Thunderstorm-generated wind shear was poorly understood until three major airline accidents compelled meteorologists and aviation experts to find solutions to the problem
On June 24, 1975, Eastern Airlines Flight 66, a Boeing 727 with 116 passengers and a crew of eight, was on final approach at John F. Kennedy International Airport in Jamaica, N.Y. Thunderstorms, some severe, were affecting parts of the airport, but the flight crew was not made aware of any particularly bad weather or that a preceding airplane had encountered serious difficulties while landing in strong winds. At an altitude of approximately 500 feet, the 727 encountered strong headwinds produced by the outflow from a severe thunderstorm. As the airliner flew directly under the storm, it quickly descended in downdrafts with an estimated negative vertical velocity of 1,000-1,500 feet per minute. Although the first officer piloting the 727 called for takeoff thrust, the airliner impacted the ground some 2,300 feet short of the runway. The crash and resultant fire killed all onboard and destroyed the aircraft.
In response to the Flight 66 crash, Horace R. Byers, director of the mid-1940s Thunderstorm Project, and T. Theodore Fujita, a brilliant young Japanese-American meteorologist, conducted an investigation into the weather conditions surrounding the accident. They concluded that it resulted from a concentrated thunderstorm downdraft—a burst of air that sank rapidly to the surface and then spread out in all directions. This “downburst,” as they termed it, covered 2½ miles or more, could last from five to 20 minutes and produced winds in excess of 40 mph.
Even more important for aircraft, they noted a downburst could have a downward speed of 12 feet per second (720 feet per minute) at 300 feet above ground level. This value corresponded to the approximate downward speed of a typical transport aircraft flying a stabilized 3-degree glide slope, and thus was capable of doubling the descent rate of the affected aircraft. It was about 10 times the downdraft velocity of a typical thunderstorm measured during the Thunderstorm Project, and it conflicted with the prevailing view that thunderstorm downdrafts weaken rapidly as they leave the cloud base. A pilot would have to recognize the situation and take immediate action to avoid potentially disastrous effects. The researchers also noted that downbursts could be extremely localized, sometimes affecting only certain parts of an airport.
Byers and Fujita used the term “wind shear” to describe the weather conditions associated with the Flight 66 crash. In meteorology, wind shear is defined as a change in wind speed and/or direction between two points, either in the horizontal or vertical plane. Soon the term would also be added to the aviator’s vocabulary.
In response to the incident, the FAA developed and began deploying a Low Level Windshear Alert System (LLWAS). The first phase of the project involved adding six anemometers to sites around airports and calculating the wind differences from the center field station. Although the system provided additional information for pilots, it supplied no vertical wind data, and thus did not solve the problem. As researchers learned more about the nature of downbursts and technology improved, later versions of the LLWAS were implemented.
To further examine the thunderstorm downdraft problem, meteorologists conducted two research projects. Project NIMROD (Northern Illinois Meteorological Research On Downbursts) in 1978 utilized Doppler radar and portable weather stations. Conventional radar sets plot thunderstorms by detecting areas of extreme rainfall rates typically associated with the storms. Other than by inference, there is nothing that specifically shows strong winds. Doppler radar, on the other hand, can actually show wind speed by referencing movement of precipitation or other airborne objects either toward or away from the radar site. It utilizes the Doppler effect, the shifting frequency of electromagnetic waves from a moving object. It can also scan the skies to detect strong winds or wind shear that cannot be measured by land-based instruments.
Ted Fujita and his colleagues determined that downbursts vary in size, from large ones called “macrobursts” to much smaller ones they named “microbursts.” The latter typically extended less than 2½ miles and lasted only two to five minutes, but they could produce wind speeds in excess of 100 mph. Their relatively small size made detection more difficult, and their intensity increased the threat they posed.
In 1982 Fujita and his University of Chicago group joined with the National Center for Atmospheric Research to intensively study thunderstorm downbursts in the Joint Airport Weather Studies (JAWS) project. The area in and around Denver’s Stapleton Airport was closely monitored over the summer for downburst occurrence. An amazing 99 events occurred within 10 nautical miles of the airport, indicating that microburst formation was more common and the threat to aircraft much greater than previously thought. Some contained rainfall (wet microbursts), but many in the dry climate of Colorado were precipitation-free (dry microbursts). These were especially difficult to see or to pick up on radar. Some microbursts were produced by showers without electrical activity.
Unfortunately, another major accident occurred during the JAWS project. On July 9, 1982, Pan Am Flight 759, a Boeing 727-235 with 138 passengers and a crew of seven, departed from New Orleans International Airport. Take-off and initial climb-out appeared normal, but then the airliner flew directly into a downburst from a heavy shower. Eyewitnesses said the 727 was in level or nose-up flight and seemed to just drop, striking a tree and crashing into a residential area beyond the airport. The impact and subsequent explosion and fire killed all 145 onboard. Eight people on the ground also perished. A post-accident analysis noted that radar had indicated a storm cell near the end of the runway, but it didn’t appear severe enough to warrant a warning. The LLWAS at the airport didn’t indicate a wind shear problem, so the pilots seemingly had no idea of the danger the storm presented.
This accident added impetus to research on microbursts. As other airports were monitored, the dangers became more and more evident. On August 1, 1983, a peak wind of 130 knots was recorded during a microburst at Andrews Air Force Base outside Washington, D.C. Other gusts to 84 knots were also noted, indicating the multiple threats from one storm. What made this observation even more compelling was the fact that Air Force One carrying President Ronald Reagan had landed at Andrews just minutes before the storm hit.
Two years later, on August 2, 1985, Delta Airlines Flight 191 was cleared for landing at Dallas/Fort Worth International Airport in Texas. The Lockheed L-1011 widebody was carrying 152 passengers and a crew of 11. Showers were reported near the airport, but nothing intense, and prior flights had landed without incident. On approach, the first officer pointed out to the captain that a cell immediately in their path had begun to produce lightning. At an altitude of 1,000 feet, the L-1011 encountered increasing headwinds from the storm. As the airliner entered a rain shaft, the captain, knowing he would lose the headwinds, throttled up the engines. The headwinds dropped 25 knots, and the plane encountered a downdraft of 1,800 feet per minute. As the aircraft exited the downburst, it was impacted by several vortices that affected both the horizontal and vertical velocity. The plane was now descending at an extreme 23-degree angle. Even at full throttle, it was at the mercy of the rapidly changing airflows. Finally, with a tailwind of 30 knots and a strong downdraft, the L-1011 impacted the ground more than a mile short of the runway. The crash and ensuing explosion and fire killed 137 people, including one on the ground, but 27 survived.
With this third major crash in 10 years, all under similar circumstances, Fujita’s once controversial theories were now accepted by both the meteorological and aviation communities. The FAA, with help from NASA, worked to address the wind shear problem. Besides the anemometer-based LLWAS, airports needed better radar coverage. Doppler weather radars were incorporated into the national radar network in the late 1980s, and today there are 154 Doppler sites in the United States. But with Doppler’s limited range and with many radar sets not located near airports, more resources were needed.
In the early 1990s, an improved Doppler radar with better resolution was developed at MIT with funding from the FAA. This unit, Terminal Doppler Weather Radar, could better detect aviation hazards such as wind shear, and was installed at 45 major metropolitan airports across the U.S. and Puerto Rico.
After the Flight 191 crash, NASA and the FAA began looking into onboard wind shear detection equipment to supplement ground detection systems under the Airborne Wind-Shear Detection and Avoidance Program. In 1993 all commercial aircraft were mandated to have forward-looking radar wind shear detectors.
By 1994 the danger of microbursts and wind shear was well known. Many major airports had LLWAS and Doppler radar detection systems, including Charlotte/Douglas International Airport in North Carolina. On July 2, a Douglas DC-9-31, USAir Flight 1016, was on final approach, with thunderstorms near the airport. Special weather observations indicating strong winds were sent out, but weren’t relayed to the pilots. When Flight 1016 encountered heavy rain and gusty headwinds, the pilots aborted the landing and began turning right for a go-around. As they turned, they encountered a peak headwind gust of 39 knots. Within seconds, they were in a tailwind of 26 knots, for a total wind change of more than 60 knots within just 15 seconds. The DC-9 contacted the ground, tail first, about a half-mile short of the runway, and 37 passengers were killed.
There were certainly failures on the part of the flight crew, who tried to land during a thunderstorm, and ATC, which didn’t keep the pilots informed of the seriousness of the situation. System failures also contributed to the accident. The DC-9 was equipped with a wind shear detection system, but it was later determined that movement of the flaps impeded the system. The LLWAS gave no warning because vegetation and new buildings impaired its view of the storm, and the closest Doppler radar unit, in Columbia, S.C., gave no indication of severe weather. All of these system problems were subsequently addressed.
Efforts to prevent thunderstorm wind shear accidents hinge on early detection and avoidance. Once an aircraft is within the region affected by thunderstorm winds, it is often too late, since conditions can exceed the aircraft’s performance capabilities. By gathering data from all sources and transmitting it directly to ATC, that information can be quickly disseminated to pilots whose takeoffs and landings might be affected.
There hasn’t been a major wind shear–related accident in the U.S. in recent years, a testament to the efforts of meteorologists and aviation officials who worked to address the problem. Although accidents still occur, especially in less developed countries that lack detection equipment, wind shear no longer poses the terrifying threat to aviation it once did.
Retired meteorology professor Ed Brotak has written extensively about the effects of natural hazards on aviation. Additional reading: The Downburst: Microburst and Macroburst, by T. Theodore Fujita.
This feature originally appeared in the January 2018 issue of Aviation History. Subscribe here!