The treacherous weather phenomenon known as "wind shear," which is considered a prime suspect as the cause of the crash of Delta Air Lines Flight 191 at Dallas-Fort Worth International Airport, has been blamed for two dozen airline accidents and 491 deaths over the past two decades.

Wind shear is a sharp or sudden shift in wind speed or direction, sometimes characterized by violent down-drafts and "microbursts" of air. It occurs so fast that it is extremely difficult for the pilot to change the plane's speed, direction or thrust to adjust for it.

It can cause a plane to descend much too rapidly. At low altitudes, particularly, with buildings, fences, towers and the ground nearby, it can cause disaster before the pilot has time to adjust.

In one case, at New York's John F. Kennedy International Airport on June 24, 1975, an Eastern Airlines Boeing 727 coming in for a landing struck the light towers on the approach path and crashed, killing 113 persons. The crash occurred because of "adverse winds associated with a very strong thunderstorm," which caused the plane to descend too rapidly, according to a National Transportation Safety Board report in 1976.

The NTSB found that four planes that had landed on the same runway just before the Eastern flight met "abrupt changes" in vertical and horizontal winds but their pilots had managed to increase jet thrust or take other action to avoid disaster. But when the Eastern plane came in, the visibility was so low that the crew did not realize how rapidly the plane was descending.

More recently, a Pan American World Airways jetliner took off at New Orleans International Airport on July 9, 1982, in "increasingly heavy rain," immediately ran into a wind shear and crashed, killing all 145 persons on board and eight persons on the ground. The crash destroyed six houses and damaged five others.

The NTSB investigation found that a "38-knot 44 mile-an-hour wind shear the flight encountered had the effect of reducing the airspeed of the airliner by about 18 knots as it reached an altitude of 100 feet. At the same time, the plane penetrated a vertical downdraft of seven feet per second.

"The board estimated that the copilot had about six seconds to react to the wind shear, raise the nose and add all available engine power to prevent descending into the trees." The records showed he probably took the right corrective action, but time was too short and the plane went down.

The Federal Aviation Administration has been involved in the testing and development of ground-based wind shear-detection systems since 1972. At present, according to a spokesman, a system called the Low Level Wind Shear Alert System (LLWSAS) is in place at 60 major airports, including Dallas. The FAA has just taken delivery on systems for use at 50 more airports.

The system at Dallas was in operation at the time of the crash Friday, officials told Washington Post staff writer T.R. Reid in Dallas yesterday.

The LLWSAS is designed to detect wind shears at low altitudes at the airport. The system consists of a series of wind direction indicators (basically weather vanes) that show wind direction and changes in wind direction, and anemometers, which use ice-cream-scoop-like devices to catch the wind and measure its velocity.

A wind-direction indicator and an anemometer are placed at various locations. In Dallas, one set is at the center of the airport and another five sets are at different runways. These teams of sensors are hooked up to a computer, which sends signals on wind speed and direction to a readout machine in the controller's booth. He can then tell the pilots of incoming and outgoing planes that there is wind shear hazard at a certain runway.

In its 1983 report on the New Orleans crash, the NTSB recommended a number of improvements in the system. One major recommendation was that the system provide more information to the pilot on the speed of head winds, tail winds and cross winds at the site of the plane. The FAA had promised a decision by July 1, 1985, but it was unclear yesterday whether it had been announced.

A more substantial reform would be to switch to a much more sophisticated system called the Advanced Doppler Weather Radar, which can, in effect, look into a storm and detect hazardous weather conditions and wind shears and measure them far better than the current system.

The FAA said yesterday that such a system is being developed for weather and wind conditions at high altitudes. If all goes well, installation could begin in 1987. Eventually it would be used at 100 airports, at a total cost of $300 million to $400 million. A spinoff from that would be a similar system for ground-level winds, which could be installed by 1988 at some airports.