The Atmosphere - UK Transport Aircraft Accidents (1975-94) PDF

The Atmosphere 1 A Definition of Meteorology...

The Atmosphere 1 A Definition of Meteorology 1 The Atmosphere “The branch of science dealing with the earth’s atmosphere and the physical processes occurring in it.” Reasons for Studying Meteorology To understand the physical processes in the atmosphere To understand the meteorological hazards, their effect on aircraft and how to minimize the risks posed by those hazards To identify the weather information that is required for each flight To interpret actual and forecast weather conditions from the documentation provided To analyse and evaluate weather information before flight and in-flight To devise solutions to problems presented by weather conditions Weather is the one factor in modern aviation over which man has no control; a knowledge of meteorology will at least enable the aviator to anticipate some of the difficulties which weather may cause. Weather-influenced Accidents to UK Transport Aircraft Table 1 Transport aircraft accidents, 1975 - 94 All accidents Aeroplanes Rotorcraft All aircraft Year Total WI Per cent Total WI Per cent Total WI Per cent 1975-79 52 17 32.69 9 4 44.44 61 21 34.43 1980-84 67 20 29.85 20 7 35.00 87 27 31.03 1985-89 95 22 23.16 20 3 15.00 115 25 21.74 1990-94 216* 25 11.58* 20 6 30.00 236* 31 13.13* 1975-94 430 84 19.53 69 20 28.98 499 104 20.84 * Includes ramp and other minor ground accidents, hence low percentage figures. WI: Weather-influenced Accidents excluding selected ramp and other occurrences Aeroplanes Rotorcraft All aircraft Year Total WI Per cent Total WI Per cent Total WI Per cent 1975-79 52 17 32.69 9 4 44.44 61 21 34.43 1980-84 67 20 29.85 20 7 35.00 87 27 31.03 1985-89 78 22 28.20 20 3 15.00 98 25 25.51 1990-94 101 25 24.75 20 6 30.00 121 31 25.62 1975-94 298 84 28.18 69 20 28.98 367 104 28.34 WI: Weather-influenced 3  1 The Atmosphere Table 2 Weather-influenced accidents to transport aircraft by element of weather, 1975 - 94 1 All Accidents Fatal Accidents The Atmosphere Element No. Percentage of No. Percentage of total total Visibility 22 21.1 10 66.7 Icing/snow 22 21.1 3 20.0 Wind and turbulence 45 43.3 2 13.3 Rain/wet runway 12 11.5 0 0 Lightning 3 2.9 0 0 All cases 104 100 15 100 For this course a knowledge of advanced physics is not required, but a knowledge of the elementary laws of motion, heating, cooling, condensation and evaporation will be useful. A Definition of the Atmosphere “The spheroidal gaseous envelope surrounding a heavenly body.” The Constituents of the Atmosphere (By Volume) Nitrogen 78.09% Argon 0.93% Oxygen 20.95% Carbon Dioxide 0.03% Plus traces of: Neon Nitrous Oxide Helium Nitrogen Dioxide Krypton Carbon Monoxide Xenon Sulphur Dioxide Hydrogen Ammonia Methane Iodine and Ozone Also present are solid particles and, in particular, water vapour which, from a meteorological point of view, is the most important gas in the atmosphere. The proportions of the constituents remain constant up to a height of at least 60 km (except for ozone), but above this the mixing processes associated with the lower levels of the atmosphere no longer exist and gravitational separation of the gases occurs. Although the trace of ozone in the atmosphere is important as a shield against ultraviolet radiation, if the whole of the layer of ozone were brought down to sea level it would only be 3 mm thick. Properties of the Earth’s Atmosphere The earth’s atmosphere varies vertically and horizontally in: Pressure. Temperature. Density. Humidity. The earth’s atmosphere is fluid, supports life only at lower levels and is a poor conductor. 4  The Atmosphere 1 The Structure of the Atmosphere 1 The Atmosphere The Troposphere: is the lowest layer of the earth’s atmosphere where temperature decreases with an increase in height. consists of ¾ of the total atmosphere in weight. contains almost all the weather. The Stratosphere is the layer above the troposphere where temperature initially remains constant to an average height of 20 km then increases to reach a temperature of -2.5°C at a height of 47 km, then above 51 km temperature starts to decrease again. The reason for the increase is the action of ultraviolet radiation in the formation of ozone. The boundary between the stratosphere and the next layer, the mesosphere is known as the stratopause. The average height of the stratopause is 50 km in temperate latitudes. The Tropopause: This marks the boundary between the troposphere and the stratosphere and is where temperature ceases to fall with an increase in height. (Practically taken as the height where the temperature fall is less than 0.65°C per 100 m (2°C per 1000 ft.) T  he height of the tropopause is controlled by the temperature of the air near the surface. The warmer the air, the higher the tropopause. The colder the air, the lower the tropopause. Therefore, temperature variations due to latitude, season, land and sea, will all cause varying heights of the tropopause. There are two locations where the tropopause abruptly changes height or “folds”. These are at approximately 40° and 60° latitude. The average height of the tropopause at the Equator is 16-18 km with an average temperature of -75°C to -80°C, and at the poles 8 km with an average temperature of -40°C to -50°C. The average value at 50°N is 11 km (36 090 ft) with a temperature of -56.5°C. T  he temperature of the tropopause is controlled by its height. The higher it is, the colder the temperature at the tropopause. The lower it is, the warmer the temperature at the tropopause. The temperature at the tropopause can be as high as -40°C over the poles and as low as -80°C over the Equator. Figure 1.1 The mean height of the tropopause at the Greenwich Meridian 5  1 The Atmosphere The Significance of Tropopause Height 1 The Atmosphere The significance of the tropopause height is that it usually marks: the maximum height of significant cloud. the presence of jet streams. the presence of Clear Air Turbulence (CAT). It is now referred to as TURB. the maximum wind speed. the upper limit of most of the weather Temperatures Temperature in the troposphere increases from the poles to the Equator. Temperature in the lower stratosphere increases from the Equator to the poles in summer but reaches max temperature in mid latitudes in winter. Atmospheric Hazards As aircraft operating altitudes increase, so concentrations of OZONE and COSMIC RADIATION become of greater importance to the aviator. Above 50 000 ft, normal concentrations of ozone exceed tolerable limits and air needs to be filtered before entering the cabin. The heat of the compressor system will assist in the breaking down of the ozone to an acceptable level. Cosmic radiation is not normally hazardous, but at times of solar flare activity a lower flight level may be necessary. Advances in meteorological forecasting and communications should result in pilots receiving prompt and accurate information regarding high altitude hazards, but it is important that they should be aware of these hazards and prepared to take the necessary re-planning action. The International Standard Atmosphere (ISA) Because temperature and pressure vary with time and position, both horizontally and vertically, it is necessary, in aviation, to have a standard set of conditions to give a common datum for: the calibration of aircraft pressure instruments the design and testing of aircraft. The standard atmosphere now used in aviation is the ICAO International Standard Atmosphere (ISA). ISA defines an ‘average’ atmosphere from -5 km (-16 400 ft) to 80 km (262 464 ft). For practical purposes we just need to look at the ISA between mean sea level and 20 km. The ICAO International Standard Atmosphere (ISA) is: a MSL temperature of +15° Celsius, a MSL pressure of 1013.25 hectopascals (hPa), a MSL density of 1225 grams / cubic metre, a lapse rate of 0.65°C/100 m (1.98°C/1000 ft) up to 11 km (36 090 ft), a constant temperature of -56.5°C up to 20 km (65 617 ft), an increase of temperature 0.1°C/100 m (0.3° C/1000 ft), up to 32 km (104 987 ft). Note: Practically we use a lapse rate of 2°/1000 ft for calculations up to the Tropopause. 6  The Atmosphere 1 1 The Atmosphere Figure 1.2 The International Standard Atmosphere (ISA). 7  1 The Atmosphere ISA Deviation 1 The Atmosphere To determine true altitude and for the assessment of performance data it is necessary to determine the temperature deviation from the ISA at any specified altitude. To do this we firstly need to determine what the ISA temperature is at a specified altitude, then calculate the deviation from the ISA. The ISA temperature at a particular pressure altitude is found by reducing the MSL temperature by 2°C for each 1000 ft above 1013 hPa datum: ISA Temperature = 15 - 2× altitude (in 1000 ft) e.g. find the ISA temperature at 18 000 ft: ISA temperature = 15 - 2 × 18 = -21°C Note: Remember the temperature is isothermal above 36 000 ft (11 km) in the ISA at -57°C. Now to find the deviation from ISA we subtract the ISA temperature from the actual temperature: ISA Deviation = actual temperature - ISA temperature So if the actual temperature at 18 000 ft is -27°C, then the deviation is: ISA Deviation = -27 - (-21) = -6° For the temperatures below, calculate the ISA deviations: Height (ft) Temperature ISA Temperature ISA Deviation (°C) 1500 +28 17 500 -18 24 000 -35 37 000 -45 9500 -5 5000 +15 31 000 -50 57 000 -67 If the limiting deviation for your aircraft at an airfield 5000 ft AMSL is ISA +10, what is the maximum temp at which you can operate? If the deviation at 3500 ft is +12, what is the ambient temperature? (Answers on page 14) 8  The Atmosphere 1 The ICAO International Standard Atmosphere 1 The Atmosphere Height (km) Height (ft) Temp (°C) Pressure Height Change Density (%) (hPa) (per hPa) 32.00 104 987 -44.7 8.9 1.1 30.48 100 000 -46.2 11.1 1.4 27.43 90 000 -49.2 17.3 2.2 24.38 80 000 -52.2 28.0 3.6 21.34 70 000 -55.2 44.9 5.8 20.00 65 620 -56.5 56.7 7.2 15.24 50 000 -56.5 116.6 15.3 13.71 45 000 -56.5 148.2 19.5 11.78 38 662 -56.5 200 103 ft 26.3 11.00 36 090 -56.5 228.2 91 ft 29.7 9.16 30 065 -44.4 300 73 ft 36.8 5.51 18 289 -21.2 500 48 ft 56.4 3.05 10 000 -4.8 696.8 37 ft 73.8 3.01 9882 -4.6 700 36 ft 74.1 1.46 4781 +5.5 850 31 ft 87.3 0 0 +15 1013.25 27 ft 100 Note: The above height change figures show how the pressure against height change equation is modified as altitude changes but the figures offered only relate to ISA conditions of Temperature and Pressure. We can assess changes outside these conditions by using the following formula: 96 ×T H= P where H = height change per hPa in feet T = Actual Absolute Temperature at that level in kelvin (K) P = Actual Pressure in hPa Note: this formula is only valid for calculating the height change per hPa change in pressure at a specified altitude; it cannot be used to calculate a change in height between two pressure levels, nor the change in pressure between two altitudes. 9