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Aircraft Fuel Systems

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Category: Flight Technical Flight Technical
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Definition

An aircraft fuel system enables fuel to be loaded, stored, managed and delivered to the propulsion system (engine(s)) of an aircraft.

General Description

Fuel systems differ greatly from aircraft to aircraft due to the relative size and complexity of the aircraft in which they are installed. In the most basic form. a fuel system will consist of a single, gravity feed fuel tank with the associated fuel line connecting it to the aircraft engine. In a modern, multi-engine passenger or cargo aircraft, the fuel system is likely to consist of multiple fuel tanks which may be located in the wing or the fuselage (or both) and, in some cases, in the empange. Each tank will potentially be equiped with internal fuel pumps and have the associated valves and plumbing to feed the engines, allow for refueling and defueling, isolate the individual tanks and, in some applications, allow for fuel dumping or for optimisation of aircraft centre of gravity.

Light Single Engine GA Aircraft

Small piston-engine powered aircraft often have a single tank fuel system. On newer aircraft, two fuel tanks, with one in each wing, are more common. A two tank system requires additional components to allow controlled provision of fuel to the single engine. Fuel tank boost pumps may or may not be incorporated depending upon the location of the tanks.

The fuel is piped from the tanks through fuel lines to a fuel control valve which is commonly referred to as the fuel selector valve. This valve serves several functions and will potentially have Left, Right, Both and Off selections. Left, Right and Both allow for fuel to be fed to the engine from either the Left tank or the Right tank individually or from Both at the same time. This facility allows the pilot to balance the fuel tanks or to "trim" the aircraft laterally. The Off selection provides for a fuel shut off valve in the event of an engine fire or to prevent unwanted fuel migration when the aircraft is not in operation. In some installations, the shut-off function is provided by a separate valve located downstream from the fuel control valve.

Light Twin Engine GA Aircraft

Adding a second engine to an aircraft, by necessity, increases the complexity of the fuel system and its management. Additional features normally found in small multi-engine aircraft include in-tank fuel pumps, a more robust fuel quantity indicating system and the provision for fuel "crossfeed". Refueling is still normally accomplished on a tank by tank basis.

Crossfeed allows for fuel from one wing tank to be burned by the engine on the other wing. In some cases, the fuel is routed directly from the tank to the engine while in others, it is transfered from one wing tank to the opposite wing tank before feeding to the engine. The crossfeed provision allows the pilot to use all of the fuel on board and to maintain lateral balance limitations in the event that a failure results in single engine operations.

Multi-engine Turbo Prop and Turbo Jet Aircraft

Increasing the size and complexity of an aircraft will normally result in corresponding changes to the fuel system. These changes are likely to include more system automation, more fuel tanks, specific Aircraft Flight Manual (AFM) requirements with respect to fuel distribution in flight and the sequence in which the tanks are to be filled on the ground or their contents used in flight, a reliable system indication and alerting system, provisions for "single point" refuelling and defuelling and, in larger aircraft, provision for fuel dumping and/or for centre of gravity optimisation through fuel movement in flight.

Enhancements to the fuel system commonly found on aircraft of this category include:

  • single point refueling/defueling - the refuelling hose is connected to a single point on the aircraft, usually located underwing or somewhere on the fuselage and all tanks are fuelled or defuelled by means of a manifold connecting to all tanks
  • fuel pump redundancy - multiple fuel pumps in each tank to ensure fuel is accessible in the event of a single pump failure
  • robust fuel management, indicating and warning systems - depending upon the aircraft, these can include:
    • fuel quantity by tank
    • total fuel quantity remaining
    • fuel used
    • estimated fuel remaining at intended destination
    • fuel temperature by tank
    • automatic selection of most appropriate fuel tank dependant upon phase of flight
    • automatic fuel transfer
    • warnings and cautions for items such as:
      • low fuel quantity
      • low fuel pressure
      • fuel pump failure
      • low fuel temperature
  • provision of fuel tanks in the outer portion of the wings to reduce wing bending. The fuel in these tanks is generally not burned until late in the flight
  • provision in the fuel system to supply an Auxiliary Power Unit (APU) (APU)
  • automated inflight transfer of fuel from the wing tanks to trim tanks in the horizontal stabiliser. Moving the fuel to the trim tank optimizes the centre of gravity and reduces the fuel burn
  • fuel dumping provisions. In the event of an unexpectedly early landing, excess fuel can be dumped to reduce the aircraft landing weight to or towards the permitted MLW

Threats

There are a number of fuel related threats to safe aircraft operation. In addition to those described in the Fuel Management article, there are several threats related to the misuse or to the malfunction of an aircraft fuel system that must also be considered. These include:

  • Fuel Leak - Fuel can leak at the engine, from the tank or anywhere in between due to fuel tank or fuel line rupture.
  • Fuel Imbalance - Fuel imbalance can occur as a result of improper refueling techniques, poor fuel management, engine failure or fuel leak.
  • Mechanical failure of a fuel pump.
  • Fuel Freezing - In gas turbine powered jet aircraft flown at high altitude for long periods, fuel temperature can be a critical factor. Minimum allowable fuel temperatures are less likely to be a factor on the operation of turboprop aircraft. The temperature at which fuel freezes will depend on the prevailing pressure and on the type and specification of fuel carried. In GA aircraft, Piston Engine Induction Icing or carburettor icing is the most common form of fuel freezing.
  • Electrical failure - may limit the availability of fuel pumps and fuel system indications
  • Fuel dumping causes two main concerns:
    • Fuel dissipation - in order for the fuel to dissipate in the air (and thus mitigate pollution on the ground) ICAO Doc 4444 (PANS-ATM) states that the level used should not be less than 6000 ft.
    • Fuel ingestion - in order to prevent other aircraft from ingesting the fuel being dumped, the following separation minima apply:
      • 10 NM horizontally, but not behind the aircraft dumping fuel
      • at least 1000 ft above or 3000 ft below for aircraft that are within 15 minutes or 50 NM behind the aircraft dumping fuel

Effects

  • A fuel leak from an engine can often be resolved by shutting down the affected engine. A tank leak due to a rupture in the tank will result in the loss of some or all of the fuel in that tank. If a fuel line is ruptured, it could result in some fuel being unuseable.
  • An uncorrected fuel imbalance can lead to difficulty in controlling the aircraft.
  • A pump failure could result in the inability to use the fuel in the affected tank. This may be mitigated by a second (or even a third) pump in the same tank.
  • Fuel freezing can lead to loss of power due to fuel starvation and potentially can result in engine failure.
  • In the event of electrical failure, some, or potentially all, fuel tank boost pumps will be lost. In most aircraft, gravity fuel feeding is only posible from some of the fuel tanks. Descent may be required to comply with the maximum allowable fuel gravity feed altitude. Diversion may be required due to unusable fuel.

Defences

  • WARNING - the misidentification or mishandling of a fuel leak can potentially lead to depletion of all fuel on board the aircraft. Use the QRH or other appropriate checklist to carefully identify and isolate the leaking component.
  • Where possible, maintain the aircraft wing to wing fuel balance within limits by referring to the QRH or other appropriate checklist
  • Fuel pump circuit breakers should NOT be reset in flight.
  • In light aircraft, use carburettor heat as appropriate. In larger aircraft at high altitude, if the fuel temperature approaches its freezing point, pilots can descend to warmer air, increase the aircraft speed to increase the Total Air Temperature or transfer fuel to a tank containing warmer fuel.

Accidents & Incidents

  • B743, vicinity Tehran Mehrabad Iran, 2015 (On 15 October 2015 a Boeing 747-300 experienced significant vibration from one of the engines almost immediately after take-off from Tehran Mehrabad. After the climb out was continued without reducing the affected engine thrust an uncontained failure followed 3 minutes later. The ejected debris caused the almost simultaneous failure of the No 4 engine, loss of multiple hydraulic systems and all the fuel from one wing tank. The Investigation attributed the vibration to the Operator's continued use of the engine without relevant Airworthiness Directive action and the subsequent failure to continued operation of the engine after its onset.)
  • B772, en-route Bozeman MT USA, 2008 (On 26 November 2008, a Boeing 777-200 powered by RR RB211 Trent 800 series engines and being operated by Delta AL on a scheduled passenger flight from Shanghai Pudong to Atlanta was in the cruise at FL390 in day VMC in the vicinity of Bozeman MT when there was an uncommanded thrust reduction or ‘rollback’ of the right engine.)
  • A346, en-route, Amsterdam Netherlands, 2005 (On 8 February 2005, a Virgin Atlantic Airways A340-600 experienced in-flight fuel management problem which led to loss of power of No 1 engine and temporary power loss of No 4. The captain decided to divert to Amsterdam where the aircraft landed safely on three engines.)
  • HUNT, manoeuvring, vicinity Shoreham UK, 2015 (On 22 August 2015 the pilot of a civil-operated Hawker Hunter carrying out a flying display sequence at Shoreham failed to complete a loop and partial roll manoeuvre and the aircraft crashed into road traffic unrelated to the airshow and exploded causing multiple third party fatalities and injuries. The Investigation found that the pilot had failed to enter the manoeuvre correctly and then failed to abandon it when it should have been evident that it could not be completed. It was concluded that the wider context for the accident was inadequate regulatory oversight of UK civil air display flying risk management.)
  • A388, en-route Batam Island Indonesia, 2010 (On 4 November 2010, a Qantas Airbus A380 climbing out of Singapore experienced a sudden and uncontained failure of one of its Rolls Royce Trent 900 engines which caused considerable collateral damage to the airframe and some of the aircraft systems. A ‘PAN’ was declared and after appropriate crew responses including aircraft controllability checks, the aircraft returned to Singapore. The root cause of the failure was found to have been an undetected component manufacturing fault. The complex situation which resulted from the failure in flight was found to have exceeded the currently anticipated secondary damage from such an event.)

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