Infrastructure characteristics also impact efficiency. In particular, delays on the ground and in the air can increase energy intensity. Extra fuel is burned on the ground during various non-flight operations, and hours spent in the air (airborne hours) do not account for more than 75-90% of the total operational hours of the aircraft (block hours). The ratio of airborne to block hours can be treated as groundtime efficiency, Zg. Similarly, non-cruise portions of the flight, poor routing, and delays in the air constitute inefficiencies related to spending fuel during the flight beyond what would be required for a great circle distance trip at constant cruise speed. This inefficiency can be measured by the ratio of minimum flight hours to airborne hours, za. Minimum flight hours are calculated with the assumption that all aircraft fly the entire route at Mach 0.80 and at an altitude of 10.7 km (no climbing, descending, or deviation from the minimum distance, the great circle route). Minimum flight hours represent the shortest time required to fly a certain stage length and reveal any extra flight time due to nonideal flight conditions. The product of zg and za gives the flight time efficiency, zft. Both za and Zg increase with stage length. The lower zft associated with short-range aircraft is related to the more than 40% of block time spent in non-cruise flight segments. Long-range aircraft operate closer to
the ideal as total flight time efficiency approaches 0.9. The impact of operational differences on Eu is evident in Fig. 3, which shows the variation of Eu with stage length for turboprop and jet-powered aircraft (both regional and large jets) introduced during and after 1980. Aircraft flying stage lengths below 1000 km have Eu values between 1.5 and 3 times higher compared to aircraft flying stage lengths above 1000 km. Regional aircraft, compared to large aircraft, fly shorter stage lengths, and therefore spend more time at airports taxiing, idling, and maneuvering into gates, and in general spend a greater fraction of their block time in nonoptimum, non-cruise stages of flight. Turboprops show a pattern distinct from that of jets and are, on average, more efficient at similar stage lengths. The energy usage also increases gradually for stage lengths above 2000 km because the increasing amount of fuel required for increasingly long stage lengths leads to a heavier aircraft and a higher rate of fuel burn.
Aircraft Ei is also improved through better utilization (e. g., load factor) and greater per-aircraft capacity (e. g., number of seats). Historically, the load factor on domestic and international flights operated by U. S. carriers climbed 15% between 1959 and 1998, all of which occurred after 1970 at an average of 1.1%/year. Figure 4 shows historical load factor evolution for both u. S. large commercial and regional aircraft. Load factor gains have been attributed to deregulation in the united States and global air travel liberalization, both of which contributed to the advent of hub-and-spoke systems. As airlines have sought greater route capacity, the
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average number of seats has also increased, by 35% between 1968 and 1998, or from 108 to 167 seats (an average of 1.4%/year), most of which occurred prior to 1980.