Aircraft Fuel Systems — Page 316, Lesson 389

I want to walk you through the start of the fuel systems chapter, and the very first thing we have to get straight is what makes a good fuel in the first place. The book opens by giving us the specification of an ideal fuel, whether it's for a gas turbine engine or a piston engine. So let's list those main requirements, because they drive everything else in this chapter. First, ease of flow under all operating conditions. That means the fuel has to move freely whether the engine is cold at start-up or hot in flight. Second, complete combustion under all conditions — we want every drop of fuel to burn fully, no matter what the engine is doing. Third, high calorific value. Calorific value is simply the amount of heat energy you get out of a given amount of fuel when it burns; the higher it is, the more energy per unit of fuel. Fourth, non-corrosive — the fuel must not attack or eat away at the metal components it touches. Fifth, no damage to the engine from combustion by-products. When fuel burns, it leaves residues and gases, and those must not harm the engine internals. Sixth, low fire hazard — we obviously want a fuel that is as safe as possible to handle and store. Seventh, ease of engine starting. And eighth, lubricity. Lubricity is the ability of the fuel to lubricate the moving parts it comes into contact with, particularly in fuel pumps and injectors. Now, here's the honest engineering reality: meeting all of these requirements perfectly is prohibitively expensive. So in practice, compromises have to be made. No real fuel is ideal on every one of these points; we accept trade-offs to get a fuel that is good enough and affordable. Now let's move to the fuels themselves, starting with piston engine fuels. Piston engined aircraft use gasoline fuels grouped under the title AVGAS, which stands for aviation gasoline. AVGAS is not just ordinary petrol; it is manufactured to conform with exacting specifications issued by the Directorate of Engine Research and Development, abbreviated DERD. The specification number for gasoline is DERD 2485. So when you see DERD 2485, that is the governing standard for aviation gasoline. The octane rating of the fuel is specified with the grade. For example, AVGAS 100 is a 100 octane fuel. Octane rating is a measure of the fuel's resistance to detonation — that is, its resistance to burning explosively instead of smoothly. Higher octane fuels are used with high performance engines having high compression ratios. A high compression ratio squeezes the air-fuel mixture more, which generates more power but also more tendency to detonate, so it needs a higher octane fuel to resist that. Now let's look at the most popular grades of AVGAS readily available today. There's AVGAS 100LL, which has a performance number of 100/130, it is coloured blue, has a specific gravity of 0.72, and it is Low Lead. Then there's AVGAS 100, also 100/130 performance number, but it is green, specific gravity 0.72, and it is High Lead. And there's AVGAS 115, performance number 115/145, green, specific gravity 0.72, and High Lead. Let me unpack a couple of those terms. The performance number, like 100/130, is a two-number rating. The first number is the lean mixture rating and the second is the rich mixture rating — the rich mixture allows more power without detonation. Specific gravity is the density of the fuel relative to water; water has a specific gravity of 1.0, so 0.72 means AVGAS is about 72 percent the density of water — it's lighter than water. And the lead content: tetraethyl lead is an additive that boosts octane, and Low Lead versus High Lead tells you how much of it is in the fuel. Now, an important note here: although AVGAS 100 and AVGAS 100LL have the same 100/130 performance number, they are easily distinguished by their colour. AVGAS 100 is green, while AVGAS 100LL is blue. That colour coding is a critical safety feature — you must never mix them up, because the lead content differs. Finally, there's MOGAS, which stands for motor gasoline — that's essentially automotive petrol. MOGAS can sometimes be used in certain airframe engine combinations, but only under the conditions specified in CAP 747 GC2, and that's because of its low octane rating. The low octane makes it unsuitable for many aircraft engines. Also, because of its higher volatility, carburettor icing and vapour locking are much more likely. Volatility is how readily the fuel evaporates; a more volatile fuel evaporates more easily, which can cause ice to form in the carburettor and vapour bubbles to form in the fuel lines — that's vapour locking. Information on the use of MOGAS can also be found in the CAA Safety Sense leaflet number 4a. So to tie it together: we have an ideal fuel specification, we have AVGAS in its grades with their colours and densities, and we have MOGAS with its restrictions. That's the foundation of the fuel system chapter.