6. One clock;
7. Any display for the powerplant parameter necessary for continued safe flight and landing; and
8. Any electrical loads unique for the airp lane 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 discharg e rate, temperature, end-point voltage, etc.; and
2. For the airworthiness requirement: the inspection schedule, useful battery life, end-of-life, etc.
Amendment 23-62 and Subsequent
In Docket No. FAA-2009-0738, the Discussion of the Regulatory Amendments stated: The ARC did not make a specific recommendation regarding § 23.1353.
However, FAA added additional battery endurance requirements depending on the airplane’s altitude performance. Section 23.1353(h) addresses the power needs of new, all electrical instruments, navigation and communications equipment, and engine controls.
When § 23.1353(h) was adopted, part 23 airplanes were mostly mechanical. FAA did not envision all-electric or almost all-electric airplanes. Previous § 23.1353(h) requires 30 minutes of sufficient electrical power for a reduced or emergency group of equipment and instrumentation. FAA considered 30 minutes adequate to reach VFR conditions to continue flying to an adequate airport, and to accomplish a safe landing for traditional part 23 airplanes.
The FAA did not envision integrated, electric cockpits when it developed § 23.1353(h). New part 23 airplanes are being certificated with all electrical instruments, including the standby instruments. This reliance on electric power increases the importance of ensuring adequate battery power until the pilot can descend and make a safe landing.
Most new engines utilize electronic engine controls. These engine controls may rely on the airplane’s electrical system for power and to control fuel and ignition. Large engines typical in part 25 airplanes will have a dedicated power source running off the engine as long as the engine is running, the electronic engine control has power.
Some of the smaller, simpler engines emerging in part 23 airplanes may not have
282 these dedicated power sources and may rely on the airplane’s electrical system to keep functioning.
The FAA believes that most new turbine powered airplanes, and some turbocharged piston-powered airplanes, will operate at high altitudes under IFR. Under these conditions, 30 minutes may not be adequate for battery power because of the time it would take to descend from maximum altitude to find visual meteorological conditions (VMC) and land, or to perform an instrument approach for a landing. For this reason, revised § 23.1353(h) would extend the battery time requirement to 60 minutes for airplanes approved with a maximum altitude above 25,000 feet.
Many new single-engine airplanes are intended for use in part 135 passenger service.
Proposed § 23.1353(h) provides consistency with the operating requirements for single engine IFR in § 135.163(i). That section requires a 60 minute battery to power all emergency equipment, as specified by the manufacturer, to allow continued safe flight and landing.
This amendment still allows 30 minutes of emergency electrical capacity for all Part 23 aircraft with a service ceiling of 25,000 feet or less, but requires 60 minutes capacity if the ceiling is above 25,000 feet.
The Final Rule adopted the comment, which was the inclusion of time to recognize the failure and take load shedding action that was deleted by mistake from the prior rule. The FAA also changed the preamble to remove the paragraph about part 135, which is not relevant to compliance to the rule. Section 23.1353 is modified as discussed.
23.1357 Circuit Protective Devices
The corresponding rules in CAR 3 are CAR 3.690, 3.691 and 3.692.
The corresponding airworthiness criteria is section 6.19, Airship Design Criteria, FAA-P-8110-2, Change 2.
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:
1. More than all anticollision lights.
2. More than all position lights.
3. 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.
Circuit Breakers/Fuses Essential to Safety Inflight
This guidance provides clarification of 14 CFR part 23, § 23.1357(d), about installed fuses or circuit breakers, including those used for either primary or secondary (inline) circuit protection. It applies to normal, utility, acrobatic, and commuter category airplanes. It also applies to non-rigid airships certificated in the normal category (14 CFR part 21, § 21.17(b)) with nine seats or fewer, excluding the pilot.
Summary
14 CFR part 23, § 23.1357(d), has stated the following since its original effective date in 1965:
“If the ability to reset a circuit breaker or replace a fuse is essential to safety in flight, that circuit breaker or fuse must be so located and identified that it can be readily reset or replaced in flight.”
The applicability of the above statement from 14 CFR part 23, § 23.1357(d), depends on whether a function is determined to be "essential to safety in flight." There are two criteria, dependent on the certification basis of the system, that are used to define “essential to safety in flight,” as used in 14 CFR part 23, § 23.1357(d). They are the following:
Change 1 AC 23-17C 7/16/2012
1. For airplane systems with a certification ba sis at Amendment 23-40 or earlier: When the function is required by the applicable airworthiness or operational requirements, as listed in 14 CFR part 23, 14 CFR part 91, or 14 CFR part 135, it is considered “essential to safety in flight;” or
2. For airplane systems with a certification basis at Amendment 23-41 or later: When the failure condition of the loss of the function is determined to be “major,” “hazardous,” or “catastrophic” [according to § 23.1309 and AC 23.1309-1E safety assessment, which also considers operational and airworthiness requirements], it has a significant impact on safety in flight and is considered “essential to safety in flight.”
To clarify, the following table summarizes the applicability of § 23.1357(d):
Certification Basis Required Equipment* Non-Required Equipment*
Amdt. 23-40 or earlier
§ 23.1357(d) applies, see statement (1) above
§ 23.1357(d) does not apply.
Amdt. 23-41 or later
§ 23.1357(d) applies if loss of function is determined to be “major,” “hazardous,” or “catastrophic.”**
§ 23.1357(d) applies if loss of function is determined to be “major,” “hazardous,” or “catastrophic.”**
* As required by operational a nd airworthiness requirements.
** According to § 23.1309 and AC 23.1309-1E safety assessment, which also considers operational and airworthiness requirements.
If the above criteria show § 23.1357(d) applies, and if the circuit protection devices are circuit breakers or fuses that cannot be reset by the pilot, an equivalent level of safety or an exemption is required.
For systems certified using the above criteria under Amendment 23-41 or later, it is acceptable for required equipment whose failure is considered “minor” under § 23.1309 to not meet § 23.1357(d). However, § 23.1357(a) still requires the applicant to show that the resulting design does not present a safety hazard.
Function Criticality and Applicability of § 23.1357(d)
Amendment 23-41 addresses the approval of more advanced and complex system designs under § 23.1309 by promoting the use of a safety assessment to assign criticality to each system function. This allows more latitude during certification of various complex system architectures in part 23 airplanes. AC 23.1309-1E, “System Safety Analysis and Assessment for Part 23 Airplanes,” documents the safety
285 assessment used to classify the failure condition for the loss of each given function in a system.
According to AC 23.1309-1E, if the failure condition is considered “major,” “hazardous,” or “catastrophic,” the circuit providing the function must have a circuit breaker or fuse that is readily accessible to be reset or replaced in flight. Therefore, the terms of § 23.1357(d) would apply.
In contrast, § 23.1357(d) would not apply for failure conditions only classified as “minor” or “no safety effect” under the terms of a safety assessment according to AC
23.1309-1E. Also, if there are several functions combined on one display and integrated under one protective device, then the total loss of all affected functions should be considered in the safety assessment to determine the criticality for that system failure.
System redundancy may be used to lessen the number of functions that require external circuit protection by reducing the criticality of some functions. However, § 23.1357(a) still requires the applicant to show that their omission does not present a safety hazard.
Example Configurations
For a system being certified under the latest amendment, see the following example of applying this policy:
For an airplane that has two electronic Primary Flight Displays (PFD), plus a Multi- Function Display (MFD) installed strictly to meet part 23 powerplant monitoring requirements, the PFDs would display the required flight critical boundaries. These include altitude, airspeed, attitude, and others.
The FAA assumes the classification for the loss of attitude on one PFD in this case is “major,” and the loss of altitude or airspeed is “minor.” For this system, § 23.1357(d) applies because of the classification of the loss of attitude. Also, both PFDs would be required to meet the airworthiness requirements of § 23.1311(a)(5).
If the MFD is used to display the powerplant limits required by § 23.1305, potential MFD failure conditions resulting in the loss of several powerplant functions at once should be addressed in the safety assessment. However, if the intended function of the MFD is to display functions that are not required by the airworthiness or operational rules and are only information for situational awareness, such as traffic, weather or terrain, then § 23.1357(d) would not apply to the MFD circuit protection devices.
For a system with two independent PFDs, with an MFD that is also capable of displaying required independent flight critical limits through a reversionary mode, the loss of attitude information on one PFD would be of a lower importance. There
286 would still be two independent sources available to display the critical flight limits.
In this case, § 23.1357(d) may not apply for some flight boundaries. A safety assessment would be needed to address other limits that may display on the MFD in a reversionary mode, such as the powerplant boundaries.
Equivalent Level of Safety or Exemption to § 23.1357 for Internal Circuit Protection Devices
Under the latest amendment, if a circuit breaker or fuse is installed, and a lack of access to that circuit breaker or fuse would result in a failure classification of “major” or worse, the circuit protection devices and equipment installation should meet the correct § 23.1309 failure probability requirements (as defined in AC 23.1309-1E). In this situation, all the correct environmental test conditions, such as indirect effects of lightning and High Intensity Radiated Fields (HIRF) standards, should be addressed for the installation. To allow approval of these installations, an equivalent level of safety or exemption to § 23.1357 would be needed to comply with the intent of § 23.1357(a) and to show that a no-hazard design has been completed.
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 FAA encourages 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 in 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
287 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 in 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.”
A circuit breaker (C/B) that is essential to safety in flight may be reset one time at the pilot’s discretion only when the operational environment requires the system and equipment. Other Essential C/Bs and Non-Essential C/Bs should not be reset in flight due to the potential for a failure that causes a fire. FAA recommends that essential to safety C/Bs be grouped together with a placard that says “Essential C/Bs.” Or mark each circuit breaker as “Essential” or “Non-Essential.”
23.1359 Electrical system fire protection
See guidance in section 23.853 of this AC.
There are no corresponding rule or airworthiness criteria in CAR 3 or the Airship Design Criteria.
See § 23.853 guidance in this AC.
Amendment 23-49 and Subsequent
A proposed revision in NPRM 94-21 explains this amendment 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 airworthiness criteria is section 6.20, Airship Design Criteria, FAA-P-8110-2, Change 2.
Amendment 23-20 and Subsequent
A proposed revision in 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 in 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 to, and retained in, the requirements because it was recognized that there are a limited number of electrical functions that are needed when the master switch is in
290 the position. The requirements of this section provide for the safe installation of these circuits. The five-ampere load restriction of a new paragraph (b)(3) was added because the FAA was made aware of an installation in which this provision was being used to circumvent the master switch arrangements by using up to four five-ampere fuses to supply a 20-ampere circuit. This restrictive provision should make it clear that such installations are not permitted.”
The proposal in NPRM 90-23 was revised by Final Rule, Docket 26344 as follows:
“This proposes to amend Sec. 23.1361 to clarify the requirement for the master switch arrangement and to permit new generations of engines to operate with the master switch turned off. No comments were received; however, an editorial revision has been made that revises the text of the last sentence from one that permits the master switch arrangement to use separate switches to text that provides requirements for the master switch arrangement if separate switches are installed.
This proposal is adopted with the aforementioned change.”
Amendment 23-49 and Subsequent
This amendment is explained by Final Rule, Docket 29806 as follows: “To harmonize with the JAR this proposal would revise Sec. 23.1361(c) by making an editorial change to remove the last two words of the paragraph that read "in flight."
This change will not alter the meaning of the requirement.”
23.1365 Electric cables and equipment
The corresponding rule in CAR 3 is CAR 3.693.
The corresponding airworthiness criteria is section 6.21, Airship Design Criteria, FAA-P-8110-2, Change 2.
Original Issue and Subsequent
Section 23.1365 requires that each cable that would overheat in a circuit overload or malfunction be at least flame resistant and not emit dangerous quantities of toxic fumes. The compliance methods for the flame resistance requirement are in AC 23-2A, “Flammability Tests.” To aid in meeting the toxic fume requirement, the FAA has removed MIL-W-5086 wire from the listing of approved wires in AC 43.13-1B, Change 1, “Acceptable Methods, Techniques, and PracticesAircraft Inspection and Repair.”
See guidance in section 23.853 of this AC for flammability.
Amendment 23-14 and Subsequent
The flame resistance and toxic fume requirements are applicable to equipment associated with the cable as well as the cable itself. A proposed revision in NPRM 71-13 explains this amendment as follows: “This proposal is considered necessary because of the higher powered electrical systems being installed in part 23 airplanes. Such systems are more likely to cause heat damage and the emission of toxic fumes in the event of malfunction.”
Amendment 23-43 and Subsequent
This amendment adopted a proposed revision in NPRM 90-23 that stated: “This proposal provides crashworthiness standards for electrical cables. It would require that electrical cables be designed to allow a reasonable degree of deformation and stretching without failure and be isolated from flammable fluid lines or must be shrouded in insulated flexible conduit.”
The proposal in NPRM 90-23 was changed by Final Rule, Docket 26344, and is explained as follows: “One commenter believes the word "isolated" used in proposed Sec. 23.1365(c)(1) is not compatible with current practices and suggests the word "separated" be used in its place. The FAA reviewed this recommended change and agrees that "separate" better describes the current practice of keeping electrical cables and flammable fuel lines spaced a part. This proposal is adopted with the aforementioned change.”
292 Amendment 23-49 and Subsequent
This amendment adopted a proposed revision in NPRM 94-21 with this explanation:
“This proposal would revise Sec. 23.1365(b) and would add three new paragraphs.
Section 23.1365(b) would be revised in relation to proposed new Sec. 23.1359(c), which would require self-extinguishing insulated electrical wires and cables. Current Sec. 23.1365(b) requires that cable and associated equipment that would overheat in the event of circuit overload or fault must be flame resistant and may not emit dangerous quantities of toxic fumes. The proposed revisions to Sec. 23.1365(b) would remove the reference to electrical cables from the flame resistance requirement since the cables would be required to have self-extinguishing insulation under Sec. 23.1359(c). The requirement for electrical cables and the associated equipment that would overheat to not emit dangerous quantities of toxic fumes has been retained.
The text of Sec. 23.1365(b) that includes the words "at least flame resistant" would also be revised by removing the words "at least". The removed words implied that there were burn requirements, other than the ones in this section that must be met.
The three paragraphs that would be added by this proposal would require: (1) The identification of electrical cables, terminals, and connectors; (2) the protection of electrical cables from damage by external sources; and (3) installation criteria for cables that cannot be protected by a circuit protection device.
As identified in the discussion of proposed Sec. 23.1359, there is an increasing use of electrical systems in part 23 airplanes. The resulting increase in the number of electrical wires used in part 23 airplanes makes proper installation difficult. The proposal for electrical cable identification would provide better assurance that the cables will be correctly installed initially and correctly reinstalled when airplane maintenance or modifications are accomplished. The other proposed new requirements would provide installation criteria that will ensure the protection of cables under circumstances that can be expected from the increased use of electrical systems.”