REVERSING RUNAWAY RUNWAY TIMES.

• Author: Chu, Jennifer
• Position: GOING PLACES

MOST FREQUENT FLIERS are familiar with long lines at airports: at the check-in counter, the departure gate, and in boarding a booked flight. Yet. even after passengers are buckled in, the waiting may continue--when a plane leaves the gate, only to sit on the tarmac, joining a long queue of flights awaiting takeoff. Such runway congestion can keep a plane idling for an hour or more, burning unnecessary fuel.

Engineers at the Massachusetts Institute of Technology have developed a queuing model that predicts how long a plane will wait before takeoff, given weather conditions, runway traffic, and incoming and outgoing flight schedules. The model may help air traffic controllers direct departures more efficiently, minimizing runway congestion. For instance, if a controller knows that a plane is unlikely to take off for half an hour, he may choose to keep the plane at the gate to avoid contributing to runway backups.

Hamsa Balakrishnan--associate professor of aeronautics and astronautics and engineering systems and an affiliate of the Institute for Data, Systems, and Society at MIT--says that, in tests at U.S. airports, the model encouraged controllers to hold flights back during certain times of day, leading to significant fuel savings. "In our field tests, we showed that there were some periods of time when you could decrease your taxi time by 20% by holding aircraft back. Each gate-held aircraft saves 16 to 20 gallons of fuel, because it's not idling, and that adds up."

Balakrishnan and former graduate student Ioannis Simaiakis have published their results in the journal Transportation Science. The team is working on airports across the country to further test the model.

A number of factors can contribute to airport congestion, which can create nose-to-tail traffic jams among departing flights. Balakrishnan analyzed departure operations at New York's John F. Kennedy International Airport, Newark (N.J.) Liberty International Airport, and Philadelphia (Pa.) International Airport, and showed that these hubs were congested 10% to 20% of the time. At Newark, for instance, passengers traveling during congested periods experienced average runway taxiing times of 52 minutes, versus 14 minutes during less-busy periods.

Balakrishnan indicates that individual decisions on when planes push back from gates can contribute to the problem. "It's mostly on the fly," she explains. "Sometimes, if there is a controller with a lot of experience or intuition, he or...