economy passengers; the ratio for first class to economy class is 9.28. Note however that, as for previous calculations, these numbers are not corrected for passenger weight in calculating the footprint. Making such corrections would tend to draw relative footprints somewhat closer together; see Table 3.1 above.

When making average carbon footprint calculations, flight distribution by flight length and aircraft type also matter. Fuel efficiency differs between aircraft, and has generally improved over time so that newer aircraft tend to have higher mileage than older ones.<sup>21</sup> Our calculations also reflect systematic relationships between average flight distance and average fuel consumption per mile flown for given equipment and passenger number. When combined with differences in equipment used we show, in Figures A3.3 and A3.5 in Appendix 3, that fuel consumption tends to be a U-shaped function of average flight distance: Fuel consumption is very high for very short flights; reaches a bottom for medium-length flights (in the range 1000-1500 nm); and with a gradual rise in fuel consumption for longer flights. The reason for such a relationship is that, for very short flights, high fuel consumption during takeoff, and the greater proportion of the entire trip flown at low altitudes (when per-mile fuel consumption is much higher), pulls the average up. For flights longer than about 1500 nm, a factor increasing fuel consumption per distance traveled is the burden of a gradually larger stock of fuel necessary to be carried on the flight. For very long flights, this factor is quite significant.<sup>22</sup>

# **5. Calculations of Carbon Footprints due to Headquarters-Based Air Travel by WBG Staff for CY-09**

# **5.1 Data Applied in 2009**

For headquarters (HQ) based activity of the WBG (including the International Finance Corporation, IFC, the WBG private-sector arm), preliminary calculations of flight records for calendar year 2009 (CY-09) were provided by the American Express (AMEX) travel office. These data consisted of 127,514 records<sup>23</sup> , containing about 189,000 trips by air, covering 447 million miles. The data are limited to trips booked by AMEX in Washington D.C., and do not include travel booked in the WBG’s country offices (CO). The best current estimate is that, overall, HQ-based booking comprises 64% of all WBG air travel in terms of traveled distance.

> 21 One may question the rationale for including such measures in standard footprint calculations. An argument for doing so is that it can give incentives to book flights to a larger degree on efficient aircraft. If many organizations do so, it may give airlines additional incentives to phase out inefficient equipment. This may be a desirable incentive effect when airlines themselves do not face the full global cost of their own emissions.

> 22 For some aircraft, at least or more than half of the total maximal gross aircraft weight is the weight of fuel when tanks are filled up. In particular, the Boeing 747-8 has a maximal take-off weight of 450 tons, of which 230 tons are the fuel carried. This excess weight implies a large “drag” on fuel consumption when tanks need to be filled up from the start of the flight.

> 23 1,685 records representing rail travel found in the Amex data were excluded in the calculations, as well as some pertaining to 2010 travel.

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