There are several stages in the distribution of aviation fuel from the oil refineries to the airport storage tanks, before it reaches the aircraft wing for re-fueling. Depending on the logistics, the fuel is transported by land in large trucks, railway wagons or by ships (tankers) for transoceanic routes and inland waterways. Distributors may have intermediate storage tank farms for longer routes.
Most of the large airports are connected through pipelines to the main receiving storage tanks at the airport terminal. These are connected by underground pipelines to several fixed fuel hydrants on the tarmac in designated places where aircrafts are parked. Mobile refueling vehicles dispense fuel to the aircrafts from the fixed hydrants. They usually have pumps, monitoring and filtering systems to control the fuel being delivered to aircrafts.
It is of paramount importance for aviation fuel to be clean, dry, and as per specification. International standard bodies publish the standards, specifications as well as accepted quality control procedures. They also continuously monitor and audit fuel handling and storage around the world and report non-conformance.
For large commercial jets, the fuel grades are JET-A and JET-A1. While JET-A is used in North America for domestic as well as international routes, JET-A1 is the predominant fuel used in the rest of the world. ASTM (American Society for Testing and Materials), AFWG (Aviation Fuels Working Group) of the IATA (International Air Transport Association) and JIG 3 (Joint Inspection Group) are some of the leading agencies working on standards and specifications.
Aviation Fuel Hazards
On 17 January 2008, a British Airways Flight-BA038 (a Boeing 777-200ER aircraft) from Beijing crashed while landing at the London Heathrow Airport. Investigations revealed that fuel lines had clogged due to ice formation from water present in the fuel and the aircraft’s engines could 4 not respond to the thrust demanded by the auto throttle during landing . On 13th April 2010, a Cathay Pacific Flight CPA780 (an Airbus A330-342 aircraft) made an emergency landing at the Hong Kong International airport due to the loss of engine thrust control. Investigations revealed that contaminations in the fuel lines had clogged the fuel valves, immobilizing them. These contaminants possibly originated from the fuel dispensing unit where the aircraft was last re-fueled4.
Though the above two incidents can possibly be attributed to design and 5 operational flaws (as reported by AAIB5 and the Hong Kong Civil Aviation Department), there is no doubt that any kind of fuel contamination is a highly critical safety situation.
Fuel quality checks are mandated and audited at every stage in the distribution, from the time it is taken from the fuel supplier’s terminal till it is pumped into the aircraft’s fuel tanks. However, despite these quality checks, there is a high possibility of contamination exceeding allowed limits.
Aviation Fuel: Effects of Off-Spec and Contamination
Some of the important properties and characteristics of aviation fuel are its specific gravity, viscosity, freezing point, flash point, calorific value/specific energy, composition (hydrocarbons, sulphur etc.) and maximum allowed contaminant concentrations. Fuels that deviate from the standard specification can cause mishaps. Depending on the extent of the deviation, it also results in suboptimal engine performance such as reduced thrust and efficiency and increased emissions. It can also cause long-term effects on the engine, leading to reduced service life. Fuel contaminations usually result in disastrous consequences. Fuel can be contaminated with water, solid particulate matter and micro-organisms
Water can enter the system due to the water separator or filter malfunction and non-conformance in the distribution, storage and re-fueling procedures. Dispersed water that enters the aircraft fuel system can cause loss of engine thrust or in the worst case, an engine flameout. As per standards, the threshold limit for water in aviation fuel is 30ppm o (at 20 C).
Excess water causes corrosion of the inner walls of the fuel tanks generating rust particles. Prolonged exposure increases the possibility of water ingress into the composite parts of modern aircrafts, endangering its structural properties.