Today, August 2, 2010 is the 25-year anniversary of the Delta Flight 191 airliner crash near the Dallas Ft. Worth Airport, which killed 135 people, including one on the ground.
I had graduated from USAF weather school just two months earlier when the crash brought aviation meteorology to the front page of newspapers around the world. With the wreckage of Flight 191 still smoldering on the field, the world was hearing terms like "microburst" and "low-level wind shear," which I had learned in school only a few short weeks before.
A thunderstorm is formed when columns of heated air move rapidly upward. As the contrast between warm and cool air becomes more dramatic with altitude, the air rushes upward even faster. While this is happening, basic cloud mechanics condense the water vapor into a visible cloud, eventually water droplets form and become large (heavy) enough to fall as rain; rain drops in the higher elevations freeze and become hail. Eventually, the rising air cools, its buoyancy decreases, its ascent slows and it begins to fall. A growing thunderstorm is comprised mainly of upward-moving columns of air, while a mature thunderstorm contains a mix of upward and downward columns of rushing air. When dissipating, a thunderstorm loses most of the upward columns and the cool air rushes downward to the Earth. Though this concept of basic meteorology is relatively easy to understand, rising and falling currents of air are only half the story, and it's the "other half" that is responsible for the downing of Flight 191.
The life of a thunderstorm. Image: Wikipedia.
Air moving up and down has to come from some place and it has to go some place; it doesn't just move up and down. As air moves upward, additional air is drawn into the lower levels to replace the air that has moved up. In a growing thunderstorm, this drawn-in air can come from miles away. If you're looking at a distant thunderstorm and the wind is at your back, it's likely the storm is in the growing stages and is literally sucking up air from all around to feed its growing tower of cloud. Conversely, a mature or dissipating storm violently throws the air downward* forcing the air to rapidly spread out from the base of the storm. If you're still looking at the same storm perhaps an hour later and you feel a burst of cool, moist air on your face, it's very likely you're feeling the air that only moments ago was within the higher levels of that same thunderstorm. Called a downburst — or when more locally confined a microburst — the winds are strong enough to topple trucks, damage buildings and knock down trees. Its effect on aircraft can be devastating.
Such was the case with Delta 191. Pilots today do not knowingly fly through thunderstorms. With ground and airborne radar, thunderstorms are easy to detect and avoid. In fact, as a result of the Delta crash, onboard radar wind-shear detectors are now standard equipment on airliners and have been for about 15 years. However, the flight crew 25 years ago made the decision to fly through a thunderstorm — a lone, innocuous-looking thunderstorm — enroute to their arrival at the Dallas/Ft. Worth airport.
NASA artist's rendering of a microburst over an airfield. Image: Wikipedia.
The crew noticed nearby lightning and within moments experienced a rapid increase in airspeed. In the illustration above the increased airspeed was a result of encountering the leading edge of the microburst. As the crew wrestled with the controls, other gusts struck the craft including side gusts and then a downward gust. Onboard flight sensors measured the airliner's rapid descent to be 5,000 feet per minute, or 83 feet per second; the plane was just 280 feet above the ground. Moments later, while passing through the center of the burst, the airspeed dropped dramatically and the rapid descent continued to the ground. The Delta flight struck the ground once, became airborne again then skidded across TX State Highway 114 killing one person on the roadway in addition to the souls lost onboard.
Since that time, the mention of low-level wind shear and microburst potential have become standard within any flight weather briefing when thunderstorms are present or expected to be present at departure, along the route or at the arrival airport. As an aviation meteorologist in Oklahoma and Texas, it was a rare day that I didn't mention wind shear to the pilot receiving my briefings. With the advent of Doppler radar, the ability to look inside a storm and analyze wind currents around a storm makes identifying wind shear possible, though it remains difficult for meteorologists to pinpoint the exact timing and location of such events in advance. Any thunderstorm has the potential for such phenomena; any pilot should assume the potential is high and give a wide berth to any thunderstorm.
Delta Flight 191 came to rest near Texas State Highway 114 north of Dallas/Ft. Worth International Airport. Image Google Maps.
This morning, on the 25th anniversary of the disaster, the Dallas/Ft. Worth International Airport dedicated a memorial to the 135 victims of Delta Air Lines Flight 191.
* As the cool, rain-laden air moves downward and spreads below the thunderstorm, evaporation further cools the air. This cooling dramatically increases the contrast between the warm/hot air surrounding the storm and the cool/cold air within the storm further accelerating the rushing air.
