Why do passenger planes fly at high altitudes and what does it have to do with the Covid-19 Pandemic?
Concorde
Covid - 19 Virus
Have you wondered why when you are travelling from one city to another, your plane first has to climb out to 35000 feet or about 6 miles above the earth’s surface before it reaches cruising altitude where it finally levels out? It then has to come back to earth only this time it is descending down. Seems like a lot of climbing up and down to get from point A to point B. Well there is a good reason for that – to try and find the sweet spot or a balanced state between two opposing forces, sort of what the country and the world is going through right now with the COVID-19 pandemic - keep everything closed to fully contain the virus and let the economy and people’s livelihood suffer or balance it with well controlled and prescribed openings.
So, what are the two opposing forces for the airplane. They are the total fuel burnt to complete the trip on one side and the power or more correctly the thrust the engine has to produce to complete the mission on the other. We want to minimize both, but Physics comes in the way and we cannot do that simultaneously and hence the need to find a balance, the sweet spot.
As the altitude increases the air becomes less dense and with that the aerodynamic drag it exerts on the airplane as it travels through it. For the airplane to maintain its speed, engines need to supply a countervailing thrust force that cancels the resistive drag and keeps the airplane cruising at about constant speed, which for most passenger jets today is around 550 miles per hour or in aviation parlance about 0.85 Mach, just a touch below the sound speed. Causally related to thrust is the fuel burn – the higher the thrust the engine has to produce, the more fuel it needs and vice-versa. Thus, flying at higher altitude reduces drag and in turn the amount of fuel burnt.
So, the next logical question is what limits the altitude? Why would the planes not fly at higher altitudes still? It is because aero engines need air to produce thrust and as the air becomes thinner, good for reducing drag, it also decreases the thrust producing capability of the engine. The engine size thus has to increase to overcome the drag which then increases the amount of fuel it burns – setting up the need to balance between reducing drag and increasing engine size, the sweet spot.
For most commercial airliners flying at speeds below the sound speed or subsonic, altitudes around 35000 feet comes close to optimum for minimizing the amount of fuel burnt. However, if the airplane speed were to increase to sonic and beyond, as was the case with the only commercial supersonic jet – the Concorde, the optimum altitude increases. Concorde which flew around Mach 2.0, more than twice as fast as the Boeing 747, cruised at altitudes close to 60000 feet. The higher altitude helped reduce the Concorde’s drag but still burnt almost five times as much fuel per person per mile as the 747. That is because drag increases by the square of the airplane speed. So, the drag for the 2 times faster plane would be 4 times higher along with the fuel it consumes. No wonder Concorde was the mode of transport either for the ultra-rich or for those on hefty expense allowance and could not remain economically viable for long.
Finding the balance for an engineering problem was hard but solvable. The search for that illusive sweet spot for the pandemic continues and may be much harder to find given we are dealing with one of the biggest social conundrums of our times which does not follow the well-established laws of motion and thermodynamics. Let us hope we find that sweet spot for COVID-19 and find it soon…
