CS 25.1351 General

1 The disconnect means required by CS 25.1351(b)(5) should be accessible to the appropriate flight-crew members in their normal seated positions. 2 The power source controls should be considered as cockpit controls and therefore also comply with CS 25.777. 3 It may not be necessary to provide disconnection controls for all power sources, for example RAT generators or engine control dedicated generators. Where it is necessary to isolate the alternate power source when normal generator power is restored, such isolation should be possible.

Each source of electrical supply (e.g. generators and batteries) should be provided with means to give the flight crew immediate warning of the failure of its output. These warning means are additional to the system indication requirements of CS 25.1351(b)(6). For multiphase systems the warning should also indicate the loss of any phase.

1 Provision should be made to ensure adequate electrical supplies to those services, which are necessary to complete the flight and make a safe landing in the event of a failure of all normal generated electrical power. All components and wiring of the alternate supplies should be physically and electrically segregated from the normal system and be such that no single failure, including the effects of fire, the cutting of a cable bundle, the loss of a junction box or control panel, will affect both normal and alternate supplies. 2 When ensuring the adequacy of electrical supplies relative to alternate power source duration and integrity, special consideration should be given to aeroplanes such as those with fly-by-wire, for which the total loss of electrical supplies could result in an immediate loss of control. 3 In considering the services which should remain available following the loss of the normal generated electrical power systems, consideration should be given to the role and flight conditions of the aeroplane and the possible duration of flight time to reach an airfield and make a safe landing. 4 The services required by CS 25.1351(d)(1) may differ between aeroplane types and roles and should be agreed with the Agency. These should normally include – a. Attitude information; b. Radio communication and intercommunication; c. Navigation; d. Cockpit and instrument lighting; e. Heading, airspeed and altitude, including appropriate pitot head heating; f. Adequate flight controls; g. Adequate engine control; and Restart capability with critical type fuel (from the standpoint of flame-out and restart capability) and with the aeroplane initially at the maximum certificated altitude; h. Adequate engine instrumentation; i. Such warning, cautions and indications as are required for continued safe flight and landing; j. Any other services required for continued safe flight and landing. 5 Consideration should also be given to the equipment and the duration of services required to make a controlled descent and forced landing in the event of failure and inability to restart all engines. 6 Alternate Power Source Duration and Integrity

6.1 Time Limited. Where an alternate power source provided to comply with CS 25.1351(d) is time limited (e.g. battery), the required duration will depend on the type and role of the aeroplane. Unless it can be shown that a lesser time is adequate, such a power source should have an endurance of at least 60 minutes, at least 30 minutes of which is available under IMC. An endurance of less than 30 minutes under IMC would not normally be acceptable. The endurances, with any associated procedures, should be specified in the Flight Manual. The endurance time should be determined by calculation or test, due to allowance being made for – a. Delays in flight crew recognition of failures and completion of the appropriate drill where flight crew action is necessary. This should be assumed to be 5 minutes provided that the failure warning system has clear and unambiguous attention-getting characteristics and where such a delay is acceptable and compatible with the crew’s primary attention being given to other vital actions. b. The minimum voltage acceptable for the required loads, the battery state of charge, the minimum capacity permitted during service life and the battery efficiency at the discharge rates and temperatures likely to be experienced. Unless otherwise agreed, for the purpose of this calculation, a battery capacity at normal ambient conditions of 80% of the nameplate rated capacity, at the one-hour rate, and a 90% state of charge, may be assumed (i.e. 72% of nominal demonstrated rated capacity at +20°C). The allowance for battery endurance presumes that adequate requirements for periodic battery maintenance have been agreed. c. For those aeroplanes where the battery is also used for engine or APU starting on the ground, it should be shown that following engine starts, the charge rate of the battery is such that the battery is maintained in a state of charge that will ensure adequate alternate power source duration should a failure of generated power occur shortly after take-off. NOTE: This may normally be achieved by ensuring that, following battery-powered starting, the battery charge current has fallen to a declared level prior to take-off. d. For those aeroplanes where the battery is used for in-flight starting of the engines or APU, it may be necessary to include limitations on the number of attempted starts, or to provide a separate dedicated battery for such purposes.

6.2 Non-Time Limited. Where an alternate electrical supply is provided by a non time limited source, e.g. APU, ram air turbine, pneumatic or hydraulic motor, due account should be taken of any limitation imposed by aeroplane speed, attitude, altitude etc., which may affect the capabilities of that power source. In considering the power source, account should be taken of the following: a. Auxiliary Power Unit (APU). An APU capable of continuous operation throughout an adequate flight envelope may be considered an acceptable means of supplying electrical power to the required services provided that its air start capability is adequate and may be guaranteed. Where, however the APU is dependent for its starting current on a battery source, which is supplying critical loads, such starting loads may prejudice the time duration of the flight if APU start is not achieved. It may be necessary therefore to include limitations on the number of attempted starts or to provide a separate battery for APU starting, if this method of supplying electrical power is adopted. Consideration should also be given to the equipment, services and duration required prior to the APU generator coming online. Common failures, which could affect the operation of all engines and the APU, should be taken into consideration, e.g. fuel supply. b. Ram Air Turbine (RAT). A ram air turbine may be utilised to provide an alternate electrical power source, but due consideration must be given to ensuring that the means of bringing the unit into use are not dependent on a source which may have been lost as a result of the original failure. This will normally necessitate independent, duplicate means of deployment. Particular attention should be given to ensuring that the RAT and its means of deployment satisfy the overall reliability requirements. The continuity of electrical power to those services which must remain operative without crew action prior to the RAT being brought into operation, may necessitate the use of a battery, unless the operation of the emergency power source is automatic and is supplying power within a timespan so as not to jeopardise the continued safety of the aeroplane in the event of failure of normal generated electrical power. c. Pneumatic or Hydraulic Motor Drive Power Source. A pneumatic or hydraulic motor driven electrical power source may be utilised subject to the same constraints on activation as the ram air turbine (see 6.2(b)). Care should be taken in ensuring that the operation of the pneumatic or hydraulic system is not prejudiced by faults leading to, or resulting from, the original failure, including the loss of, or inability to restart all engines. d. Regaining of Main Generators. In the event of a major loss of electrical power, provision may be made for regaining the output of one or more generators using separate control and switching arrangements on the generator side of the normal generator line contactor. Such a system would not normally be acceptable on aeroplanes with less than three engine-driven generators, as the probability of the loss of all engine-driven generators is unlikely to meet the requirements of CS 25.1351(d). To comply with CS 25.1351(d)(2) the system should be designed such that the loss of both the main and alternate means of control and distribution is Extremely Improbable. Consideration should be given to the services and duration required prior to the activation of the system and to enable a descent and forced landing to be made, in the event of the inability to restart all engines. e. Usage of a battery system to ensure continuity of electrical power. This subparagraph applies if a battery system is used to ensure the continuity of electrical power when the non-time-limited alternate electrical power source(s) is(are) not providing electrical power. When establishing the minimum battery endurance requirements, the following conditions should be considered: It should be shown that following the loss of normal electrical power, and during the time periods when the non-time limited alternate electrical power source(s) does(do) not provide electrical power (per design), the battery system provides an adequate electrical power supply to those services which are necessary to make a controlled descent and landing, stop and complete a safe evacuation of the aeroplane (CS 25.1351(d) and 25.1362). The applicant should take into account the transient time period between the loss of normal electrical power and the non-time limited alternate electrical power source being operational, as well as other time period(s) when the non-time limited alternate electrical power source is not available. For example, the time period between when the RAT electrical generator goes off-line and when the aeroplane is stopped on ground and a safe evacuation of the aeroplane is performed. The most critical configuration, from a battery system point of view, should be considered. The loss of normal electrical power is usually associated with one of the following conditions: either the all-engine out case or the loss of power coming from the primary power centre. In the second case, the proximity of a battery to the power centre should be taken into account. Any battery located near this power centre should be considered as part of the normal electrical power generating system (ref. CS 25.1351(d)(1). The time periods corresponding to the intended usage of the battery system in the emergency scenario will need to be substantiated, with a due margin taken for any uncertainty. Any permanent load on the battery system (i.e. a hot bus) will also have to be accounted for. For determining the capacity of the battery system, Section 6.1(b) of this AMC, on time-limited power sources, applies. The capability of the backup battery system to provide adequate power for the required minimum duration should be demonstrated by actual testing or demonstrated equivalent means. Instructions for Continued Airworthiness for the battery system should be provided. These instructions should ensure that adequate battery power is available between maintenance cycles. There should be a means for the flight crew or maintenance personnel to determine the actual battery system charge state prior to take-off.

[Amdt 25/26]