Most of the time, the weather gives a sign that it’s brewing trouble. You don’t have to know much meteorology when you show up at the airport for a scheduled flight to find a thunderstorm’s winds rocking the tied-down airplanes. No question, you’ll have to wait until later to go flying. A forecast for poor visibility on a planned route of flight tells you to stay on the ground unless you have an instrument rating.
Unfortunately, the atmosphere doesn’t always give obvious clues that it’s cooking up potentially deadly winds. This can affect experienced airline flight crews as well as a student who’s had only a couple of flying lessons. Now and then, what looks like a harmless rainshower can prove deadly to an experienced crew and its passengers on a large airliner.
A tragic example is the August 2, 1985, crash of Delta Air Lines Flight 191, a Lockheed L-1011 TriStar at the Dallas-Fort Worth International Airport that killed eight of the 11 crewmembers, 126 of the 152 passengers, and one person on the ground.
CLOUDS CAN HIDE VIOLENCE. Five seconds of the National Transportation Safety Board’s transcript from the cockpit area microphone (CAM) during the two minutes before the flight crashed reveals the Delta pilots saw what we now know was a clue to possible trouble—but neither the pilots nor controllers recognized it.
From the transcript:
18:04:18 CAM-2: “There’s lightning coming out of that one.”
18:04:19 CAM-1: “What?”
18:04:21 CAM-2: “There’s lightning coming out of that one.”
18:04:22 CAM-1: “Where?”
18:04:23 CAM-2: “Right ahead of us.”
(Flight 191 hit the ground at 18:05:52.)
“That one” was a towering cumulus cloud producing rain that was falling on the runway on which the L-1011 had been cleared to land. The shower became a thunderstorm as soon as lightning came from it. Pilots learn early in their training that they should avoid thunderstorms, but as the Dallas-Fort Worth crash shows, a towering cumulus on the way to becoming a thunderstorm can also hide danger.
The danger hidden by the cloud was a microburst—a phenomenon that no one really knew about until the mid-1970s and wasn’t named until 1978.
THE DISCOVERY OF MICROBURSTS. At the time of the 1985 Flight 191 crash, a few atmospheric scientists, pilots who were especially interested in weather, and others who were well aware of the microburst danger were developing programs to train pilots how to recognize and avoid them. They were working on Doppler radars that could help controllers alert pilots about potential microbursts. Until the Flight 191 crash, however, neither the FAA nor many others in the aviation industry saw a need to spend much time or money trying to reduce microburst crashes.
The growing awareness of the microburst danger had begun with the June 24, 1975, crash of an Eastern Air Lines Boeing 727 at New York’s Kennedy International Airport, which killed 112 of the 124 people aboard. Theodore “Ted” Fujita of the University of Chicago, a highly regarded severe storms expert, conducted a detailed study of the New York City area’s weather at the time of the crash and concluded that air blasting down from a thunderstorm—which he called a “downburst”—caused the crash. He later sharpened his conclusion by coining another new term, “microburst”—a downburst concentrated in an area 2.5 miles or less in diameter—to refine his explanation of the crash’s cause. (Today’s Enhanced Fujita Scale of tornado strength is named for Ted Fujita.)
Fujita’s extensive study of the Flight 191 crash found that the L-1011 had encountered a 50-knot downward tailwind as it flew through the downburst, which was 1.9 nautical miles in diameter. As descending air hit the ground it formed three concentric vortex rings—somewhat like horizontal tornadoes—which the airplane flew through.
How could microbursts have been missed until 1975? Fortunately, they are rare and the odds are small that one will happen to hit a weather station. Even if one did, the station’s single anemometer would show only the wind in one place, not the starburst pattern of winds hitting the ground that Fujita visualized as he studied the data from the New York crash.
Until Fujita advanced his theory, meteorologists had blamed thunderstorm gust fronts for the quick changes in wind speed and direction that sometimes occur in the vicinity of showers and thunderstorms. A gust front is the air that descends and spreads out as rain begins falling from a shower or thunderstorm. They are dangerous to aircraft taking off and landing, but are easier to detect than microbursts and aren’t as strong as the worst microbursts. Until the Dallas-Fort Worth crash, microbursts had been causing an airline accident in the United States approximately every 18 months, but these crashes had not spurred investment in the measures some scientists and pilots had been urging.
The Flight 191 crash was the turning point. Among other things it led to the decision to deploy terminal Doppler weather radars (TDWR), which are designed to detect early signs of a developing microburst and alert controllers who can warn pilots. These, along with pilot training on how to avoid microbursts and how to respond if caught in one, have meant that since 1985 a microburst has caused only one air carrier accident: the July 2, 1994, crash of a USAir DC-9 at Charlotte, North Carolina, which killed 37 people. At the time, Charlotte’s TDWR had not yet been installed.
Since most general aviation takeoffs and landings aren’t at airports covered by TDWR, you can’t count on radar help to avoid microbursts. The good news is that staying out of the way of all thunderstorms, no matter how small, reduces the odds that you’ll fly into a microburst while taking off or landing.
But, as the Flight 191 crash showed, not all microbursts come from obvious thunderstorms. To play it really safe you’d want to avoid flying under any cumulus cloud with rain falling from it. The bigger the cloud, the greater the danger. The top of the cloud that caused the Flight 191 microburst was only 23,000 feet high, which isn’t high compared to most violent thunderstorms. But, it is a respectable towering cumulus cloud.
In the western United States, where the humidity is generally lower than in the East, that base of clouds is usually higher. Often you’ll see virga—rain that evaporates before reaching the ground—falling from Western clouds; it’s seen less often in the East. Virga can cause microbursts, some of them strong. When such a “dry” microburst hits the ground it can kick up a ring of dust. Dust cloud or not, you shouldn’t fly at low altitude under a cloud that’s producing virga. In the more humid East, most microbursts are “wet,” which means they are hidden in falling rain. A TDWR is the only way to spot rain clouds that are or are about to produce a microburst. Your safest bet is to avoid flying at low altitudes under any cumulus cloud that’s producing any rain.
This diagram shows one of the principal dangers of encountering a microburst during takeoff or landing: a sudden shift from a headwind to a tailwind with a resulting loss of lift. Unlike in the diagram, this occurs inside a maelstrom of heavy rain and turbulence.
