SUBPART J – AUXILIARY POWER UNIT INSTALLATIONS
Large Aeroplanes (CS-25) · CS-25
CS 25J901 Installation
(See AMC 25J901) (a) For the purpose of this subpart, the APU installation includes:
AMC 25J901(c)(2) Assembly of Components
The objectives of CS 25.671(b) should be satisfied with respect to APU systems, where the safety of the aeroplane could otherwise be jeopardised.
AMC 25J901(c)(4) Electrical Bonding
Where the APU is not in direct electrical contact with its mounting the engine should be electrically connected to the main earth system by at least two…
CS 25J903 Auxiliary power unit
Each APU must meet the appropriate requirements of CS-APU for its intended function:
CS 25J939 APU operating characteristics
APU operating characteristics must be investigated in all aeroplane operating conditions from APU start until shutdown to determine that no adverse…
CS 25J943 Negative acceleration
No hazardous malfunction of an APU or any component or system associated with the APU may occur when the aeroplane is operated at the negative…
AMC 25J943 APU Operating Characteristics
1 Compliance with CS 25J943 should be shown by design analysis and flight tests.
CS 25J951 General
Each fuel system must be constructed and arranged to ensure a flow of fuel at a rate and pressure established for proper APU functioning under each…
CS 25J952 Fuel system analysis and test
Proper fuel system functioning under all probable operating conditions must be shown by analysis and those tests found necessary by the Agency.
CS 25J953 Fuel system independence
Each fuel system must allow the supply of fuel to the APU: Through a system independent of each part of the system supplying fuel to the main engines;
CS 25J955 Fuel Flow
(See AMC 25J955) (a) Each fuel system must provide at least 100 percent of the fuel flow required by the APU under each intended operating condition and…
AMC 25J955(a)(2)(iii) Fuel Flow
The word "blocked" should be interpreted to mean "with the moving parts fixed in the position for maximum pressure drop".
CS 25J961 Fuel system hot weather operation
For essential APUs: The fuel supply of an APU must perform satisfactorily in hot weather operation.
CS 25J977 Fuel Tank Outlet
For essential APUs: There must be a fuel strainer for the fuel tank outlet or for the booster pump.
CS 25J991 Fuel Pumps
(See AMC 25J991) For essential APUs: Main pumps. Each fuel pump required for proper essential APU operation, or required to meet the fuel system…
AMC 25J991 Fuel Pumps
If the fuel supply to the APU is taken from the fuel supply to the main engine, no separate pumps need be provided for the APU.
CS 25J993 Fuel system lines and fittings
Each fuel line must be installed and supported to prevent excessive vibration and to withstand loads due to fuel pressure and accelerated flight…
CS 25J994 Fuel system components
Fuel system components in an APU compartment or in the fuselage must be protected from damage which could result in spillage of enough fuel to constitute…
CS 25J995 Fuel valves
In addition to the requirements of CS 25J1189 for shut-off means, each fuel valve must be supported so that no loads resulting from their operation or…
CS 25J997 Fuel strainer or filter
For essential APUs: There must be a fuel strainer or filter between the fuel tank outlet and the inlet of either the fuel metering device or an APU…
CS25J1011 Oil System General
Each APU must have an independent oil system that can supply it with an appropriate quantity of oil at a temperature not above that safe for continuous…
CS 25A1017 Oil lines and fittings
Each oil line must meet the requirements of CS 25J993 and each oil line and fitting in any designated fire zone must meet the requirements of CS 25J1183.
CS 25J1019 Oil Filter
Where there is a filter in the APU lubrication system through which all the oil flows, it must be constructed and installed so that oil may flow at an…
CS 25J1021 Oil system drains
A drain (or drains) must be provided to allow safe drainage of the oil system.
CS 25J1023 Oil radiators
Each oil radiator must be able to withstand, without failure, any vibration, inertia, and oil pressure load to which it would be subjected in operation.
CS 25J1025 Oil valves
Each oil shut-off must meet the requirements of CS 25J1189. Each oil valve must have positive stops or suitable index provisions in the "on'' and "off''…
CS 25J1041 General
(See AMC 25J1041.) The APU cooling provisions must be able to maintain the temperatures of APU components and fluids within the temperature limits…
AMC 25J1041 General
The need for additional tests, if any, in hot climatic conditions should take account of any tests made by the APU constructor to establish APU…
CS 25J1043 Cooling tests
General. Compliance with CS 25J1041 must be shown by tests, under critical conditions.
CS 25J1045 Cooling test procedures
Compliance with CS 25J1041 must be shown for the critical conditions that correspond to the applicable performance requirements.
CS 25J1091 Air intake
The air intake system for the APU: Must supply the air required by the APU under each operating condition for which certification is requested, (b) May…
CS 25J1093 Air intake system icing protection
(See AMC 25J1093) (a) Each non-essential APU air intake system, including any screen if used, which does not comply with CS 25J1093(b) will be restricted…
AMC 25J1093(b) Essential APU air intake system de-icing and anti-icing provisions
General In establishing compliance with the requirements of CS 25J1093(b), reference should be made to AMC 25.1093(b).
CS 25J1103 Air intake system ducts
Each air intake system duct must be: Drained to prevent accumulation of hazardous quantities of flammable fluid and moisture in the ground attitude.
CS 25J1106 Bleed air duct systems
For APU bleed air duct systems, no hazard may result if a duct failure occurs at any point between the air duct source and the aeroplane unit served by…
CS 25J1121 General
Each exhaust system must ensure safe disposal of exhaust gases without fire hazard or carbon monoxide contamination in any personnel compartment.
CS 25J1123 Exhaust piping
Exhaust piping must be heat and corrosion resistant, and must have provisions to prevent failure due to expansion by operating temperatures.
CS 25J1141 APU controls
Means must be provided on the flight deck for starting, stopping, and emergency shutdown of each installed APU.
CS 25J1163 APU accessories
APU mounted accessories must be approved for installation on the APU concerned and use the provisions of the APU for mounting.
CS 25J1165 APU ignition systems
Each APU ignition system must be independent of any electrical circuit except those used for assisting, controlling, or analysing the operation of that…
CS 25J1181 Designated fire zones
Any APU compartment is a designated fire zone. Each designated fire zone must meet the requirements of CS 25J1185 through CS 25J1203.
CS 25J1183 Lines, fittings and components
Except as provided in sub-paragraph (b) of this paragraph, each line, fitting, and other component carrying flammable fluid in any area subject to APU…
CS 25J1185 Flammable fluids
No tank or reservoir that is a part of a system containing flammable fluids or gases may be in a designated fire zone unless the fluid contained, the…
CS 25J1187 Drainage and ventilation of fire zones
There must be complete drainage of each part of each designated fire zone to minimise the hazards resulting from failure or malfunctioning of any…
CS 25J1189 Shut-off means
(See AMC 25.1189) (a) Each APU compartment specified in CS 25J1181(a) must have a means to shut-off or otherwise prevent hazardous quantities of…
CS 25J1191 Firewalls
Each APU must be isolated from the rest of the aeroplane by firewalls, shrouds, or equivalent means.
CS 25J1193 APU compartment
Each compartment must be constructed and supported so that it can resist any vibration, inertia, and air load to which it may be subjected in operation.
CS 25J1195 Fire extinguisher systems
(See AMC 25J1195) (a) There must be a fire extinguisher system serving the APU compartment.
AMC 25J1195(b) Fire extinguisher systems
Acceptable methods to establish the adequacy of the fire extinguisher system are laid down in FAA Advisory Circular 20 – 100, with reference to Halon…
CS 25J1197 Fire extinguishing agents
Fire extinguishing agents must: Be capable of extinguishing flames emanating from any burning of fluids or other combustible materials in the area…
CS 25J1199 Extinguishing agent containers
Each extinguishing agent container must have a pressure relief to prevent bursting of the container by excessive internal pressures.
CS 25J1201 Fire extinguishing system materials
No material in any fire extinguishing system may react chemically with any extinguishing agent so as to create a hazard.
CS 25J1203 Fire-detector system
There must be approved, quick acting fire or overheat detectors in each APU compartment in numbers and locations ensuring prompt detection of fire.
CS 25J1207 Compliance
Unless otherwise specified, compliance with the requirements of CS 25J1181 through CS 25J1203 must be shown by a full scale test or by one or more of the…
CS 25J1305 APU instruments
The following instruments are required for all installation:
CS 25J1337 APU instruments
Reserved (b) Reserved (c) Reserved (d) There must be a stick gauge or equivalent means to indicate the quantity of oil in each tank.
CS 25J1501 General
Reserved (b) The operating limitations and other information necessary for safe operation must be made available to the crew members as prescribed in CS…
CS 25J1521 APU limitations
The APU limitations must be established so that they do not exceed the corresponding approved limits for the APU and its systems.
CS 25J1527 Ambient air temperature and operating altitude
The extremes of the ambient air temperature and operating altitude for which operation is allowed, as limited by flight, structural, APU installation…
CS 25J1549 APU instruments
For each APU instrument either a placard or colour markings or an acceptable combination must be provided to convey information on the maximum and (where…
CS 25J1551 Oil quantity indicator
Each oil quantity indicator must be marked with enough increments to indicate readily and accurately the quantity of oil.
CS 25J1557 Miscellaneous markings and placards
Reserved (b) APU fluid filler openings. The following applies:
CS 25J1583 Operating limitations
APU limitations established under CS 25J1521 and information to explain the instrument markings provided under CS 25J1549 and CS 25J1551 must be furnished.
Consolidated from the EASA Easy Access Rules (revision 26 Jan 2023, extracted 17 Aug 2026) for convenience. Not the official publication — verify against the Official Journal of the European Union and the EASA publications before operational use.
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