We're starting a brand-new chapter now: Gas Turbines – Fuels — Page 367, Lesson 462

We're starting a brand-new chapter now: Gas Turbines – Fuels. This is where we get into what actually goes into the tanks of a jet aircraft, and why it behaves the way it does. Let me start with the big picture. The chapter opens by asking what an ideal fuel would look like, whether for a gas turbine or a piston engine. There are eight main requirements, and I want you to hold onto all of them because they define everything we'll discuss. First, ease of flow under all operating conditions — the fuel has to move freely whether it's freezing cold at altitude or hot on the ground. Second, complete combustion under all conditions — we want every drop burned, no waste. Third, high calorific value — that's the energy content per unit of fuel; more energy means more thrust for less weight. Fourth, non-corrosive — it must not eat away at the metal components it touches. Fifth, no damage to the engine from combustion by-products — the stuff left after burning shouldn't harm the engine internals. Sixth, low fire hazard — we don't want a fuel that ignites easily. Seventh, ease of engine starting — it should light off readily. And eighth, lubricity — the fuel itself must provide some lubrication to moving parts like fuel pumps. Now, here's the honest engineering reality: meeting all of these perfectly is prohibitively expensive. So in practice, we make compromises. The fuel we use is a balance of these eight ideals, not a perfect fulfilment of any single one. So what do we actually use? Gas turbine aircraft run on kerosene fuels. There are two main types in common civilian use, and I want you to know their exact names and numbers. The first is JET A1, also called AVTUR, which stands for Aviation Turbine fuel. It's a kerosene-type fuel with a nominal specific gravity — that's SG — of 0.8 at 15°C. Nominal means the typical or stated value. It has a medium flash point of 38.7°C and a waxing point of -50°C. I'll explain flash point and waxing point in a moment, but note those numbers. The second main type is JET A. It's similar to JET A1, but its waxing point is -40°C instead of -50°C. And importantly, JET A is normally only available in the USA. There's also a third fuel mentioned, JET B, also called AVTAG, which stands for Aviation Turbine Gasoline. This one is different — it's a wide-cut gasoline/kerosene mix. Its nominal SG is 0.77 at 15°C, it has a low flash point of -20°C, a wider boiling range than JET A1, and a waxing point of -60°C. Now, JET B can be used as an alternative to JET A1, but here's the safety catch: with that low flash point of -20°C, it's a very flammable fuel. For reasons of safety, it is not generally used in civilian aircraft. So you see the trade-off — JET B has the lowest waxing point, which is good for cold weather, but that low flash point makes it dangerous. Let me make sure you understand flash point and waxing point, because these are professional terms you'll be tested on. Flash point is the lowest temperature at which the fuel gives off enough vapour to ignite in the presence of an ignition source. A low flash point means the fuel is more flammable — it can ignite at colder temperatures. That's why JET B at -20°C is dangerous compared to JET A1 at 38.7°C. Waxing point, on the other hand, is the temperature at which wax crystals begin to form in the fuel. If the fuel waxes, it can clog filters and stop flowing. So a lower waxing point is better for cold operations — JET B at -60°C is best in that regard, JET A1 at -50°C is good, and JET A at -40°C is the least cold-tolerant of the three. Now, one more thing about appearance. Turbine fuels are not dyed. They retain their natural colour, which can range from a straw yellow to completely colourless. That's important for identification — if you see a coloured fuel, it's not a turbine fuel. So to tie it together: we have JET A1, the standard civilian kerosene with SG 0.8, flash point 38.7°C, waxing point -50°C. JET A, the US-only variant with a warmer waxing point of -40°C. And JET B, the wide-cut gasoline mix with SG 0.77, flash point -20°C, and waxing point -60°C — flammable, and therefore avoided in civilian service. The chapter will go on to cover fuel colour, cloudy fuel, additives, water in the fuel, waxing, boiling, and the effects of specific gravity — but those are the foundations we're building on.