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Meteorology
“No pilot can consider himself a safe and competent operator unless he
can read and appreciate meteorology”
- Air Safety Digest No.119 -
1. Turbulence is a sudden change of airflow speed and/or direction.
2. Usually happens in cloud due to up draughts and down draughts caused by
rising and falling air masses.
3. Clear Air Turbulence (CAT) is usually created by mountain waves (airflow
moving across mountain tops, creating “waves”) or a change in air mass. It is
difficult to avoid as it cannot be seen visually. It is also impossible to detect
CAT by means of weather radar.
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4. Planes that encounter any detectable turbulence would attempt to avoid it.
 Passengers should fasten their seat belts and pilots would slow down
the aircraft to minimise possible structural damage of the aircraft.
1. Jet Streams are strong upper level winds and can serve to explain the
reason why the flight time to a destination is different from that of the
coming back journey.
2. The flight time for Hong Kong (VHHH)-Vancouver (CYVR) is shorter
than that of Vancouver-Hong Kong. The plane is facing the “tailwind” in
the first sector, and against the “headwind” while flying back to Hong
Kong.
3. Jet streams vary in position and strength according to seasonal
changes.
They flow from West to East and are usually strongest in
winter. The flight time from Hong Kong to Vancouver is, therefore, shorter
in the winter.
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1. Cloud is formed when moist air is forced to rise and cool to a temperature below
the dew point (Condensation Level). When moist air is cooled down to below
the dew point temperature, condensation occurs and clouds are formed. Dew
point is the temperature when the air mass is 100% saturated.
2. The air mass experiences a change in temperature due to pressure
changes rather than temperature changes of the surrounding
environment.
3. This vertical movement may result from any one (or a combination of two
or more) of the following:
a. turbulence
b. orographic ascent (Mountains or high ground)
c. convection (Ground heated by the sun)
d. widespread ascent, such as that caused by air converging at a
warm, cold or occluded front
CLOUD TYPES:
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1. DEFINITION:
Windshear is defined as a sudden change in wind direction/or speed over a small
area. Windshear can subject an aircraft to violent updrafts and downdrafts, as well
as abrupt changes to the horizontal movement of the aircraft. It is especially
dangerous during take-off and landing as the aircraft does not have much time or
altitude to recover from this sudden upset.
2. TYPES OF WINDSHEAR:
a) Convective windshear:
 The most severe windshear is associated with convective
precipitation or rain from thunderstorms.
b) Mechanical Windshear
 The intensity of the windshear associated with ground obstructions
depends on the size of the obstacle and the primary velocity of the
wind.
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3. HAZARDS OF WINDSHEAR:
a) Turbulence induced injury or damage.
b) Sudden change in the angle of wind flow over the aircraft wing, resulting in a
stall.
c) Dramatic decrease in forward airspeed, resulting in loss of Lift and a rapid
descent that is greater than the aircraft’s performance can counter.
d) Powerful downdrafts causing excessive descent rates.
4. TAKE-OFF PRECAUTIONS:
a) Use maximum engine thrust available.
b) Use the longest suitable runway that is clear of the likely windshear.
c) Increase the take-off speed.
d) Be alert for any airspeed fluctuations during take-off and initial climb.
5. LANDING PRECAUTIONS:
a) Establish a stabilized approach no lower than 1,500FT above the ground, to
improve windshear recognition capability.
b) Use the most suitable runway that avoids the area of suspected windshear
and is compatible with crosswind and tailwind limitations.
c) Use autopilot and auto-throttle.
d) Avoid large engine thrust reductions in response to sudden airspeed
increases, as these may be followed by airspeed decreases.
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1. Typhoons develops from a cluster of convective clouds that intensifies, first to a
Tropical disturbance (wind speed less than 20 KT), then into a Tropical Depression
(wind speed 20 – 34 KT), a Tropical Storm (35 – 64 KT) and then finally into a
Tropical Revolving Storm (>64 KT) or Typhoon. (1 KT = 1.852km/h).
2. A Typhoon can only form where certain definite conditions are fulfilled. Among
these are:
a) Warm and humid surface, water temperature of at least 27° C. (24°C for
survival.)
b) Uniform winds at all levels, otherwise the thunderstorm-clouds will be
scattered.
3. It is quite clear from what has been said above that the tropical cyclones can´t form
over land, because it is too dry.
4. The name and time of occurrence varies over the earth;
a) In the Atlantic and eastern Pacific N.H. they are called hurricanes, occur
mainly in the period July to November, frequency about 10 times a year.
b) In the Indian Ocean they call them Cyclones, occur April – May and Oct.- Nov.
c) Off the coast of East Africa they also use Cyclone, Nov. – April, occasionally.
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d) In the western Pacific N.H. and South China Sea, the name is
Typhoon,
June – November, this is the peak tropical rotating storm (TRS) area with a
frequency of about 20 systems a year and a small TRS possibility the whole
year around.
e) In the western Pacific S.H. they are known as Cyclones, Nov. - April.
5. Most typhoons form between 8 – 15° latitude north of the Equator. They are then
caught by the prevailing easterly flow in the tropics and carried away northwestwards according to the arrows on the map at a speed of about 10 to 20 KT.
When the TRS passes the 25-30° latitude they ”re-curve” pole-wards in a north,
later north-east direction.
6. When reaching a continent or cooler water surface the radius increases, the wind
speed decreases.
7. If the storm hits land before the whirl has exhausted itself, not only the hurricanewinds but also a flooding will cause destruction. A pillar of water, the Storm Surge
or ocean swell, is created by the strong circulation in the system and the pressure
from the on-shore wind will do the rest. The flooding may rise to 15 to 30 feet or
more above the normal water level, and in regions near the coast the most common
cause of death is drowning, while most people survive the gusts of wind. Ocean
swells travels outwards from the system at an average speed of about 1600 KM a
day which is several times faster than the typhoons movement.
8. Typhoons from The Pilot’s View
From a pilot’s point of view all any flight through a typhoon should be avoided,
because there is always a potential risk of severe turbulence in and around the
thunderstorm-spiral bands and especially in the border around the eye. At low
levels devastating winds and turbulence close the airports. The change to lighter
winds occurs at heights above 30 000 feet or so in the outflow region from the
typhoon.
Due to the cyclonic circulation in a typhoon, a flight with the eye on your left side will
result in a tailwind component. This also means that the most intense parts of a
typhoon is found in the right front quarter of the storm where the storm movement
and winds interact while the weakest portion is in the left rear quarter.
In the tropical regions there are often extensive areas with high-clouds. These are
no real threat to flight but it is vital, when operating in these areas, to distinguish
between these clouds and the spiral band thunderstorms.
The areas with the strongest winds are found 95 to 190 KM outside the eye. The
wind speed also depends on what side of the hurricane’s direction of movement you
are. The highest speeds are on the side coinciding with the direction of movement
and this is the area that sailors try to avoid. This is the right side of a typhoon.
Speeds in excess of 275km/h are not uncommon, while the system itself moves at
about 18km/h.
Typhoons may prevent an aircraft from taking off or landing, due to the amount of
windshear associated with them. There are times when we can take-off and land in
a typhoon Nr 8 but not Nr 3. This would be the case when there is no windshear in
a typhoon nr 3.
9. The end of a Typhoon
When the typhoon moves in over a cooler surface (less than 24C) the winds slow
down and the eye of the cyclone is filled up with clouds before the whole system
dissolves.
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1. Hailstones form readily in thunderstorm cloud, and large stones can cause
significant damage to the leading and upper surfaces of an aircraft.
2. As a rule, the hailstones decrease in both size and intensity towards the top of the
cloud, and so penetration, if unavoidable, should be made at high level.
3. Hail should be assumed to exist at any level in a thunderstorm.
4. Although airframes have been designed to absorb various types of impact,
encountering hail will undoubtedly damage the structural integrity of an
aircraft, reducing its performance.
AIRCRAFT DAMAGED BY HAIL
a) The low temperatures at high altitude and the water droplets inside clouds
can be a dangerous combination. Not only is there a possibility that the
airframe will lose performance due to the deformation of the aerodynamic
design and added weight, but the engines may also ingest ice, causing
them substantial damage and in extreme cases cease their operation.
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b) Ice formation on the airframe with result in:
a) Increase in the aircrafts weight, lowering the climb rates and cruise altitudes.
b) Increase in the aircrafts drag; decrease in lift; decreasing airspeeds and angles
of climb; increasing stall speeds.
c) Possible restrictions of control surface movement.
d) Reduction of visibility through windscreens.
e) Blockage of unprotected aircraft flight instrument sensors..
f) Interference with radio communications and radio navigation aids when ice
forms on aerials.
1. Lightning strikes on aircraft are thought to be most likely to occur at levels where
the temperature is between -10C and +10C, that is to say within about 5000 ft
above or below the freezing level.
2. Providing that the aircraft is properly bonded there should be little damage other
than burn marks at the points of entry and exit of the lightning strike. External
aerials are of course insulated from the airframe rather than bonded to it. Should
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lightning strike such an aerial, it is likely that the heat generated across the
insulating material will burn off the aerial as effectively as a welding torch.
3. Magnetic compasses will become totally unreliable following a lighting strike. The
large deviations observed immediately following the strike will decay fairly rapidly.
Smaller but significant residual deviations will however remain for long periods and
it will be necessary to check and probably recalibrate the compasses before the
next flight
4. Lightning flashes may cause short term partial loss of vision at a time when the pilot
needs it least. Antiglare glasses (or sunglasses) should be worn and flight deck
lights (or the normal internal lighting) should be turned full up to minimise the effect.
1. DEFINITION:
Icing conditions can be defined by a situation where the ambient temperature
is 10 degrees Celsius or below with visible moisture (in the form of clouds or
rain) present.
2. The aircrafts wing and fuselage must be clear of any icing before the aircraft
can take-off. The aircraft is de-iced before take-off by special ground
equipment spraying a hot fluid mixture onto the aircraft. Thereafter the aircraft
is spayed with a special ant-icing fluid, to prevent the buildup if ice, while the
aircraft is still on the ground.
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3. There are several things that we can do to minimise the impact of icing on
aircraft in these circumstances:
a)
Use of anti-ice system:

This is a system using hot air produced from the engines to
prevent the build up of ice on the airframe and in the engines.
Therefore, we turn this system on prior to entering icing
conditions.
b)
Use of the de-icing system:

This system is used to remove ice already present on the
airframe. The most common usage is on the ground when
snow and/or ice have formed on the aircraft skin and a special
de -icing fluid is used to remove them.
4. During winter in some countries, snow and ice may cover the runway to great
depths. This hinders an aircraft’s take -off and landing performance greatly.
There are legal requirements as well as aircraft manual data in place to help
us determine whether we can take -off and land in the distance available on
these contaminated runways .
5. CX’s definition on the different types of runway surface
conditions are:
a) Dry Runway:
 The runway surface is dry or damp with no measurable depth of water.
b) Wet Runway:

The runway is well soaked but without significant areas of standing
water, or the depth of standing water is 3 mm or less.
c) Contaminated Runway:
 Standing water in excess of 3mm, Slush, Loose snow, or Compacted
snow, cover more than 25% of the runway surface area.