Quote of the Day

The majority of men are bundles of beginnings.

— Ralph Waldo Emerson. I understand this saying well as I have three brothers, and I am the father of two sons.

## Introduction

My youngest son and his wife are going to have a baby girl in November – my first grandchild. Since they live in Montana, I will soon be doing some long-distance traveling. As I am famously cheap, I usually drive the 1000+ mile distance to visit them. I have started to think about flying there because the drive to their home is 16 hours of extreme boredom.

As I looked at the cost of airline tickets versus driving, I became curious as to how much fuel would be used to fly me to a Montana airport like Bozeman or Butte. I was surprised to learn that airliners can be quite fuel efficient compared to cars. This post contains my analysis.

## Background

### Data Sources

I am going to base the airliner portion of this work on long-haul and turbo-prop aircraft data available on the Wikipedia.

The car fuel economy data available from the US Department of Energy. There is fuel economy data available on hundreds of cars – I limited my view to data from Honda and Subaru, my two favorite brands.

This post assumes that all the airliner seats are occupied. Appendix A shows the load factors of various airlines. I will also ignore any energy differences that exist between gasoline and jet fuel – I am only looking at volume of fuel.

### Unit Conversions

Equation 1 shows how to convert between kg per km and L per 100 km using the density of aviation fuel (0.81 gm/cm^{3}). I should note that the Wikipedia page on airliner fuel usage also provides a "mileage" in terms of fuel volume, but it looks like each manufacturer used a different density value. I decided to use an average fuel density that I applied to all aircraft.

Eq. 1 |

## Analysis

### Airliner Fuel Economy Data

Figure 2 shows how are converted the airliner data from kg/km to L per 100 km.

Figure 3 shows my table of long-haul and turbo-prop airliner fuel usage, which range from 2.31 to 6.11 L per 100 km per seat.

### Automobile Fuel Economy

The fuel economy data was given in an Excel workbook, so I just did my unit conversion work using a pivot table and a calculated field. Figure 4 shows a screenshot of my pivot table of Honda and Subaru highway-driving, fuel economy data, which has a range from 5.88 to 10.23 liters per 100 km.

## Conclusion

I found that fully loaded long-haul and turbo-prop airliners have a fuel economy between 2.31 to 6.11 liters per 100 km per seat. My favorite brands of automobiles had fuel economies of between 5.88 to 10.23 liters per 100 km. So many fully loaded airliners use substantially less fuel per km than a car carrying a single passenger.

## Appendix A: Airliner Occupancy Levels

Figure 5 shows the load factor (i.e. percentage of seats occupied) by airlines for various years – it looks like most airlines operate at ~85%. I know that most of my flights are fully occupied, i.e. load factor =100%.

Mark, you've nicely shown the fuel usage per airline seat. I wonder if fuel usage per average airline passenger seat filled might be another useful metric which would provide a higher number for fuel usage as not all seats are filled. According to the below webpage, "Load factors on US domestic flights are at an all-time high, with an average load factor of 80-85%. "

Ref: https://www.quora.com/How-many-empty-seats-are-there-on-the-average-US-domestic-flight

Hi Ronan,

That would be a nice addition! I knew I was assuming 100% utilization and this was not correct. I will update the page later this week.

Thanks!

Mark

Hi Mark,

Interesting estimations. I add a couple of comments though.

On average European cars are more economical than similar size american models. It is mostly due to smaller engines and proliferation of diesels in Europe. Here is my example from our recent holiday. We took our 2009 Citroen C5, which is a large family car but with just 1.6 liter diesel, from the UK to Czech republic and back. The total trip was 2950 km with 5.1 L/100km consumption. This is for 3 people. A similar trip last year with 4 people had 5.4 L/100km consumption. Consumption for more modern diesel cars is at the scale of 4.5 L/100km

My second comment is about the aircraft types you used in your estimations. They are mostly the long distance models. I think the list should be extended with short range models and in particular with turboprops such as for instance Bombardier Dash 8. There are many similar planes operated by airlines at the local level in USA and Canada.

Regards,

Artemi.

I grabbed the long-range ones just because I am most familiar with those. I will try to augment the data with turbo-props as I have time – I certainly have the data.

Thanks for the comments.

Mark

Mark, according to https://en.wikipedia.org/wiki/Gasoline, the average density of automobile gasoline is 0.71–0.77 kg/L or 0.71–0.77 g/cm^3. I believe you assumed 1 g/cm^3 for gasoline.

Hi Ronan,

Actually, you don't need density to change the units of MPG to L/100 km – you just need to compute 1/MPG and then convert the units (Mathcad does the unit conversion automatically. Here is a quick analysis with no density used.

Mark, you are amazing fast at putting info together and with colored arrows. I was thinking of the density factor for your kg/km metric in figure 3. Although as you are comparing liters/100 km for both autos and aircraft, it's probably not needed.