Impact of Congestion on Taxi Times, Fuel Burn, and Emissions at Major Airports

Operations on the airport surface include those at the gates, the ramp areas adjoining the gates, the taxiways, and the runway system. Each component could be subject to queuing delays and is also influenced by downstream factors such as the terminal airspace. On the basis of observations by Idris (4), four regions were identified on the surface where queues primarily form during the departure process: gates (before pushback), ramp, taxiways, and runways. Conceptually, the departure process can be described as follows: First, aircraft request pushback from their gates. They wait to be cleared for pushback; this waiting time is modeled by the pushback queue. After pushback, they enter the ramp and then the taxiway system; then they taxi to the departure queues that form at the runway(s) to wait for takeoff. While taxiing out, different aircraft may interact with each other. For example, aircraft queue up to cross an active runway (runway-crossing queue) or to enter a taxiway segment where another aircraft is taxiing. The different queues that form on the surface have different characteristics and costs associated with them. This paper primarily focuses on taxiing delays, fuel burn, and the aircraft emissions during taxi-out. As a result, the queues of particular interest are the ramp queue, the taxi queue, the runway-crossing queue, and the departure queue. Delays incurred at these queues are a direct consequence of congestion and contribute to excess fuel burn and emissions. Therefore, to estimate the total impact of congestion on taxiing aircraft, the waiting times in all four queues are added. Key variables that affect the taxi time of a departing flight include runway configuration, weather conditions, downstream restrictions, gate location, and queuing delays (5). The concept of a segment, which is defined as a particular combination of runway configuration and weather conditions, is adopted here (6). The reported meteorological conditions [denoted either visual meteorological conditions (VMC) or instrument meteorological conditions (IMC)] are used as the indicator of weather conditions. The runway configuration is characterized by both the runways used for arrivals and those used for departures. A segment is denoted as (weather conditions; arrival runways departure runways). Impact of Congestion on Taxi Times, Fuel Burn, and Emissions at Major Airports