As an alternative, the required energy can be supplied by ground-based equipment that delivers electrical power at 400 Hz and preconditioned air to the aircraft. At Zurich Airport in 1997, about 68,000 aircraft used terminal A and B, which has the facility to provide preconditioned air and 400 Hz power. However, fuel used by APUs is only a relatively small part of the total fuel use of an aircraft. Various scenarios have been discussed and attempted to reduce these emissions. Summary of Other Operational Factors Aviation passenger mobility efficiency is very dependent on the type of aircraft, the configuration, the load factor, and the distance flown.
Old aircraft use much more fuel per passenger-km than new aircraft of similar size.
The fuel efficiency of different aircraft is examined in Chapter 7 . Airlines have generally optimized energy use per passenger-km, largely because of economic pressures and the requirement within the industry to minimize operational costs. Thus, with or without environmental considerations, market and cost considerations are drivers for airlines to optimize the utilization of an aircraft as much as possible. CO 2 intensity for rail transport also depends on factors such as energy source, type of locomotive, and load factor, and emissions of CO 2 range between Other operational factors to reduce aircraft fuel burn include optimization of cruise speed, reduction of tankering, reduction of additional weight, and energy savings at the airport such as limitations on the use of APUs and reduced taxi times. The relative contribution of each factor is indicated in Table 8-6 .