Aviation - Atmospheric Science (Microburst)

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1304BPS: Atmospheric Science

Assessment Item:| Report|
Due Date:| 5pm, 23-9-2011|
Student: | |
Student No. : | |
Course Convenor: | |

1.0 Introduction| 2|
2.0 Microburst| 2|
- 2.1 Crash Analysis (Microburst)| 4|
3.0 Aircraft Icing| 5|
* 3.1 Crash Analysis (Aircraft Icing)| 8|
4.0 Conclusion| 9|
5.0 References| 9|

Executive Summary
Many Meteorological phenomena have been identified as being directly hazardous to aircraft in flight. This report focuses on two of these: Microbursts and Aircraft Icing. The first being a localised wind shear event caused by rapid cooling and descent of a column of air, the second being the build-up of ice on an aircraft as it flies through certain conditions. Each weather condition is explained in detail, with emphasis on how and why it is dangerous in the context of aviation. Two aircraft crashes, each relating to one of the weather conditions, are analysed and the meteorological factors that led to the crash examined. The official National Transport Safety Board (NTSB) documents are used as a reference.

1.0 Introduction
Meteorological conditions have an obvious influence on aviation and can be extremely hazardous in certain situations. The experience or ability of a pilot can often become irrelevant if the aircraft gets caught in a one of the many dangerous weather phenomena known to cause aviation accidents. Thunderstorms, lightning, wind shear and icing were deemed responsible for downing 4927 general aviation aircraft between 1994-2003 in the USA alone (National Transportation Safety Board, 2004). This report will focus primarily on two weather phenomena; Microbursts (Wet/Dry) and Aircraft Icing. Each meteorological condition will be explained in detail and an example of an aviation accident relating to the two types of weather condition will be analysed.

2.0 Microburst
A microburst is a rapidly descending column of air generally caused by a temperature differential associated with rain clouds. Tetsuya Fujita, a leading weather expert, defines a microburst as ‘affecting an area less then 4km in diameter’, separating a microburst from the more common ‘wind shear’, which generally affects far larger areas. Cool air descends beneath cloud cover, accelerating towards the ground before spreading linearly outward, resulting in localised high wind speeds in a radial pattern, with damage converging on the point of initial contact with the ground (see Figure. 1). For airliners and smaller aircraft alike, microbursts pose a significant threat especially at take-off and landing. Sudden and drastic variance in vertical/horizontal wind speed due to microbursts has been attributed to at least 59 general aviation crashes in the period between 1994 and 2003 (NTSB, 2004).

Figure 1: The rapidly moving cold air hits the ground and spreads out horizontally. These phenomena can last anywhere between a few seconds and several minutes. Sudden wind gusts of up to 270km/h have been recorded as a direct result of microbursts. A microburst develops in three distinct phases: 1. Downburst: A body of air underneath a cloud is cooled by precipitation or virga (rain that evaporates before hitting the ground) and begins to descend. This downdraft accelerates and eventually makes contact with the ground. 2. Outburst: Having hit the ground, the air spreads out horizontally, driven by the column of air descending behind it. 3. Cushion: The air that is in contact with the ground begins to slow due to friction while the air above continues to push down and accelerate outward. Microbursts can be further categorized into ‘wet’ and ‘dry’ varieties. A wet microburst is produced as a result of warm and humid air being suddenly cooled. Precipitation or hail cools the column of air through which it is falling, resulting in a downdraft and subsequent outburst of wet air. A wet microburst is partially driven by the downward...
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