Air travel and energy use

Last February, I attended The Energy Conference 2013: Energy At The Crossroads, which was held by NERI. Over the next month or two, I’m going to talk about some of the work presented at the conference – starting today with a presentation by Robert Vale from Vic University. You can find his Powerpoint slides here, along with the slides used by most of the other speakers.

The aircraft vs. the dove pigeon

Robert’s presentation, poetically titled “Oh for the wings of a dove”: the future of flight in a resource constrained world, looked at the history of passenger aviation over the last century or so, and the likely future. He started off with an interesting analogy: how does the efficiency of modern planes compare with the efficiency of that well-known flying rat, the domestic pigeon?

Pigeon vs plane


It turns out that pigeons have an energy efficiency of about 7.5 kJ/kg/km – that is, it takes 7.5 kilojoules to move one kilogram of bird over a distance of one kilometre. Robert assumes that the average weight of a passenger plus baggage is 90 kg. As such, using the pigeon’s energy efficiency, 0.7 MJ (megajoules) is needed to move a single passenger plus baggage for 1 km.

So, how does that compare with modern aircraft? Modern jets average around 1.5 MJ per “available seat kilometre” – ASKs being a common measure in the aviation industry, and fairly self-explanatory. This is more than twice the energy used by the pigeon! Note that planes aren’t usually full, so actual energy use per passenger would be slightly higher again.

The latest generation aircraft, like the Airbus A380 and Boeing 787 “Dreamliner”, represent another step forward in efficiency – by my rough calculations, about 1.2 MJ per available seat kilometre. But it’ll be some years before these kinds of aircraft are dominant globally.


Aircraft fuel efficiency over time

Robert makes the point that “airliners now are no more efficient, in terms of fuel used per passenger-km travelled, than airliners in the 1950s”. This essentially comes down to the shift from propeller-driven aircraft to jet aircraft – and the higher speeds that have gone along with this shift. The first jet airliners used horrendous amounts of fuel, but they were much faster than propeller aircraft.

Aircraft fuel efficiency over time

Of course, passengers have benefited massively from the switch from propellers to jets. The higher speeds have made air travel much more attractive, and made long-haul international flights much more viable. So no one’s saying that today’s planes aren’t better than those of the ’50s. But it’s interesting that it’s taken jets 50 years to to match propeller aircraft for fuel efficiency. (and, beginning with the A380 and Boeing 787, to beat propeller aircraft for fuel efficiency).

Robert also compares the energy efficiency of the ill-fated Hindenburg airship: at 3.44 MJ/ passenger kilometre, it’s much worse than the other aircraft discussed here – its figure includes hydrogen gas which had to be vented during flight. He sums up with the following slide:

Comparing various aircraft

Note: despite the figure given in the slide above, the ‘average’ fuel consumption for the Boeing 747-400 is 1.4 MJ/ passenger kilometre, as given later in Robert’s presentation

This slide raises another relevant issue, which is that much of the weight carried by aircraft (or road vehicles, for that matter) is essentially dead weight – including the aircraft itself. The ultimate goal of the flight is to move passengers and baggage, and nothing else really matters. This is why airlines keep a close eye on their seat kilometres, and passenger kilometres, and of course their loadings. Vehicles can be made more efficient by upping the percentage of ‘useful’ weight, and this is an important consideration for aircraft and car engineers alike.

42 comments to Air travel and energy use

  • Starnius

    Ironically, the first thing I thought when I saw this is that is why Amazon drone delivery of your package is a stupid thing from a sustainability viewpoint. Helicopters have even worse fuel efficiency than planes, presumably, because there’s no gliding involved, you constantly have to expend energy on lift.

    Wheels on ground, with as little friction as possible (railroad wheels!) remain one of the most energy-efficient means of transport ever.

    • grantb

      I thought about that, but the Amazon drone idea (not that I think it will occur anytime soon) will probably only operate in particular niches.

      Something like a 5-10kg drone carrying a ~2kg payload might be inefficient on some measures, but if the alternative for a task like delivering pizza locally, is to use a 1500kg car + driver which needs to drive several kilometers pausing at intersections, versus a small electric drone that simply flies directly from pizza shop to house and back, then the drone will win on total cost/energy consumed regardless of efficiency per KG/KM figures.

      In the same way, a Airbus 380 might be more efficient on the measures given, but not if you are transporting people back and forth between Devonport and the CBD.

      Railways win on efficiency – when there are rails.

      • Starnius

        “In the same way, a Airbus 380 might be more efficient on the measures given, but not if you are transporting people back and forth between Devonport and the CBD.”

        I don’t know. It it floats, it could be a pretty good ferry. Plus, if we allow walking across the wings, and extend the container terminal a bit further, like Ports of Auckland wants to, then one could walk across. That’s quite efficient, walking.

  • Logan

    We could be on to something here! Let’s talk capacity–how many coconuts can a pigeon carry? (I’m assuming here that he grips it by the husk.)

    • Starnius

      But then the piegon can’t be counted as passenger anymore, he’s the delivery vehicle. That drops the 100% efficiency right there.

      Also, I don’t think you can count this in coconuts per piegeon (CPP). This needs to be counted in piegons per coconut (PPC).

  • Sanctuary

    It’s not a question of where he grips it! It’s a simple question of weight ratios! A five ounce bird could not carry a one pound coconut.

  • Phil

    Where to start really 🙂 first of all a propeller aircraft uses different fuel to a jet engined aircraft so on a calorific value per passenger kilometre the results would differ from your chart.
    Secondly, modern jet aircraft move a lot of freight that the Hindenburg and piston engined aircraft didn’t.
    Third the Comet IV shown in the graph is a jet engine, it is neither piston engined or a modern jet so rather shoddy graphics.
    I think you are mistaken on the influence the 787 will have on the global aviation fleet. Being made of a lot more composites it is far more energy efficient than previous all alloy aircraft. Airlines react very quickly to advances in fuel economy as proven by the success of the 787. That aircraft is more efficient than the Jumbo because it flies straight and level with less drag whilst the uneven weight distribution of a 747 upper deck meant those aircraft always flew at an angle.
    There are also some pretty exciting advances in aviation fuels about to enter into the market which will mean efficiency gains. Maybe the Energy conference 2014 should pay me a tonne of dosh to make a presentation 🙂

    • Ran Derson

      1. Not necessarily – turboprop aircraft also use Jet A1, but have the benefit of better fuel economy. It’s no surprise that Air NZ’s regional fleet is all turboprop – very efficient for those short routes where a jet wouldn’t be more than a few minutes faster.
      2. Good point. The global freight-only aircraft fleet is falling with high freight capacity passenger aircraft such as the B777 and A380.
      3. The Comet IV is pointing to the point of inefficency on the graph and isn’t pointing at the piston engine part of the graph.

    • James

      1. The chart is an index of caloric value (MJ is a measure of energy expenditure) per passenger kilometre; all conversions have been made. How would you graph it differently?
      2. The “success of the 787”? I’m guessing you’re not an Air NZ insider 😉
      3. The point the presenter is making is that a lot of energy is spent transporting the vehicle as opposed to the people/freight. So no matter how “efficient” aviation fuel becomes, that problem will always exist until humans or freight learns to fly long distances.

    • TheBigWheel

      Agree, plenty more to come aviation technology generally.. fuels, engines, aircraft design, flight controls, air traffic control. Efficiency gains all the more valuable of course because of the continuing high cost of oil. I confidently predict the pigeon’s days at the top of the efficiency chart are numbered..

      Good idea to change the topic to Friday afternoon fun to close out the week 🙂

    • nonsense

      you should stick to your car trolling…

  • George D

    I wish he’d done the research and updated his figures. The two aircraft he illustrates the presentation with (an Airbus a340-500 and a 747-400) are both particularly inefficient in comparison to aircraft that entered into service subsequently. An a380 is already ahead of a pigeon at low passenger configurations (at high configurations that efficiency increases). (a comparison of wide-bodied aircraft).

    I will add that the humble pigeon is not optimised for long-distance flight. An invaluable explanation of the dynamics involved comes from Richard de Crespigny, the pilot of QF32 (the near catastrophic a380 engine failure). An albatross or petrel would have better described the theoretical optimum for joules per distance.

    Air New Zealand has almost finished decommissioning its 747s, and would have done so earlier had the 787-9 not been delayed.

  • Piston engines are much less reliable than jets, with higher maintenance costs. If you are going to try to do a reasonable comparison you need to consider all costs, not just fuel.

    • Mike

      This is specifically a comparison of fuel efficiencies: maintenance (or any other) costs are not relevant to this comparison, and would just confuse the issue.

      • grantb

        So I wonder how something like solarimpulse ( this would fit it to the chart?

        Sure, not a lot of cargo carrying capacity or speed (but still better than the pigeon) but effectively zero fuel cost.

        Still, somewhat disappointed with the transport blog posters. Its a Friday afternoon, and nobody has yet asked the all important question.

        What is the air-speed velocity of an unladen swallow?

        • Max

          Before or after you hit it with a coconut?

        • African or European? 😉

        • Eric D

          We have just watched a pair of swallows fledge their second brood of the season outside our kitchen window.
          They fly into the porch at about 30 kph, and pull a 180 turn on a radius not much more than a metre.
          They defacate on the wing as it is said. Not true – they defacate on our window,involuntarily as they are pulling about 9g.
          I suspect they weigh in at about half a kilo on the tight turns.

  • Duncan

    This technology sounds awesome if it all proves right it’ll be by far the biggest advancement in the jet engine since it was created.

    Chief engineer Alan Bond explains how the ability to cool air entering the new “Sabre” engine system by more than 1,000 degrees Celsius in .01 seconds would allow a jet engine to run at higher power than what is possible today.

    More power = more speed. Enough to fly at Mach 5, five times the speed of sound, “pretty easily,” Bond says.

    • Steve D

      The Concorde was a dramatic commercial failure, and the United States canned supersonic spyplanes like the SR-71 on the grounds of being redundant with spy satellites. There’s even less appetite for noise pollution and giant subsidies to aerospace companies today.

      Hypersonic travel is inherently going to be more fuel-hungry than subsonic aircraft, and so more expensive to fly on. There’s clearly just not a big enough niche of wealthy people for whom a door-to-door travel time of even as little as 8 hours is going to be worth paying a premium over 12 or 24 hours. It’s possibly more likely for private jets, but there aren’t any at the moment that are even supersonic.

      If a jet engine like that has any real potential, it’s probably as part of a multi-stage spaceplane like ones Scaled Composites/Virgin Galactic are building (assuming the spaceplanes themselves happen, too).

      • Duncan

        The jet will use 1/20th of the fuel current planes do cause it will be flying approximately 100km up the materials etc will be expensive but they put tickets at current rates for business class travel. All new technologies start out being for the rich until it reaches the mainstream etc, upon reading the article it puts London > Sydney at 4.5 hours, the aircraft is significantly cheaper to transport materials into outer space unlike conventional rocket launches etc. Its amazing tech.

  • Steve B

    Two things. 1., Pigeons fly point-to-point. Not saying your analysis is wrong, it’s just limited to enroute energy consumption. Take that same 90kg passenger (I think that’s a bit low) and get him from home to the airport then from the airport to the office/hotel/tennis court/whatever, and the energy cost goes up even further. The pigeon has none of that extra cost.
    2., The Comet’s energy consumption was probably less than indicated because so many of them never got to their destination.
    (3., Pigeons have excellent flying ability. Next time you see one, watch how incredibly maneuverable and graceful it is. This is common to pigeons of all species, and not all birds are created equal in flying ability, cf. kiwi and kokako.)

    • Point to point?! Every pigeon loft in my last hometown of Geordieland had stacks of them flying round in circles!! Not sure Robert factored in the truck miles required up ship me to the start line either. And as for pollution…..eeew.

  • One thing to consider with the Hindenberg, it was extremely salubrious accomodations. More akin to a oil sheiks private plane than cattle class on Jetstar.

    The passengers as total weight was 3.6% because they were too busy hauling around private cabins, grand pianos and more service staff than paying customers. Flying cruise ship.

    Would be interesting to see what a modern day passenger service dirigible would look like.

    • I don’t think that word means what you think it means and even then the way you’re using it isn’t accurate in this case. The accommodations on the Hindenburg were fairly basic. Mainly due to the need for everything to be extremely light. The cabins were tiny and basic as most people only used them to sleep in.

      The Grand Piano was specially made to be lightweight. The number of crew was similar to the number of passengers. There were about 50 crew and originally 50 berths for passengers although it was later increased to 72 berths. That’s total crew so actual service staff was certainly less than the number of passengers.

      Was definitely an upper class way to travel though!

      • Max

        Awesome link, thank u!

      • Nick R

        The second meaning:

        adjective: salubrious
        health-giving; healthy.
        “odours of far less salubrious origin”
        synonyms: healthy, health-giving, healthful, beneficial, good for one’s health, wholesome, salutary More
        antonyms: unhealthy
        2. (of a place) pleasant; not run-down.
        “an over-priced flat in a none too salubrious area”
        synonyms: pleasant, agreeable, nice, select, upmarket, high-class, leafy, fashionable, expensive, luxurious, grand, fancy; More

        • I’ve only ever seen the first usage but I agree your usage is correct based on the second. It definitely couldn’t be called run down!

  • ‘The new planes allow the airlines to save on fuel, their biggest cost…’

    This retooling of the air fleet is similar to what is happening, slowly but steadily, to cities. Well the smart ones anyway, there is a necessary reaction to the rise in cost (in money, time, and frustration) of movement in our auto-dependent cities and a generally desire by people to live and move in different ways. And just as this is working out well for Boeing and Airbus this is working out well for apartment developers and holders, even bike shops are booming.

    This is a period of change not stasis.

  • john polkinghorne

    Thanks for the python references, chaps! As for the 787 and 380, no those weren’t in the chart above as it was based on slightly older data. Hence my rough calculations, based on lufthansa’s actual experience with the new planes. Still not at pigeon level, i think!

    • John Polkinghorne

      Now I’m back at a proper computer, and in case I want to come back to it later: calculation based on fuel consumption from at around 3.4 litres per 100 passenger km, assuming jet fuel energy content of 35 MJ/ litre, giving 1.2 MJ/ passenger kilometre. Fuel consumption per ASK would actually be a little lower, and depend on what Lufthansa’s loadings are… I didn’t look into those.

  • Peter Olorenshaw

    The Vales figures for Airships are based on 1930’s technology engines and don’t take into account that airships could easily be photovoltaic ally powered so could be zero carbon powered. Even with low efficiency PVs they have a huge surface area to generate electricity and they aren’t using any energy for lift. They no longer need to vent gas with tiltable propellers. OK they only fly at 100-200kph but that is much faster then ships and being vertical takeoff and landing do have potential efficiencies point to point. And of course using inflammable helium, they aren’t going to go up in flames like the Hindenburg. We just need those fusion reactors to give us plentiful supply of helium.

  • Martin

    All in all, for those of us experienced in Aviation (I’m a senior Engineer for British Aerospace), see big holes/inaccuracies in the stats provided and as some have pointed out a Swift or Albatross would make a better comparison then the proverbial flying rat.