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Civil Aviation Authority advisory circular contains guidance and information about standards, practices, and procedures that the Director has found to be an acceptable means of compliance with the associated rules and legislation.
However, the information in the advisory circular does not replace the requirement for participants to comply with their obligations under the Civil Aviation Rules, the Civil Aviation Act 1990 and other legislation.
An advisory circular reflects the Director’s view on the rules and legislation. It expresses CAA policy on the relevant matter. It is not intended to be definitive. Consideration will be given to other methods of compliance that may be presented to the Director. When new standards, practices, or procedures are found to be acceptable they will be added to the appropriate advisory circular. Should there be any inconsistency between this information and the rules or legislation, the rules and legislation take precedence.
An advisory circular may also include guidance material generally, including guidance on best practice as well as guidance to facilitate compliance with the rule requirements. However guidance material should not be regarded as an acceptable means of compliance.
An advisory circular may also include technical information that is relevant to the rule standards or requirements.
This advisory circular provides guidance material for the preparation of an electrical load analysis (ELA).
This advisory circular relates specifically to Civil Aviation Rules Part 91General Operating and Flight Rules and Part 21 Certification of Products and Parts.
Revision 1 provides clarity around the expected content of an ELA, its place within the aircraft records, and its intended purpose.
This advisory circular cancels AC21-11 & AC91-23 Revision 0 dated 2 August 2016.
Summary of Changes
02 August 2016
This was the initial issue of this advisory circular.
10 December 2019
This revision provides clarity around the expected content of an electrical load analysis, its place within the aircraft records, and its intended purpose.
1.1 An electrical load analysis (ELA) is a fundamental document part of the aircraft records verifying the airworthiness of an aircraft by providing evidence that the aircraft electrical generation, storage and distribution systems have sufficient capacity to power all installed electrically-powered equipment in both normal and emergency flight conditions. As such, an ELA has two main purposes—
1.2 From a regulatory perspective, there are two areas where the required performance may be stipulated—
1.3 An ELA provides a means of showing compliance against these requirements.
1.4 This advisory circular has been generated with reference to MIL-E-7016F and ASTM F2490-05.
1.5 The initial ELA generated for type certification provides the baseline for subsequent changes and this should be kept updated with all future configuration changes. Where role equipment is to be used, the impact of this equipment should be immediately available so the operator can determine safety, suitability and any operating limitations prior to departure.
2.1 This advisory circular is applicable to all aircraft and is specific to the actual configuration of the aircraft. If at any stage avionics or electrically controlled/actuated equipment is added or removed, or the generating/storage devices altered, the ELA should be revised accordingly.
2.2 In the absence of an ELA, practical testing is a suitable method of verifying electrical loads.
3.1 The definitions specifically pertaining to an ELA are included in Appendix 2 to this advisory circular.
4.1 An ELA does not compensate or cover design or installation. Any changes to the aircraft for which the ELA was created or updated still need to be done in accordance with the required design standards and specifications pertaining to the aircraft type and operation. (ie any changes to the aircraft require acceptable technical data per rule 21.503.
4.2 The ELA is a living document and should stay with the aircraft and be updated to constantly reflect the aircraft configuration. Where role equipment is permissible for the aircraft, the effect of installing that role equipment should be readily summarised and be able to be applied to the overall aircraft ELA.
4.3 This advisory circular has been generated at a high level to provide an overview and means of compliance. In some instances it may be necessary to provide greater detail and determine more specific details of the aircraft’s electrical system, such as the discharge and recharge characteristics of the battery. In these instances or where a greater level of understanding is desired by the designer/person generating the ELA, reference to MIL-E-7016F and ASTM F2490-05 should be made.
5.1 An ELA is essentially a summation of the electrical loads applied to the electrical system during specified operating conditions and then the demand analysed with respect to the supply. The ELA requires the listing of each item of equipment or system, the power requirement for that system/equipment and identification of when the item is used during the phases of flight.
5.2 To determine the overall evaluation of power requirements, it is necessary to consider the transient demands of equipment and determine whether these require inclusion. The in-rush currents on motors and momentary/intermittent operation of relays are not included unless considered significant by the person compiling the ELA.
5.3 Load shedding may be applied to reduce the power requirement on the battery in an emergency. Unless the aircraft system provides automatic load shedding it is assumed a five‑minute period is required before any manual load shedding is completed by the flight crew after a generating capacity failure. If there is no clear unambiguous attention getting warning of the generating failure the time required can be as much as 10 minutes after failure. If there are reasonable assumptions that this time is less than 5 minutes, other than automatic load shedding, these reasons and assumptions must be clearly documented in the ELA.
5.4 Any load shedding applied in the ELA should be referenced in the document. Consideration should be given to the minimum required equipment in Part 91 General Operating and Flight Rules (and certificated operator rules, where applicable) for the kind of operation the ELA is being prepared for. Load shedding in the ELA can only be applied to systems that have automatic load shedding or can be shed by the flight crew. Systems that do not have a switch or a circuit breaker that can be manually tripped (pulled) within normal reach should remain operational throughout the entire flight. Any load shedding applied must not invalidate any procedures or requirements of the aircraft flight manual or associated supplements.
Note: Where Part 91 requires more than one identical system as part of the minimum required equipment for the operation being carried out, only one of those systems is required to be kept operational after an electrical generation failure.
5.5 If load shedding is applied to systems not covered in, or additional to the flight manual, or where the aircraft flight manual refers to descriptively vague procedures like “reduce electrical loads as required” it should be made clear in the ELA and included on ELA summary CAA form 24021-20 which systems were load shed.
Note: Where the aircraft flight manual emergency procedures load shed a system that is required under the minimum equipment requirements under Part 91, the flight manual procedures have precedent but reference to the manual by part number and revision should be included in the ELA.
6.1 ELA should contain the following information as a minimum.
6.1.2 Assumptions and criteria
6.1.3 Table of values (an example is included in Appendix 1 to this advisory circular)
Note: Transient/ intermittent loads (such as valves and relays) are not included. In-rush currents on motors are not included. The overload design ratings of the sources should however be adequate to cope with these.
6.1.4 Emergency and standby operations
184.108.40.206 Where standby power is provided by a non-time-limited source - such as RAT, APU or pneumatic/hydraulic motor, the emergency loads should be listed and evaluated to ensure that demand does not exceed capacity.
220.127.116.11 Where standby power is provided by an equipment specific backup (internal) battery this battery power may be used to load shed the equipment of the main battery during emergency procedures, where the power source can be manually selected by the operator and this source is certified to provide power for a time greater than that the main battery is required to last. The device is still to be included in preload shedding consumption as it uses the battery prior to switching over.
18.104.22.168 Where the backup battery switching is automatic by a logic interface without input of the operator, this battery power may only be used to load shed the equipment of the main battery calculation where the switching happens at a voltage greater than fully charged battery voltage and lower than the nominal generating-device voltage and the battery is certified to provide power for a time greater than 30 minutes.
Note: For example; Model 182S/182T/T182T Maintenance Manual (rev 20) 24-00-00 states “If there is an alternator failure, the standby battery controller will not let the standby battery discharge to the G1000 essential bus until the depletion or failure of the main battery.” As such this does not relieve the G1000 system as a battery load for the purpose of the ELA.
6.1.5 Calculations and results
22.214.171.124 Once the results have been tabulated, the following calculations can be completed.
6.1.6 Summary and conclusion
126.96.36.199 The summary should provide evidence that for each operating condition the available power can meet the loading requirements, with adequate margin for both peak loads and maximum continuous loads under normal and abnormal conditions. For AC systems these summaries should include power factor and phase loadings.
188.8.131.52 The conclusion should include statements confirming that the power sources can satisfactorily supply electrical power to necessary equipment during abnormal /emergency operations under the highest-demand conditions.
7.1 In the absence of sufficient data to enable a comprehensive analysis, when validation of data is required or when it is considered more desirable by the operator to decide, testing is a suitable alternative to analysis. The principle of the testing is the same as the analysis. To prove that the aircraft’s generation system can supply the equipment needs and to ensure that sufficient emergency capacity exists in the event of generator failure.
7.2 To conduct the test, the participant should develop a similar table as per the template in Appendix 1 to this advisory circular, in which the participant identifies the equipment that will be operating during each phase of flight. By means of measuring the current at a suitable location with a calibrated device, determine the current draw of individual systems where possible, and combine system loads where individual current cannot be practicably determined4. The rest of the process can then be followed as per an ELA by analysis. Identify in the ELA whether current values are obtained by measurement or documented data, especially when a mixture of both exist in the ELA.
7.3 To measure the current draw, it is important to ensure that the current being measured is purely related to the equipment current draw. As such, the battery needs to either be isolated (if testing with generator) or current draw assessed purely off battery. It is preferable to use external current measuring as most aircraft ammeters do not require calibration and cannot be guaranteed as accurate.
7.4 Aircraft with air/ground sensing, and/or retractable gears need to be on jacks to accurately determine in-flight system loads. When measuring communication loads ensure the radio is tuned to an active station while measuring current draw while receiving and measure transmitting current while modulating the carrier wave. Transponder and/or DME equipment need to be measured while receiving/replying to interrogations (by use of ground stations or suitable test equipment). Similar considerations should be made for all equipment on board, “does my ground measurement equal in-flight condition?”
7.5 If it is considered that in-flight forces will affect the current draw on the system (eg. control surfaces), a factor may be applied to the measured result and this needs to be identified and explained in the assumptions that accompany the test results.
8.1 A load analysis may show the aircraft is not meeting the requirements for the kind of aircraft and operation the ELA was prepared for. This may happen after installation of additional or new equipment or if the operation requirements change. It may also show on older aircraft that have been certified before design criteria were amended or better defined.
8.2 These non-compliances usually come to light when the aircraft is being modified and being altered from the original design, which may mean the electrical system may need to be modified to meet current design standards.
8.3 There are multiple ways of upgrading the electrical system to meet ELA requirements as not only avionics technology evolved but also electrical technology. Some old systems draw a lot of current especially when compared to modern equivalents. The following are some changes that are worth considering.
Note: This advisory circular does not constitute approved technical data to make these changes.
9.1 In addition to the electrical load analysis itself being retained with the aircraft records5, a summary of the ELA should be completed on CAA form 24021-20. This form informs the crew of the conclusions from the ELA. It also informs the crew of what systems were load shed to reach the conclusion. This can aid in situations where the flight manual procedures are descriptively vague and uses terms like “as required”. This form should be kept with the aircraft in a place accessible to the flight crew such as the folder containing the flight manual.