Air Navigation_Part 5

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AIR NAVIGATION
Part 5
Weather
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LEARNING OUTCOMES

• On completion of this unit, you should
• Be able to carry out calculations to determine
  aircraft distance, speed and time
• Understand the principles of vectors and the
  triangle of velocities to establish an aircrafts
  track and ground speed

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LEARNING OUTCOMES

• Understand the principles of the 1 in 60 rule
• Understand the types of compass systems used for
  air navigation, how they work and their
  limitations
• Know the hazards that weather presents to
  aviation

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Weather
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Introduction
You will have previously studied the weather as
it relates to walking in the hills.
Icing is a far more serious problem for an
aircraft than it is for a walker
It is the same weather that affects aircraft
operations but with one major difference
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Meteorological Conditions
Simple aircraft such as basic trainers are not
equipped with instruments to enable them to
safely fly in cloud or fog
The student pilot does not have the experience to
fly in fog or cloud.
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Meteorological Conditions
Consequently, it is necessary to define the
weather conditions in which beginners may fly.
These are called
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Visual Met Conditions
a simplified version of the rules are set out in
the following table
VMC
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ABOVE 3000 BELOW 3000
Visibility - 8 KM
Visibility - 5 KM
distance FROMcloud 1000 vertically 1500m
horizontally
distance FROMcloud 1000 vertically 1500m
horizontally
NB AIRCRAFT FLYING BELOW 140 KTS AND IN SIGHT OF
THE GROUND MAY USE KM VISIBILITY AND MERELY
KEEP CLEAR OF THE CLOUD
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It follows that if an aircraft flies in weather
worse than shown in the table, it must have the
necessary instruments to fly in or near to cloud
or in poor visibility.
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This weather is known as Instrument Met Conditions
Only aircraft with suitable equipment and pilots
with suitable instrument ratings may fly in IMC
IMC
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The Visual Circuit
In the early stages of flying, a trainee pilot
will not want to lose sight of the runway when
flying circuits in order to practice take-offs
and landings
To achieve this, VMC is needed and normally the
aerodrome controller will decide if the weather
is good enough
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If the circuit height is 1000 then the lowest
cloud base will need to be above this (usually
1500) and the visibility will need to be good
enough to be able to see the runway from anywhere
in the circuit (usually 5 km)
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Surface Wind
We have already looked at the effects of wind
drift, when transiting from A- B.
On the airfield we must also note the effect of
surface wind.
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Surface Wind
If conditions are not completely calm, we need to
know the wind direction strength, so we take
off land into the wind
You hopefully will remember that takeoffs
landings into the wind are shorter !
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Wind Component
Normally the wind will blow partly across the
runway, so we need to calculate cross wind
headwind
It is very rare to find the wind blowing exactly
along the runway (even thought runway
directions are chosen along the line of the
prevailing wind)
To find this you can draw a vector, use a table
or a simple mental method, as we shall see.
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THE VECTOR
6 0
Angle Off
TAKE OFF
HEAD WIND 15 KNOTS
40
SURFACE WIND 130/20 KNOTS
50
90- Angle Off
2 7
CROSSWIND COMPONENT 13 KNOTS
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• .

Angle between wind direction runway heading for
crosswind component
THE TABLE
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40
50
60
70
80
90
20
30
W i n d s p e e d i n K n o t s
5 1 2 2 3 4 4 4 5
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This is a standard table to enable you to work
out the wind component
10 2 3 5 6 7 8 9 9 10
15 3 5 7 9 11 13 14 14
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20 3 7 10 13 15 17 18 19 20
25 4 8 12 16 19 22 23 24 25
30 5 10 15 19 23 26 28 29 30
Note these angles are from the vector triangle
shown minus angle off
80 70 60 50 40 30
20 10 0
For headwind component - Angle between wind
direction and runway heading
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To use the table you need the angle between the
runway heading the wind direction (angle off)
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If it is 40 degrees you obtain the crosswind
component you use the top row of angles, find the
40 degree column, follow it until you get to
the windspeed, in this case 20 knots.
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This gives the cross wind component as 13 knots
40
20
You use the bottom angles if you know the headwind
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The Quick Method
This is somewhat easier and definitely quicker
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Shallow Fog
As fog starts to form in the early evening, there
is often a shallow layer, a few feet thick, next
to the ground.
However once in the approach on the glide slope
the fog will appear to be much thicker, prevent
the aircraft from landing as the runway or light
will no longer be visible.
A pilot in the circuit, especially at night may
not even notice this as the ground lights are
clearly visible
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Shallow Fog
This slant visibility can be measured if the
runway visual range ( RVR ) is under 800 metres a
safe landing is unlikely.
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Precipitation
It causes the following problems
This is a fancy word for rain! Covers rain,
sleet, snow, hail etc
Leaks into aircraft on the ground
Once a fluid has frozen on the airframe it must
be removed with de icing fluid
Floods runways
If it is frozen it can stick to the airframe and
cause takeoff problems
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Airborne Hazards
Apart from thunderstorms, the main hazard is ice
Even in VMC icing can form on an airframe at
certain temperatures.
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TEMPERATURE
ICE !
This can be fatal, but why ?
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In a car the main problem on a frosty morning is
the frozen windscreen
In an aircraft this is easily cured by heating
the windscreen.
But you cannot heat the whole of the airframe
So the ice will stick to the surface.
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On the wings this means the shape of the wing
changes will eventually cease to be an aerofoil
Ice on leading edge
WING
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However this is not all. As the ice gathers on
the airframe the weight increases
This means that lift will be decreasing,
eventually the aircraft will fly like a brick
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Icing can also affect other aspects of the
aircrafts operations, such as undercarriages,
controls surfaces, and radio aerials
It will also affect engine operation, so the best
advice is to stay away from icing
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