Flight Profile and Distance
The civil aircraft fleet average for speed and cruise altitude is 575 miles/hour and 8 miles, respectively (IPCC, 1999). Just as a car experiences different fuel efficiencies at different speeds and under different conditions, aircraft experience different burn rates in various flight profiles: taxi, takeoff, climb, cruise, landing approach, and landing.
The rate of fuel burned is proportional to the drag, the force that resists motion and which therefore has to be balanced by the thrust of the engine. The takeoff phase requires full engine thrust, and thus the most fuel. As the aircraft ascends to higher altitudes, drag decreases and so does the rate of fuel use. Offset calculators have to take into account the variety of flight profiles encountered throughout a given flight.
Flight distance is an essential factor in determining fuel consumption. Generally speaking, the farther the route, the more fuel burned. However, since takeoff and landing demand higher fuel burn rates than level flight, shorter routes where takeoff and landing comprise a larger portion of the overall flight tend to be less efficient (i.e. require more fuel per mile). Takeoff and landing are smaller portions of the overall flight for medium range routes, so they are generally more efficient. In addition, over very long distances the fuel use per mile increases because of the greater amount of fuel that has to be carried during the early stages of flight.
Offset calculators use different methods for calculating total flight distance. Some use only the great circle distance (the shortest distance between two points on the globe) between two airports. Some account for routing and delays. Since an aircraft's route is normally not an exact great circle due to flight path routing, detours around weather, and delays due to traffic, some calculators add an extra amount to the overall distance.
A study calculated the excess distances of flights as the difference between the actual flight path length and the direct route length (great circle distance). Their results indicate that intra-European flights fly around 10 percent excess distance compared to direct routes. In the US, the inefficiency was found to be around 6- 8 percent. The study further determined that about 70 percent of the total excess distance flown takes place within terminal airspace.
Yet the study did not address regional differences. Some airports are heavily congested and experience regular delays, such as those in the New England corridor (Boston to DC), the Chicago/Great Lakes area, or the Southern California corridor. Also, these delays can vary significantly depending on the time of day, the season, and the weather.
Congestion is a serious problem that airports and the communities they serve typically seek to address for social, environmental, and economic reasons. Congestion also leads to increased GHG emissions. Yet calculators cannot realistically incorporate these differences or use real time data. Rather, entities that would prepare a greenhouse gas inventory for a specific airport or aviation in general would capture these conditions. It therefore makes sense for carbon offset calculators to use an averaged multiplier to account for routing and delays.