Wind shear is a sudden change in wind speed or direction. A microburst is one of its most dangerous forms: a downward rush of air from a thunderstorm that spreads out in every direction when it hits the ground. An aircraft on final approach that flies through one first gains airspeed, then is pushed down, then loses airspeed, all within a few hundred feet of the ground. On 2 August 1985 that sequence brought down Delta Air Lines Flight 191 short of the runway at Dallas/Fort Worth.
What wind shear is
The FAA describes wind shear as a sudden change in wind speed and direction, and notes that it is common in thunderstorms because of microbursts. It matters most near the ground, where an aircraft on approach or just after take-off is slow and has little height to lose.
What a microburst does to an aircraft on approach
A microburst is cool, heavy air falling out of a cumulonimbus cloud. When it reaches the ground it spreads out, like water from a tap hitting a sink. It is not always visible. An aircraft flying through it meets three things in turn:
- A headwind. The air flowing out towards the aircraft raises its airspeed and lift, and it rises above the glide path.
- A downdraft. In the core, the air is falling. The aircraft is pushed down and loses lift.
- A tailwind. On the far side, the air is flowing away from it. The aircraft loses airspeed, and lift, just as it has been pushed low.
The danger is the swing from the first phase to the third. On Delta 191, the NTSB measured a horizontal wind change of at least 73 knots across the microburst.
Delta 191 at Dallas/Fort Worth, 1985
Delta Air Lines Flight 191 was a Lockheed L-1011-385-1 TriStar, registration N726DA, flying from Fort Lauderdale to Los Angeles with a stop at Dallas/Fort Worth. It carried 152 passengers and 11 crew. The aircraft type has its own page, Lockheed L-1011 TriStar.
The storm. Between 17:53 and 18:00 Central Daylight Time, a storm cell north of the airport grew from weather radar level 1 to level 4. The NTSB found the centre weather service meteorologist was away from his station, and nobody passed the change on. Approach control described only a rain shower north of the field.
The approach. At 18:04:18 the first officer, who was flying, remarked on lightning in the cell ahead. The approach continued. Just after the 1,000-foot call, the aircraft entered the rain shaft. The first officer held the aircraft through the first gust with a high nose-up attitude and near take-off thrust. Seventeen seconds before impact, it met rapid reversals in the wind. The stick shaker and the ground proximity warning sounded, and the captain called for go-around thrust.
The impact. At 18:05:52 the TriStar touched down in a field about a mile and a quarter short of runway 17L. It became airborne again, struck a car on State Highway 114 and then water tanks on airport property, and broke up in a fire. The NTSB reported that 134 of the 163 people on board were killed, along with the driver of the car. Twenty-six passengers and three flight attendants survived, most of them in the rear of the aircraft.
Why the alarm did not help. Dallas/Fort Worth had a low-level wind shear alert system (LLWAS), made up of wind sensors around the airport. The microburst was north of the airport, outside the sensor network, and the NTSB concluded that the system could not have warned this crew in time.
The probable cause. The NTSB (report AAR-86/05) found that the probable cause was the crew’s decision to begin and continue the approach into a cumulonimbus cloud they had seen to contain lightning. Two other factors were the lack of specific guidance, procedures and training for avoiding and escaping low-altitude wind shear, and the lack of real-time wind shear hazard information. Together they led to an encounter with a severe, microburst-induced wind shear at low altitude.
Earlier warnings
Delta 191 was not the first microburst accident. These two do not have event pages on this site yet.
- Eastern Air Lines Flight 66, New York JFK, 24 June 1975. A Boeing 727 on an instrument approach to runway 22L flew through a very strong thunderstorm sitting on the approach path and struck the approach lights. Of the 124 people on board, 113 died. The NTSB found the probable cause was adverse winds associated with the thunderstorm. It noted that the winds might have been too severe for a successful approach even with a fast, correct response. The continued use of runway 22L was a contributing factor.
- Pan Am Flight 759, New Orleans, 9 July 1982. A Boeing 727 lifted off into showers east of the airport, climbed to between 95 and about 150 feet, then descended into trees and a residential area. All 145 people on board and 8 people on the ground were killed. The NTSB found that the aircraft met a microburst-induced wind shear just after lift-off. A contributing factor was the limited ability of the ground-based wind shear detection of the time to warn pilots and controllers.
What changed
Each of these reports asked for the same things: detect the wind shear, warn the crew, and train them to escape it.
- Research. After a Congressional mandate to the FAA, a decade of FAA, NASA, industry and university research followed. NASA’s Langley Research Center developed microburst sensors, including a Doppler radar that looks ahead of the aircraft to measure wind shear.
- Wind shear systems on board. Under 14 CFR 121.358, turbine airliners built after 2 January 1991 must carry an approved airborne wind shear warning and flight guidance system, an approved detection and avoidance system, or a combination of the two. Older turbine airliners had to have at least an airborne wind shear warning system.
- Doppler radar at airports. Terminal Doppler Weather Radar (TDWR) was developed by the Massachusetts Institute of Technology’s Lincoln Laboratory in response to the wind shear crashes of the 1970s and 1980s. It detects microbursts, gust fronts and wind shifts in the approach and departure corridors, and passes warnings to controllers and pilots. The FAA says it has been in service since 1994.
- Ground sensors alongside radar. Airports kept their LLWAS wind sensor networks next to the new radar. The USAir 1016 report examines how both were used.
- Escape training. The recommendations after Delta 191 led to a standard escape manoeuvre: maximum thrust, pitch to a target attitude, accept the stick shaker intermittently, and keep thrust up until clear of the shear.
Has it worked?
The FAA says the last wind shear accident in the US was USAir Flight 1016 at Charlotte, North Carolina, on 2 July 1994, before the airport’s TDWR was installed and working. The DC-9 collided with trees and a house after a missed approach in a thunderstorm, and 37 passengers were killed. The NTSB’s probable cause included the crew’s decision to continue into severe convective activity “conducive to a microburst”. It also cited the crew’s late recognition of the wind shear, an escape attitude and thrust setting that were not kept up, and the lack of real-time wind shear warnings from air traffic control. The accident showed that the equipment needed controllers and crews to use the information they had.
Terrain and collision warnings followed the same pattern; see TCAS and Terrain Warnings. Weather can also bring down an aircraft through ice on the wings; see Aircraft Icing.
Every accident here had more than one cause, which is the subject of the Swiss cheese model. To read the NTSB’s reasoning in full, How to Read an Air Accident Final Report shows where to look. The TriStar’s other accidents are in Lockheed L-1011 TriStar Crashes. More weather accidents are on the weather or wind shear topic page.
Four wind shear accidents compared
| Flight | Aircraft | Date and place | Phase of flight | Deaths | What the NTSB found |
|---|---|---|---|---|---|
| Eastern 66 | Boeing 727-225 | 24 June 1975, New York JFK | Approach | 113 of 124 on board | Adverse winds from a strong thunderstorm on the approach path |
| Pan Am 759 | Boeing 727-235 | 9 July 1982, New Orleans | Take-off and initial climb | 145 on board, 8 on the ground | Microburst-induced wind shear just after lift-off |
| Delta 191 | Lockheed L-1011 TriStar | 2 August 1985, Dallas/Fort Worth | Final approach | 134 of 163 on board, 1 on the ground | Approach continued into a storm with lightning; no wind shear training or real-time information |
| USAir 1016 | Douglas DC-9-31 | 2 July 1994, Charlotte | Missed approach | 37 passengers | Approach continued into convective activity; wind shear recognised late; no real-time warnings |
Sources
- National Transportation Safety Board: Aircraft Accident Report NTSB/AAR-86/05, Delta Air Lines Flight 191, Lockheed L-1011-385-1, N726DA, Dallas/Fort Worth International Airport, August 2, 1985
- National Transportation Safety Board: Aircraft Accident Report NTSB-AAR-76-8, Eastern Air Lines Flight 66, Boeing 727-225, John F. Kennedy International Airport, June 24, 1975
- National Transportation Safety Board: Aircraft Accident Report NTSB/AAR-83/02, Pan American World Airways Clipper 759, Boeing 727-235, New Orleans International Airport, Kenner, Louisiana, July 9, 1982
- National Transportation Safety Board: Aircraft Accident Report NTSB/AAR-95/03, USAir Flight 1016, DC-9-31, Charlotte/Douglas International Airport, July 2, 1994
- US Code of Federal Regulations (eCFR): 14 CFR 121.358, Low-altitude windshear system equipment requirements
- Federal Aviation Administration: Terminal Doppler Weather Radar (TDWR)
- NASA: Technology for Safer Skies (NASA Spinoff, 1995)
Last checked 31 October 2026.