FAA should not be interpreted as an endorsement to install an unlimited number of circuits that bypass the master switch. This provision was added 277