Refuelling and Defuelling Risks

Refuelling and Defuelling Risks

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Description

Aircraft refuelling and defuelling are accompanied by attendant hazards which must be managed sufficiently for their mitigation to acceptable levels. The issues are much the same whether the fuel source is a tanker/bowser or a fuel hydrant system. Pressure refuelling is normal for multicrew transport aircraft and business jets, but gravity refuelling of these types may be available as a backup system. The kerosene fuel used by turbine engine aircraft has a higher flash/ignition point than the aviation gasoline used by piston engine aircraft, but there are still potential hazards.

The primary risk is unintended ignition of fuel vapour, which can occur by a single spark. A sufficient quantity of fuel vapour to create a high risk of ignition may result from spillage arising from procedural errors, leaks, aircraft tank venting or failure of pressurized fuel lines or their couplings. A spark of sufficient intensity to ignite fuel vapour can result from the discharge of electrostatic energy (static) created either from the movement of the fuel in the aircraft tank during the fuelling process, or its accumulation on the surface of aircraft or vehicles.

Fuel movement during refuelling or defuelling may lead to the a static charge building up in the fuel. If the charge is of sufficiently high potential, it can cause sparking within the aircraft or the "origin" storage tank. The charge density in the fuel and the possibility of sparks inside the tanks are not affected by bonding. However, the use of static dissipater additives in fuel can contribute materially to reducing the risk involved.

Accumulation of a surface static charge may occur on either an aircraft or its fuelling vehicle under certain conditions. Electrical bonding must be used to eliminate this hazard. Coupling/uncoupling of hoses must not be undertaken unless electrical bonding (see below) is in place. Refuelling should not take place during active electrical storms/thunderstorms in the immediate vicinity of the airport.

Czech Airlines Airbus A310 being fueled in Prague - Source: Kristoferb, 2010 (Wikicommons)

Electrical Bonding

There must be a cable to link to designated points or to clean unpainted metal surfaces on the chosen airframe. Bonding cables should connect the installation delivering the fuel with the aircraft or installation receiving the fuel. All connections should be made before filler caps are removed prior to the start of fuelling and then not broken until fuelling is complete and the filler caps have been replaced. On no account should either the fuelling vehicle (including hydrant dispenser) or the aircraft be bonded to a fuel hydrant pit. It should be noted that fuel hoses, including so-called “conductive” hoses, are not suitable substitutes for dedicated clips and bonding wires.

Static Dissipater Additive

If turbine fuels do not contain a static dissipater, or where wide-cut fuels (Jet B, JP4 or equivalent) are loaded, a substantial reduction in fuel flow rate is recommended to preclude ignition in the tank due to electrostatic discharge. Wide-cut fuel is considered to be involved when it is being supplied or when it is already present in the aircraft tanks. It is recommended that when wide-cut fuel has been used, the next two uplifts of fuel should be treated as though they too were wide-cut. Mixtures of wide-cut and kerosene turbine fuels can result in the air-fuel mixture in the tank being in the combustible range at common ambient temperatures during fuelling.

PED use during refuelling

There is a risk that a PED (personal electronic device) may create or induce a spark of sufficient intensity to ignite fuel vapour released during fuelling but it is extremely remote under normal circumstances. A particular concern is the proliferation of below-specification mobile telephone batteries that have the potential to fail dangerously. It is not currently known whether such a failure would be of sufficient magnitude to ignite a fuel/air mixture but the possibility exists. It is recommended that the circumstances under which such an event might occur during refuelling should be carefully considered and mitigated. It also appears that PEDs close to or on board modern aircraft can interfere with fuel gauges and some navigation equipment and may cause false fire warnings in cargo/baggage holds. Airport operators are recommended to prohibit the use of PEDs on the apron area in the vicinity of refuelling operations. Passengers boarding or disembarking an aircraft other than via an airbridge should be discouraged from using PEDs.

Refuelling with Passengers on Board

Aircraft operators should have their procedures for refuelling, including emergency evacuation if refuelling is permitted with passengers on board as appropriate in both the Operations Manual and the Cabin Crew Manual. Crew stations and duties should be clearly defined as should appropriate passenger communications. Procedures for emergency passenger evacuation should consider availability of airbridge access and in-position ground stairs in the selection of exits to be used. Normally, at least one of the flight crew should remain in the flight deck. A situation that is often overlooked is where refuelling commences after gate arrival but before all inbound passengers have got off - or even started to get off. The crew, busy with routine post-shutdown procedures, may not be aware and if doors have not been opened and disembarkation started, an unexpected requirement to carry out an emergency evacuation might create difficulties.

Defuelling

This is a special and infrequent operation; the personnel involved, including any flight crew, should be careful to refer to the necessary procedural documentation. Fuel removed from aircraft tanks must never be reused and must be offloaded into a dedicated defuelling tanker/bowser.

Accidents and Incidents

The following events on the SKYbrary database involved Fuel Loading:

A333, en-route, near Gaborone Botswana, 2022

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 pumps 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. 

B738, en-route, south of Kalgoorlie Australia, 2021

On 25 October 2021, a Boeing 737-800 had just reached its cruise altitude after takeoff from Perth when a fuel imbalance message was displayed on the system panel. Despite specified indications for a fuel leak as the cause of this message not being met, it was determined that the left engine should be shut down. A ‘PAN’ was declared and a diversion to Kalgoorlie completed. Inspection there found the fuel imbalance was within normal limits and that crew diagnosis of a fuel leak had been flawed. Non-standard closure of the crossfeed valve was suspected as the origin of the imbalance.

B773, Dhaka Bangladesh, 2016

On 7 June 2016, a GE90-115B engined Boeing 777-300 made a high speed rejected takeoff on 3200 metre-long runway 14 at Dhaka after right engine failure was annunciated at 149KCAS - just below V1. Neither crew nor ATC requested a runway inspection and 12 further aircraft movements occurred before it was closed for inspection and recovery of 14 kg of debris. The Investigation found that engine failure had followed Super Absorbent Polymer (SAP) contamination of some of the fuel nozzle valves which caused them to malfunction leading to Low Pressure Turbine (LPT) mechanical damage. The contaminant origin was not identified.

C550, en-route, north of Savannah GA USA, 2019

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.

B772, en-route, northern Indian Ocean, 2014

On 16 April 2014, a pre-flight concern about whether a Boeing 777-200ER about to depart Singapore had been overfuelled was resolved by a manual check but an en-route fuel system alert led to close monitoring of the fuel system. When a divergent discrepancy between the two independent fuel remaining sources became apparent, an uneventful precautionary air turnback was made and overfuelling subsequently confirmed. The Investigation found that a system fault had caused overfuelling and that the manual check carried out to confirm the actual fuel load had failed to detect it because it had been not been performed correctly.

A333, Hong Kong China, 2010

On 13 April 2010, a Cathay Pacific Airbus A330-300 en route from Surabaya to Hong Kong experienced difficulty in controlling engine thrust. As these problems worsened, one engine became unusable and a PAN and then a MAYDAY were declared prior to a successful landing at destination with excessive speed after control of thrust from the remaining engine became impossible. Emergency evacuation followed after reports of a landing gear fire. Salt water contamination of the hydrant fuel system at Surabaya after alterations during airport construction work was found to have led to the appearance of a polymer contaminant in uplifted fuel.

B772, Denver CO USA, 2001

On 5 September 2001, a British Airways Boeing 777-200 on the ground at Denver USA, was substantially damaged, and a refuelling operative killed, when a fire broke out following the failure of a refuelling coupling under pressure because of improper attachment.

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Further Reading

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Ground Operations, Operational Fires

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