ppl_04_e2
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F O R EW O R D
JAA Theoretical Examination Papers |
Corresponding Oxford Book Title |
Air Law and Operational Procedures |
Volume 1: Air Law |
Human Performance and Limitations |
Volume 2: Human Performance |
Navigation and Radio Aids |
Volume 3: Navigation |
Meteorology |
Volume 4: Meteorology |
Aircraft (General) and Principles of Flight |
Volume 5: Principles of Flight |
|
Volume 6: Aeroplanes |
Flight Performance and Planning |
Volume 5: Aeroplane Performance |
|
Volume 6: Mass and Balance |
JAR-FCL Communications (PPL) |
Volume 7: Radiotelephony |
R e g u l a t o r y C h a n g e s .
Finally, so that you may stay abreast of any changes in the flying and ground training requirements pertaining to pilot licences which may be introduced by your national aviation authority, be sure to consult, from time to time, the relevant publications issued by the authority. In the United Kingdom, the Civil Aviation Publication, LASORS, is worth looking at regularly. It is currently accessible, on-line, on the CAA website at www.caa.co.uk.
Oxford,
England
August 2011
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P R EF A C E T O M ET EO R
TO THE PILOT.
A pilot is required by law, before he goes flying, to obtain an appropriate aviation weather forecast, either for the local area or for his route and destination airfield, and to assure himself that, given the weather conditions, his qualifications, and his aircraft’s performance and equipment, the planned flight can be carried out safely.
From the earliest days of aviation, pilots have needed to acquire a deep knowledge of the winds and weather in order to fly safely.
In the days before specialised aviation forecasts, it was crucial that pilots should know how to assess the present and developing weather situation by analysing variations in atmospheric pressure and reports from meteorological observation posts, before getting airborne. Once in the air, pilots had to be capable of interpreting changes in the wind, and cloud formations, in order to judge whether or not it was prudent to continue with their flight. It should not, therefore, be in any way surprising that, since the very beginnings of manned-flight, Meteorology has been a core groundschool subject in pilot training.
Even in today’s world of advanced technology where aircraft can almost fly and navigate themselves, and where accurate weather forecasts are available to all pilots, amateur as well as professional, a thorough understanding of the weather phenomena that an aircraft may encounter in flight is no less important to the pilot than it was at the dawn of aviation.
Advances in weather forecasting, and their increasing availability to all pilots, should have almost eliminated aircraft accidents occurring because pilots fly, unawares, into deteriorating weather conditions. But this has not been the case. The factor that has not changed over the passing years is, of course, that of human fallibility, and so, despite increasingly accurate and available weather forecasts, between 15% and 20% of all aircraft accidents and fatalities, in general aviation, are weather related. This proportion of weather-related accidents has remained constant over very many years, and suggests that it is the pilot’s understanding of weather phenomena and interpretation of the developing meteorological situation which have not kept pace with advances in weather forecasting.
General aviation pilots operating light aircraft remain particularly vulnerable to adverse weather conditions, because of a combination of factors. There is the light structure and basic instrumentation of the aeroplane, the probable lack of formal, programmed training of the pilot, and the sometimes basic nature of any operational support from air traffic control and meteorological services.
Pilot error is, however, by far the most significant factor in aircraft accidents. Pilot error accounts for almost 75% of all aircraft accidents, world-wide, and, within that category, the proportion of accidents caused by weather-related pilot error is high.
A study of general aviation accidents in the USA, conducted by the Aircraft Operators’ and Pilots’ Association (AOPA), over a period of 11 years, found that the four most common causes of weather-related accident were connected with the pilot’s inadequate appreciation of, or handling of, wind, visibility, airframe and induction icing, and thunderstorms. Over the period, in the USA, there were 5 894 fixed-wing light aircraft accidents related to the weather categories just mentioned, of which
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P R EF A C E T O M ET EO R O L O G Y
more than 1 700 were fatal. Of the fatal accidents, the unplanned passage of VFR flight into Instrument Meteorological Conditions (IMC) was the leading cause. Most accidents occurred between 9 am and 6 pm on Saturdays and Sundays, which points to the heavy toll borne by the recreational pilot.
Perhaps the most telling aspect of the VFR into IMC accidents was that two thirds of the pilots involved had had access to weather briefings which indicated that the weather was unsuitable for VFR flight.
It would seem, then, that the determination of some pilots to get airborne despite adequate warning of unsuitable weather conditions must be added to lack of understanding of the subject of Meteorology as a major cause of weather-related accidents.
It should, therefore, go without saying that all pilots need to strive to acquire a deep understanding of, and respect for, the weather, if they are to be confident of planning and conducting their flying in a safe and expeditious manner.
This book has been written with that aim in mind by instructors of CAE Oxford Aviation
Academy, one of the world’s leading professional flying training schools.
The book’s primary aim is to provide both student and qualified general aviation pilots with pilot-oriented instruction in the theory of Aviation Meteorology and to teach them how to use their knowledge of weather theory to interpret meteorological forecasts and reports in order to carry out effective flight planning.
A further important aim of the book is to help student pilots prepare for the theoretical knowledge examination of the Part-FCL/EASA Private Pilot’s Licence (PPL) in the subject of Meteorology.
It is hoped, too, that this book will constitute a sound introduction to the subject of Aviation Meteorology for those students preparing for professional pilot examinations.
A wise man once said that whereas aviation, in itself, is not inherently dangerous, it is, to an even greater extent than the sea, terribly unforgiving of any carelessness, incapacity or neglect. There is no aspect of flying to which this truth is more applicable than to that of the pilot’s need to acquire a thorough understanding of the weather.
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CHAPTER 1
THE ATMOSPHERE
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Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.comC H A P T ER 1 : T H E A T
INTRODUCTION.
An understanding of the Earth’s Atmosphere is fundamental to a pilot’s training. The atmosphere is not just the air he breathes, but also the environment in which he flies. This environment can be benign, but may also be very hazardous.
The total depth of the atmosphere is difficult to determine. As altitude increases, the atmosphere becomes slowly thinner and thinner, gradually merging with inter-planetary space. What may be stated with some certainty, though, is that spacecraft, returning to Earth, begin to experience atmospheric effect at an altitude of approximately 120 kilometres (400 000 feet).
Figure 1.1 The Earth is surrounded by a gaseous envelope which we call the “atmosphere”.
An atmosphere, in planetary terms, is defined as being “the gaseous envelope surrounding a celestial body”. The Earth’s atmosphere is the collection of gases which is held around the Earth by gravity. There are over twenty gases making up the Earth’s atmosphere, some of which are much more abundant than others.
THE COMPOSITION OF THE ATMOSPHERE.
By far the most common gas in the Earth’s atmosphere is Nitrogen, which accounts for about 78% of the atmospheric gases. Oxygen, which is essential to sustaining life and the proper functioning of internal combustion engines, makes up just 21% of the atmosphere. Argon makes up less than 1% of the atmosphere. Carbon Dioxide and trace gases are the remaining gases. Trace gases include Carbon Monoxide, Helium, Methane, Ozone and Hydrogen.
One of the gases present in the atmosphere in variable concentrations is water vapour (H2O), a combination of the gases Hydrogen and Oxygen. By volume, water vapour can make up between 0% and 7% of the atmosphere.
Water vapour has by far the greatest effect on our weather. Without water vapour, there would be no clouds or precipitation. Water vapour is invisible as a gas; however, it becomes visible when it is in the liquid (water) or solid (ice) state. Most of the atmosphere’s water vapour is contained in its lowest layer (the Troposphere), near the Earth’s surface.
The
atmosphere consists of
78% Nitrogen,
21% Oxygen, with 1% of other gases, including Carbon Dioxide and Argon.
Water vapour
can make up between
0% and 7%
of the atmosphere. Water vapour is mainly found in the Troposphere. Without water vapour, there would be no weather.
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Clouds and rain are examples of water vapour’s liquid state; snow and hail are examples of its solid state.
The proportions of the atmospheric gases are fairly constant up to about 60 km above the Earth; thereafter, gravitational separation alters the proportions of these gases. However, further explanation of this phenomenon is beyond the scope of this book.
The atmosphere is composed of various layers, which have discrete properties. In order to define the structure of the atmosphere, a property which varies with height must be identified. Though several properties of the atmosphere vary with height, the defining property, for meteorologists, is temperature.
STRUCTURE OF THE ATMOSPHERE.
The atmosphere has a layered structure, as depicted in Figure 1.3. Generally speaking, the light aircraft pilot is concerned only with the Troposphere because of the limit to the operating altitude of his aircraft; but brief mention, too, will be made of the Stratosphere, the layer above the Troposphere.
Figure 1.3 In the Troposphere, temperature decreases with height.
Almost all of the Earth’s
weather occurs in the
Troposphere.
T h e T r o p o s p h e r e .
The lowest layer of the atmosphere is known as the Troposphere, from the Greek word tropos meaning mixing or turning, which undoubtedly refers to the fact that it is in the Troposphere that temperature and pressure changes cause the meeting and mixing of air which gives rise to our weather. Almost all of the Earth’s weather occurs in the Troposphere.
The troposphere has two defining characteristics.
Firstly, the troposphere exhibits a marked decrease in temperature with height, throughout its vertical extent. This variation of temperature within the Troposphere, together with the fact that the Troposphere contains the major part of the atmosphere’s water vapour, causes instability within the lower atmosphere, resulting in clouds, precipitation (e.g. rainfall or snowfall) and weather systems. Atmospheric instability is explained further in Chapter 8.
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Secondly, the Troposphere contains over 70% of the total mass of the atmosphere. This is because air is compressible, and has mass. Air is subject to the effects of the Earth’s gravitational field, with most of the air mass being held close to the
Earth’s surface where the force of gravity is greatest. Furthermore, the air near the Earth’s surface is compressed by the weight of the air above it. This means that air pressure and air density are greatest near the surface of the Earth and decrease with increasing altitude.
The upper limit of the Troposphere is defined as the point where temperature no longer decreases with height. This upper boundary is called the Tropopause.
The thickness of the Troposphere, and therefore the height of the Tropopause, is not constant around the Earth. (See Figure 1.4.) Above the Earth’s geographical North and South Poles, the height of the Tropopause is about 8 km, whereas over the Equator the Tropopause can be twice that height, at around 16 km. This height difference in the Tropopause is caused by variations in the surface temperature of the Earth. Where the Earth is warmest, near the Equator, the Tropopause is at its highest. Over the poles, where the air is very cold, the Tropopause is at its lowest. The mechanisms for these variations in temperature are explained in greater detail in Chapter 4.
Figure 1.4 The Troposphere is thicker over the Equator than over the Poles.
The average height of the Tropopause, is around 11 km, or 36 090 feet. This is the height of the Tropopause at about 45° latitude, and is also the average height assumed in the ICAO Standard Atmosphere (ISA) which will be described later in this chapter.
The higher the Tropopause, the lower is its temperature. Over the Equator, the Tropopause temperature can be -80°C, whereas over the Poles its temperature is about -40°C. The average temperature of the Tropopause is approximately -56°C. This temperature prevails in the mid-latitudes, where, as previously mentioned, the height of the Tropopause is approximately 11 km.
The height of
the Tropopause varies from
16 km over
the Equator to 8 km over the poles. The average height of the Tropopause is 11 km at 45º of latitude. 11 km is the height of the tropopause
assumed in the ICAO Standard Atmosphere.
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T h e S t r a t o s p h e r e .
The atmospheric layer above the Tropopause is called the Stratosphere. (See Figure 1.5). The Stratosphere, unlike the Troposphere, is characterised by the lack of any significant change in temperature with height, especially in its lower layer. (See Figure 1.3).
The Tropopause
generally marks the upper limit of
the Earth’s weather. Most of the Earth’s weather occurs in the Troposphere.
Figure 1.5 The Stratosphere is the layer above the tropopause.
This absence of any significant temperature variation within the lower Stratosphere gives the air of the Stratosphere stability. In other words, vertical movement of air within the Stratosphere is severely restricted, because temperature variation, which is the mechanism for this vertical movement, is no longer present. Cloud development, therefore, is curtailed, and so the Tropopause marks the upper limit of the majority of weather phenomena experienced within the atmosphere.
The temperature of the Stratosphere, in fact, rises a little in the upper levels, due to the presence of Ozone. Further explanation of this phenomenon, however, is outside the scope of this book.
The upper boundary to the Stratosphere is called the Stratopause. Above the Stratopause, temperature begins to increase with height. The Stratopause is usually found around 50 km above the surface of the Earth.
At altitudes greater than 50 km, we enter the realms of the atmosphere reserved for rocket-propelled craft and astronauts. So we will end our account of temperature variation with altitude at the Stratopause.
THE ICAO STANDARD ATMOSPHERE.
Changes of air pressure, air density, air temperature and humidity within the atmosphere greatly affect the performance of an aircraft in flight, as well as the readings of certain flight instruments. In the real atmosphere, of course, these properties are changing continuously with altitude, with passing time, and from place to place. In order, therefore, that aerodynamicists, aircraft manufacturers and engineers might have a set of standard values for pressure, density, temperature etc, against which to measure aircraft performance and to calibrate instruments, a so-called standard atmosphere has been defined by the International Civil Aviation
Organisation (ICAO).
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