Aircraft Fuel Tanks | Aircraft Systems

Aircraft Fuel Tanks


Each fuel tank must be able to withstand, without failure, the vibration, inertia, fluid, and structural loads to which it may be subjected in operation. Fuel tanks with flexible liners must demonstrate that the liner is suitable for the particular application. The total usable capacity of any tank(s) must be enough for at least 30 minutes of operation at maximum continuous power. Each integral fuel tank must have adequate facilities for interior inspection and repair. Additionally, each fuel quantity indicator must be adjusted to account for the unusable fuel supply.

Fuel Tank Tests

Aircraft fuel tanks must be able to withstand the forces that are encountered throughout the entire spectrum of operation. Various tank testing standards exist. A main focus is to ensure that tanks are strong enough to remain fully operational and not deform when under various loads. Vibration resistance without leaking is also a concern. Tanks are tested under the most critical condition that may be encountered. Fuel tank supporting structure must be designed for the critical loads that could occur during flight or when landing with fuel pressure loads.

Fuel Tank Installation

Various standards exist for fuel tank installations. No fuel tank may be on the engine side of a firewall, and there must be at least ½-inch of clearance between the fuel tank and the firewall. Each tank must be isolated from personnel compartments of the aircraft by a fume-proof and fuel-proof enclosure that is vented and drained to the exterior of the airplane. Pressurization loads should not affect the tank(s).

Each tank compartment must be ventilated and drained to prevent the accumulation of flammable fluids or vapors. Compartments adjacent to tanks must also be ventilated and drained. Aircraft fuel tanks must be designed, located, and installed to retain fuel when subjected to inertia loads resulting from ultimate static load factors, and under conditions likely to occur when the airplane lands on a paved runway at a normal landing speed with the landing gear retracted. They must also retain fuel if one of the gear collapses or if an engine mount tears away.

Many aircraft have fuel tanks that are not metal. Bladder fuel tanks have their own standards of construction and installation. As with metal tanks, there must be pads to prevent any chafing between each tank and its supports. The padding must be nonabsorbent or treated to prevent the absorption of fuel. Bladders must be supported so they are not required to support the entire fuel load. Surfaces adjacent to the liner must be smooth and free from projections that could cause wear. A positive pressure must be maintained within the vapor space of each bladder cell under any condition of operation, or it should be shown not to collapse under zero or negative pressure. Siphoning of fuel or collapse of bladder fuel cells should not result from improper securing or loss of the fuel filler cap. Bladder-type fuel cells must have a retaining shell at least equivalent to a metal fuel tank in structural integrity.

aircraft gear up accident retain fuel
Aircraft fuel tanks must be designed to retain fuel in the event of a gear-up landing. The fuel system drain valve should be located to prevent spillage

Fuel Tank Expansion Space

Each fuel tank must have an expansion space of not less than two percent of the tank capacity. This is waved if the tank vent discharges clear of the airplane, in which case no expansion space is required. It must be impossible to fill the expansion space inadvertently with the airplane in the normal ground attitude.

Fuel Tank Sump

Keeping contaminants out of the fuel delivered to the engine begins with the proper construction and installation of the fuel tank(s). Each tank must have a drainable sump with an effective capacity, in the normal ground and flight attitudes, of 0.25 percent of the tank capacity, or 1/16 gallon, whichever is greater. Each fuel tank must allow drainage of any hazardous quantity of water from any part of the tank to its sump with the airplane in the normal ground attitude. Reciprocating engine fuel systems must have a sediment bowl or chamber that is accessible for drainage. Its capacity must be 1 ounce for every 20 gallons of fuel on board. Each fuel tank outlet must be located so that water drains from all parts of the tank, except the sump, to the sediment bowl or chamber in the normal flight attitude.



Fuel Tank Filler Connection

Each fuel tank filler connection must be specifically marked. Aircraft with engines that use only gasoline fuel must have filler openings no larger than 2.36 inches in diameter. Turbine fuel aircraft filler openings must be no smaller than 2.95 inches. Spilled fuel must not enter the fuel tank compartment or any part of the airplane other than the tank itself. Each filler cap must provide a fuel-tight seal for the main filler opening. However, there may be small openings in the fuel tank cap for venting purposes or for the purpose of allowing passage of a fuel gauge through the cap. Fuel filling points must have a provision for electrically bonding the airplane to ground fueling equipment (except pressure fueling connection points).

Fuel Tank Vents and Carburetor Vapor Vents

To allow proper fuel flow, each fuel tank must be vented from the top part of the expansion space. Vent outlets must be located and constructed in a manner that minimizes the possibility of being obstructed by ice or other foreign matter. Siphoning of fuel during normal operation must not occur. Venting capacity must allow the rapid relief of excessive differences of pressure between the interior and exterior of the tank. The airspaces of tanks with interconnected outlets must also be interconnected. There must be no point in any vent line where moisture can accumulate either on the ground or during level flight (unless drainage is provided by an accessible drain valve). 

Fuel tank vents may not terminate at a point where the discharge of fuel from the vent outlet constitutes a fire hazard or from which fumes may enter personnel compartments. The vents must be arranged to prevent the loss of fuel when the airplane is parked in any direction on a ramp having a one-percent slope. Fuel discharged because of thermal expansion is allowed.

Each carburetor with vapor elimination connections and each fuel injection engine employing vapor return provisions must have a separate vent line to lead vapors back to the top of one of the fuel tanks. If there is more than one tank and it is necessary to use these tanks in a definite sequence for any reason, the vapor vent line must lead back to the fuel tank to be used first, unless the relative capacities of the tanks are such that return to another tank is preferable.

For acrobatic category airplanes, excessive loss of fuel during acrobatic maneuvers, including short periods of inverted flight, must be prevented. It must be impossible for fuel to siphon from the vent when normal flight has been resumed after any acrobatic maneuver for which certification is requested.

Fuel Tank Outlet

There must be a fuel strainer for the fuel tank outlet or for the booster pump. On reciprocating-engine aircraft, the strainer must have 8 to 16 meshes per inch. The clear area of each fuel tank outlet strainer must be at least five times the area of the outlet line and the strainer diameter must be at least that of the fuel tank outlet. It must also be accessible for inspection and cleaning. Turbine-engine aircraft fuel strainers must prevent the passage of any object that could restrict fuel flow or damage any fuel system component.

Pressure Fueling Systems

Pressure fueling systems are used on many large, high-performance, and air carrier aircraft. Each pressure fueling system fuel manifold connection must have means to prevent the escape of hazardous quantities of fuel from the system if the fuel entry valve fails. A means for automatic shutoff must be provided to prevent the quantity of fuel in each tank from exceeding the maximum quantity approved for that tank. A means must also be provided to prevent damage to the fuel system in the event of failure of the automatic shutoff means prescribed in this section. All parts of the fuel system up to the tank that are subjected to fueling pressures must have a proof pressure of 1.33 times and an ultimate pressure of at least 2.0 times the surge pressure likely to occur during fueling.

Fuel Pumps

Fuel pumps are part of most aircraft fuel systems. Standards exist for main pumps and emergency pumps. Operation of any fuel pump may not affect engine operation by creating a hazard, regardless of the engine power or thrust setting or the functional status of any other fuel pump. On reciprocating engines, one main fuel pump must be engine-driven and there must be at least one for each engine. Turbine engines also require dedicated fuel pumps for each engine. Any pump required for operation is considered a main fuel pump. The power supply for the main pump for each engine must be independent of the power supply for each main pump for any other engine. There must also be a bypass feature for each positive displacement pump.

Emergency pumps are used and must be immediately available to supply fuel to the engine if any main pump fails. The power supply for each emergency pump must be independent of the power supply for each corresponding main pump. If both the main fuel pump and the emergency pump operate continuously, there must be a means to indicate a malfunction of either pump to the appropriate flight crew member.

Fuel System Lines and Fittings

Even aircraft fuel system fluid lines and fittings have standards to ensure proper fuel system operation. Each fuel line must be installed and supported to prevent excessive vibration and to withstand loads due to fuel pressure and accelerated flight conditions. Lines connected to components of the airplane, between which relative motion could exist, must have provisions for flexibility. Flexible hose assemblies are used when lines may be under pressure and subject to axial loads. Any hose that is used must be shown to be suitable for a particular application. Where high temperatures may exist during engine operation or after shutdown, fuel hoses must be capable of withstanding these temperatures.

Fuel System Components

Fuel system components in an engine nacelle or in the fuselage must be protected from damage that could result in spillage of enough fuel to constitute a fire hazard as a result of a wheels-up landing on a paved runway.

Fuel Valves and Controls

There must be a means to allow appropriate flight crew members to rapidly shut off the fuel to each engine individually in flight. No shutoff valve may be on the engine side of any firewall. There must be means to guard against inadvertent operation of each shutoff valve and means to reopen each valve rapidly after it has been closed. Each valve and fuel system control must be supported so that loads resulting from its operation, or from accelerated flight conditions, are not transmitted to the lines connected to the valve. Gravity and vibration should not affect the selected position of any valve.

Fuel valve handles and their connections to valve mechanisms must have design features that minimize the possibility of incorrect installation. Check valves must be constructed to preclude incorrect assembly or connection of the valve. Fuel tank selector valves must require a separate and distinct action to place the selector in the OFF position. The tank selector positions must be located in such a manner that it is impossible for the selector to pass through the OFF position when changing from one tank to another.

Fuel Strainer or Filter

In addition to fuel tank strainers already discussed, there must be a fuel strainer, or filter, between the fuel tank outlet and the inlet of either the fuel metering device or an engine-driven positive displacement pump, whichever is nearer the fuel tank outlet. This fuel strainer, or filter, must be accessible for draining and cleaning and must incorporate a screen or element that is easily removable. The fuel strainer should have a sediment trap and drain, except that it need not have a drain if the strainer or filter is easily removable for drain purposes. The fuel strainer should also be mounted so that its weight is not supported by the connecting lines. It should have the capacity to ensure that engine fuel system function is not impaired when fuel is contaminated to a degree that is greater than that established for the engine during its type certification. Commuter category airplanes must have a means to automatically maintain the fuel flow if ice clogs a filter.

Fuel System Drains

Aircraft fuel systems must be fitted with at least one drain to allow safe drainage of the entire fuel system with the airplane in its normal ground attitude. The drain must discharge the fuel clear of all parts of the aircraft. A readily accessible drain valve that can easily be opened and closed is required. It must have a manual or automatic means for locking in the closed position, and it must be observable that it is closed. Fuel should be collectible from the system drain valve so it can be examined. The location of the valve should be such that spillage is prevented should a gear up landing be made.

Fuel Jettisoning System

If an aircraft’s design landing weight is less than that of the maximum takeoff weight, a situation could occur in which a landing is desired before sufficient fuel has burned off to lighten the aircraft. Fuel jettisoning systems are required on these aircraft so that fuel can be jettisoned in flight to avoid structural damage cause by landing the aircraft when it is too heavy. Fuel jettisoning systems are also referred to as fuel dump systems.

Boeing 767 fuel jettison system
The fuel jettison panel on a Boeing 767

Fuel jettisoning systems must meet several standards. The average rate of fuel jettisoning must be at least 1 percent of the maximum weight per minute, except that the time required to jettison the fuel need not be less than 10 minutes. Fuel jettisoning must be demonstrated at maximum weight with flaps and landing gear up and in a power-off glide at 1.4 VS1. It must also be demonstrated during a climb with a critical engine inoperative and the remaining engines at maximum continuous power. Finally, the fuel jettisoning system must be performed during level flight at 1.4 VS1 if the glide and climb tests show that this condition could be critical.

During the demonstration of the fuel jettisoning system, it must demonstrate that it operates without fire hazard. No fuel or fumes can enter any part of the aircraft. The fuel must discharge clear of any part of the aircraft and the jettisoning operation must not adversely affect the controllability of the airplane. The system must be designed so that any reasonably probable single malfunction in the system does not result in a hazardous condition due to unsymmetrical jettisoning of, or inability to jettison, fuel. The fuel jettisoning valve must be designed to allow flight crew members to close the valve during any part of the jettisoning operation.

Fuel being jettisoned free of the airframe on a transport category aircraft
Fuel being jettisoned free of the airframe on a transport category aircraft

On reciprocating-engine aircraft, the jettisoning system must be designed so that it is not possible to jettison the fuel in the tanks used for takeoff and landing below the level allowing 45 minutes of flight at 75 percent maximum continuous power. However, if there is an auxiliary control independent of the main jettisoning control, the system may be designed to jettison all the fuel. For turbine engine powered airplanes, the jettisoning system must be designed so that it is not possible to jettison fuel from the tanks used for takeoff and landing below the fuel level that would allow climb from sea level to 10,000 feet plus 45 minutes cruise at a speed for maximum range. If certain flight control configurations negatively affect jettisoning the fuel, a placard stating so must be posted next to the actuation control in the cockpit.

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