Aircraft Fuel System Requirements | Aircraft Systems

Aircraft Fuel System Requirements

Fuel System Independence

Each fuel system for a multiengine airplane must be arranged so that, in at least one system configuration, the failure of any one component (other than a fuel tank) does not result in the loss of power of more than one engine or require immediate action by the pilot to prevent the loss of power of more than one engine.

If a single fuel tank (or series of fuel tanks interconnected to function as a single fuel tank) is used on a multiengine airplane, independent tank outlets for each engine, each incorporating a shut-off valve at the tank, must be provided. The shutoff valves may serve as firewall shutoff valves, which are also required. However, note that if the line between the valve and the engine compartment contains more than one quart of fuel (or any greater amount shown to be safe) that can escape into the engine compartment, an additional firewall shutoff valve is needed. Lines and any components from each tank outlet to each engine must be completely independent of each other.

The fuel tank must have at least two vents arranged to minimize the probability of both vents becoming obstructed simultaneously. The filler caps must be designed to minimize the probability of incorrect installation or in-flight loss.


Fuel System Lightning Protection

The fuel system must be designed and arranged to prevent the ignition of fuel vapor within the system by direct lightning strikes or swept lightning strokes (where highly probable). Swept strokes occur when the lightning strike is deformed by interaction with aerodynamic forces and propagates in a unique manner due to the material and shape of the airframe surfaces. Corona and streamering must also be inhibited at fuel vent outlets since they may ignite the fuel-air mixture. A corona is a luminous discharge that occurs as a result of an electrical potential difference between the aircraft and the surrounding area. Streamering is a branch-like ionized path that occurs in the presence of a direct stroke or under conditions when lightning strokes are imminent.

Fuel Flow

The ability of the fuel system to provide fuel at a rate of flow and pressure sufficient for proper engine operation is vital in aircraft. Moreover, the fuel system must deliver the fuel at the aircraft attitude that is most critical with respect to fuel feed and quantity of unusable fuel. Tests are performed to demonstrate this performance. Fuel flowmeters are installed on most aircraft. During testing, the flowmeter is blocked and fuel must flow through or bypass the meter and still supply the engine at sufficient rate and pressure.

For gravity-flow fuel systems, the fuel flow rate must be 150 percent of the takeoff fuel consumption of the engine. For fuel pump systems, the fuel flow rate for each pump system (main and reserve supply) for each reciprocating engine must be 125 percent of the fuel flow required by the engine at the maximum takeoff power. However, the fuel pressure, with main and emergency pumps operating simultaneously, must not exceed the fuel inlet pressure limits of the engine. Auxiliary fuel systems and fuel transfer systems may operate under slightly different parameters. Turbine engine fuel systems must provide at least 100 percent required by the engine under each intended operating condition and maneuver.

On aircraft with multiple fuel tanks, performance is monitored when switching to a new tank once fuel has been depleted from a tank. For reciprocating, naturally aspirated, single-engine aircraft in level flight, 75 percent maximum continuous power must be obtainable in not more than 10 seconds. For turbocharged aircraft, 20 seconds is allowed. Twenty seconds is also allowed on multiengine aircraft.


Flow Between Interconnected Tanks

In a gravity feed fuel system with interconnected tank outlets, it must be impossible for enough fuel to flow between the tanks to cause an overflow of fuel from any tank vent. If fuel can be pumped from one tank to another in flight, the fuel tank vents and the fuel transfer system must be designed so that no structural damage to any airplane component can occur because of overfilling of any tank.

Unusable Fuel Supply

The unusable fuel supply for each tank must be established. It cannot be less than that quantity at which the first evidence of malfunctioning appears under the most adverse fuel feed condition occurring under each intended operation and flight maneuver involving that tank. The effect on the usable fuel quantity as a result of a failure of any pump is also determined.

Fuel System Hot Weather Operation

Each fuel system must be free from vapor lock when using fuel at its critical temperature, with respect to vapor formation, when operating the airplane in all critical operating and environmental conditions for which approval is requested. For turbine fuel, the critical temperature must be 110 °F, -0°, +5 °F or the maximum outside air temperature for which approval is requested, whichever is more critical.


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