AIR NAVIGATION
Part 5
Weather
LEARNING OUTCOMES
On completion of this lesson, you should:
- Know the hazards that weather presents to
aviation.
Weather
Introduction
You will have previously studied the weather
as it relates to walking in the hills.
It is the same weather that affects aircraft
operations but with one major difference
Icing is a far more serious
problem for an aircraft than it is for
a walker
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.
Consequently, it is necessary to define the
weather conditions in which beginners may
fly.
Meteorological Conditions
These weather conditions are called
Visual Met Conditions
VMC
A simplified version of the rules are set out
in the following table
ABOVE 3000’
Visibility - 8 KM
distance FROM
cloud:
1000’ vertically
1500m horizontally
BELOW 3000’
Visibility - 5 KM
distance FROM
cloud:
1000’ vertically
1500m horizontally
NB AIRCRAFT FLYING BELOW
140 KTS AND IN SIGHT OF THE
GROUND MAY USE 1 KM
VISIBILITY AND MERELY KEEP
CLEAR OF THE CLOUD
Meteorological Conditions
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.
This weather is known as Instrument Met
Conditions
IMC
Only aircraft with suitable equipment and
pilots with suitable instrument ratings may fly
in IMC
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
The Visual Circuit
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)
THE VISUAL CIRCUIT 2
1500’ CLOUDBASE
5 KM VISIBILITY
26
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.
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 !
Surface Wind
It is very rare to find the wind blowing exactly along
the runway.
(even though runway directions are chosen along
the line of the prevailing wind)
Normally the wind will blow partly across the
runway, so we need to calculate cross wind &
headwind
To find this you can draw a vector, use a table
or a simple mental method, as we shall see.
THE VECTOR
60
Angle Off
TAKE OFF
Runway 09
SURFACE WIND
130/20 KNOTS
40
50
CROSSWIND
COMPONENT
13 KNOTS
27
HEAD
WIND
15 KNOTS
90°- Angle Off =
Angle between wind direction & runway heading for crosswind
component
THE TABLE
This is a
standard
table to
enable you
to work out
the wind
component
10
W
i
n
d
s
p
e
e
d
i
n
K
n
o
t
s
20
30
40
50
60
70
80
90
5
5
1
2
2
3
4
4
4
5
10
2
3
5
6
7
8
9
9 10
15
3
5
7
9
11 13 14 14 15
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
Wind Component
To use the table you need the angle between
the runway heading & the wind direction
(angle off)
If it is 40 degrees, to 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.
This gives
the cross
wind
component
as 13 knots
Angle between wind direction & runway heading for crosswind
component
10
W
i
n
d
s
p
e
e
d
i
n
You use the bottom
angles if you want to
know the headwind
K
n
o
t
s
20
30
40
40
50
60
70
80
90
5
1
2
2
3 4
4 4
5 5
10
2
3
5
6
7
8
9 10
15
3
5
7
9
11 13 14 14 15
20
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
9
80
70
60
50
40
30
20
10
0
For headwind component - Angle between wind direction and
runway heading
The Quick Method
This is somewhat easier & and definitely quicker
ANGLE BETWEEN WIND
DIRECTION AND RUNWAY
HEADING FOR CROSS WIND
COMPONENT
DEGREES
0-15
15-30
30-45
45-60
60-90
ZERO
1/4 WIND STRENGTH
1/2 WIND STRENGTH
3/4 WINDSTRENGHT
FULL WIND STRENGTH
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.
A pilot in the circuit, especially at night may not
even notice this as the ground & lights are
clearly visible
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 lights will no longer be visible.
Shallow Fog
This slant visibility can be measured & if the
runway visual range ( RVR ) is under 800
metres a safe landing is unlikely.
UNDER 800 METRES? ABORT!
Precipitation
This is a fancy word for rain! Covers rain,
sleet, snow, hail etc
It causes the following problems:
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
Aircraft De-Icing
Airborne Hazards
Apart from thunderstorms, the main
hazard is ice
Even in VMC icing can form on an
airframe at certain temperatures.
Icing
TEMPERATURE
ICE !
This can be fatal, but why ?
Icing
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.
Icing
On the wings this means the shape of the
wing changes & will eventually cease to be
an aerofoil
Ice on leading edge
WING
Icing
However this is not all. As the ice gathers on the
airframe the weight increases
This means that lift will not support the weight of
the aircraft plus the ice & eventually the aircraft
will fly like a brick
Icing
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