Alaska Airlines could lose its position as the most fuel-efficient US carrier on domestic routes as a result of its merger with the country's least efficient airline, Virgin America, according to a recent study carried out by the International Council on Clean Transportation (ICCT).

In the latest edition of its annual report on US domestic airline fuel efficiency, the ICCT ranks Alaska Airlines in first place for the seventh year running. However, the carrier could lose its crown in next year's survey if it allows perceived inefficiencies at Virgin America – which the ICCT ranked last out of a total of 12 carriers – to drag it down.

"I'd expect Alaska's fuel efficiency to go down when it merges with Virgin America, but how much depends on how it integrates Virgin's operations and planes," ICCT programme director for aviation Dan Rutherford, a co-author of the report, tells FlightGlobal.

"We ran a sensitivity analysis of this a while back. It suggests that Alaska would have lost its fuel efficiency advantage over Frontier, Spirit and Southwest in 2016 if the merger had gone ahead then. But Alaska's future efficiency will depend on how it actually operates Virgin's leased [Airbus] A320s, and also for how long."

The ICCT uses what it describes as a "deterministic frontier model" to evaluate the fuel efficiency of the airlines listed in its report. This approach benchmarks carriers using a fuel per transport service metric, based on data reported by airlines to the US Department of Transportation's Bureau of Transportation Statistics (BTS). It says this approach "allows for the comparison of airlines, irrespective of business model, in a fair and transparent manner".

According to the study, Alaska had a fuel efficiency score of 1.13 in 2016 (the industry average being 1.00). With a score of 0.90, Virgin America burned 26% more fuel per unit transport service than Alaska, a wider gap than the 25% difference recorded in 2014, when Virgin America was also ranked as the least-efficient US domestic airline.

Virgin America's bottom-of-the-table ranking "may be due to the carrier retaining a relatively static business model while its competitors adopt strategies to increase their revenue and, correspondingly, their fuel efficiency on a passenger-mile basis", the ICCT says in its report. It says Virgin America operated its A320s up to 15% less efficiently than number two-ranked Frontier, "in part because the average Virgin America flight carried 30-plus fewer passengers".

While US carriers increased their average seating densities by 4% between 2010 and 2016, the report says Virgin America's average seating density increased by just 1%, and the carrier "continues to use single-aisle Airbus A320 aircraft on cross-country routes on which other carriers would generally use larger aircraft".

Increased demand and the resulting growth in revenue passenger-miles (RPMs) is outstripping any efficiency gains made by US carriers, according to the ICCT. This, it says, does not bode well for the industry's carbon-neutral growth aspirations. The report points out that while the average fuel efficiency of US domestic carriers improved by 3% between 2014 and 2016, domestic RPMs increased by 10% and fuel use was up 7%.

"This sharp increase raises questions about how the US aviation industry can meet its goal of carbon-neutral growth from 2020 without further policy intervention," the report says.

However, it adds that the continued expansion of ultra-low-cost carriers such as Spirit "may have large implications for fleet-wide efficiency", while the introduction of more efficient aircraft such as the Airbus A320neo, Boeing 737 Max and Bombardier CSeries "should influence the efficiency of early adopters".

Rutherford's hope is that passengers will use the results of the survey to vote with their wallets and reward the most efficient carriers with their custom. "We certainly would like to see passengers use the ranking to reward less polluting airlines with their business. That would enable an overall race to the top in terms of fuel efficiency," he says.

"Results will vary from flight to flight, though, so what is ultimately needed is carbon footprinting provided to the consumer at the time of ticket purchase at the level of individual flights or itineraries. We're thinking more now about how to get that in place."

Alaska Airlines did not respond to a request for comment.