Downburst at Johannesburg Intranational Airport

A cold front with thunderstorm activity was in the process of passing over Johannesburg International Airport at about 8 o' clock on the morning of Thursday the 8th October 1998 when the early morning flights from other domestic airports and overnight international flights began arriving.

During its approach to land the pilot of a Jetstream 41 aircraft reported 35 knot windshear. An Airbus A300, flight from Durban, following about 5 minutes behind suddenly encountered a +1.78 G force south of the airport, the radio altimeter registered large height fluctuations and the aircraft radar indicated heavy rain. Heavy rain in thunderstorm conditions is a sign that there is a strong downdraft ahead. The pilot decided that safety was the first option, initiated a climb and diverted back to Durban. Following shortly behind, a Boeing 737 suddenly encountered heavy rain and the aircraft fell 100 feet. The pilot also climbed away from the situation, but thereafter made a circuit of the airfield and landed at the second attempt. Altogether 5 flights diverted, two of them long distance international flights.

The news media got wind of fragmented bits of information and began to talk about a mircoburst and an aircraft dropping 2000 feet over Heidelberg. This paper attempts to discover exactly what happened. Did the aircraft encounter a strong downburst often associated with thunder activity. or was it a more intense microburst? Fujita (1985:9) divides downbursts into a 400 - 4 kilometre macroburst and a 4 kilometre to 4 metre microburst.

The incident received considerable media coverage at the time and is believed to be the first of its sort investigated in South Africa. The study looks at synoptic data, model output and flight recording data from one of the aircraft.

(NB. This is not a final abstract for photcopy preprints. The final copy will include an image or two in html. This abstract is understood to be for approval or rejection purposes only)