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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, the empange. Each tank will 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 optimization of aircraft centre of gravity.

Light Single Engine GA Aircraft

Small piston-engined 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 of 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 will include advances in automation, the number and location of fuel tanks, specific direction from the manufacturer on fuel distribution and the sequence in which the tanks are to be filled or emptied, a robust indicating and warning system, provisions for "single point" refuelling and defuelling and, in some applications, provisions for fuel dumping and/or for centre of gravity optimization.

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

  • single point refueling/defueling - the refueling hose is connected to a single point on the aircraft and all tanks are fuelled or defuelled by means of a manifold connecting the tanks
  • fuel pump redundancy - multiple fuel pumps in each tank to ensure fuel is accessable in the event of a single failure
  • robust fuel management, indicating and warning systems - depending upon the aircraft, these can include:
    • fuel quantity by tank
    • total fuel quantity
    • fuel used
    • 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
  • provisions in the fuel system to support an Auxiliary Power Unit (APU)
  • automated inflight transfer of fuel from the wing tanks to trim tanks in the horizontal stabilizer. Moving the fuel to the trim tank optimizes the centre of gravity and reduces the fuel burn
  • fuel dumping provisions. In the event of a diversion, fuel can be dumped to reduce the aircraft landing weight

Threats

There are a number of fuel related threats to safe aircraft operation. In addition to those described in the Fuel article, there are several fuel system threats 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 Inbalance - Fuel imbalance can occur as a result of improper refueling techniques, engine failure or fuel leak.
  • Mechanical failure of a fuel pump.
  • Fuel Freezing - In GA aircraft, Piston Engine Induction Icing or carburettor icing is the most common form of fuel freezing. On high altitude, long range aircraft, fuel temperature can be a critical factor. Fuel freezing temperature is dependant upon the type of fuel carried.
  • Electrical failure

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 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

  • In all cases comply with the manufacturer's limitations and recommendations as published in the AFM and Quick Reference Handbook (QRH)
  • 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 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.

Typical Scenarios

  • Three hours into an eight hour flight, a passenger suffers a heart attack necessitating an immediate diversion. Fuel is dumped to reduce the mass of the aircraft to comply with its maximum landing weight.
  • A light, single engine piston aircraft is descending from cruising altitude into warmer moist air. On approaching ciruit altitude, power is applied but the engine does not respond. The pilot suspects carburettor icing and selects the carburettor heat on. Initially the situation worsens, engine power declines further and the aircraft loses height. Then, after about 30 seconds, engine power increases and the aircraft is able to regain height and speed.
  • During an ETOPS flight, an engine is shut down due to a fire indication and an immediate diversion is initiated. During the transit to the diversion airfield, the pilots use the fuel crossfeed system in accordance with the QRH procedure to maintain the fuel imbalance within the manufacturer's limitations.

Accidents & Incidents

  • A388, en-route Batam Island Indonesia, 2010 (LOC AW) (On 4 November 2010, a Rolls Royce Trent 900-powered Airbus A380 which had just departed from Singapore was climbing through 7,000 ft when the No 2 engine suddenly suffered an uncontained failure. After careful preparation due to the complex collateral damage, the augmented flight crew made a successful air turnback. After coming to a stop, there were difficulties with engine shutdown which delayed passenger disembarkation. Investigation is ongoing but the engine failure has been attributed to a component manufacturing fault which the engine manufacturer had failed to identify.)
  • AT72, en-route, Mediterranean Sea near Palermo Italy, 2005 (AW LOC HF) (On 6 August 2005, an ATR 72-202 operated by Tuninter ditched into the sea off the coast of Capo Gallo (Palermo) after an uncommanded shutdown of both engines caused by fuel exhaustion.)
  • B772, en-route Bozeman MT USA, 2008 (AW LOC) (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.)
  • B772, London Heathrow UK, 2008 (LOC AW) (On 17 January 2008, a Boeing 777‑236ER crash-landed at London Heathrow after a loss of engine power on short final. This reduction of thrust (rollback) was the result of ice causing a restriction in the fuel feed system and subsequently led to a loss of airspeed and the aircraft touching down some 330 m short of the paved surface of Runway 27L at London Heathrow.)
  • A332, en-route, North Atlantic Ocean, 2001 (HF LOC AW FIRE) (On 24 August 2001, an Airbus A330-200 on a flight from Toronto to Lisbon, operated by Air Transat, experienced complete fuel exhaustion. After a flame out of both engines, the crew diverted the flight, making a successful glide of 65 nm120.38 km from FL345 to land at an alternate aerodrome - Lajes Air Force Base in the Azores.)
  • … further results


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