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| Fig. 1: Image of an aircraft. (Source: Wikimedia Commons) |
I love flying: the view, feeling in my stomach, and the in-flight entertainment. The first time I flew was in 2015. Since then, I've been on at least 10 other flights. So, it surprised me then, when it recently occurred to me that I have no idea what powers planes. The first time I sat at a window seat right behind the wing, I wondered how planes worked, how the motion of the lever-like parts of the wing affected the motion, what turbulence was, and especially how safe I was. I wasted no time in looking all these up as soon as I got out of the airport, but I had never wondered what the fuel energy source is for airplanes. We know what powers cars, we know where that comes from and how renewable and environmentally safe these are for the world. Why do we have no conversations (in my experience) about aviation fuels?
All aviation fuels commonly used today are distilled fractions from crude oil. There are two types of aviation fuels: jet fuels, which are made from and similar to the kerosene fraction obtained from crude oils, and avgas-gasoline used in planes. [1] There is also the wide-cut fuel type, which is a mixture of kerosene and gasoline (naptha) type fuels. [1]
Engines that use avgas are different from those that use jet fuels. Jet fuels are used in turbine-based engines whilst avgas is used in piston engines. [1] Today, jet fuels are the more commonly used type of aviation fuel because jet / turbine engines are more efficient than piston engines. In 2010, jet fuels accounted for 8% of transportation fuels used worldwide. [1] There are different types/ grades of jet fuel. The clearest/ most obvious difference between the different grades of jet fuels is the location where it is used. When it comes to the two most common grades: Jet A and Jet A-1, the primary difference is in their freezing points. Jet A fuel which is mostly used in North America has a higher freezing point, -40 degrees Celsius than Jet A-1 fuel which freeze at -47 degrees Celsius. [1] Jet A-1 fuel is used outside North America. Due to its lower freezing point, it is more suitable for longer/ international flights. Wide cut fuel, though not as commonly used, is also known as Jet B fuel, and is used in colder regions of North America. [1]
Despite the earlier notion that jet engines were relatively insensitive to fuel properties, an aviation fuel must fulfill some basic requirements. it must be high energy - this is why aviation fuels are hydrocarbons - specifically, alkanes, alkenes and cycloalkanes. [2] Flights are usually over relatively long distances, meaning planes have to be powered for quite an amount of time, but as we all know from baggage requirements and the exorbitant cost of extra baggage-storage space and amount of weight planes can carry is rather limited, so aviation fuels have to produce a relatively large amount of energy per their weight/volume - between 41,000 kJ/kg to 50,000 kJ/kg. [2] Energy content of aviation fuels is measured both by energy produced per mass-gravimetric and energy produced per volume-volumetric. [1]
Aviation fuels must not be too volatile - high energy hydrocarbons tend to be rather volatile. An aviation fuel that is not volatile enough will not evaporate to form the ignitable vapor needed for combustion but a highly volatile aviation fuel will not only evaporate too quickly at high altitudes but is usually also more flammable; in fact, wide-cut fuels are not commonly used because of their high volatility - this volatility means increased chances of fire during plane crashes and thus decreased chances of plane crash survival. [1,2]
A lower freezing point is also very useful when it comes to aviation fuels because at the commercial jet cruising height of 35,000 ft above sea level, the outside temperature can fall below -50 degrees Celsius. [3]
Aviation fuels must retain their fluidity at low temperatures because the fuel tanks are kept in the wings and must stay fluid enough in the low temperature environment of airplanes to flow from the storage tanks to the engine. Jet fuels also must have good lubricating properties since, they are responsible for the lubrication of some engine parts. [1] Another very important property is that they must not react with the material used to make the storage tanks, they must be non-corrosive. [2] It definitely would be problematic if the aviation fuel corroded the storage tanks and proceeded to leak out, leaving a plane full of people stranded at outrageously high altitudes mid-flight.
Well, there are other answers apart from the obvious one that knowledge is amazing! Aviation fuels affect our entire experience of flying. First and most obvious of all, airplanes cannot run on fumes (on nothing), they need to be powered and the way they are currently powered is with chemical energy stored in aviation fuels. When we fly, we put ourselves in a metal container far, far above the earth trusting that we will arrive safely but how do we know we truly will when we dont know how that flight is even powered?
Secondly, aviation fuels are of key importance in flight safety. Flying safety depends on the safety of the fuels being used. I have already mentioned two examples of this: fuel volatility and chances of plane crash survival and fuel corrosiveness.
Most importantly for me however, aviation fuels affect the pricing of flights. Flying home to West Africa is very expensive from San Francisco and now I know one of the major reasons why. Aviation fuels are expensive and aviation fuel prices directly affect flight ticket prices. When aviation fuel prices go up, so do ticket prices. The effect of fuel prices on airline costs is so much so that between 2004 and 2008, when aviation fuel prices spiked, airlines reported to even flying slower, to save on fuel consumption and be able to offer very slightly cheaper tickets. [4]
The recent literature on aviation fuels is all about environmentally friendly aviation fuels. We have been using basically the same kind of aviation fuel since the first successful turbine engine flight in 1939. As you probably assumed, these fuels are not quite environmentally friendly. As it turns out, flying alone accounts for approximately 5% of the total climate change impact of human activity and the emissions due to flying are directly related to the mass of fuel burnt. [5] The world currently cares a lot about emissions and climate change and aviation fuels are a part of that. If we dont even know what we fly on, how can we know about the climate impacts of aviation fuels?
The good news is that this contraption called the aeroplane - as seen from Fig. 1- that we fly in, does not fly on fumes. We fly on hydrocarbon-based fuels that are distilled fractions from crude oil. These fuels affect our entire experience of flying, though they play a behind-the-scenes role. These fuels are not particularly friendly but the aviation industry is currently exploring more environmentally friendly aviation fuels - and you can find out more about this in my next report.
© Nana Ansuah Peterson. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] S. P. Srivastava and Jenõ Hancsók, Fuels and Fuel-Additives (Wiley, 2014).
[2] J. G. Speight, Handbook of Petroleum Product Analysis: Second Edition (Wiley, 2015).
[3] H. Morris, "Why Do Planes Have to Fly So High?" The Telegraph, 9 Mar 17.
[4] M. S. Ryerson and M. Hansen, "Capturing the Impact of Fuel Price on Jet Aircraft Operating Costs with Leontief Technology and Econometric Models," Transport. Res. C-Emer. 33, 282 (2013).
[5] R. H. Moore et al., "Biofuel Blending Reduces Particle Emissions From Aircraft Engines at Cruise Conditions," Nature 543, 411 (2017).
