CS 25J951 General

ED Decision 2010/005/R

(a) 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 likely operating condition, including any manoeuvre for which certification is requested and during which the APU is permitted to be in operation.

(b) For essential APUs:

Each fuel system must be arranged so that any air which is introduced into the system will not result in flameout.

(c) For essential APUs:

Each fuel system for an essential APU must be capable of sustained operation throughout its flow and pressure range with fuel initially saturated with water at 26.7 °C and having 0.20 cm3 of free water per liter added and cooled to the most critical condition for icing likely to be encountered in operation.

[Amdt 25/1]

[Amdt 25/9]

CS 25J952 Fuel system analysis and test

ED Decision 2005/006/R

(a) Proper fuel system functioning under all probable operating conditions must be shown by analysis and those tests found necessary by the Agency. Tests, if required, must be made using the aeroplane fuel system or a test article that reproduces the operating characteristics of the portion of the fuel system to be tested.

(b) The likely failure of any heat exchanger using fuel as one of its fluids may not result in a hazardous condition.

[Amdt 25/1]

CS 25J953 Fuel system independence

ED Decision 2005/006/R

Each fuel system must allow the supply of fuel to the APU:

(a) Through a system independent of each part of the system supplying fuel to the main engines; or

(b) From the fuel supply to the main engine if provision is made for a shut-off means to isolate the APU fuel line.

[Amdt 25/1]

CS 25J955 Fuel Flow

ED Decision 2016/010/R

(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 manoeuvre. Compliance must be shown as follows:

 (1) Fuel must be delivered at a pressure within the limits specified for the APU.

 (2) For essential APUs:

(i) The quantity of fuel in the tank may not exceed the amount established as the unusable fuel supply for that tank under the requirements of CS 25.959 plus that necessary to show compliance with this paragraph.

(ii) Each main pump must be used that is necessary for each operating condition and attitude for which compliance with this paragraph is shown, and the appropriate emergency pump must be substituted for each main pump so used.

(iii) If there is a fuel flowmeter, it must be blocked and the fuel must flow through the meter or its bypass. (See AMC 25J955(a)(2)(iii).)

(b) For essential APUs:

If an APU can be supplied with fuel from more than one tank, the fuel system must, in addition to having appropriate manual switching capability, be designed to prevent interruption of fuel flow to that APU, without attention by the flight crew, when any tank supplying fuel to that APU is depleted of usable fuel during normal operation, and any other tank, that normally supplies fuel to that APU, contains usable fuel.

[Amdt 25/1]

[Amdt 25/18]

AMC 25J955(a)(2)(iii) Fuel Flow

ED Decision 2005/006/R

The word "blocked" should be interpreted to mean "with the moving parts fixed in the position for maximum pressure drop".

[Amdt 25/1]

CS 25J961 Fuel system hot weather operation

ED Decision 2005/006/R

For essential APUs:

(a) The fuel supply of an APU must perform satisfactorily in hot weather operation. It must be shown that the fuel system from the tank outlet to the APU is pressurised under all intended operations so as to prevent vapour formation. Alternatively, it must be shown that there is no evidence of vapour lock or other malfunctioning during a climb from the altitude of the airport selected by the applicant to the maximum altitude established as an operating limitation under CS 25J1527, with the APU operating at the most critical conditions for vapour formation but not exceeding the maximum essential load conditions. If the fuel supply is dependant on the same fuel pumps or fuel supply as the main engines, the main engines must be operated at maximum continuous power. The fuel temperature must be at least 43°C at the start of the climb.

(b) The test prescribed in sub-paragraph (a) of this paragraph may be performed in flight or on the ground under closely simulated flight conditions. If a flight test is performed in weather cold enough to interfere with the proper conduct of the test, the fuel tank surfaces, fuel lines, and other fuel system parts subject to cold air must be insulated to simulate, insofar as practicable, flight in hot weather.

[Amdt 25/1]

CS 25J977 Fuel Tank Outlet

ED Decision 2005/006/R

For essential APUs:

(a) There must be a fuel strainer for the fuel tank outlet or for the booster pump. This strainer must prevent the passage of any object that could restrict fuel flow or damage any fuel system component.

(b) The clear area of each fuel tank outlet strainer must be at least five times the area of the outlet line.

(c) The diameter of each strainer must be at least that of the fuel tank outlet.

(d) Each finger strainer must be accessible for inspection and cleaning.

[Amdt 25/1]