Saturday, March 19, 2011

More on Airplane MPG

Since we don't have EPA labels for planes, it is a bit of mystery how efficient they are.  Theoretically, they seem to have the potential for great efficiency because of the density of passengers. A 747 holds over 400 passengers.  However, lifting a 250 ton steel object into the air and keeping it there is energy intensive. In addition, airplanes have to carry a great deal of extra weight in fuel to power the aircraft.  How does passenger density versus aircraft weight net out?  

 The Wall Street Journal ran an article on airplane MPG last year.  This graphic shows the range of MPG across airlines:

 The Wall Street Journal used a per seat figure for airplanes but a vehicle MPG for cars.  Thus, these airplane MPGs assume a full plane--if the plane were only 80% full (a number reported recently by JetBlue and by Continental), the real MPG per passenger would be reduced by 20%. The car MPGs shown above assume 1 occupant (the driver). Once you add 4 people to the Prius, the equivalent per passenger MPG is 200.  And 6 people in the Suburban gets you to 108 MPG--far superior to the airplanes.  A counterintuitive implication is that, for long trips, families may use less fuel driving a low MPG car than flying together. Of course, if the family owns a Prius, the margin in favor of the car is huge.


I always have conflicting hopes when I board a plane.  I love having an empty seat next to me, but I also want a full plane to boost the MPG. But if the plane is too full, I start thinking about the fact that increased demand (in the aggregate over many flights) will lead the airline to schedule more flights. 

This is an indirect way of raising the "average" versus "marginal" issue in assessing emissions from plane flights.  The WSJ used a per seat figure to calculate MPG.  But if a plane has empty seats, isn't my marginal fuel consumption (and CO2 emissions) zero because the plane was going to fly anyway?  This analysis holds up in a "static analysis" (in the language of economics), but falls apart if you think about airlines responding to demand overtime.  Tufts University has a nice article on the pros and cons of flying that puts it well (the whole article is worth a read if you are interested in this issue):
This does not take into account the issue of marginal versus average carbon emissions. In other words, you could argue that the plane flies anyway, no matter if you are on it or not. Looked at it from this angle, adding one more person to the plane will only very marginally increase the amount of fuel the plane uses. From this view, it is always better to fly than to drive, because you'll avoid the emissions from your car.
Yet this argument is flawed. Planes only fly because there are people who want to get to places. During the Y2K scare around the turn of this century, hundreds of flights were canceled, because people chose not to fly. The consumer drives the supply.
Flying versus Driving

 We can't all be the marginal passenger and "fly for free."  It makes sense to focus on per seat MPG and then shrink by the unoccupied seats. analyzes different modes of transportation adjusted for average occupancy--airplanes and cars end up remarkably close given average occupancy rates.

A more interesting and practical question is deciding between modes of travel for longer trips.  In that case, the wise GHG choice is directly dependent on the number of people you are travelling with.  One travel partner can easily tip your decision to driving.  And driving is better for a family by a wide margin.  This terrapass calculator will let you see theCO2 emissions created when flying between two airports for 1 or more passengers.  If you click "details," you can see comparisons to driving and to taking the train.  (Warning:  The carpool item is confusing. To use this calculator, it appears that, if you are travelling with 2 people, take the CO2 from driving alone and divide it by 2 to get an accurate estimate for carpooling. Same for 3, 4, etc.)
The Future of Flying

What does the future hold for airplane MPG?  Recent history has shown a steady improvement in efficiency (due in part, of course, to competitive pressures and high jet fuel prices).

Here is a collection of stories about future efficiency in airplane fuel consumption:


MIT is designing a plane to replace the 737 that uses 70% less fuel

Projections and solutions

The CAFE Foundation

A plug in plane

What countries account for the most airplane fuel use?  This video is quite revealing.   Look to the skies--they tell the story.