Last Update June 12, 2022
During a webinar it became clear Diederik Samsom, the Head of the Cabinet of Euro Commissioner Frans Timmermans, has no recollection of the law of the conservation of energy. Which is a bit strange, given that Diederik studied applied physics for eight years and conservation of energy is a rather fundamental part of physics.
It is also unfortunate, because due to this lack of knowledge, the European Commission continues its push to replace all flights within 500 km with high-speed rail. A plan that will not only cost at least 500 billion euros but will also lead to more instead of less CO2-emissions.
As so often, perception is the problem. Flying over short distances must be bad, so the argument goes, because it takes a lot of energy to bring a heavy object such as an aircraft to a height of eight or ten kilometers. That argument is a standard part of the framing of aviation as the big bad wolf in the world of climate change. However, as is also often the case, this frame is nonsensical and unfortunately policy based on nonsense tends to end badly.
The lack of knowledge became apparent during a webinar about the consequences for aviation of the Fit for 55 program of the EU. Diederik Samsom is intelligent and charming, and the discussion was friendly and respectful. But towards the end, Diederik allowed the participants a revealing insight into his inner thoughts. He stated:
“I do think indeed, one way or the other, that we get to the situation that short-haul journeys are made by transport modes that keep you on the ground. Defying gravity for 500 kilometers, going five miles up, to go 500 kilometers, in a horizontal line ...”
The discussion then turned to another topic, and I continued to stare at my screen flabbergasted.
Indeed, the energy consumption per passenger per kilometer (pkm) over distances shorter than 500 km is somewhat higher, about 20%, than for flights over longer distances with the same aircraft. However, that has nothing to do with the energy required to climb to the flight altitude. Due to the law of conservation of energy, that energy is almost completely recovered during the descent. Anyone who has ever been on a roller coaster knows this from experience.
The Roller Coaster
A set of roller coaster cars does not have an engine. They are hoisted to the highest point of the track and reach their maximum potential energy there. They then start a steep descent, converting that potential energy into kinetic energy, into velocity. Thereafter, through spectacular turns and loops, the conversion of kinetic energy into potential and vice versa takes place again and again. Part of the energy is needed to overcome the drag, both rolling and air, and the height that can be reached therefore becomes lower and lower. Finally, the last remnant of kinetic energy is also converted into heat when upon arrival the brakes are applied.
It works the same with an aircraft, although the descent here is intended to be as unspectacular as possible. When an airplane flies at an altitude of ten kilometers and the power is reduced to idle for the descent, it uses the potential energy to maintain the speed and it can cover at least another 200 km in that descent. It is precisely on the short distances that the potential energy built up during the climb is almost completely recovered.
A bit less is recovered on longer distances. The weight of the fuel that is taken up also represents potential energy, but the weight of the fuel consumed during the cruise flight is removed from the aircraft. The potential energy associated with that fuel can obviously no longer be recovered by the aircraft. So, the shorter the flight, the more completely the energy used for the climb is recovered.
The Air Density
The fact that aircraft use more energy per pkm over short flights than over longer flights is due to another aspect of nature: air density. At a height of ten kilometers, it is only a third of that at sea level. The air resistance is directly proportional to the air density and is therefore also only one third. As a result, only a third of the energy that would be needed at sea level is required. Which is one of the reasons why flying is so efficient.
However, short flights are almost always performed at a somewhat lower altitude, around 8 to 9 km, so with a slightly higher density. Moreover, proportionally to the length of the whole flight, they only fly at that height for a short time and therefore they benefit less from that low density. Together this accounts for the about 20% more energy use per pkm than for longer flights with the same aircraft. Nevertheless, climbing as high as possible is the most energy-efficient strategy for short flights too.
Simple Calculations will do
Knowledge of the laws of physics and the structure of the atmosphere helps to understand why airplanes are so extremely fuel efficient. But you don’t need that knowledge to determine which mode of transport produces the least emissions in a specific situation. For this simple calculations will do. How much energy do you use, how many passengers do you transport, and over what distance? Sometimes high-speed rail wins, but in almost all cases where people go by air, flying is indeed the best choice. See the post ‘HSR-syndrome‘ for more on this.
The question that got stuck in my head is this. How is it possible that someone with the intelligence and education of Diederik Samsom has no apparent knowledge of the law of the conservation of energy and does not feel the need to do some simple calculations either?
The answer may have to do with his first work experience. Diederik cut his teeth working for Greenpeace in 1995 as an activist and campaign manager. That was also the year of the Brent Spar-affair and that affair prompted me to cancel my membership of Greenpeace. Because I think facts matter.
But Diederik apparently embraced another life lesson: perceptions and emotions are far more important than facts and analysis. And I am afraid he is right. Certainly in the short run and in the virtual world of politicians and policy.
My concern, however, is that ultimately facts will determine what happens in the physical world. Then it will again be society that pays for the damage caused by bad policies. In this case, in terms of costs alone, we are talking about at least 500 billion euros. A sizeable budget that could also be used to do sensible things.