The Calculators of Calm

Just how far out of their way will airlines go to give you a smooth ride?

The DC-8 lost its left outboard engine and 19 feet of wing and fell 500 feet in 10 seconds, but landed safely. (NCAR)
Air & Space Magazine

It’s a typical January day in Chicago—overcast skies, snow on the ground, a high in the 20s—but Terry O’Toole is far more interested in some menacing thunderstorms boiling above the eastern Pacific. O’Toole stares at weather data displayed on a desktop computer screen in United Airlines’ flight operations center, near O’Hare International Airport. A dispatcher, O’Toole devises and assigns flight plans, which include routes and altitudes. Like United’s passengers and crews—and, perhaps most of all, the company’s accountants—O’Toole wants United flights to avoid turbulence as much as possible.

From This Story

Thunderheads can, of course, be easily seen by both pilots and radar, but not the violent winds that sometimes swirl for miles downwind of a storm. In slightly less than two hours, United flight 52 will depart Honolulu for Los Angeles. O’Toole clicks up a satellite image of the Pacific on his other monitor. A box superimposed on the image warns him of the area in which airliners can expect to encounter turbulence between 28,000 and 38,000 feet. The box lies directly across every possible route O’Toole can assign UAL 52.

Though the storm may dissipate before the flight enters the area, residual turbulence can linger for a long time, causing choppiness. The best O’Toole can do is find an altitude either above or below the “chop.” It’s the start of a complicated numbers game.

“Rarely do we fly below 28,000 feet,” says O’Toole. Ideally, an airliner will climb to a higher altitude, where the air might be smoother. But to minimize fuel consumption, airliners typically fly as high as weight limits allow, so they’re already at their ceiling. To escape turbulence, most airliners have to descend; that causes them to burn more fuel, which eats into company profits.

O’Toole clicks on icons of United flights already in the alert area and keys a message to a UAL aircraft flying at 37,000 feet: “How’s the ride so far?” He hits the return key, and the message is transmitted via satellite to a computer screen in the airborne jet’s cockpit.

As O’Toole waits for a reply, he inputs weather and fuel parameters into a computer that will plot the most economical flight plan for UAL 52, a Boeing 777 that will carry an almost-full complement of passengers and cargo. Minutes later a dot-matrix printer cranks out the results: the recommended altitude and route, and a wealth of weather information and figures relevant to other routes and altitudes UAL 52 could be assigned.

“At 33,000 feet, we’ll be at five hours and 32 minutes—which is considerably higher than our target time. And it’ll require 60,000 pounds of fuel,” says O’Toole, looking over the computer’s calculations. By contrast, “if I run him at 24,000 feet, we’ll be on target time because the tailwinds are so strong there. But you’re also looking at burning 8,000 pounds more fuel”—costing about $3,000. “It’s nice to make it in less time, but instead of that 8,000 extra pounds of fuel, you could be carrying 8,000 more pounds of cargo or passengers…. But if the air is bad, we go down lower,” he assures me. “We don’t give it a second thought.”

A response comes in from the United flight crew already in the area: “Light chop on and off most of the way; seat belt sign on.” According to the printout, if UAL 52 flies at 37,000 feet, it will actually burn slightly more fuel than it would at 33,000 feet, because the tailwinds aren’t as strong higher up. But the air seems to be smoother up there. O’Toole decides to take the hit on fuel and schedules UAL 52 for a flight at 37,000 feet. Chances are it’s less than the hit—or hits—the airline could take if the flight runs into severe turbulence.

According to statistics compiled by the National Transportation Safety Board, between 1987 and 2000, only two fatal accidents (involving one death each) aboard U.S. commercial airliners were attributed to turbulence, and the phenomenon is believed to have caused the crash of just one U.S. airliner—some 40 years ago—mostly because the pilots failed to respond properly when severe winds struck.

While the majority of encounters with turbulence are not lethal, rough air wreaks havoc on the airline industry. “Probably the least of our worries is that the airplane is going to fall out of the sky,” says Lou Andelmo, a dispatcher for U.S. Airways. “It’s the injuries we’re most worried about.” Turbulence is the leading cause of nonfatal passenger and crew injuries, which result in work time losses as well as overtime paid to other crews to fill in. Passenger injuries can also result in lawsuits and settlements. And if a turbulence encounter is severe enough, Federal Aviation Administration rules mandate the airline conduct an immediate, unscheduled (i.e., costly) inspection of the aircraft for damage or stress before the airplane can return to service.

Comment on this Story

comments powered by Disqus