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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 frictional force of the falling precipitation and therefore relies on a high saturation level in order to form. These microbursts are generally easy to spot as strong shafts of precipitation can be seen hitting the ground and then spreading horizontally. Dry microbursts on the other hand are generally caused by virga underneath high level cumulus clouds with little to no surface precipitation. Moisture in and underneath these clouds cools an otherwise dry and warm column of air to a point where the temperature differential creates enough negative buoyancy to begin the downburst phase.
Both wet and dry microbursts depend on mid-level moisture, the adiabatic lapse rate in the sub-cloud layer and relative humidity at ground level (Wakimoto, 985). These variables are complicated to model and forecast, making microbursts extremely difficult to predict. This poses a particular risk to aircraft at lower altitudes as dry microbursts can occur in Visual Meteorological Conditions (VMC) with little to no warning, resulting in loss of control due to sudden crosswind or severe increase/decrease in lift due to either headwind of tailwind (depending on direction of flight relative to the microburst).
The smaller scale of microbursts translates to much steeper wind shear gradients and therefore a much more hazardous situation then standard wind shear. 2. 1 Crash Analysis (Dry Microburst) On 24th July 2005 a single-engine Beach N35 was destroyed shortly after take-off from Salt Lake City municipal airport, Utah. All three passengers on board at the time sustained fatal injuries as a result of the crash (NTSB Incident DEN05FA114, 2005). The aircraft was found to be approximately 50kg over normal operating weight, but still within maximum parameters.
According to the NTSB investigation, both the pilot and the pilot-rated passenger had checked the local weather briefing from the flight services station, and believing there to be minimal risk, proceeded to attempt take-off. Witnesses in the area reported that the plane took off without incident but stalled during climb approximately 150ft above the ground. The plane then rolled sharply, lost altitude and impacted with terrain within the aerodrome boundary. Autopsy results from the passengers ruled out any edical cause for the crash and mechanical failure was ruled out based on witness reports and examination of the wreckage. At the time of the crash, the local weather briefing included a warning for high level thunderstorms in the area; however weather data confirmed the complete absence of precipitation and minimal wind until the time of the accident. The NTSB report states that conditions were extremely favourable for the formation of dry microbursts, and goes on to state the crash was likely caused by ‘a dry microburst from high level thunderstorms in the area’ (NTSB Probable Cause Report DEN05FA114, 2006).
Although the conditions did not seem dangerous based on weather briefings and pilot judgement, an unexpected microburst occurred in the area of the aerodrome during take-off. This likely caused a sudden increase in tailwind, stalling the aircraft and resulting in loss of height and eventual collision with the ground (See Figure 2). There is very little the pilot could have done in this situation as there was no real warning of this event occurring. This crash is a prime example of the unpredictability and hazardous nature of microbursts.
Figure 2: Impact of microburst on aircraft during take-off. (Cotton, W. R, 2010). 3. 0 Aircraft Icing Icing can be a major issue for aircraft of all sizes/types and according to the NTSB, resulted in 350 general aviation accidents between 1994-2003 (NTSB, 2004). Icing will only occur in specific meteorological conditions where the atmosphere contains droplets of supercooled liquid water which will instantly form ice crystals upon contact with an aircraft. The likelihood of icing is defined quantitatively by the average droplet size, Liquid Water Content (LWC) and local atmospheric temperature.
This makes icing conditions fairly straightforward to forecast given adequate weather recording systems in the area. Most aircraft are not rated to fly in known icing conditions and local weather services are obligated to warn against flying into such situations. Icing can occur in a number of ways on an aircraft. The most common is structural icing, where ice crystals form on the exterior surfaces of the aircraft. This can cause a considerable decrease in performance as the ice adds weight and inhibits the flow of air over the wing.
Severe icing can also restrict movement of control surfaces and cause a pilot to lose control. Icing can also affect engine performance through carburettor or inlet icing. Icing, accelerated by the venturi effect as humid air enters the air inlet, can impede airflow and restrict the mechanical workings within the carburettor (See Figure 3). Fuel-injected systems, although not directly susceptible to carburettor icing, can still suffer from air intake icing which can slow down or completely block airflow into the engine. This can have a negative impact on engine performance and eventually lead to loss of engine function.
Figure 3: Build up of Icing in Induction System (Civil Aviation Authority, 2000) According to the Terry T. Lankford in his book ‘Aircraft Icing: A Pilot’s Guide’ (1999) there are several different types of icing that can occur on an aircraft, all of which have the potential to hamper sustained flight: * Clear Ice: Supercooled liquid droplets strike the aircraft and then slowly freeze into large ice formations that can protrude out from the aircraft. * Rime Ice: Supercooled liquid droplets strike the aircraft and freeze instantly into a thin layer of ice that coats the exterior of the aircraft. Runback Ice: Droplets strike the aircraft and collect in unprotected areas of the aircraft, forming solid areas of ice that can hamper the free movement of control surfaces. * SLD Ice: Super-cooled Large Droplet ice forms when above-average size droplets strike the aircraft and freeze in a similar way to Clear Ice. This icing can occur very quickly and form large ice crystals anywhere on the aircraft. Icing can be avoided by simply not flying into the meteorological conditions known to produce ice. However, many aircraft manufacturers include anti-icing systems that can be used in-flight in the event of unexpected icing.
External surfaces can be heated by routing engine gases through ducts in the structure of the aircraft or via electrical heating. Another method is the use of ‘deicing boots’. These consist of a sealed rubber membrane stretched over the leading edge of a wing, which can inflate pneumatically in flight and break the ice off the aircraft. A more drastic method in some larger aircraft is an anti-icing fluid distribution system, which delivers fluid directly to areas where icing is occurring via tubes and spray nozzles.
Within the air inlet, exhaust gases are rerouted or carburettor heat applied to minimise icing. In many aircraft, an alternate air inlet is also available in the event of complete blockage. Extreme icing can contribute to the crash of an aircraft by reducing performance to the point where sustained flight becomes impossible. Structural or carburettor icing, or in some cases a combination of the two, is a dangerous situation for any pilot if the correct actions are not taken to avoid an accident. 3. 1 Crash Analysis (Icing)
On 16th of February 2007 a twin-engine Cessna 340A crashed into a snowy mountainside 3nm South-Southeast of Council Bluffs Municipal Airport, Iowa. At the time of the crash Night Instrument Meteorological conditions prevailed in the area. The pilot and three passengers sustained fatal injuries as a result of the crash. NTSB records indicate that the pilot had obtained the local weather briefing, which stated the possibility of ‘moderate icing’ and ‘moderate turbulence’ in the area. This was confirmed by other pilots and witnesses, who had reported icing below 9000ft earlier in the day.
The Current Icing Potential (CIP) for the area was recorded as being a ‘70% probability of severe icing between 9000ft and 3000ft’ (NTSB Probable Cause Report CHI07FA073). As such, the aircraft likely encountered severe icing and possibly wind shear on descent, resulting in loss of control and subsequent collision with the ground. The aircraft, a 1977 Cessna 340A, was equipped with inflatable deicing boots to prevent major icing on the leading edge of the wings. However, post-crash analysis of the wreckage reported that the system had either not been deployed or was not functioning correctly during the flight prior to the crash.
In this case, the pilot failed to acknowledge the potential for icing and take the proper precautions in conditions that had been determined very likely to produce severe aircraft icing. The accident occurred at night in low visibility conditions, further hampering the pilot’s ability to safely land the aircraft. It is clear that icing is at least partially responsible for the accident, with icing on the external surfaces and air inlets likely contributing to the eventual crash. The accident occurred at night in low visibility conditions, further hampering the pilot’s ability to safely land the aircraft.
The accident was probably avoidable if the weather warnings had been acknowledged and the planned rote revised to minimise the risk of icing. 4. 0 Conclusion Both of these meteorological conditions, as well as all other hazardous weather phenomena, can be extremely hazardous if weather warnings are ignored or proper actions taken to minimise risk while flying. In some cases these conditions cannot be accurately predicted, and the responsibility therefore falls onto the pilot to determine whether it is safe to fly in the area.
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