Aircraft Engine Fuel System Requirements

Electronic engine controls have allowed great increases in controlling the metered fuel flow to the engine. Engine fuel systems have become very accurate at providing the correct mixture of fuel and air to the engines. Gas turbine fuel controls have also greatly improved the ability to schedule (meter) the fuel correctly during all flight regimes. Improvements in electronics and the use of digital computers have enabled the aircraft and engines to be electronically interfaced together. By the use of electronic sensors and computer logic built in to electronic controls, the engines can be controlled with much more accuracy. Fuel cost and availability have also become factors in providing engines with fuel systems that are efficient and very precise in scheduling fuel flow to the engine. Many engines use an interactive system that senses engine parameters and feeds the information to the onboard computer (electronic engine control). The computer determines the amount of fuel needed and then sends a signal to the metering device. This signal sent to the metering device determines the correct amount of fuel needed by the engine. Electronic controls have become quite common with gas turbines and have increased the capabilities of the fuel system, making it less complicated for the technician and decreasing maintenance problems.

Engine fuel systems can be fairly complicated, yet some are quite simple, such as on small aircraft with a simple gravity-feed fuel system. This system, consisting of a tank to supply fuel to the engine, is often installed in the overhead wing and feeds a small float-type carburetor. On multiengine aircraft, complex systems are necessary so that fuel can be pumped from any combination of tanks to any combination of engines through a crossfeed system. Provisions for transferring fuel from one tank to another may also be included on large aircraft.

Vapor Lock

All fuel systems should be designed so that vapor lock cannot take place. Older gravity-feed systems were more prone to vapor lock. The fuel system should be free of tendency to vapor lock, which can result from changes in ground and in-flight climatic conditions. Normally, the fuel remains in a liquid state until it is discharged into the air stream and then instantly changes to a vapor. Under certain conditions, the fuel may vaporize in the lines, pumps, or other units. The vapor pockets formed by this premature vaporization restrict the fuel flow through units which are designed to handle liquids rather than gases. The resulting partial or complete interruption of the fuel flow is called vapor lock. The three general causes of vapor lock are the lowering of the pressure on the fuel, high fuel temperatures, and excessive fuel turbulence.

At high altitudes, the pressure on the fuel in the tank is low. This lowers the boiling point of the fuel and causes vapor bubbles to form. This vapor trapped in the fuel may cause vapor lock in the fuel system.

Transfer of heat from the engine tends to cause boiling of the fuel in the lines and the pump. This tendency is increased if the fuel in the tank is warm. High fuel temperatures often combine with low pressure to increase vapor formation. This is most apt to occur during a rapid climb on a hot day. As the aircraft climbs, the outside temperature drops, but the fuel does not lose temperature rapidly. If the fuel is warm enough at takeoff, it retains enough heat to boil easily at high altitude. The chief causes of fuel turbulence are sloshing of the fuel in the tanks, the mechanical action of the engine-driven pump, and sharp bends or rises in the fuel lines. Sloshing in the tank tends to mix air with the fuel. As this mixture passes through the lines, the trapped air separates from the fuel and forms vapor pockets at any point where there are abrupt changes in direction or steep rises. Turbulence in the fuel pump often combines with the low pressure at the pump inlet to form a vapor lock at this point.

Vapor lock can become serious enough to block the fuel flow completely and stop the engine. Even small amounts of vapor in the inlet line restrict the flow to the engine-driven pump and reduce its output pressure. To reduce the possibility of vapor lock, fuel lines are kept away from sources of heat; also, sharp bends and steep rises are avoided. In addition, the volatility of the fuel is controlled in manufacture so that it does not vaporize too readily. The major improvement in reducing vapor lock, however, is the incorporation of booster pumps in the fuel system. These booster pumps, which are used widely in most modern aircraft, keep the fuel in the lines to the engine-driven pump under pressure. The pressure on the fuel reduces vapor formation and aids in moving a vapor pocket along. The boost pump also releases vapor from the fuel as it passes through the pump. The vapor moves upward through the fuel in the tank and out the tank vents. To prevent the small amount of vapor that remains in the fuel from upsetting its metering action, vapor eliminators are installed in some fuel systems ahead of the metering device or are built into this unit.

14 CFR Part 23

The fuel systems of aircraft certified or registered in the US must be designed to meet specific operating requirements in 14 CFR Part 23 of the Federal Aviation Regulations. Some of these requirements are:
  • Each fuel system must be constructed and arranged to ensure fuel flow at a rate and pressure established for proper engine and auxiliary power unit functioning under all likely operating conditions.
  • Each fuel system must be arranged so that no pump can draw fuel from more than one tank at a time, or provisions must be made to prevent air from being drawn into the fuel supply line.
  • Turbine-powered aircraft must be capable of sustained operation with 0. 75 cubic centimeter of free water per gallon of fuel at 80 degrees Fahrenheit. In addition, an engine must be capable of sustained operation when the fuel is cooled to its most critical condition for icing.
  • Each fuel system of a multiengine aircraft must be arranged so that the failure of any one component (except a fuel tank) will 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.
  • If a multiengine airplane has a single tank or assembly of interconnected tanks, each engine must have an independent tank outlet with a fuel shutoff valve at the tank.
  • A way to rapidly shut off fuel in flight to each engine of a normal category aircraft must be provided to appropriate flight crewmembers. The engine fuel shutoff valve cannot be located on the engine side of any firewall.
  • On multiengine aircraft, the closing of an individual fuel shutoff valve for any engine shall not affect the fuel supply to the other engines.
  • Tanks used in multiengine fuel systems must have two vents arranged so that is unlikely that both become plugged at the same time.
  • All filler caps must be designed so that they are unlikely to be installed incorrectly or lost in flight.
  • The fuel systems must be designed to prevent the ignition of fuel vapors by lightning.
  • The fuel flow rate of a gravity-feed system must be 150 percent of the takeoff fuel flow when the tank contains the minimum fuel allowable. The same requirement exists when the airplane is in the attitude that is most critical for fuel flow.
  • The fuel flow rate of a pump-feed fuel system for each reciprocating engine must be 125 percent of the takeoff fuel flow required.
  • If an aircraft is equipped with a selector valve that enables an 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 multiengine) aircraft between the time one tank runs dry and the time fuel is supplied by the other tank.
  • A turbine-powered aircraft must have a fuel system that will supply 100 percent of the fuel required for operation in all flight attitudes with uninterrupted flow as the fuel system automatically cycles through all of its tanks (or fuel cells).
  • If a gravity-feed system has interconnected tank outlets, it should not be possible for fuel from one tank to flow into another and cause it to overflow.
  • The amount of unusable fuel in an aircraft must be determined and 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.
  • The fuel system must be designed with a means to prevent vapor lock (fuel vapor that blocks flow) when fuel is at critical temperature (with respect to vapor formation) under the most critical operating conditions.
  • Each fuel tank compartment must be adequately vented and drained to prevent the accumulation of explosive vapors or liquid.
  • No fuel tank can be on the engine side of a firewall; it must be at least one-half inch away from the firewall.
  • Each fuel tank must have an expansion space of at least two percent that cannot be filled with fuel. However, if a fuel tank vent discharges clear of the airplane, no expansion space is required.
  • Each fuel tank must be vented from the top part of its expansion space. In addition, if more than one fuel tank has interconnected outlets, the airspace above the fuel must also be interconnected.
  • Each fuel tank must have a drainable sump located where water and contaminants will accumulate when the aircraft is in its normal ground attitude. In addition, each reciprocating engine fuel system must have a drainable sediment bowl with a capacity of one ounce for every 20 gallons of fuel.
  • Provisions must be made to prevent fuel spilled during refueling from entering the aircraft structure.
  • For aircraft with reciprocating engines, the filler opening of an aircraft fuel tank must be marked at or near the filler opening with the word "Avgas" and the minimum grade of fuel. For turbine-powered aircraft, the tank must be marked with the word "Jet Fuel" and with the permissible fuel designation. If the filler opening is for pressure fueling, the maximum permissible fueling and defueling pressures must be specified.
  • All fuel tanks are required to have a strainer at the fuel tank outlet or at the booster pump inlet. For a reciprocating engine, the strainer should have an element of 8 to 16 meshes per inch. For turbine engines, the strainer should prevent the passage of any object that could restrict fuel flow or damage any of the fuel system components.
  • For engines that require fuel pumps, each engine must have one engine-driven fuel pump.
  • At least one drain must be available to permit safe drainage of the entire fuel system when the airplane is in its normal ground attitude.
  • 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.
  • The fuel-jettisoning valve must be designed to enable personnel to close the valve during any part of the jettisoning operation.

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