By Gerald S. Schatz
EARLY ACCOUNTS of the crash of Delta Flight 191 at Dallas-Fort Worth Airport on Aug. 2 seem to have taken for granted the notion that aircraft normally fly through thunderstorms.
Some do, most don't.
Thunderstorms vary in intensity, but all pack violent updrafts and downdrafts, and pilots are urged early in their training to avoid thunderstorms altogther. Don R. Dickson, in "Weather And Flight: An Introduction to Meteorology for Pilots": "The smart pilot will always avoid all thunderstorms, because it is far easier to get into trouble than it is to get out of it!"
The Federal Aviation Administration and National Atmospheric and Oceanic Administration recognize that pilots may get caught in thunderstorms but advise, in "Aviation Weather": "Avoiding thunderstorms is the best policy . . . . Don't land or take off in the face of an approaching thunderstorm. A sudden wind shift or . . . turbulence could cause loss of control." The U.S. Air Force, in "Weather for Aircrews," recognizes that aircraft may get caught in thunderstorms but advises: "Best policy -- AVOID THUNDERSTORMS !"
Although it is too early to pinpoint the causes of the crash, the understandings and misunderstandings that have ensued suggest inquiries long and fatally overdue. The airplane, a Lockheed L-1011 on approach to the airport, flew into a thunderstorm.
There was no shortage of simple explanations of what happened next.
Initial, speculative illustrations of the descent showed the aircraft's encounter with wind shear -- abrupt change in wind speed or direction -- when it was low, slow and had almost no time and very little altitude to recover flying speed. Later news stories described a shear of the microburst type, and hinted that Flight 191's having to slow to avoid overtaking a Learjet also on approach was a causal factor.
It was noted, too, that Flight 191 wasn't warned by the airport's wind shear alert system or by its Automatic Terminal Information Service, the recorded radio announcements that report at-the-airport weather and runways in use.
The sequence of events that led to the crash of Flight 191 is more complicated, with a strong psychological component. This does not mean that the flight crew was emotionally unfit; it does mean that the complete pathology of Flight 191 must take into account how the crew was trained and what Flight 191's crew, the Lear pilot, the Dallas controllers and the Federal Aviation Administration expected of themselves, the weather, the airplanes, the Air Traffic Control system and each other and, equally important, what they thought was expected of them.
Wind shears are not necessarily dramatic events. They are turbulence, and some shears are worse than others. The New Orleans crash of Pan American Flight 759 in 1982 was the first clearly documented air- transport disaster attributed by the National Transportation Safety Board to wind shear associated with the phenomenon now termed a microburst -- a powerful, relatively narrow downdraft that strikes the ground and spreads outward.
To visualize it, imagine an upside- down mushroom.
A microburst generally is only a few miles wide. The wind velocity can differ 70 knots or more from one side to the other; the aircraft with a 35-knot (40-miles-per-hour) headwind on entry into a microburst can go quickly into a region of 35- knot tailwinds.
In other words, an airplane that stalls -- loses lift -- at an airspeed under 90 knots could enter a microburst at 120 knots, suddenly be going at an airspeed of 155 knots (far too fast for approach to landing), just as suddenly leave the headwind, enter the downdraft, recover and in the reversal of wind direction be moving at an airspeed well under 90 knots and so stall and fall.
No one can see or predict microbursts and no one knows their cause. They are sometimes associated with moisture and thunderstorms, sometimes not, although strong wind shears are part of all thunderstorms.
They intensify over five or six minutes and dissipate just as quickly. They are too localized to be always detected by typical airport equipment for measuring wind differentials and alerting pilots to low-altitude wind shear. Doppler radar, more expensive, is more effective in identifying small but intense shears, but the nation's major airports are not being equipped with Doppler- radar arrays.
Scientists of the National Center for Atmospheric Research have advised pilots since the Pan Am crash to break off their landing approaches immediately on encountering or hearing of unexpected turbulence. But often there are only seconds in which to act.
See the dangerous weather out the window? Transports and business jets don't provide much visibility. And landing time is busy time.
Further, thunderstorms may be embedded in seemingly innocuous clouds. As for radar, moisture may hide thunderstorms from detection.
Close airports when weather threatens? It has long been discussed and long opposed. To some extent there is a cry-wolf problem. Many pilots have survived bad weather. Conditions often are ripe for awful weather, the warnings are issued and the predicted weather doesn't occur. Forecasting is an uncertain art.
There is gossip among pilots that airlines differ in their policies regarding pressure to make schedules. Certainly, pilots differ in their judgments of weather they can handle.
If pilots are under pressure from pride or employer to fly into severe weather, it ought to be known and countered. If airports will not close in thunderstorms, they might reconsider. Controllers may need more authority, which pilots can be expected to fight.
Early reports indicate the Lear pilot did not report the turbulence -- indeed, landing is a demanding exercise, and the Lear pilot may have been wholly preoccupied.
Telling one aircraft to slow when behind another on approach is not unusual, and the Delta L-1011 was so instructed. But in this weather the L-1011 needed more airspeed.
The specifics of what happened to Delta Flight 191 are being examined very carefully. Beyond the specifics, there are critical questions for policy: What are pilots expected to do? What should they expect of themselves? How and when are the warning systems to be improved? When should airports and Air Traffic Control intervene to bar flight into hazardous weather?
Most pilots are prudent and highly skilled. Modern transports can take a good deal of punishment. The United States' system of air commerce and aviation regulation is remarkably safe and reliable. It is also fallible. The system's operators may expect too much. The human factor, the National Transportation Safety Board has observed, is "one of the oldest -- but least understood -- causes" of aircraft accidents.