# 5 Planes

Imagine that you make one intercontinental trip per year by plane. How much energy does that cost?

A Boeing 747-400 1 with 240 000 litres of fuel carries 416 passengers about 8 800 miles (14 200 km). And fuelâ€™s calorific value is 10 kWh per litre. (We learned that in Chapter 3.) So the energy cost of one full-distance roundtrip on such a plane, if divided equally among the passengers, is

[\begin{matrix} {\frac{\text{2\ Ă—\ 240\ 000\ litre}}{\text{416\ passengers}} \times \text{10\ kWh/litre}} \ {\simeq \text{12\ 000\ kWh\ per\ passenger}} \ \end{matrix}]

If you make one such trip per year, then your average energy consumption per day is

[\frac{\text{12\ 000\ kWh}}{\text{365\ days}} \simeq \text{33\ kWh/day}]

14 200 km is a little further than London to Cape Town (10 000 km) and London to Los Angeles (9000km), so I think weâ€™ve slightly overestimated the distance of a typical long-range intercontinental trip; but weâ€™ve also overestimated the fullness of the plane, and the energy cost per person is more if the planeâ€™s not full. Scaling down by 10 000 km/14 200 km to get an estimate for Cape Town, then up again by 100/80 to allow for the planeâ€™s being 80% full, we arrive at 29 kWh per day. For ease of memorization, Iâ€™ll round this up to 30 kWh per day.

Figure 5.1. Taking one intercontinental trip per year uses about 30 kWh per day.

Letâ€™s make clear what this means. Flying once per year has an energy cost slightly bigger than leaving a 1 kW electric fire on, non-stop, 24 hours a day, all year.

Figure 5.2. Bombardier Q400 NextGen [www.q400.com](http://www.q400nextgen.com)

Just as Chapter 3, in which we estimated consumption by cars, was accompanied by Chapter A, offering a model of where the energy goes in cars, this chapterâ€™s technical partner (Chapter C), discusses where the energy goes in planes. Chapter C allows us to answer questions such as â€śwould air travel consume significantly less energy if we travelled in slower planes?â€ť The answer is no: in contrast to wheeled vehicles, which can get more efficient the slower they go, planes are already almost as energy-efficient as they could possibly be. Planes unavoidably have to use energy for two reasons: they have to throw air down in order to stay up, and they need energy to overcome air resistance. No redesign of a plane is going to radically improve its efficiency. 2 A 10% improvement? Yes, possible. A doubling of efficiency? Iâ€™d eat my complimentary socks.

## Queries

Energy per distance (kWh per 100 p-km)

Car (4 occupants)

20

Ryanairâ€™s planes, year 2007

37

Bombardier Q400, full

38

747, full

42

747, 80% full

53

Ryanairâ€™s planes, year 2000

73

Car (1 occupant)

80

Table 5.3. Passenger transport efficiencies, expressed as energy required per 100 passenger-km.

#### Arenâ€™t turboprop aircraft far more energy-efficient?

No. The â€ścomfortably greenerâ€ť Bombardier Q400 NextGen, â€śthe most technologically advanced turboprop in the world,â€ť according to its manufacturers [www.q400.com], uses 3.81 litres per 100 passenger-km (at a cruise speed of 667 km/h), which is an energy cost of 38 kWh per 100 p-km. The full 747 has an energy cost of 42 kWh per 100 p-km. So both planes are twice as fuel-efficient as a single-occupancy car. (The car Iâ€™m assuming here is the average European car that we discussed in Chapter 3.)

#### Is flying extra-bad for climate change in some way?

Yes, thatâ€™s the expertsâ€™ view, though uncertainty remains about this topic [3fbufz]. Flying creates other greenhouse gases in addition to CO2, such as water and ozone, and indirect greenhouse gases, such as nitrous oxides. If you want to estimate your carbon footprint in tons of CO2equivalent, then you should take the actual CO2 emissions of your flights and bump them up two- or three-fold. This bookâ€™s diagrams donâ€™t include that multiplier because here we are focusing on our energy balance sheet.

The best thing we can do with environmentalists is shoot them.

Michael Oâ€™Leary, CEO of Ryanair [3asmgy]