Edited by Saurish Kapoor.
An introduction to the intelligent systems that run a commercial aircraft, and what the future brings for environmentally friendly air travel
Ever since the renowned flight of the Wright brothers on December 17, 1903, air travel has consistently achieved numerous milestones in the advancement of aircraft, whether it be through the modification of the exterior to enhance aerodynamics or the modification of instruments in the flight deck [1] to facilitate in improving pilots' tasks. Internationally recognized commercial aircraft manufacturers, such as Airbus and Boeing, have been at the forefront of developing new ideas and implementing ways to substantially improve aircraft – not only by improving flight controls but also by being more environmentally friendly. The burning of aviation fuel is infamously known for severely damaging the surrounding environment and not being sustainable. Over time, aviation fuel can also cause detrimental damage to Earth and the ozone layer due to CO₂ pollution. As a matter of fact, big aircraft such as the “Jumbo Jet” (E.g. Boeing 747-400) burns approximately “10 to 11 tonnes of fuel an hour when cruising – this equates to roughly 1 gallon (4.55 liters) of fuel every second.” To put this into perspective, a car, on average, burns “a quarter of a gallon of gas for every fifteen minutes you idle²,” hence meaning that a car takes about 1 hour to burn that same gallon of fuel. Excluding cruising, which is usually the flight process that takes the least amount of fuel per minute due to the engines being idle, issues arise when trying to accurately calculate exactly how we can avoid this problem since many factors come into play. These are mainly composed of on-ground scenarios, such as when planes may be waiting for long periods of time when taxiing, or having to taxi long distances before reaching the assigned runway. However, when there is an unlikely weight-related emergency, dumping fuel from the aircraft’s fairings [2] contributes to the factor of artificially caused environmental damage.
Figure 1: A diagram showing the primary compounds that undergo combustion and the after-effects – provided by https://www.researchgate.net/figure/Schematic-representation-of-emissions-from-aircraft-combustion-and-their-potential_fig1_312193345 To understand how the burning of aircraft fuel differs from standard road vehicles, we need to look at the aircraft combustion chamber, situated within the engines; a complex process and framework enables the engines to create power. Well known by many aviation enthusiasts, the simplest way to describe combustion in 4 steps is “Suck, Squeeze, Bang, Blow.” In other words, firstly, an engine goes through the process of intake, where it takes in air from outside (the engine’s rapidly spinning turbines act as a suction here). Due to the fact that gases can be compressed, the engine proceeds to do just that, in order to obtain an appropriate density for the next two processes. Following this, the engine combusts the air, leading to a massive build-up of heat and ultimately releasing the air in the exhaust, as properties of air change with an increase in altitude (such as temperature, t, and density, ρ), the engine can no longer operate optimally, which is why aircraft have maximum operating altitudes (MOA). Another reason for MOAs is cabin pressure – since going too high would cause a lack of oxygen for passengers – combat jets avert this since pilots have oxygen masks on at all times.
Figure 2: A cross-section of how an engine creates power: source is https://www.quora.com/How-does-an-airplane-turbine-work Formulae can also be applied to calculate the uses of aircraft fuel such as maximum flight time, distances, etc. As an example, to calculate the fuel flow rate of an aircraft, we can use the formula:
mf = Fuel Mass Flow Rate fc = Fuel Consumption
F = Net Thrust With regard to this, we can also calculate maximum flight time using mf :
tmax = Maximum Flight Time mf = Fuel Mass Flow Rate
M = Total Fuel Load As an example, we could use the data from the newest commercial aircraft: the A350-1000 XWB. When cruising at Mach ≈0.8, the fuel flow rate of this aircraft (at a weight of 571,000 lb) is “6.8t (15,000 lb / 6803 kg) per hour ³”, whilst it was carrying approximately, 250200 lb (ca. 113,489 kg) of fuel:
This is roughly equal to 9000+ nautical miles worth of flight (16688 km).
As time progresses, aircraft manufacturers are trying to create a plane with optimal fuel efficiency, so it can fly 24 hours reliably, and carry 300, or more, passengers, which will be a new milestone for aviation. There are other models of this aircraft such as A350-900 ULR / XWB, therefore results can vary.
Figure 3: A front profile view of the Lufthansa Airbus A350-900 / A359, obtained from https://wallpaperaccess.com/airbus-a350 I previously mentioned that cruising takes up the least amount of fuel, but what takes up the most? I’m sure that the first idea that comes to mind is takeoff and climb to cruising altitude – Due to the fact that maximum engine power is used to accelerate down the runway. Fuel is marginally saved during takeoff since pilots engage flaps [2], contrary to what aviators thought prior to the 21st century. What may surprise you is that landing is also one of the most fuel-heavy parts of a flight, due to a process called “reverse thrust.” Reverse thrust is a mechanism in the engine that, when engaged, serves the opposite function of forward thrust, as the name implies, meaning that all thrust is now facing the direction contrary to the nose of the aircraft. This enables an aircraft to slow down on the runway, in addition to braking devices like wheel brakes and spoilers [3]. The process can sometimes use even more fuel than forward thrust, primarily in larger or heavier aircraft, because they would need more power to successfully stop. On the other hand, occasionally, reverse thrust can be so powerful that an aircraft manages to fully reverse, reaching speeds of up to 15 knots, which is roughly the same speed of taxiing, but backward!
So we’ve looked at the issue, applications for formulae in regard to fuel efficiency, and a few other elements, but what are the solutions?
Though it has been proven that a fully renewable energy-based seaplane can take off, moving to a fully renewable engine source is highly unlikely at this stage and remains merely an intention or idea.⁴ However, simple measures such as using GPU rather than engine power [4] when pushback [5] occurs can undeniably save massive amounts of fuel and carbon emissions. Some airlines choose to use one engine when taxiing rather than two. In this case, both the environment and airports/airlines benefit since they can save fuel, especially taking into consideration that the fuels used are finite.
As previously mentioned, engineers are experimenting with more fuel-efficient designs, including the creation of winglets that optimise aerodynamic performance (L/D ratio) [6], the abandonment of 4-engine aircraft, the maximisation of passenger and freight capacity, and the exploration of alternative, environmentally-friendly methods to power the cabin. Additionally, making an aircraft as lightweight as possible whilst simultaneously remaining durable will definitely help. Apart from working with planes, organisations, for instance, the International Air Transport Association (IATA) and the International Civil Aviation Organisation (ICAO), are attempting to find optimal flight routes/plans from one place to another. This will certainly be a challenge for all planes to follow due to the fact that thousands take flight daily; as quoted “doing some rough maths based on that estimate, it's likely that there are anywhere between 7,782 and 8,755 commercial planes in the air on average at any given time these days”. ⁵
Perhaps the present feels like there are many boundaries for electric aeroplanes, but in a matter of time, similar to other forms of transport, ideas that appear unrealistic can still become reality. The Concorde, infamously known for its retirement, unsustainable nature, and expensive ticket price, is still regarded today as the milestone in aviation that it achieved. Currently, similar, more eco-friendly concepts are being developed to enable travel at higher speeds, to allow passengers to reach their desired designated, much quicker than ever before. Nevertheless, humans took a century to come this far, and there’s no doubt that as time progresses, this timeframe will only be halved before a new era of aeronautical technology arises.
Glossary (subscripts):
Flight deck – An alternative term for the cockpit of an aircraft; the area where pilots fly a plane
Fairings – Canoe-shaped placements under the wing that help stabilise flaps, slats (hinged panels on the wing, front wing for slats, back wing for flaps), and moderate wing flex (the process of the wings bending upwards to generate lift) – usually 3 to 5 of these are placed under each wing depending on the size of an aircraft
Spoilers – Placements of rectangular-shaped brakes on the wing that move upward when engaged during landing which redirects the air hence slowing the plane
GPU – Acronym for Ground Power Unit; powers air conditioners and other cabin features when a plane is parked at a gate with engines shut off; it does use fuel but significantly less than engines
Pushback – The process of a tow truck pushing a parked plane out of its respective gate
L/D ratio – The ratio between lift and drag of an aircraft
References (superscripts):
How Much Fuel does the Jumbo Jet Burn? | FlightDeckFriend.com. (n.d.). Flight Deck Friend. Retrieved August 20, 2022, from https://www.flightdeckfriend.com/ask-a-pilot/how-much-fuel-does-a-jumbo-jet-burn/
Curious About How Much Gas Is Used While Idling Car. (2011, January 31). CBS News. Retrieved August 20, 2022, from https://www.cbsnews.com/boston/news/curious-about-how-much-gas-is-used-while-idling-car/
Airbus A350. (n.d.). Wikipedia. Retrieved August 20, 2022, from https://en.wikipedia.org/wiki/Airbus_A350
Business Insider. (2021, April 9). Electric Planes Are the Future of Aviation, but They Haven't Taken Off. Business Insider. Retrieved August 20, 2022, from https://www.businessinsider.com/electric-planes-future-of-aviation-problems-regulations-2020-3
Waldek, S. (2022, March 16). Here's How Many Planes Are in the Air at Any Given Moment. Travel + Leisure. Retrieved August 20, 2022, from https://www.travelandleisure.com/airlines-airports/number-of-planes-in-air
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