Precursors

July 23, 1973 Ozark Airlines Flight 809

Ozark Airlines Flight 809, a Fairchild-Hiller 227B, crashed near St. Louis Airport, killing 38 of the 44 persons on board. Probable cause was the aircraft's encounter with a downdraft following the captain's decision to initiate and continue an instrument approach into a thunderstorm. Shortly before the crash, the flight crew was informed by the tower controller of a heavy rain shower moving right across the approach end of the runway. The crew acknowledged that they could see the shower but elected to continue the approach. After passing the outer marker, the aircraft began to descend below the glide slope until it struck the ground.

December 17, 1973 Iberia Flight 933

Iberia Flight 933, a McDonnell Douglas DC-10, approaching Boston's Logan International airport in bad weather (rain, fog, two mile visibility) struck approach lights 500 feet short of the threshold and collided with a dike. The right main gear was sheared off, and the aircraft skidded and came to a rest 3,000 feet from the threshold. The accident caused injuries but no fatalities. Information from the aircraft's flight data recorder helped investigators establish the presence of windshear. Subsequent study of this accident led to a new understanding and awareness of the windshear hazard. In this accident the pilot did not recognize, and may have been unable to recognize, the increased rate of descent in time to arrest it before the aircraft struck the approach light piers. The increased rate of descent was induced by an encounter with a low-level windshear at a critical point in the landing approach where the pilot was transitioning from automatic flight control under instrument flight conditions to manual flight control with visual references. The pilot's ability to detect and arrest the increased rate of descent was adversely affected by a lack of information as to the existence of the windshear and the marginal visual cues available.

June 24, 1975 Eastern Airlines Flight 66

Eastern Airlines Flight 66, a Boeing 727, struck approach lights and crashed during an attempted landing at New York's Kennedy airport. Of the 124 occupants on board the aircraft, 113 were killed. Despite being aware of an L-1011 crew who had just abandoned their approach to the same runway due to "severe windshear," the tower controller cleared the 727 to land and the crew elected to continue their approach despite the presence of a very strong thunderstorm located over the final approach course. During transition from instrument to visual conditions, the aircraft experienced a high descent rate due to windshear which caused the aircraft to contact the approach light towers, ultimately severing the left wing which ignited a severe fire and caused the aircraft to roll into a steep left bank, ultimately leading to destruction of the aircraft. The flight crew's delayed recognition and correction of the high descent rate were probably associated with their reliance upon visual cues rather than on flight instrument reference. However, the adverse winds might have been too severe for a successful approach and landing even if they had relied upon, and responded rapidly to, the indications of the flight instruments. Contributing to the accident was the continued use of runway 22L when it should have become evident to both air traffic control personnel and the flight crew that a severe weather hazard existed along the approach path.

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Photo of Continental 426, Denver Stapleton, 1975
Photo of Continental 426, Denver Stapleton, 1975

August 7, 1975 Continental Flight 426

Continental Flight 426, a Boeing 727, attempting to takeoff from Denver's Stapleton airport runway 35L, reached approximately 100 feet altitude then crashed near the departure end of the runway. At the time of the accident, a thunderstorm with associated rain showers was moving over the northern portion of the airport. The National Transportation Safety Board determined that the probable cause of this accident was the aircraft's encounter, immediately following takeoff, with severe windshear at an altitude and airspeed that precluded recovery to level flight. The windshear caused the aircraft to descend at a rate which could not be overcome, even though the aircraft was flown at or near its maximum lift capability throughout the encounter. The windshear was generated by the outflow from a thunderstorm which was over the airplane's departure path. The 134 persons on board the aircraft survived the crash though 15 were severely injured.

July 9, 1982 Pan Am Flight 759

Pan Am Flight 759, a Boeing 727, attempted to depart the New Orleans International Airport during a severe thunderstorm. The aircraft climbed to approximately 100 feet when it began to descend, striking trees past the end of the runway and crashing into a residential area. All 145 people on board the aircraft along with eight people on the ground were killed in the accident. Probable cause was identified as "the airplane's encounter during the liftoff and initial climb phase of flight with a microburst-induced windshear which imposed a downdraft and a decreasing head wind, the effects of which the pilot would have had difficulty recognizing and reacting to in time for the airplane's descent to be arrested before its impact with trees. Contributing to the accident was the limited capability of current ground-based low-level windshear detection technology to provide definitive guidance for controllers and pilots for use in avoiding low-level windshear encounters."

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