Tuesday, December 7, 2010
Beware The Downburst
An insidious risk that can undercut airspeed and drive you into the ground
That’s what happened to Delta Airlines flight 191, a Lockheed L-1011, approaching DFW airport in August, 1985. The captain was anything but unwary. A 14,000-hour professional, he was well respected as one of Delta’s best, and his actions during the flight suggest he knew exactly what was happening. That accident was associated with thunderstorms in the area, but it presented a classic case of the dangers of a microburst, a phenomenon not always associated strictly with thunderstorms.
The Delta flight was following a Learjet from four miles out on the ILS to runway 17 when the copilot, flying the approach, called the captain’s attention to lightning coming out of a thunderstorm slightly above and straight ahead. The Learjet landed safely without any reported problems, so the Delta crew continued the approach. Curiously, as the copilot attempted to keep the needles caged on the ILS, the airspeed began to increase, suggesting updrafts.
As the aircraft passed 800 feet AGL with speed still increasing, the captain recognized what was about to happen and ordered the copilot to add full power. “You’re going to lose it all of a sudden,” he said. “There it is. Push it up, way up, way up, all the way up.”
The brief period of updrafts was followed by a series of violent downdrafts. The L-1011 lost 40 knots in about eight seconds, and the flight crew couldn’t recover from the descending air, at one point speeding downhill at 5,000 fpm. The captain’s final word was, “TOGA,” (“Takeoff/Go Around Power”). Full power from all three engines did arrest some of the aircraft’s descent, but the Delta flight finally crashed to the ground short of the runway, impacting a highway with a final descent rate of 600 fpm and an airspeed over 200 knots. After touchdown, the big Lockheed slammed into two water tanks and disintegrated, killing 136 passengers and crew.
Granted, most general aviation pilots don’t fly anywhere near thunderstorms, but airline flight crews sometimes do. This exposes large passenger jets to more risk from thunderstorms, but all pilots should learn to recognize the symptoms of a downburst. In its simplest form, a thunderstorm-generated downburst is a flow of air that can hide inside a rain shaft, though relatively dry downbursts, like the one I flew through in Albuquerque, are possible. Dry downbursts sometimes originate from clouds at high altitude that seem to pose no threat to aircraft flying at lower levels.
Rain may fall as virga and affect the atmosphere below. When rain or moisture drops below the cloud base or mixes with dry air, it begins to evaporate and cool. Since warm air rises and cool air descends, the colder air falls progressively faster as it approaches the ground.
Downbursts go through three distinct stages; the actual descent or downburst toward the ground, the outburst during which the air is deflected by terrain and the cushion effect associated with rising air at the edges of the phenomenon. When a downburst actually arrives at the surface, its advancing edge spreads abruptly in all directions, the signature of a downburst. The air also effectively “splashes” back up toward the outside of the rain shaft. This accounts for the updraft phenomenon that may precede violent downdrafts as the air currents swirl and shift in the sky.
A microburst is simply a small downburst, usually no more than 2.5 miles in diameter, and it may generate temporary surface winds as high as 100 mph in all directions, usually highly localized and lasting no more than 10 minutes. Thunderstorms generally travel fairly quickly across the ground, so conditions can change in a few minutes. The winds may be erratic and almost unmanageable, shifting from head- to tailwinds in an instant, making it virtually impossible for a pilot to fly a consistent, stabilized approach.
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