On the roads, refuelling involves minimal choice – there’ll be two or three pumps if you’re lucky. In aviation it’s another matter: a whole range of factors may determine the most suitable fuel. As well as the type of aircraft, engine, air speed and altitude, there are factors like storage conditions, temperature tolerance, pumpability, safety, emissions and renewability.
Several bodies define standard fuel blends, but these can be further blended or improved with additives. However, when one element of a fuel specification is changed, other characteristics may be affected, so fuel formulation is a complex safety critical operation. Suppliers like Coryton excel in this specialist area.
The International Civil Aviation Organisation (ICAO) defines several basic fuels, including Jet A and Jet A-1. These are common choices for gas turbines and based on kerosene – which we used to call paraffin. They are much safer than the diesel or petrol fuels (aviation spirit) used in most piston and other compression engines. A high flashpoint makes them safe to store on the ground. Good cold tolerance keeps them pumping through northerly blizzards or at high altitude.
British civilian aircraft often use a high-performance alternative called DEF STAN 91-91, and Jet B is another kerosene fuel designed for colder climates like Canada and Alaska.
Jet fuel suppliers in Russia and central Asia conform to a different set of standards called GOST (GOsudarstvennyy STandart). These impose much the same requirements as the ICAO. Under GOST, the standard fuel for subsonic jets is TS-1.
Flights across central Asia are often long and subject to extremes of climate, so TS-1 is a little more volatile. This means it flows well at even colder temperatures, both in the air or during ground storage. Its volatility also makes it combust at higher and more efficient temperatures, maximising flight times and range.
Aviators often need good figures describing how a fuel will behave in a particular jet on a particular route in particular conditions. Like blending, accurately measuring a fuel’s performance requires great expertise, as it depends on the engine, air speed, temperature, pressure and many other variables. Suppliers like Coryton control test environments scrupulously in order to deliver tailored fuels and tailored performance predictions.
If an additive with a desirable characteristic is found to cause other problems, the negative effect can often be controlled by others. Since an additive that improves combustion might negatively affect viscosity, pumpability is an important characteristic to monitor. A fuel that becomes hard to pump, under any combination of conditions, could stall the aircraft.
Environmental pressures are another major consideration for airfields and flight operators. Today’s jet fuel suppliers are increasingly adept at blending low-emission fuels from renewable sources to keep neighbours, regulators and environmental lobbies happy.
While aviation fuel suppliers can deliver consistently in bulk, they can also tailor a fuel supply to very specific customer requirements.