It is of critical importance that the fuel taken onboard at uplift is not contaminated in any way since the effects of any such contamination are likely to affect all engines and this may not be evident until after an aircraft has become airborne.
The scope of this article is a review the management of the contamination risk insofar as it arises after delivery of fuel of the correct specification to an airport supplier or to a shared airport storage and distribution system. It does not directly address the importance of training or the technical detail which the essential quality control and quality assurance must be based on. Other risks involved in fuel storage and handling at airports are not considered and the focus is primarily on the integrity of jet fuel - although the issues in general apply equally to aviation gasoline.
The Regulatory Context
In common with one other subject also critical to aircraft operational safety, aircraft ground de/anti icing, the quality of fuel ‘into-plane’ has been omitted from the ICAO system of safety oversight and there are no corresponding SARPs. Some National Regulators have in recent times reacted to this situation by re-iterating the ultimate responsibility of aircraft operators to ensure that they adequately address fuel contamination risk with adequate “Policies, Standards and Procedures” (PSPs) without continuing very far into the presentation of guidance or the specification of requirements. Fortunately, the subject of fuel quality and the prevention of contamination has been the subject of very comprehensive attention from both the supplier and consumer parts of the industry and this activity and the purposes it serves have now been summarised in the first ICAO Document dedicated to fuel quality, "Doc 9977 Manual on Civil Aviation Jet Fuel Supply" (2012). This publication acknowledges that its content does little more than highlight, based on industry assistance, the way that existing non statutory PSPs have acted collectively as a fuel quality management system. Appendix 2 ‘Regulatory Practices in States’ seeks “examples of effective regulatory oversight of the supply chain” from States and says that “on receipt of this information ICAO will consider including information on State regulatory roles in an amendment to, or a future edition of, this Manual”.
Maintaining Fuel Quality
Once refined to specification and delivered as such to an on-airport storage facility, aviation fuel is drained on a daily basis to remove any water which may have resulted from condensation so as to minimise the chances of microbial proliferation. Before being taken from bulk storage and uplifted by an aircraft, fuel is then filtered as least twice to ensure that it is free from particulate matter which could affect aircraft fuel systems and to ensure that any remaining traces of free water are removed.
Large airports, where multiple fuel suppliers are likely be present, have since the 1970s seen the sharing by fuel supply companies of storage tanks and, where installed, hydrant systems. This trend was the direct trigger for the main oil companies to form the “Joint Inspection Group” (JIG) to develop a single set of standards to govern the operation of such shared facilities and ensure that they supported the maintenance of fuel quality. The JIG is a not-for-profit company with over 60 members which defines aviation fuel Standards and operates a quality assurance inspection process which validates their application. JIG 1 covers operating standards for into-plane fuelling services and JIG 2 covers operating standards for airport depots. Both are endorsed by IATA which, along with many new entrants to the aviation fuel supply business who are not also major oil refiners (who are the core JIG membership) is an Associate Member of the Group. Associate Members now form the majority of the JIG membership. Standards JIG 1 and JIG 2 have been complimented by JIG 3 which covers operating standards prior to airport delivery and an additional Standard, JIG 4, which covers operating standards at smaller airports, has recently been issued.
In 2011, JIG Standards were being directly applied to shared facilities at about 180 of the world’s major airports as well as at many smaller airports where Member Companies are the sole suppliers or where the Standards are used as a reference. The result is that an estimated 2500 airports and approximately 40% of the world aviation fuel supply is covered by JIG Standards. The JIG has noted that their standards have effectively become the de facto global standard for aviation fuel quality control.
In recent years, the Energy Institute (EI) has become more involved in aviation fuel standards and in a new joint initiative which now effectively enhances and replaces JIG 3, the EI and the JIG have developed and issued (November 2013) a new EI/JIG Standard 1530 on “Quality Assurance requirements for the manufacture, storage and distribution of aviation fuels to airports”. This comprehensively addresses all aspects of fuel quality upstream of airports and includes both mandatory requirements and best practice guidance. Whilst this new Standard has been prepared in the context of the European legislative and regulatory framework, it is noted that its provisions “can be similarly applied in other countries providing national and local statutory requirements are complied with”. It is broadly compatible with equivalent Standards published by the American Petroleum Institute (API).
Sources of potential fuel contamination
The most common contaminants are particulates, water, other petroleum products or their residues and microbial growth:
- Particulates - despite the increasing use of protective coatings on the interior surfaces of fuel tanks and pipes predominantly made of steel or its alloys, the main source of particulate contamination is rust and scale. The presence of even small quantities of water ensures that almost any distribution process will be the source of some rust contamination. Other sources of particulates include airborne solids that enter through tank vents or slip past the seals of floating roof tanks (dust and pollen), solids entering through from damaged hoses and filters (especially rubber particles and fibres) and solids from microbial infestation (cellular debris and microbial by-products).
- Water - the accumulation of water is almost inevitable in stored aviation fuels even if it has a low water content at airport delivery because of a number of opportunities for moisture to be taken up. These include free water in low spots in a pipeline, rain water leaking past the seals in floating-roof tanks and moist outside air entering the vents of fixed roof storage tanks. Air flowing in and out of a fixed-roof tank as fuel is added or removed may also change the moisture content of air in contact with fuel. The temperature of the fuel itself may change in association with the diurnal variation in outside air temperature or for other reasons so that the chances of water condensing out (or not being absorbed) are altered. Also, if fuel from underground tanks or pipelines is cooled during surface transmission into a fuel bowser or via a fuel dispensing vehicle linked to a hydrant system directly into an aircraft and the fuel involved is near to water saturation, then excess water may condense out.
- Other Petroleum Products - if a batch of aviation fuel is found to have been contaminated with another petroleum product to the extent that the specification requirements are no longer met then there is no remedy and the fuel concerned must be returned to a refinery for reprocessing. However, there are situations where very minor amounts of product mixing may occur without the specification being compromised but even here, if the contaminant is a surfactant , caution is required because the effect can be to degrade water separability. One contaminant which has been encountered in airport supply systems in recent years is Fatty Acid Methyl Ester (FAME) which is most likely to be found when biodiesel fuel has passed through a common unsegregated fuel distribution system.
- Microbial growth - although aviation fuels are sterile when first produced, they inevitably become contaminated with micro-organisms that are omnipresent in both air and water Micro-organisms found in fuels include bacteria and fungi. Solids formed by microbial growth are very effective at plugging fuel filters and some micro-organisms also generate acidic by-products that can accelerate metal corrosion. Since most micro-organisms need (at least) free water to grow, microbial growth is most commonly found at any fuel-water interface that may exist. Higher ambient temperatures favour microbial growth. The most effective way to prevent microbial contamination is by minimising the presence of free water. The use of biocides may sometimes be an option if this type of contamination reaches problem levels, but their use is not necessarily an appropriate or complete response.
- Accidental Contamination - incidents have occurred in which Diesel Exhaust Fluid (DEF) was inadvertently used to service the icing inhibitor tank on a fuel bowser. Refer to the article Diesel Exhaust Fluid (DEF) as a Fuel Contaminant for full details.
Detection and Removal of Contaminants
Three main methods are used sequentially to detect and remove any contaminants. Filtering through a pleated paper or synthetic fibre screen removes particulates, then passage though a filter/separator, water-absorbing media and a salt drier removes water and finally, clay treatment removes any surfactants. One or more of these processes may be used at the inlet or outlet of airport storage tanks and in equipment used to dispense fuel to aircraft.
Control of the fuel contamination risk
Control of fuel quality at airports and ultimately its integrity at uplift by aircraft has historically been achieved by a combination of the fuel supplier standards and methods already discussed and by aircraft operators working cooperatively. From the perspective of aircraft operators, it is almost always the case that refuelling will be achieved by the services of a contractor. This means that the Quality Assurance process which aircraft operators are required to have by aviation safety regulators must cover all such contractors. The basis for audit/inspection is variously JIG 1and the IATA publication “Standard into-plane fuelling procedures”. In the Americas, the ATA publication “Specification 103 - Standard for Jet Fuel Quality Control at Airports” and the ASTM (American Society for Testing and Materials) publication “Aviation Fuel Quality Control Procedures” provide similar technical guidance.
In the face of an absence of specific requirements set by aviation regulators, this has, just as in the similar case of aircraft ground de/anti icing, led to a joint audit system for all refuelling agents used by more than one aircraft operator at a particular airport. In the case of fuel quality, the member organisation is the IATA Fuel Quality Pool (IFQP) which, although supported by the IATA Fuel Technical Group, is not restricted to IATA Members. Whilst motivated by cost savings, the ITQP has crucially proved to be a means to achieving reliable results from inspections which, by definition, must be done by specially trained inspectors. Reports on IFQP inspections are confidential and are only shared amongst participating airlines through a secure website. Observations made following an inspection are only provided to the respective supplier insofar as this is necessary to ensure that corrective action is taken. The scale of the IFQP Programme can be judged by the number of active airline members and their associated airlines (listed as 116 at the beginning of 2014) and the number of trained and accredited inspectors (listed as 156 at the beginning of 2014).
Given that there is very little that an aircraft commander can do ‘in the field’ to assure himself that the fuel being uplifted is to specification and therefore free of contamination, the importance of the airport fuel supplier and installation to aircraft operators cannot be underestimated.
The application of Safety Management System principles to aviation fuel supply means that it is incumbent on both aircraft and airport operators to ensure that they are formally notified in good time of any activity that could generate new or changed fuel hazards, especially a potential for contamination. ICAO note in "Doc 9977" that examples of such changes are:
- major fuel system modification
- a fuel supply system being taken out of service (including intrusive scheduled maintenance)
- new, additional, replacement or modified refuelling equipment
Accidents and Serious Incidents
With such a generally effective industry response to the risk of uplifting contaminated fuel, serious incidents caused by it are rare but the case below shows not only just how important all the various ways of controlling and detecting contamination usually are. It also shows how it was still possible for entirely avoidable primary contamination to overwhelm the filtration process at fuel uplift and lead to the generation of a hazardous secondary contaminant. It was this event that led to the development of the first ICAO Manual on this subject and informed some of its content.
- 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.
- 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.
- A321, vicinity London Gatwick, UK 2020 - On 26 February 2020, after a difficult Airbus A321 left engine first flight of the day start, the same happened on the third sector with en-route engine abnormalities then affecting both engines. With no fault found during post flight maintenance inspections and despite similar engine starting problems, both engines then malfunctioned after takeoff from Gatwick. A MAYDAY return followed. Investigation found that the cause was fuel system contamination by addition of approximately 38 times the correct quantity of biocide during earlier scheduled maintenance and that the release of the aircraft to service for the flight had followed inadequate troubleshooting action.
- ^ Fuel which is “on specification” or “meeting the specification” is defined by the Energy Institute/Joint Inspection Group (see later for an explanation of this organisation) as “meeting the various maximum/minimum limits for fuel property tests and also satisfying all other aspects of the specification such as material composition, approved additives, quality assurance, management of change, cleanliness, traceability, etc.”
- ^ Surfactants can be introduced as a contaminant by adsorption from multi-product pipeline walls and pumping equipment or be introduced from soaps or detergents used in equipment cleaning.
- ^ Microbial contamination in Avgas is much less common than in Jet fuel, possibly due to the toxicity of tetraethyl lead, although it can occur.