One weird trick to improve airline efficiency - gain belly weight!

Posted Thursday, 22 February 2018, 09:00

In our recent transpacific airline fuel-efficiency ranking, we quantified the passenger-based fuel efficiency of 20 airlines after correcting for cargo carried on passenger flights ("belly freight"). To do so, we used actual freight data reported by each airline to the U.S. Department of Transportation's Bureau of Transportation Statistics (BTS). Belly freight increases the absolute fuel burn of a given flight, but improves the fuel efficiency per unit of mass moved, because the aircraft is operated closer to its maximum payload.

Hainan Airlines and All Nippon Airways (ANA) were the most fuel-efficient airlines on transpacific operations in 2016. However, they achieved the same overall fuel efficiency using very different strategies. Hainan's ranking mostly reflected its very fuel-efficient fleet of aircraft. ANA, in contrast, operated aircraft that use fuel less efficiently, but their planes carried more payload, especially cargo. The Japanese carrier transported approximately three times as much belly freight per passenger as Hainan, equaling 48% of total payload carried.

The study focused only on passenger aircraft and excluded dedicated freighter service. While similar to passenger aircraft in terms of design and technology, dedicated freighters offer an alternative way of moving freight. This raises two questions. First, would our fuel efficiency rankings change if dedicated freighters were taken into account? Second, did the ranking incorrectly reward airlines for carrying belly freight when putting that cargo on a dedicated freighter would have saved more fuel overall?

We decided to test these questions by analyzing the fuel efficiency of four airlines that provide both dedicated freighter operations and passenger service between the same origin and destinations: Asiana, Cathay Pacific, EVA Air, and Korean Air.

Freight carriers have an important operational decision to make: carry more fuel and less cargo in order to increase flight range but sacrifice revenue, or carry more cargo and less fuel to increase revenue but sacrifice range. For Asian carriers, the U.S. government made the decision-making a little easier. In 1996, the U.S. Department of Transportation exempted Alaska's Anchorage International Airport and Fairbanks International Airport from the Jones Act, which prohibits foreign airlines from transporting cargo between U.S. cities. Now, foreign carriers are allowed to refuel their planes in Alaska before continuing on to an American airport or returning home. Because of this, Ted Stevens Anchorage International Airport now handles approximately 80% of all air cargo traffic between Asia and North America.

Cathay Pacific, EVA Air, and Korean Air all fly from Asia to Anchorage first, then on to their destination in the United States. More information on the dedicated freight operations of these four airlines is provided in Tables 1 and 2. Asiana and Korean Air are in direct competition on the routes between Seoul and Chicago, Los Angeles, and New York. Korean Air flies to Anchorage first to refuel and transfer cargo, which adds between 250 and 550 km compared to the great circle distance flown by Asiana on the same route. But because Korean Air is flying to Anchorage first, it can load more cargo onto its planes, as shown by the higher freight load factors in Table 2.

Table 1. Selected transpacific dedicated freight operations, 2016

Airline Origin Destination Flights Great Circle
Distance [km]
Asiana Seoul Chicago 158 10,669 Boeing 747-400F
Los Angeles 433 9,773
New York 89 11,239
Cathay Pacific Hong Kong Anchorage 1,738 8,302 Boeing 747-8F
Anchorage Chicago 172 4,680
Los Angeles 460 3,874
New York 356 5,549
EVA Air Taipei Anchorage 832 7,657 Boeing 747-400F
Anchorage Los Angeles 113 3,874
Korean Air Seoul Anchorage 1,090 6,239 Boeing 747-400ERF,
Boeing 747-8F,
Boeing 777F
Anchorage Atlanta 227 5,599
Chicago 283 4,680
Dallas 149 4,997
Los Angeles 326 3,874
New York 46 5,549

Table 2. Load factors for selected transpacific passenger/cargo and dedicated freight operations, 2016

Airline Origin Destination Passenger/Cargo Dedicated
% Max
Freight LF
Asiana Seoul Chicago 83% 28% 55% 50%
Los Angeles 89% 15% 61% 74%
New York 81% 10% 56% 87%
Hong Kong Chicago 88% 15% 56% 88%
Los Angeles 87% 28% 66% 81%
New York 87% 14% 55% 81%
EVA Air Taipei Los Angeles 84% 27% 67% 76%
Seoul Atlanta 75% 26% 61% 96%
Chicago 77% 35% 64% 97%
Dallas 80% 27% 56% 93%
Los Angeles 74% 17% 58% 92%
New York 72% 18% 56% 91%

LF: load factor %
Max Payload: ratio of total passenger and cargo payload to the maximum structural payload of the aircraft (maximum zero fuel weight - operational empty weight)

The impact of routing strategy on dedicated-freighter fuel efficiency is shown in Table 3. Overall, it is more fuel efficient to move a tonne of payload with dedicated freight operations than with a mixed passenger/cargo service, especially with a stop in Anchorage. For Asiana, dedicated freight operations consumed 4% to 52% less fuel than combined passenger and cargo operations, depending on route, to transport a tonne of payload. Korean Air's dedicated freight operations moved a tonne of payload and burned 51% to 66% less fuel than its combined passenger and cargo operations.

Table 3. Transpacific airline fuel efficiency on selected routes for passenger and freight operations

Airline Origin Destination Fuel Efficiency [tonne-km/L]
      Passenger Freight
Asian Seoul Chicago 3.67 3.82
Los Angeles 3.54 5.72
New York 3.12 6.45
Cathay Pacific Hong Kong Chicago 3.54 8.31
Los Angeles 4.16 8.12
New York 3.45 8.13
EVA Air Taipei Los Angeles 4.74 7.62
Korean Air Seoul Atlanta 3.39 8.55
Chicago 4.20 8.65
Dallas 3.69 8.27
Los Angeles 2.91 8.54
New York 2.94 8.40

On average, freighters are more fuel efficient than passenger aircraft on a mass basis. This means that incorporating those extra aircraft into our ranking would tend to move up those airlines that operate dedicated freighters. (Note that not all airlines do, which is one reason we left them out in the first place.) But what about the separate question: given a choice of putting cargo on a dedicated freighter or as belly freight on a passenger aircraft, which is more environmentally friendly? In either case, increasing payload to aircraft's structural maximum increases the fuel efficiency of each flight.

This effect helps explain ANA's first-place finish on our airline ranking. But it's well known that aircraft are the most carbon-intensive means of moving freight, higher by a factor of ten than trucks and by a factor of one hundred than ships and trains. Aside from fuel efficiency, how would overall fuel use and carbon emissions change as a result of shifting payload between dedicated freighters and passenger aircraft?

We ran a quick sensitivity analysis to determine the difference in fuel efficiency and overall fuel burn if payload was shifted from dedicated freight operations to combined passenger and cargo. As shown in Table 4, shifting 1000 kg of freight from dedicated freighters to passenger aircraft would increase the fuel efficiency of each airline's passenger flights by 2% to 3%. The payload shift has a smaller impact on the fuel efficiency of dedicated freight operations. Shifting 5000 kg of freight to passenger operations would mean even better fuel efficiency, with increases of 9% to 14%, while only decreasing the dedicated freighters' fuel efficiency by 3% to 4%. There is minimal effect on the total fuel burn for one passenger operation and one dedicated freighter operation by shifting freight, as shown in Table 5. For Asiana, the degradation in freighter efficiency is higher because its baseline fuel efficiency is lower because it doesn't stop in Anchorage.

Table 4. Sensitivity analysis of transpacific airline fuel efficiency by shifting freight from dedicated freight operations to passenger operations

Airline Origin Destination 1,000 kg shift 5,000 kg shift
      Passenger Freight Passenger Freight
Asiana Seoul Chicago +3% -2% +14% -10%
Los Angeles +2% -1% +11% -6%
New York +2% -1% +10% -5%
Chicago +3% -1% +14% -3%
Los Angeles +2% -1% +11% -4%
New York +3% -1% +14% -4%
EVA Air Taipei Los Angeles +2% -1% +9% -4%
Seoul Atlanta +2% -1% +12% -3%
Chicago +2% -1% +11% -3%
Dallas +3% -1% +13% -3%
Los Angeles +3% -1% +12% -3%
New York +2% -1% +12% -4%

Table 5. Sensitivity analysis of transpacific airline fuel burn [tonnes] by shifting freight from dedicated freight operations to passenger operations

Airline Origin Destination Fuel burn [tonnes]
      Baseline (no shift) 5,000 kg shift Change
Asiana Seoul Chicago 197.5 197.2 -0.15%
Los Angeles 224.4 224.0 -0.18%
New York 291.4 291.0 -0.13%
Chicago 270.9 271.1 +0.04%
Los Angeles 253.8 253.9 +0.02%
New York 284.7 284.8 +0.01%
EVA Air Taipei Los Angeles 240.8 240.4 -0.16%
Seoul Atlanta 266.7 266.6 -0.03%
Chicago 222.5 222.4 -0.03%
Dallas 227.1 227.0 -0.02%
Los Angeles 238.1 238.0 -0.03%
New York 268.2 268.1 -0.02%

As shown in Table 5, total fuel burn (passenger plus freighter) doesn't change significantly from shifting the maximum 5 tonnes of cargo between the two aircraft types.

So where does that leave us on our second question; that is, were we incorrect in crediting airlines carrying a lot of belly freight with greater fuel efficiency because putting that cargo on a dedicated freighter would have saved fuel after all? Here, thankfully, the answer is no. At the margin, belly and dedicated freight have the same fuel efficiency on a mass basis, which suggests that the main ranking results are robust. That means that a strategy where ANA, for example, decided to shift freight from a passenger aircraft to a dedicated freighter to carry more passengers should produce no material increase in overall emissions.

Belly freight, it would appear, is indeed one weird trick for improving airline efficiency.