Aircraft Fuel Systems (Turbine Engine)
Aircraft Fuel Systems (Turbine Engine)
Definition
An aircraft fuel system enables fuel to be loaded, stored, managed, and delivered to the engine(s) of an aircraft.
General Description
Multi-engine turboprop and turbojet aircraft normally have much more complex fuel systems that those found on smaller piston-engine aircraft. These complex systems are likely to include more tanks and more system automation. There are usually specific Aircraft Flight Manual (AFM) requirements with respect to fuel distribution in flight and the sequence in which tanks are to be filled on the ground and used in flight. Turbine aircraft may include an indication and alerting system that presents warning, caution, advisory, and status messages in plain text. Enhancements to the fuel systems commonly found in transport-category aircraft include:
- Single-point refueling/defueling - The refueling hose is connected to a single point on the aircraft, usually located underwing or somewhere on the fuselage. All tanks are fueled or defueled 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.
- Fuel filtration, with capability for fuel to bypass a filter that becomes clogged.
- Robust fuel instrumentation - Depending on the aircraft, this can include: fuel quantity by tank, total fuel quantity, fuel used, fuel flow rate, estimated fuel remaining at destination, and fuel temperature by tank. In aircraft with an Engine Indicating and Crew Alerting System (EICAS), warnings and cautions may be generated for malfunctions such as low fuel quantity, low fuel pressure, fuel pump failure, low fuel temperature, and fuel imbalance.
- Provision for 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 flight.
- Provision for fuel supply to an Auxiliary Power Unit (APU).
- Automated in-flight transfer of fuel from wing tanks to trim tanks in the horizontal stabiliser. Moving fuel to the trim tanks optimises the centre of gravity and reduces fuel burn.
- Fuel dumping provisions - In the event of an unexpectedly early landing, excess fuel can be dumped to reduce landing weight to or towards the permitted maximum landing weight.
Other system features can include collector tanks in the inboard section of the wing tanks. Fuel pumps are normally contained within these collector tanks. The purpose of the collector tanks is to keep the fuel pumps submerged during all phases of flight, ensuring a steady fuel supply to the engines.
The pumps within the collector tanks may include a primary ejector pump to feed the respective engine. Ejector pumps have no moving parts and operate on the motive flow principle. They are backed up by electrical boost pumps. Scavenge ejector pumps may also be present within a collector tank. These pumps keep the collector tank filled.
As another subsection of the wing tank, a surge tank may be installed at the outboard end of the wing tank. Surge tanks collect fuel that may enter the vent system during wing-down or uncoordinated maneuvers.
In addition to the usual fuel quantity indicating systems, magnetic level indicators may be present. These indicators, accessible from outside the aircraft, allow ground personnel to confirm fuel quantity if the electronic indicating system fails.
The electrical fuel boost pumps in turbine aircraft are normally powered by different electrical busses, so the failure of one bus does not cause failure of all the pumps. As an additional redundancy, some systems include DC-powered pumps as well as AC-powered pumps.
Combinations of fuel boost pumps and crossfeed valves, separation valves, and isolation valves allow for maximum flexibility in working around system failures. Multiple valves and pumps can provide multiple paths for getting fuel from a tank affected by a malfunction.
In some of the most sophisticated fuel systems, an inerting system may be included to reduce the risk of fire. As fuel is burned, the volume of air above the fuel level increases. This area is known as the "ullage," and it can contain highly flammable fuel vapor. However, an inerting system can replace oxygen in the fuel vapor with a non-flammable inert gas such as nitrogen.
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 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 from anywhere in between due to fuel tank or fuel line rupture.
- Fuel imbalance: Fuel imbalance can occur due to improper refueling techniques, poor fuel management, engine failure, or fuel leak.
- Mechanical failure of a fuel pump.
- Fuel contamination: Poor storage by a vendor can allow water or other contaminants to enter fuel in ground-storage tanks.
- Fuel freezing: In turbine-powered aircraft flown at high altitude for long periods, fuel temperature can be a critical factor. The temperature at which fuel freezes will depend on the prevailing pressure and on the type and specification of fuel carried. Some aircraft are equipped with a fuel heating capability.
- Electrical failure: Electrical failure may limit the operability of fuel pumps, valves, and system indications.
- Fuel dumping causes two main concerns: fuel dissipation and fuel ingestion. To help ensure the dissipation of dumped fuel, aircraft may have recommended minimum altitudes for fuel dumping. To prevent another aircraft from ingesting dumped fuel, certain separation minima may apply.
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 becoming unusable.
- 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 a loss of power due to fuel starvation.
- If electrical failure causes the loss of enough boost pump capability, gravity fuel feeding may be the only way to use fuel in the affected tanks. Descent may be required to comply with the maximum allowable gravity feed altitude. Diversion may be required due to unusable fuel.
Defences
- In all cases, comply with the limitations in the Aircraft Flight Manual (AFM) and in the Operations Manual (OM) or 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 carefully identify and isolate the leaking component.
- Where possible, maintain aircraft wing-to-wing fuel balance within limits by referring to the QRH or other checklist. As circumstances permit, it is advisable to keep multi-tasking at a minimum when crossfeeding or transferring fuel. Inattention at this stage can make the problem worse.
- Fuel pump circuit breakers should NOT be reset without consulting maintenance.
- If fuel temperature reaches its freezing point, pilots can descend to warmer air, increase speed to increase the total air temperature, or transfer fuel to a tank containing warmer fuel.
Accidents & Incidents
On 14/15 April 2022, refuelling of an Airbus A330-300 in Accra was delayed by multiple automated interruptions but resolved by changing from tanker to hydrant. Departure to Johannesburg was delayed to the following day. During the cruise at FL410, a right wing fuel pump low pressure annunciation prompted descent to FL190 to activate gravity fuel feed. An ‘ENGINE 2 STALL’ annunciation then appeared and could only be removed by manually controlling thrust at below-normal level. The fuel pump low pressure annunciation remained after landing. Initially suspected fuel contamination with water in both cases was eliminated during the investigation.
On 31 October 2021, a "Fuel Imbalance" message occurred on a Boeing 787-9 soon after departing Bangkok at night but attempted fuel transfer was unsuccessful. A "Fuel Disagree" message subsequently appeared, and use of available system checklists indicated that there was a fuel leak from the left engine or tank. Left engine shutdown was therefore accomplished and a MAYDAY diversion to an overweight landing at Goa, India, followed. The Investigation determined that the leak was actually from the right fuel tank and attributed crew misdiagnosis to limited fuel system malfunction checklists and gaps in crew guidance and training on fault diagnosis.
On 31 August 2019, all six occupants of an Airbus AS350 B3 being used for a sightseeing flight in northern Norway were killed after control was suddenly lost and the helicopter impacted the terrain below where the wreckage was immediately consumed by an intense fire. The Investigation found no airworthiness issues which could have led to the accident and concluded that the loss of control had probably been due to servo transparency, a known limitation of the helicopter type. However, it was concluded that it was the absence of a crash-resistant fuel system which had led to the fatalities.
On 27 July 2019, a fuel configuration advisory was annunciated on a Boeing 767-300 about to depart Auckland as a result of wing tank imbalance. Having established there was no evidence of a fuel leak, they planned to correct the imbalance in flight but then delayed this until it had exceeded the permitted limits. The fault was only verbally reported after flight and the aircraft continued to operate without centre tank use with maintenance remaining unaware of the fault for several days. The cause of imbalance was a fuel system fault subject to a crew response which was not followed.
On 9 May 2019, a Cessna 550 level at FL 350 experienced an unexplained left engine rundown to idle and the crew began descent and a diversion to Savannah. When the right engine also began to run down passing 8000 feet, an emergency was declared and the already-planned straight-in approach was successfully accomplished without any engine thrust. The ongoing Investigation has already established that the likely cause was fuel contamination resulting from the inadvertent mixing of a required fuel additive with an unapproved substance known to form deposits which impede fuel flow when they accumulate on critical fuel system components.
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