Smart flying can outwit microbursts

How scenario replays and science have teamed to tame these intense events.

By Mike Smith
CEO, WeatherData Services

Artist’s conception of an aircraft on approach as a deadly microburst slams onto the runway.

Twenty-five years ago, on Aug 2, 1985, Delta Flight 191 crashed short of the runway at DFW in a microburst, killing 137 people.

Conversation recorded on the L1011’s CVR shortly before the accident included the following exchange:

FO: Lightning coming out of that one.
Capt: What?
FO: Lightning coming out of that one.
Capt: Where?
FO: Right ahead of us.

Between 1973 and 1995, airline accidents and major incidents related to microburst windshear were occurring at 16-month intervals. Since that horrible Dallas evening, there has been just 1 commercial airline crash due to a microburst—USAir Flight 1016 at CLT on Jul 2, 1994.

Instead of 16-month intervals between accidents and incidents, it has been 187 months from that crash to the present time. What changed? And how are those changes pertinent to today’s pilots? While the danger posed by extreme windshear seems obvious today, it was hardly obvious that evening in Dallas.

Initial news reports speculated that lightning was to blame for the crash. Eleven days after the crash, coverage in The New York Times mentioned neither microbursts nor windshear. For the 8 years prior to that night, a battle had been raging within the science of meteorology.

Ted Fujita had hypothesized that a form of windshear he called a “downburst” had downed Eastern Air Lines Flight 66 at JFK in 1975. He called a smaller version of a downburst a “microburst.” Most in the fields of meteorology and aviation were deeply skeptical of Fujita’s theory, even though confirmatory photographic and radar data confirmed their existence within 2 years.

The skepticism vanished within months of the Delta crash. Post-accident investigation confirmed a level of windshear caused by the micro­burst that overwhelmed the flight­crew and caused the L1011 to crash short of the runway. During the 2 years following the accident, efforts were mounted to teach pilots how to recognize and avoid microbursts and how to escape an inadvertent encounter.

The results have been amazingly successful. Even in the 1 commercial airline crash that has occurred since 1985, the crew was discussing the potential for windshear and received a windshear alert before entering the microburst and crashing.

There are 2 primary defense systems in the war against windshear. The first is the Low Level Wind­shear Alert System (LLWAS)—an array of 6–32 aerovanes (which look like wingless miniature airplanes on a stick) that measure wind at the airport and on the approach courses. When the LLWAS detects windshear, it flashes an alert to the control tower which is relayed to approaching or departing aircraft as appropriate.

The second defensive system is Terminal Doppler Weather Radar (TDWR), which is deployed at 44 US airports chosen on the basis of traffic and windshear risk. The TDWR is able to measure wind using the Doppler technique with greater precision than the WSR88D radars.

If we look at the base reflectivity data from the ICT WSR88D—9 miles closer to the EWK microburst—there is no easily discernible indication of a microburst. (See lower illustration)

(L) Wichita TDWR reflectivity data is displaying a tiny echo on the west side of EWK (Newton KS) at 18:04 CDT which at first glance seems unlikely to be a danger. (R) In the next 15 minutes, the small cell collapses and produces a downburst which is well detected by the ICT TDWR. The small circle of yellow echo with green on the leading (southern) edge is the downburst at 18:22. This would represent a hazard to an aircraft on approach to EWK.

Airports with a combination of TDWR and LLWAS are relatively safe from microbursts. However, airports using LLWAS and the WSR88D are less safe because the 88D radar does not detect micro­bursts well.

Despite this scientific triumph, there are danger signs that the lessons of Delta 191 are being “unlearned” by a new generation of pilots who are either not familiar with or are unaware of the danger of windshear accidents.

Two studies by Dale Rhoda et al—conducted at DFW and MEM in 1999 and 2000, respectively—reveal an apparent increase in thunderstorm penetrations and a reluctance to be the first aircraft to deviate when thunderstorms are on or near the approach course.

These findings were sufficently robust that NASA’s human factors research continues to cite them in presentations to aviation groups. The question is why competent pilots would put themselves in jeopardy by penetrating thunderstorms on approach.

Bizav applications

Ring of downburst outflow continues to spread around EWK. Greens indicate winds blowing toward the radar, yellows away. Even a small echo can indicate deadly windshear.

Some of the basic lessons learned from research on microbursts conducted during the 1970s and 1980s apply to today’s corporate aircraft operations. Based on data conducted during project Nimrod in Illinois and JAWS in Denver CO, we know the basic structure of a microburst.

On entering the microburst, an aircraft encounters increased headwind. In many cases, the PIC would lower the nose to get back on the glideslope, only to encounter the sinking air, then tailwind, and the aircraft would crash short of the runway. This same headwind/sink/tailwind sequence also caused aircraft to crash on takeoff.

Examples include Continental Flight 426 in Denver CO in Aug 1975 and Pan Am Flight 759 at New Orleans LA in Jul 1982. There are 3 mechanisms for keeping today’s aircraft safe from wind­shear events—training, avoidance and warnings.



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