A new study ranks the fuel efficiency of 15 US mainline carriers in 2010, and places Alaska Airlines first and Allegiant Air last - with a 26% gap between them. Here is how the 15 airlines stack up:
All graphics: ICCT
The report (here), by the International Council on Clean Transportation (ICCT), attempts to do more than just take the fuel consumption reported by airlines to the US Bureau of Transportation Statistics and simply divide the gallons of fuel burned by the passenger miles flown - as past rankings have done.
Instead it uses a new metric, developed by University of California, Berkeley, which tries to recognize that airlines burn fuel to provide a "transport service" that involves both mobility (passanger miles flown) and access (frequency of service and number of airports served).
How this works is that, on a simple fuel-burn/passenger-mile basis, Virgin America with its coast-to-coast flights to a handful of destinations does disportiately well. But, applying the new metric, Southwest ranks higher because of the better access provided by its higher frequencies to more airports.
The metric also tries to reflect the efficiency of airlines' operating practices by taking into account factors such as the "circuity" of the transport service provided - how many of the miles flown are productive, measured by the ratio between distance from origin to destination and actual distance flown.
What this does is reward carriers for burning fuel to provide better access via higher frequencies to more airports, while penalizing them for wasting fuel to take passengers the long way from A to B by routing them via a hub.
The analysis also - for the first time, ICCT says - attributes the transport service provided and fuel consumed by affiliate airlines to the mainline carriers. Adding in the highly fuel-efficient all-turboprop operations of Horizon Air boosted Alaska from third to first in the rankings, ICCT says.
But why some airlines are more efficient that others is not entirely clear. A third of the variance is attributable to the fuel-efficiency of the aircraft, with new fleets, winglets and other advances giving some airlines an edge. But the rest comes from a range of factors, including how well airlines actually use the fuel-efficiency potential of their aircraft.
The above chart from the report attempts to show that graphically. The chart plots fuel efficiency (vertical axis) against technology utlization (horizontal axis). Not surprisingly Alaksa comes out best at top right and Allegiant worse at bottom left. But it also shows that second-ranking Spirit is way outperforming its technology while second-to-last American is seriously under-performing on what its aircraft are capable of.
One final data point, for which ICCT admits it has no clear explanation - Allegiant may have had the worst fuel efficiency, but it was the most-profitable US airline in the three years to 2010. But the report's authors note Allegiant does do lots of things efficiently, like filling its aircraft and flying point-to-point.