Fuel Savings of Optimally Routed Flights
This article was published in the Journal of Air Traffic Control, Summer 2012, Vol. 54, No. 3.
Over the last four decades, the price of jet fuel has soared. In 1971, fuel accounted for just 12 percent of the operational costs for airlines, but by the end of 2011, it had risen to 31 percent, eclipsing all other costs, including labor. Consequently, one of the benefits most anticipated by the aviation community of the Federal Aviation Administration’s (FAA’s) modernization of the National Airspace System (NAS)—“NextGen”—is a reduction in air carrier fuel use. One key element of this modernization is an overhaul of the method of routing flights.
The present NAS practice for routing flights involves the traversal of fixed airways, a method of air traffic management that dates to the beginning of the FAA. The FAA currently allows pilots to file flight plans with more direct routes, but these amendments must be requested after a plane is airborne and are not guaranteed. Advances in technology should one day allow for 4-D routing of flights, with each route being uniquely designed to minimize the fuel used and time in transit, while maintaining adequate separation from other flights. The current NextGen development plan contains elements to achieve this goal.
In the meantime, questions remain. What are the quantitative benefits of these more efficient flight paths? What are the primary causes of increased efficiency, and can these causes be exploited now, within the confines of the present NAS architecture? This paper presents results that will help to answer these questions.
Since all commercial passenger airplanes for the foreseeable future will be designed for minimum fuel use at a constant (high) altitude and airspeed, an ideal 4-D route will still consist of three distinct phases of flight: ascent, cruise, and descent. On average, most flights expend the most fuel in the cruise phase. Therefore, this article will focus on determining the efficiency of the cruise portion of flights in the NAS and calculating the expected increase in efficiency based on more ideal routes.
Efficiency will be measured by comparing the time flown and distance traveled on the cruise portion of planned and actual routes with the time and distance of cruise paths that are optimized for shortest transit time (i.e., “wind optimal”) and those that are optimized for shortest distance (i.e., “great circle”). For the cruise phase, fuel burned is proportional to time flown, so a wind optimal route is also one of least fuel use.
