Aircraft Systems: Aircraft Fuel System

Aircraft Fuel System

All powered aircraft require fuel on board to operate the engine(s). A fuel system consisting of storage tanks, pumps, filters, valves, fuel lines, metering devices, and monitoring devices is designed and certified under strict Title 14 of the Code of Federal Regulations (14 CFR) guidelines. Each system must provide an uninterrupted flow of contaminant free fuel regardless of the aircraft’s attitude. Since fuel load can be a significant portion of the aircraft’s weight, a sufficiently strong airframe must be designed. Varying fuel loads and shifts in weight during maneuvers must not negatively affect control of the aircraft in flight.

Each Federal Aviation Administration (FAA) certified aircraft is designed and constructed under FARs applicable to that type of aircraft. The certification airworthiness standards are found in 14 CFR as follows: 14 Part 23—Normal, Utility, Acrobatic, and Commuter Category Airplanes
  • 14 Part 25—Transport Category Airplanes
  • 14 Part 27—Normal Category Rotorcraft
  • 14 Part 29—Transport Category Rotorcraft
  • 14 Part 31—Manned Free Balloons

Additional information is found in 14 CFR part 33. It addresses airworthiness standards for engines and pertains mainly to engine fuel filter and intake requirements.

Under each 14 CFR part for a specific aircraft to be certified, paragraphs 951 through 1001 address very specific design criteria required to ensure the fuel system functions properly. These paragraphs from 14 CFR part 23, Normal, Utility, Acrobatic, and Commuter Category Airplanes, are summarized below. Airworthiness standards specified for air carrier and helicopter certification are similar. Although the technician is rarely involved with designing fuel systems, a review of these criteria gives insight into how an aircraft fuel system operates.

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 (APU) functioning under each likely operating condition. This includes any maneuver for which certification is requested and during which the engine or APU may be in operation. Each fuel system must be arranged so that no fuel pump can draw fuel from more than one tank at a time. There must also be a means to prevent the introduction of air into the system.

Each fuel system for a turbine engine powered airplane must meet applicable fuel venting requirements. 14 CFR part 34 outlines requirements that fall under the jurisdiction of the Environmental Protection Agency (EPA). A turbine engine fuel system must be capable of sustained operation throughout its flow and pressure range even though the fuel has some water in it. The standard is that the engine continues to run using fuel initially saturated with water at 80 °F having 0.75 cubic centimeters (cm) of free water per gallon added to it and then cooled to the most critical condition for icing likely to be encountered in operation.