1.2.4.2.3. Evaporative cooling, virga and downbursts

  • 10 months ago
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  • Author: SW101

[ADVANCED] x.6. Downbursts and aircraft safety

Downbursts present a particular hazard to air traffic and have been responsible for a number of major aircraft accidents. Aircraft caught within the rapidly descending air close to the ground on take off or landing experience could rapid and unexpected descent rates as well as strong turbulence which may result in loss of control of the aircraft and crash. Major crashes involving downbursts in the 1980s have resulted in aircraft now being equipped with on-board sensors to successfully and reliably detect and avoid downbursts.

NASA artist’s rendering of a microburst.

How a downburst affects an aircraft at low altitude

An aircraft entering a downburst will initially encounter strong headwinds from the expanding gust front close to the ground. The indicated airspeed will increase; a pilot on approach, wanting to to fly at a set airspeed may reduce power. As the airraft passes through the downburst, the headwind becomes a tailwind the indicated airspeed and lift drops. Aircraft performance is decreased and descent rate increased. Also, the rapidly downward moving airmass may exert a significant downward force on the aircraft, increasing its descent rate. Very rapid descent rate combined with poor visibility may result in a crash. Also, very strong tailwind may result in the aircraft stalling and crashing.

Over the past several decades, a number of major aircraft accidents have been attributed to downbursts/microbursts:

  • Malév Ilyushin Il-18 crash at Copenhagen Airport – August 28, 1971
  • Eastern Air Lines Flight 66, Boeing 727 at John F. Kennedy International Airport, New York – June 24, 1975 [111 fatalities, 13 survivors; crashed on approach to JFK airport over 700 m short of the runway]
  • Pan Am Flight 759, Boeing 727, New Orleans International Airport, Louisiana – July 9, 1982 [153 fatalities – 145 on the aircraft, 4 on the ground; aircraft crashed on takeoff, reaching 30-45 m altitude and then crashing into trees and houses just 1400 m from the end of the runway. Winds reported as “gusty and swirling”.]
  • Delta Air Lines Flight 191, Lockheed L-1011 TriStar, Dallas/Fort Worth International Airport, Texas – August 2, 1985 [137 fatalities, 27 survivors; crashed on approach at Dallas/Fort Worth International Airport; aircraft encountered a downburst, descending at up to 15 m/sec, missing the runway and crashing into Texas State Highway 114. Crash investigation resulted in NASA testing a Doppler weather radar on board an airplane, ultimately resulting in the airborne wind shear detection and alert system. FAA now mandates that all commercial aircraft must have on board windshear detection capability]
  • Martinair Flight 495, McDonnell Douglas DC-10, Faro Airport, Portugal – December 21, 1992 [56 fatalities, 284 survivors]
  • USAir Flight 1016, Douglas DC-9, Charlotte/Douglas International Airport, North Carolina – July 2, 1994 [37 fatalities, 20 survivors; aircraft entered rapid descent, crashed 800 m from the runway]
  • Bhoja Air Flight 213, Boeing 737-200, Islamabad International Airport, Pakistan – April 20, 2012 [127 fatalities; aircraft entered successive strong downfrafts, on exit from the final downdraft the crew maintained high pitch angle, stalling the aircraft and crashing]

Several other aircraft accidents have also been attributed to downbursts.