Alisdair Clarke, aviation fuels research and development manager.
You probably know there are many types of aviation fuel, but what makes them different from one another? We find out from Alisdair Clarke, our aviation fuels research and development manager, so you can be ready should anyone ask…
Q: What are the differences between Jet A, Jet A-1, and Jet B?
A: The only difference between Jet A and Jet A-1 is the freezing point. It is a maximum of -40 degrees Celsius for Jet A and a maximum of -47 degrees Celsius for Jet A-1. The slightly more lenient specification for Jet A fuel is beneficial during the manufacturing process, which helps us meet the demand for fuel in large markets like the US. Both Jet A and Jet A-1 requirements are detailed in ASTM D1655 Standard Specification for Turbine Fuels.
Jet B requirements are detailed in the ASTM D6615 Standard Specification for Wide-Cut Aviation Turbine Fuels. The term ‘wide-cut’ refers to the volatility of the product, which is in the gasoline range. Compared with Jet A and Jet A-1, this gives Jet B some vapor pressure (14 – 21 kPa), a lower density range (751 – 802 kg/m3), and a lower freezing point (–50 degrees Celsius maximum). The downside is Jet B’s flash point is typically below ambient temperature (the vapor is always flammable) whereas Jet A and Jet A-1 has to reach >= 38C to form a readily flammable vapor.
Q: Where are these fuels used and when would an operator want to use a specific type?
A: Jet A is typically used in the US to meet market demand. Elsewhere, Jet A-1 is often prevalent, usually to the UK Defence Standard 91-091 specification. Russia uses TS-1 and China uses Jet Fuel 3.
Jet B is rarely used these days. In earlier years, its low flash point was beneficial for very cold regions like north Canada, but it has been largely superseded by newer engine technology. This is good news, as the low flash point means the product isn’t as safe as Jet A and A-1.
Q: How does adding Prist to Jet A differ from using Jet B in terms of fuel properties and performance?
A: ‘Prist’ is a trade name for an aviation fuel system icing inhibitor, Diethylene Glycol Monomethyl Ether (DiEGME). This is an approved additive which may be added to Jet fuel (Jet A, A-1 or B) to prevent free water from forming ice and potentially blocking engine filters.
DiEGME is very water soluble and readily combines with any free water present to depress the freezing point of the water phase. It does not significantly alter the freezing point of the Jet fuel, which relates to the hydrocarbons present. As an oxygen-containing chemical, it is very different to the hydrocarbon mixture which forms jet fuels.
DiEGME also acts as a biostat to prevent microbial contamination in fuel systems by making any trace-free water less suitable for growth.
Q: What types of jet fuel are available in different parts of the world? Might I encounter other types? If so, what are they and what would be the operational implications?
A: Four Jet specifications dominate the civil aviation market, these are: ASTM D1655 (Jet A and Jet A-1); Defence Standard 91-091 (Jet A-1); GOST 10227 TS-1 (Russia’s jet fuel grade) and GB 6537 No. 3 Jet Fuel (China’s jet fuel grade). In addition, there can be country specifications, often closely linked to the above, for example CAN/CGSB 3.23 (for Jet A and Jet A-1 in Canada).
Aircraft and engine manufacturers work with IATA to ensure all specifications are appropriate to keep global aviation transport operating. This can be challenging for the more obscure regions of the globe.
Q: How do civilian fuels differ from military jet fuels like JP-4, JP-5 and JP-8? Could I use military fuel in an emergency?
A: There are detailed differences between military and civil fuels, but, very broadly, JP-4 is similar to Jet B fuel plus a military additive package. JP-5 is similar to Jet A-1 but with a maximum freezing point of -46 degrees Celsius, a 60 degrees Celsius minimum flash point and a military additive package. JP-8 is similar to Jet A-1 but with a military additive package.
Aircraft and engine manufacturers set out detailed requirements regarding which fuel can or cannot be used. Some products may be permitted for limited operations and require a specific entry in the aircraft log. This usually results in an early overhaul and potentially added expense. As such, obtaining the correct Grade(s) is important and the industry works 24/7 to maintain supply as best as possible across the globe.