BASIC FUEL SYSTEM REQUIREMENTS
The requirements for the fuel system design are specified in detail in the parts of the Federal Aviation Regulations under which the aircraft was built. Since the vast majority of airplanes in the general aviation fleet are built under FAR Part 23, "Airworthiness Standards: Normal
1. No pump can draw fuel from more than one tank at a time, and provisions must be made to prevent air from being drawn into the fuel supply line. (23.951)
2. Turbine-powered aircraft must be capable of sustained operation when there is at least 0.75 cc. of free water per gallon of fuel, and the fuel is cooled to its most critical condition for icing. The system must incorporate provisions to prevent the water which precipitates out of the fuel freezing on the filters and stopping fuel flow to the engine.
3. Each fuel system of a multi-engine aircraft must be arranged in such a way that the failure of any one component (except the fuel tank) will not cause more than one engine to lose power. (23.953)
4. If multi-engine aircraft feed more than one engine from a single tank or assembly of interconnected tanks, each engine must have an independent tank outlet with a fuel shutoff valve at the tank. (23.953)
5. Tanks used in multi-engine fuel systems must have two vents arranged so that they are not likely to both become plugged at the same time. (23.953),
6. All filler caps must be designed so that they are not likely to be installed incorrectly or lost in-flight. (23.953)
7. The fuel systems must be designed to prevent the ignition of fuel vapours by lightning. (23.954)
8. A gravity feed system must be able to flow 150% of the takeoff fuel flow when the tank contains the minimum fuel allowable, and when the airplane is positioned in the attitude that is most critical for fuel flow. (23.955)
9. A pump feed fuel system must be able to flow 125% of the takeoff fuel -flow required for a reciprocating engine. (23.955)
10. If the aircraft is equipped with a selector valve that allows the engine to operate from more than one fuel tank, the system must not cause a loss of power for more than ten seconds for a single-engine or twenty seconds for a multi-engine airplane, between the times one tank is allowed to run dry and the time at which the required power is supplied by the other tank. (23.955)
11. Turbine-powered aircraft must have a fuel system that will supply 100% of the fuel required for its operation in all flight attitudes, and the flow must not be interrupted, as the fuel system automatically cycles through all of the tanks or fuel cells in the system. (23.955)
12. If gravity feed system has interconnected tank outlets, it should not be possible for fuel feeding from one tank to flow into another tank and cause it to overflow. (23.957)
13. The amount of unusable fuel in an aircraft must be determined and this must be made known to the pilot Unusable fuel is the amount of fuel in a tank when the first evidence of malfunction occurs. The aircraft must be in the attitude that is most adverse for fuel flow. (23.959)
14. The fuel system must be so designed that it is free from vapour lock when the fuel is at a temperature of 110 °F under the most critical operating conditions. (23.961)
15. Each fuel tank compartment must be adequately vented and drained so no explosive vapours or liquid can accumulate. (23.967)
16. No fuel tank can be on the engine side of the firewall, and it must be at least one-half inch away from the firewall. (23.967)
17. No fuel tank can be installed inside a personnel compartment of a multi-engine aircraft. (23.967)
18. Each fuel tank must have a 2% expansion space that cannot be filled with fuel, and it must also have a drainable sump where water and contaminants will normally accumulate when the aircraft is in its normal ground attitude. (23.969 and 23.9_71)
19. Provisions must be made to prevent fuel spilled during filling the tank from entering the aircraft structure. (23.973)
20. The filler opening of an aircraft fuel tank must be marked with the word "FUEL " and, for aircraft with reciprocating engines, with the minimum grade of fuel. For turbine-powered aircraft, the tank must be marked with the permissible fuel designation. If the filler opening is for pressure fuelling, the maximum permissible fuelling and defuelling pressure must be specified. (23.1557).
21. If more than one fuel tank has interconnected outlets, the airspace above the fuel must also be interconnected. (23.975)
22. If the carburetor or fuel injection system has a vapour elimination system that returns fuel to one of the tanks, the returned fuel must go to the tank that is required to be used first. (23.975)
23. All fuel tanks are required to have a strainer at the fuel tank outlet or at the booster pump. For a reciprocating engine, the strainer should have an 8 to 16 mesh element, and for turbine engines, the strainer should prevent the passage of any object that could restrict the flow or damage any of the fuel system components. (23.977)
24. For engines requiring fuel pumps, there must be one engine driven fuel pump for each engine. (23.991)
25. There must be at least one drain that will allow safe drainage of the entire fuel system when the airplane is in its normal ground attitude. (23.999)
26. If the design landing weight of the aircraft is less than that permitted for takeoff, there must be provisions in the fuel system for jettisoning fuel to bring the maximum weight down to the design landing weight. (23.1001)
27. The fuel jettisoning valve must be designed to allow personnel to close the valve during any part of the jettisoning operation.
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