ASTM F2490-20

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Designation: F2490 − 05 (Reapproved 2013) F2490 − 20
Standard Guide for
Aircraft Electrical Load and Power Source Capacity
Analysis
This standard is issued under the fixed designation F2490; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This guide covers how to prepare an electrical load analysis (ELA) to meet Federal Aviation Administration (FAA)
requirements.
1.2 This guide is intended to address aircraft level electrical load analysis. Electric propulsive power load analysis was not
considered in the development of this guide.
1.3 The values givenstated in SI units are to be regarded as the standard.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.5 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 FAA Aeronautics and Space Airworthiness Standards:
14 CFR 23.1309 Normal, Utility, Acrobatic, and Commuter Category Airplanes—Equipment, Systems, and Installations
14 CFR 23.1351 Normal, Utility, Acrobatic, and Commuter Category Airplanes—General
14 CFR 23.1353 Normal, Utility, Acrobatic, and Commuter Category Airplanes—Storage Battery Design and Installation
14 CFR 23.1419 Normal, Utility, Acrobatic, and Commuter Category Airplanes—Ice Protection
14 CFR 23.1529 Normal, Utility, Acrobatic, and Commuter Category Airplanes—Instructions for Continued Airworthiness
14 CFR 91 General Operating and Flight Rules
14 CFR 135.163 Operating Requirements: Commuter and On Demand Operations and Rules Governing Persons on Board Such
Aircraft—Equipment Requirements: Aircraft Carrying Passengers under IFR
3. Terminology
3.1 Definitions of Terms Specific to This Standard:
3.1.1 abnormal electrical power operation (or abnormal operation), n—occurs when a malfunction or failure in the electric
system has taken place and the protective devices of the system are operating to remove the malfunction or failure from the
remainder of the system before the limits of abnormal operation are exceeded.
3.1.1.1 Discussion—
The power source may operate in a degraded mode on a continuous basis when the power characteristics supplied to the using
equipment exceed normal operation limits but remain within the limits for abnormal operation.
3.1.2 alternate source, n—second power source that may be used instead of the normal source, usually on failure of the normal
source.
This guide is under the jurisdiction of ASTM Committee F39 on Aircraft Systems and is the direct responsibility of Subcommittee F39.01 on Design, Alteration, and
Certification of Electrical Systems.
Current edition approved July 1, 2013June 1, 2020. Published September 2013July 2020. Originally approved in 2005. Last previous edition approved in 20052013 as
ε1
F2490 – 05 (2013). . DOI: 10.1520/F2490-05R13.10.1520/F2490-20.
Available from U.S. Government Printing Office Superintendent of Documents, Publishing Office (GPO), 732 N. Capitol St., NW, Mail Stop: SDE, Washington, DC
20401.20401, http://www.gpo.gov.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F2490 − 20
3.1.2.1 Discussion—
The use of alternate sources creates a new load and power configuration and, therefore, a new electrical system that may require
separate source capacity analysis.
3.1.3 cruise, n—condition during which the aircraft is in level flight.
3.1.4 electrical source, n—electrical equipment that produces, converts, or transforms electrical power.
3.1.5 electrical system, n—consists of an electrical power source, the electrical wiring interconnection system, and the electrical
load(s) connected to that system.
3.1.6 emergency electrical power operation (or emergency operation), n—condition that occurs following a loss of all normal
electrical generating power sources or another malfunction that results in operation on standby power (batteries or other emergency
generating source such as an auxiliary power unit (APU) or ram air turbine (RAT)) only, or both).
3.1.7 ground operation and loading, n—time spent in preparing the aircraft before the aircraft engine starts.
3.1.7.1 Discussion—
During this period, the APU, internal batteries, or an external power source supplies electrical power.
3.1.8 landing, n—condition starting with the operation of navigational and indication equipment specific to the landing approach
and following until the completion of the rollout.
3.1.9 nominal rating, n—this rating of a unit power source is its nameplate rating and is usually a continuous duty rating for
specified operating conditions.
3.1.10 normal ambient conditions, n—typical operating conditions such as temperature and pressure as defined by the
manufacturer’s technical documentation.
3.1.11 normal electrical power operation (or normal operation) , operation), n—assumes that all the available electrical power
system is functioning correctly with no failures or within the Master Minimum Equipment List (MMEL) limitations, if a MMEL
has been approved (for example, direct current (DC) generators, transformer rectifier units, inverters, main batteries, APU, and so
forth).
3.1.12 normal source, n—provides electrical power throughout the routine aircraft operation.
3.1.13 takeoff and climb, n—condition starting with the takeoff run and ending with the aircraft leveled off and set for cruising.
3.1.14 taxi, n—condition from the aircraft’saircraft’s first movement under its own power to the start of the takeoff run and from
completion of landing rollout to engine shutdown.
4. Significance and Use
4.1 To show compliance with 14 CFR 23.1351, you must determine the electrical system capacity.
4.2 14 CFR 23.1351(a)(2) states that:
4.2.1 For normal, utility, and acrobatic category airplanes, by an electrical load analysis or by electrical measurements that
account for the electrical loads applied to the electrical system in probable combinations and for probable durations; and
4.2.2 For commuter category airplanes, by an electrical load analysis that accounts for the electrical loads applied to the
electrical system in probable combinations and for probable durations.
4.3 The primary purpose of the electrical load analysis (ELA) is to determine electrical system capacity (including generating
sources, converters, contactors, bus bars, and so forth) needed to supply the worst-case combinations of electrical loads. This is
achieved by evaluating the average demand and maximum demands under all applicable flight conditions. A summary can then
be used to relate the ELA to the system capacity and can establish the adequacy of the power sources under normal, abnormal,
and emergency conditions.
NOTE 1—The ELA should be maintained throughout the life of the aircraft to record changes to the electrical system, which may add or remove
electrical loads to the system.
4.4 The ELA that is produced for aircraft-type certification should be used as the baseline document for any subsequent changes.
When possible, the basic format of the original ELA should be followed to ensure consistency in the methodology and approach.
4.5 The original ELA may be lacking in certain information, for instance, time available on emergency battery. It may be
necessary to update the ELA using the guidance material contained in this guide.
5. Basic Principles
5.1 A load analysis is essentially a summation of the electric loads applied to the electrical system during specified operating
conditions of the aircraft. The ELA requires the listing of each item or circuit of electrically powered equipment and the associated
power requirement. Note that the power requirement for an item may have several values, depending on the utilization for each
phase of aircraft operation.
F2490 − 20
5.2 To arrive at an overall evaluation of electrical power requirement, it is necessary to give adequate consideration to transient
demand requirements, which are of orders of magnitude or duration to impair system voltage or frequency stability, or both, or
to exceed short-time ratings of power sources, that is, intermittent/momentary and cyclic loads. This is essential, since the ultimate
use of an aircraft’saircraft’s ELA is for the proper selection of characteristics and capacity of power-source components and the
resulting assurance of satisfactory performance of equipment under normal, abnormal, and emergency operating power conditions.
5.3 A large majority of general aviation aircraft uses only DC power. If an aircraft also uses AC power, the ELA will have to
include the AC loads as well.
6. Procedure for Preparation of Electrical Load Analysis
6.1 Content—The load and power source capacity analysis report should include the following sections:
6.1.1 Introduction,
6.1.2 Assumptions and Criteria,
6.1.3 Load Analysis—Tabulation of Values,
6.1.4 Emergency and Standby Power Operation, and
6.1.5 Summary and Conclusions.
6.2 Introduction:
6.2.1 The introduction to the ELA report should include information to assist the reader in understanding the function of the
electrical system with respect to the operational phases of the aircraft.
6.2.2 Typically, the introduction to the ELA should contain the following:
6.2.2.1 Brief description of aircraft type, which may also include the expected operating role for the aircraft;
6.2.2.2 Electrical system operation, which describes normal, abnormal, and emergency operations, bus configuration with
circuit breakers, and connected loads for each bus. A copy of the bus wiring diagram or electrical schematic should also be included
in the report;
6.2.2.3 Generator, alternator, and other power source description and related data (including such items as battery discharge
curves, inverter, emergency battery, and so forth). Typical data supplied for power sources would be as shown in Table 1;
6.2.2.4 Operating logic of system (for example, automatic switching, loading shedding, and so forth); and
6.2.2.5 List of installed equipment.
6.3 Assumptions and Criteria—All assumptions and design criteria used for the analysis should be stated in this section of the
ELA. For example, typical assumptions for the analysis may be identified as follows:
6.3.1 Most severe loading conditions and operational environment in which the airplane will be expected to operate are assumed
to be night and in icing conditions;
6.3.2 Momentary/intermittent loads, such as electrically operated valves, that open and close in a few seconds are not included
in the calculations;
6.3.3 Motor load demands are shown for steady-state operation and do not include starting inrush power. The overload ratings
of the power sources should be shown to be adequate to provide motor starting inrush requirements;
6.3.4 Intermittent loads such as communications equipment (radios, for example, VHF/HF communication systems) that may
have different current consumption depending on operating mode (that is, transmit or receive);
6.3.5 Maximum continuous demand of the electrical power system must not exceed 100 % of the load limits of the alternator(s)
or generator(s) that are equipped with current monitoring capability;
6.3.6 Cyclic loads such as heaters, pumps, and so forth (duty cycle); and
6.3.7 Estimation of load current, assuming a voltage drop between bus bar and load.
TABLE 1 Typical Data for Power Sources
Identification 1 2 3
Item DC Generator Inverter Battery
Number of units 2 1 1
Continuous rating 250 A 300 VA (total) 35 Ah
(Nameplate) . . . . . . . . .
5-s rating 400 A . . . . . .
5 s rating 400 A . . . . . .
2-min rating 300 A . . . . . .
2 min rating 300 A . . . . . .
Voltage 30 V 115 VAC 24 VDC
Frequency . . . 400 Hz . . .
Power factor . . . 0.8 . . .
Manufacturer ABC XYZ ABC
Model number 123 456 789
Voltage regulation ±0.6 V ±2 % . . .
Frequency regulation . . . 400 Hz ± 1 % . . .
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6.4 Load Analysis—Tabulation of Values—A typical load and power source analysis would identify the following details in
tabular form:
6.4.1 Connected Load Table—See Appendix X1.
6.4.1.1 Aircraft Bus—Identify the appropriate electrical bus being evaluated. In a multiple bus configuration, there will be a set
of tables for each bus (that is, DC Bus 1, DC Bus 2, AC Bus 1, Battery Bus, and so forth).
6.4.1.2 Condition of Power Sources—Normal, abnormal (abnormal conditions to be specified, for example, one generator
inoperative, two generators inoperative, and so forth), and emergency.
6.4.1.3 Aircraft Operating Phases—The following aircraft operating phases should be considered for the ELA. Assume “night”
conditions as the worst-case scenario.
NOTE 2—Icing conditions should be considered for worst-case scenarios if the aircraft is approved for flight into known icing in accordance with 14
CFR 23.1419. However, in some cases, the icing system is deactivated or not installed, so icing may not always be the worst-case.
6.4.1.4 Permissible Nonserviceable Conditions—The analysis should also identify permissible nonserviceable conditions likely
to be authorized in the MMEL, if approved, during the certification of the airplane and should include calculations appropriate to
these cases. All MMEL items must be accounted for in the load analysis to ensure that the electrical system capacity is not exceeded
when all items are functional.
6.4.1.5 Circuit Breaker—Identify each circuit breaker by circuit name or identification number.
6.4.1.6 Load at Circuit Breaker—The ampere loading for each circuit.
6.4.1.7 Operating Time:
(1) The operating time is usually expressed as a period of time (seconds/minutes) or may be continuous, as appropriate.
Equipment operating time is often related to the average operating time of the aircraft. If the “on” time of the equipment is the
same or close to the average operating time of the aircraft, then it could be considered that the equipment is operating continuously
for all flight phases.
(2) In such cases in which suitable provisions have been made to ensure that certain loads cannot operate simultaneously, or
there is reason for assuming certain combinations of load will not occur, appropriate allowances may be made. Adequate
explanation should be given in the summary.
(3) In some instances, it may be useful to tabulate the data using a specified range for equipment operating times, such as
follows:
5-s Analysis All loads that last longer than 0.3 s
should be entered in this column.
5 s Analysis All loads that last longer than 0.3 s
should be entered in this column.
5-min Analysis All loads that last longer than 5 s
should be entered in this column.
5 min Analysis All loads that last longer than 5 s
should be entered in this column.
Continuous Analysis All loads that last longer than 5
min should be entered in this column.
(4) Alternatively, the equipment operating times could be expressed as actual operating time of equipment in seconds or
minutes or as continuous operation. In the example given in Appendix X1, the approach taken is to show either continuous
operation or to identify a specific operating time in seconds/minutes.
6.4.1.8 Condition of Aircraft Operation—Phase of preflight and flight (such as ground operation and loading, taxi, takeoff,
cruise, and land). For aircraft, the conditions in Table 2 could be considered.
TABLE 2 Condition of Aircraft Operation
Ground operations and loading 15 min typically
Engine start 5 min typically
Taxi 10 min typically
Takeoff and climb 20 min typically
Cruise as appropriate for aircraft type
Landing 20 min typically
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6.4.2 Calculations:
6.4.2.1 The following equations can be used to estimate total current, total current rate, and average demand for each of the
aircraft operating phases (ground operation and loading, engine start, taxi, takeoff and climb, cruise, and landing):
Total Current A 5 Sum of All Current Loads (1)
~ !
Operating at a Given Time
Total Current Rate ~A2 min!5 (2)
Number of Units Operating Simultaneously 3Current per Unit ~A!3
Operating Time min
~ !
Average Demand or Average Load A 5 Total Current A2 min ÷Duration of Ground or Flight Phase min (3)
~ ! ~ ! ~ !
6.4.2.2 It can be considered that at the start of each operating period (for example, taxi, takeoff, and so forth), all equipment
that operates during that phase is switched “on,” with intermittent loads gradually being switched “off.”
6.4.3 Additional Considerations for Non-Ohmic or Constant Power Devices (for example, Inverters)—In some cases, the
currents drawn at battery voltage (for example, 20 VDC to 24 VDC) are higher than at the generated voltage (for example, 28
VDC) and will influence the emergency flight conditions on battery. However, for resistive loads, the current drawn will be reduced
because of the lower battery voltage.
6.4.4 System Regulation:
6.4.4.1 The system voltage and frequency should be regulated to ensure reliable and continued safe operation of all essential
equipment while operating under the normal and emergency conditions, taking into account the voltage drops that occur in the
cables and connections to the equipment.
6.4.4.2 The defined voltages are those supplied at the equipment terminals and allow for variation in the output of the supply
equipment (for example, generators, alternators, and batteries), as well as voltage drops caused by cable and connection resistance.
NOTE 3—Voltage drop between bus bar and equipment should be considered in conjunction with bus bar voltages under normal, abnormal, and
emergency operating conditions in the estimation of the terminal voltage at the equipment (that is, reduced bus bar voltage in conjunction with cable volt
drop could lead to malfunction or shutdown of equipment).
6.4.5 Load Shedding—Following the loss of a generator/alternator, it is assumed a 5-min 5 min period will elapse before any
manual load shedding by the flight crew, provided tha
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