A Very-Good-News Story: Microbursts and U.S. Aviation

If you’re one of the millions of Americans who plan to take at least one flight this summer, take heart: you are remarkably well protected from weather during your flight, especially considering the risks that U.S. passengers faced not that long ago. From the 1970s to the 1990s, more than 800 fliers perished in U.S. commercial airline crashes that were linked to microbursts, small but intense downdrafts generated by thunderstorms. It took years of persistence from scientists to raise awareness of the issue and solve the problem. But solve it they did, by developing warning systems that took advantage of Doppler radar, surface wind sensors, and sophisticated software. Thanks to these systems, the last fatal crash of a U.S. passenger plane attributed to a microburst was on July 2, 1994, when a US Airways flight crashed near the Charlotte-Douglas International Airport, killing 37. As of this weekend, we’ll have put that disaster 22 years behind us.


Figure 1. A microburst emerges from a thunderstorm downdraft. Image credit: NWS/Birmingham, AL.


Figure 2. Depending on the amount of moisture in a thunderstorm and its environment, a microburst may be “dry” (left) or “wet” (right). Both types can be dangerous to aircraft. A video by photographer Brian Snider shows the formation of a dramatic wet microburst near Tucson in August 2015. Image credit: NOAA.

A stealthy villain
Microbursts emerged as a major threat when the mid-century boom in consumer aviation put thousands more flights aloft. It was in the 1970s that the eminent storm researcher T. Theodore Fujita analyzed the risk from localized downdrafts related to convection (the upward motion related to instability that leads to showers and thunderstorms). First trained as a mechanical engineer, Fujita took a research flight in 1945 over the debris left by the bombs that struck Hiroshima and Nagasaki and observed starburst damage patterns emanating outward from the greatest impact points of the bombs. Later, while surveying damage from the Super Outbreak of tornadoes of April 3, 1974, Fujita recognized similar starburst patterns, and he concluded that some of the damage must have resulted from descending wind bursts.

The next year, an Eastern Airlines flight crashed while landing at John F. Kennedy International Airport in New York on June 24, 1975, killing 113. Fujita was enlisted to help analyze the disaster. In a 1976 report, he remarked on “four to five cells of intense downdrafts which are to be called ‘downburst cells’. Apparently, those aircraft which flew through the cells encountered considerable difficulties in landing, while others landed between the cells without even noticing the danger areas on both sides of the approach path.” Fujita estimated that the peak downdraft speed several hundred feet above ground was up to eight times higher than conventional thinking would have predicted. Alas, he added, “there is no way of predicting the occurrence of these phenomena both in time and space.”

Even with very limited data, Fujita’s legendary analytic skills enabled him to see the core process at work: “In general, the air near the ground spreads out violently from the ‘outburst center,’ the spreading center above the ground. Unless a heading correction is made immediately, an aircraft in the crosswind burst will drift away from its expected course. If an aircraft flies straight into the outburst center, its indicated airspeed will increase momentarily followed by a high rate of sink. Before the aircraft can break out of the downburst cell, its indicated air speed will drop suddenly, due to an increase in the tailwind component.”


Figure 3. Diagram showing how a microburst could bring down an aircraft that flies directly through it. Modern warning systems now allow air traffic controllers to steer passenger planes safely around microbursts. Image credit: NCAR Research Applications Laboratory.

From dismissal to acceptance
Fujita’s ideas faced widespread skepticism, as noted by Josh Chamot (National Science Foundation, or NSF): “Until the mid-1970s, most researchers believed that downdrafts would substantially weaken before reaching the ground and not pose a threat to aircraft. They blamed tornadoes and gust fronts as the primary causes of storm damage.” After the JFK crash, the FAA developed a simple cluster of anemometers called LLWAS that could detect wind shear related to large-scale phenomena such as a frontal passage, but the LLWAS sensors were not close enough together to detect microbursts and other crucial small-scale features.

Meanwhile, Fujita and colleagues at the National Center for Atmospheric Research (NCAR) carried out several field campaigns with NSF support, designed to convince scientists and policymakers of the hazard and figure out how to address it. The first of these was Project NIMROD (Northern Illinois Meteorological Research on Downburst). On May 29, 1978, Fujita and NCAR’s James Wilson became the first scientists to detect a microburst on radar. “It was right on top of us,” Wilson told me in a 2010 interview. After this event, Fujita created a new category, microbursts, to denote downbursts less than 4 kilometers (2.5 miles) across.

Project NIMROD detected about 50 microbursts. A follow-up project in 1982, JAWS (Joint Airport Weather Studies) found dozens more near Denver’s now-defunct Stapleton International Airport. When JAWS kicked off its operations that summer, “the FAA was still not ready to admit there was such a thing as a microburst,” according to Wilson. But midway through the project, on July 9, 1982, a Pan Am aircraft was forced down by a microburst in a residential neighborhood in Kenner, Louisiana, killing all 145 people on board and 8 others on the ground. That calamity helped jump-start FAA funding for the remainder of the summer. The agency’s involvement in research accelerated further after a microburst-related 1985 Delta crash near Dallas-Fort Worth International Airport took 137 lives.


Figure 4. NCAR’s John McCarthy spent much of his time during the summers of 1984 and 1985 in the control tower of Denver’s Stapleton International Airport during the CLAWS project (Classify, Locate, Avoid Wind Shear). Image credit: UCAR.


Several more field projects followed, as did NCAR’s development of an enhanced version of the LLWAS system that’s now in place at more than 100 U.S. airports. NCAR and MIT’s Lincoln Laboratory also began collaborating with the FAA to develop a radar-based warning system called Terminal Doppler Weather Radar. TDWR systems were deployed at 45 U.S. airports in the late 1980s and 1990s, parallel with the NWS NEXRAD deployment. Along with the warning system, there was newfound pilot awareness of the wind shear hazard. All of the world’s commercial jet pilots were soon required to take part in a wind shear training program, and a powerful series of videotapes featured pilots sharing their stories of microburst encounters. Together, these innovations led to the essential vanquishing of microburst-related accidents on U.S. passenger planes since the mid-1990s.

Outside the U.S., “most international airports have no wind shear protection,” said Bruce Carmichael, director of NCAR’s Aviation Applications Program. “However, the new generation of aircraft radar systems include forward-looking wind shear warnings, so there is a good deal of capability inherent in the aircraft.”

In the realm of general aviation (all civilian flights outside of scheduled passenger service), wind shear remains a deadly threat, because most of those flights involve smaller aircraft and airports that lack the means of detecting wind shear. Still, there is only an average of about 10 shear-related fatalities per year in U.S. general aviation, compared to the 400-plus general aviation deaths related to weather hazards as a whole.


Figure 5. An intense hailstorm bears down on the NWS NEXRAD radar located at Front Range Airport, just southeast of Denver International Airport, on May 21, 2014. A separate radar at DIA that focuses on smaller-scale phenomena is part of the FAA’s Terminal Doppler Weather Radar network. The storm was part of a multiday outbreak of severe weather from the Rockies to the Northeast U.S. (May 18-23) that caused $4 billion in damage. Image credit: Bob Henson.

Safer skies overall: just one fatal passenger crash in nearly a decade
Not only are U.S. airline passengers far safer from microbursts than they were decades ago, but they’ve been remarkably safe from other weather hazards—and just about all hazards, for that matter. On August 27, 2006, all 47 passengers aboard a Delta Connection flight were killed in a crash at the Blue Grass Airport near Lexington, KY, that was attributed to pilot error. Since then, there has only been one fatal accident on a U.S. passenger plane: a Continental Connection flight that crashed near Buffalo, NY, on February 12, 2009, killing all 45 passengers. This crash was also attributed to pilot error. For the nation to go nearly a decade with just one fatal accident on U.S. passenger planes would have been unthinkable as recently as the 1990s, when fatalities were a near-annual occurrence.

Americans are also much safer on the road: highway deaths are down by close to 20,000 per year compared to the 1970s and by about 10,000 per year compared to the 1990s. Unfortunately, a big spike occurred in 2015, when traffic deaths rose by an estimated 8% over the previous year, equating to some 2800 additional deaths. This was the largest year-over-year percentage increase in 50 years, according to the National Safety Council. While we don’t know exactly what caused this spike (increased traffic and growing phone-related distraction are possibilities), we do know that weather continues to be a major factor in highway safety. Roughly 3400 people are killed each year when driving in rain, and roughly 5700 on wet roads, according to the Federal Highway Administration.

The take-home message for travelers this summer: don’t worry too much about weather hazards if you’re a passenger on a commercial flight--but when you’re on the road, do everything that you can to drive safely and defensively, especially during adverse conditions. That text message can wait!

I'll be back with a new post by Friday afternoon, including updates on the tropical cyclone likely to form off the coast of Mexico in the next few days and the potential for very heavy rains across the Central Plains and Midwest into next week. PS: If you're hearing claims of "unprecedented" jet stream flow from the Northern to the Southern Hemisphere, check out the excellent debunking just published by Jason Samenow at Capital Weather Gang.

Bob Henson