Storm Data Abound; Rules for Flying in One Are Few

Thunderstorms were strung out east to west Tuesday evening along much of southern Arkansas when American Airlines Flight 1420, delayed for two hours by another storm system, finally left Dallas-Fort Worth International Airport, bound for Little Rock, Ark.

With on-board weather computers and radios, as well as a weather radar in the nose of their MD-80 aircraft, veteran pilot Richard Buschmann and First Officer Michael Origel certainly knew the weather around Little Rock was bad.

Although the cause of the crash has not been determined, weather data and eyewitness accounts suggest the plane may have touched down in the middle of a storm with hurricane-force wind gusts, raising the question: What weather information did the cockpit crew have at that critical moment, shortly before midnight, when they decided to land the plane in the wind and the rain?

“If in fact they knew they were landing under the influence of a thunderstorm, that was a mistake, unless there were some other extenuating circumstances we don’t know about,” said retired pilot Barry Schiff, who flew for 34 years with TWA. “There are no official rules [for bad-weather landings], there are things you just don’t do--and one of them is fly into a storm.”

American Airlines Executive Vice President Bob Baker said regular weather updates were relayed to the plane.

The flight data and cockpit voice recorders, which were taken to Washington for analysis, should reveal exactly what controllers told the pilots about the weather conditions, investigators said.

With a system of Doppler radar blanketing the United States, two geostationary weather satellites overhead, counters recording lightning strikes and thousands of paid and unpaid weather watchers phoning in everything from hail size to dew-point to the National Weather Service, the tiniest atmospheric squall can hardly sneak unnoticed across the country.

In larger cities, where many television news stations operate Dopplers of their own, rain can be tracked literally by the block.

Despite advances in meteorology, pilots still radio in to let others know where to find a patch of smooth air along a bumpy route or to warn of puddles at the end of a runway. They still watch the clouds as much as the radar scope to find storms and routes around them.

And when it comes to flying in bad weather, the reams of Federal Aviation Administration rules say, in effect: It’s up to the pilot.

There are guidelines.

Pilots of most commercial jets must have 600 feet of visibility before taking off.

On most aircraft, pilots are supposed to be able to see half a mile before landing, though some planes carry autopilot equipment that allows for less visibility. Cockpit crews with additional training and certification can land without being able to see the runway, so long as they have 150 feet of visibility once the wheels touch down.

For each plane, manufacturers set guidelines, recommending the plane not land in a crosswind stronger than an average of about 45 knots (about 52 mph). Airlines sometimes adjust those parameters slightly, depending on their own testing and the precise equipment on board a specific plane.

It was not clear what the precise recommendations were for the American plane, which was built in 1983, but pilots said crosswinds of more than 50 knots, or 57.5 mph, are considered too strong for almost all commercial craft.

But weather changes quickly, pilots and meteorologists point out, and in the end, pilots make the call.

“Pilots land at their own discretion, fly at their own discretion,” Schiff said. “The only thing the control tower does is coordinate traffic. They may say, ‘There’s a thunderstorm in progress over the airport,’ but they can’t say, ‘You’re forbidden from landing.’ ”

Tuesday’s crash combined two of the most precarious elements of flying: landing and weather.

According to a Boeing study, 36% of the world’s fatal crashes from 1988 to 1997 occurred during landings, when planes are less stable because they are flying more slowly and are especially vulnerable to wind shear as they power down.

The National Transportation Safety Board, meanwhile, cited weather as a factor in nearly a third of 582 accidents from 1983 to 1996. Wind shear, specifically, has been blamed for several of the worst accidents in recent decades, including that of a Delta Airlines L-1011 that crashed on approach to Dallas-Fort Worth in 1985, killing 154.

Like many spring thunderstorms common to the central United States, the system in Arkansas on Tuesday night pounded one neighborhood with hailstones 2 inches in diameter and left another nearby with stones hardly bigger than BBs; it whipped up winds of 87 mph at one end of the airport approximately six minutes after the crash and barely bent the tulips not far to the north.

“It was, by definition, a severe thunderstorm,” said Mike Smith of the private meteorology firm WeatherData Inc., meaning it produced hail of at least three-quarters of an inch in diameter or winds of 50 knots.

“But relating the storm to the plane is very difficult because we don’t know precisely where the plane was at what time,” Smith said.

That is likely to be the first order of business for the NTSB investigators.

“What will be very important as this investigation unfolds is what were the conversations between the tower and the crew,” said one airline industry official, who asked not to be identified. “What did the tower tell them? Did the tower know the weather conditions at the farthest edges of the airport?”

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Researcher John Beckham in Chicago contributed to this story.