It's one of the toughest calls a pilot can make: when, if ever, to try to beat the weather and land a plane.
Such decisions are the prerogative of the pilot, in consultation with ground-control officials.
But the decision is not always easy. A pilot has to weigh whether it is safer to stay in the air or try to land a jetliner before a storm gets worse. Then there are other considerations: crew and passenger fatigue, fuel levels, changing weather reports.
How pilots make these decisions is coming under increased scrutiny in the wake of the crash of an American Airlines jetliner in Little Rock, Ark., in a blinding storm late Tuesday.
While it's too early to tell what caused the crash that resulted in at least nine fatalities and dozens of injuries, weather is likely to have played a major part.
"You try to get in before the storm gets too bad," says David Stempler of the Air Travelers Association, a passenger advocacy group in Washington. "You think, 'Can I make it? Is there enough braking capability on the runway?' And apparently there was not."
On the night the plane landed at Little Rock National Airport, winds were gusting up to 90 m.p.h. across the runway. Lightning flashed, and hail and rain pounded. Visibility was estimated to be zero.
"The plane had landed, and apparently it collided with a support structure that holds runway lights, which caused the plane to rotate and end up on a slight embankment a few yards from the Arkansas River," says George Black, a National Transportation Safety Board member on scene. "The plane broke in half. And a fuselage fractured in pieces."
Of the 145 people on board, at least 80 people were injured. The fatalities were the first on a major United States airline in almost two years. Investigators are combing through the wreckage and interviewing survivors looking for clues to the crash.
Waiting for the investigation
Experts caution that what may appear to be the most obvious reason for the accident, in this case the storm, may not in the end turn out to be the cause.
Still, with the plane having been recently serviced, the early focus was on the line of thunderstorms that delayed the flight from Dallas-Ft. Worth to Little Rock for more than two hours late Tuesday night.
Aviation experts say that with all of the sophisticated technology that predicts weather patterns and advanced radar that can track storms, pilots have better tools than ever to help them make weather-related decisions.
In fact, airlines and pilots regularly fly with confidence through inclement conditions, and airports seldom shut down.
Yet, for pilots, the decision of whether to land in stormy weather is rarely black-or-white. "It's in that gray zone in the middle," says Mr. Stempler.
The Little Rock airport is also not equipped with "terminal weather radar," also known as Doppler radar. It can look at the end of a runway and pickup so-called microcells that might result in wind shear.
And while experts say early indications are that wind shear may not have been a factor here, the Doppler radar could have given the pilot a better indication about the severity of the storm and it's proximity.
Though most television stations in urban areas use Doppler radar for weather forecasting, only 38 airports were equipped with it as of January.
The FAA says simply it's in the process of fact gathering about the latest crash. "What was the weather?" asks Les Dorr, an FAA spokesman. "What was air-traffic control saying? What are the pilot records? Anything on the airplane?" All the information will be sent to the NTSB, which is responsible for the investigation.
While it's been awhile since there has been a weather-related incident, they do occur. The NTSB cited weather as a cause or a factor in nearly one third of the 582 accidents involving scheduled flights from 1983 through 1996.
One example occurred on July 2, 1994, in Charlotte, N.C. A McDonnell Douglas DC-9 operated by US Air collided with trees and a house after the plane missed the runway.
According to NTSB documents, the flight crew was not provided with weather information that indicated a severe microburst over the approach end of the runway.
Doppler radar was supposed to have been installed at the Charlotte airport but was not in place.
The NTSB concluded that the radar would have provided controllers with information about the severity of the weather. But the NTSB also said crew errors contributed to the crash. Thirty-seven people were killed and 20 injured.
Airline legislation on Hill
The difficulty of predicting weather patterns also raises questions about some of the proposals on Capitol Hill that are designed to protect passengers in today's booming airline market. But they may end up having unintended results.
One of the proposals in so-called Passengers Bill of Rights pending before Congress would penalize airlines for keeping passengers hostage on the ground.
Stempler and his organization oppose that because most of the delays that keep planes on the ground are weather, or air-traffic control, or mechanical delays - all safety related.
"You don't want to create financial pressures on an airline or a pilot to say you better land that airplane or you better take off," says Stempler.
Indeed, experts say the quixotic nature of some weather fronts adds to the importance of ensuring that both pilots and air-traffic controllers feel free to make decisions that are safety-related first.
"With this kind of line of storms, these little cells can just pop up so quickly that you can think you've got a nice opening," and then it disappears, says Clint Oster, an aviation expert at Indiana University in Bloomington. "That's just a characteristic of these really fast-moving, violent kinds of fronts."
Evidence on the ground
The severity of the storm that hit Little Rock was evident, according to people on the scene. Debris and damage were visible at Raytheon Aircraft, which is just a few feet away from the airport tower. Air-traffic controllers were warned about the thunderstorms.
"Apparently some significant winds could have had something to do with this crash," says Philip Launius, spokesman for the Little Rock airport.
The Arkansas crash brings to an end one of the longest and safest runs for commercial aviation in the United States. In 1998 there was not a single fatality as a result of a commercial flight.
"It's a terrible way to end what was a good run for commercial aviation," says Mr. Oster.