Carbon Emissions from Planes: I eat my words

A Boeing 747: How much fuel does it use?

At the end of March I will be attending the 9th International Temperature Symposium, a conference that takes place once every 10 years. Sadly the event takes place at DisneyLand, California, roughly 5,500 miles away :-(. I am looking forward to spending a week discussing arcane details of temperature measurement and taking morning coffee in the Magic Kingdom East Foyer, but I am concerned at just how much carbon dioxide I will emit travelling to the conference.

I had worked this out before – and my answer was very roughly that the flight to LA from Heathrow emitted around 2 tonnes of carbon dioxide per passenger. But at the last Protons for Breakfast someone questioned this – and said it was nearer to half that! It took me ages to figure out that the reason we disagreed was simply in our estimation of how much fuel a Boeing 747 uses. I have looked hard to find the answer and… – I was wrong. The flight will ‘only’ emit around 1 tonne of carbon dioxide per passenger each way. Great!

The key information that led me to this conclusion was a post on this chat site that sounded trustworthy – it seemed to be from a 747 crew member.

Fuel burn (planning) 747-200s or 747-300s

  • Taxi/Takeoff : 1 tonne
  • Assuming a heavy aircraft at 377 tonne takeoff weight, it will burn some 15 tonnes to initial cruise.
  • First hours of cruise, expect some 13 tonnes per hour
  • As the plane gets lighter, this will decrease to under 10 tonnes per hour.
  • Descent, approach, landing, generally burn 3 tonnes.
  • Taxi-in will be anywhere from 0.500 to 1 tonne
  • Our company policy is to plan landing at destination with 14 tonnes reserve.
  • The capacity of these airplanes is generally 155 tonnes with the 7 tanks configuration, or 165 tonnes  with 9 tanks…

So for a flight to LA from Heathrow taking roughly 11 hours and 5500 miles. Fuel use is therefore:

  • Taxi/Takeoff : 1 tonne 
  • 15 tonnes to initial cruise.
  • 9 hours of cruising at an average of 11 tonnes per hour = 99 tonnes
  • Descent, approach, landing, 3 tonnes.
  • Taxi-in 1 tonne
  • Total is approximately 120 tonnes 

I had previously assumed that the fuel used was nearer to 200 tonnes. We can work out the carbon dioxide emissions from this much fuel in several stages as follows:

  • Fuel 120 tonnes = 120,000 kg
  • Fuel is kerosene which has an approximate formula C12H26
  • Molecular weight is 12 x 12 + 1 x 26 = 170
  • Carbon fraction is roughly 144/170 ~ 84.71% by weight
  • Mass of Carbon in fuel 101,640 kg
  • Mass of CO2 from carbon in fuel 373,019 kg. This is a factor 44/12 = 3.67 larger than the mass of carbon in the fuel because of the addition of oxygen.
  • Passengers on the plane is typically 350
  • CO2 per passenger 1066 kg, or just over 1 tonne.

After thoughts. Firstly, I had checked that calculation several times, but my error was in the very first step: I >am< an idiot. Secondly, one tonne is still a large amount of carbon dioxide to emit in 11 hours. And thirdly, it’s important to get these things right. If the Protons’s attendee is reading this: sorry.

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