Photograph by Michael Dwyer, Alamy

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Planes may be grounded at airports during winter storm Boreas, which is passing through the East Coast over the busy Thanksgiving travel period.

Photograph by Michael Dwyer, Alamy

Storm Warnings: How Do Airlines Know If It's Safe to Fly in Bad Weather?

With extreme weather targeting the East Coast, travelers wonder if their holiday flights will be canceled.

As a ferocious winter storm takes aim at the East Coast later today, bringing a mix of rain, wind, snow, and ice, Thanksgiving air travelers may wonder if their holidays will be ruined.

Several hundred flights have already been canceled because of inclement weather in the Midwest, with Texas-based American Airlines and its regional carrier, American Eagle, hardest hit.

First the good news: Your plane is certified to fly in "extreme conditions," according to Boeing spokesman John Dern—probably far more extreme than anything this nor'easter can throw your way.

The Federal Aviation Administration (FAA) has a long list of flight requirements for commercial aircraft, which manufacturers generally exceed. Many of the rules are designed to ensure that your plane can operate in wind and snow.

Weather-related flight cancellations seldom happen because a plane can't handle the prevailing conditions, according to veteran airline captain and aviation analyst John Nance. "Usually your flight can't operate because the airport shut itself down, or the airline decided to ground a large number of flights for operational reasons," he says.

Which brings us to the bad news: Just the anticipation of problems and long delays can spur airlines to cancel.

So if you're flying later today or early tomorrow, chances are the "go/no-go" decision has already been made by your airline in consultation with the FAA's Command Center in Vint Hill, Virginia. Check your itinerary online or call the carrier to see if your flight's been affected.

What are the factors going into such decisions? Each weather condition presents different dangers or difficulties:


In clear weather, winds are rarely a factor, and then only if they're blowing across, not down, the runway. "If you have a dry runway, you can take a crosswind of 25 knots in many commercial aircraft," says Nance.

But in rain or snow—or any condition that slickens the runway—tolerance for a crosswind decreases. It becomes that much harder for a pilot to stay on the centerline of the runway without skidding, particularly when landing a large plane at 150 miles an hour. Brakes are the vital element, because a pilot can't bring a plane to a stop using reverse thrust alone.

Each airline has FAA-approved formulas and tables it uses to determine the maximum crosswind. If a severe winter storm hits and causes ice on the runways, even a moderate crosswind could exceed the limits and make a takeoff or landing unsafe.

"You can reach the point that even a five-knot crosswind could blow you off a frictionless ice-covered runway," says Nance.


Light or moderate snow will not stop operations, says Nance. "A heavy snowfall, however, can cause cancellations."

Even planes that have been de-iced need to reach the runways, which often require heavy plowing.

And pilots could face difficulties generating flying speed in slush or standing water over a half-inch deep, or when there's too much snow accumulation. In such conditions, the drag on the aircraft tires can be so great that a plane is unable to take off before the runway ends. Visibility can also be a factor.

Bottom line? A few snowflakes won't cause a cancellation, but a blizzard might.

Freezing rain

"One thing we don't fly in is freezing rain," says Nance. "If an airport is hit with rain or drizzle when temperatures hover near the freezing point, airport authorities will consider shutting it down and most definitely the airlines will start canceling flights."

The reason? In such conditions, aircraft can gather ice faster than de-icing equipment can remove it, and ice can also wreak havoc on every other part of airport operations. An aircraft can lose traction in the ice, making it difficult to control. Critical equipment can freeze, and braking can become uncertain if not impossible.

New strategies

Airlines, airports, and the FAA do what they can to prevent weather from stranding passengers. But their strategies have changed in the last few years, particularly in light of the Department of Transportation's 2009 rules on airport-tarmac delays, which fine airlines for keeping passengers on a parked plane for more than three hours.

"It's better to inconvenience a passenger at their point of origin," says Nance. "Having a sky full of airplanes that can't land and are sucking up fuel is not a good strategy."