AIR NAVIGATION
Part 5
Weather
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 aircraft’s track and ground speed
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
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.
Meteorological Conditions
Consequently, it is necessary to define the
weather conditions in which beginners may fly.
These are called
Visual Met Conditions
VMC
a simplified version of the rules are set out in the following table
ABOVE 3000’ BELOW 3000’
Visibility - 8 KM Visibility - 5 KMdistance FROMcloud:1000’ vertically1500m horizontally
distance FROMcloud:1000’ vertically1500m horizontally
NB AIRCRAFT FLYING BELOW 140 KTS AND IN SIGHT OF THE GROUND MAY USE KM VISIBILITY AND MERELY KEEPCLEAR OF THE CLOUD
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 achievethis, VMC is needed
and normally the aerodrome controller
will decide if the weather is good enough
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
2 6
5 KM VISIBILITY
1500’ CLOUDBASE
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 !
Wind Component
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)
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.
2 7
6 0
TAKE OFF
SURFACE WIND130/20 KNOTS
CROSSWIND COMPONENT13 KNOTS
HEADWIND15 KNOTS
50
40
THE VECTOR
Angle Off
90°- Angle Off =
• .
10 40 50 60 70 80 9020 30
Angle between wind direction & runway heading for crosswindcomponent
Windspeed
in
Knots
For headwind component - Angle between wind direction andrunway heading
5 1 2 2 3 4 4 4 5 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
80 70 60 50 40 30 20 10 0Note: these anglesare from the vectortriangle shownminus angle off
THE TABLE
This is a standard table to
enable you to work out
the wind component
To use the table you need the angle between the runway
heading & the wind direction (angle off)
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.
This gives the cross
wind component as 13 knots
10 40 50 60 70 80 9020 30
Angle between wind direction & runway heading for crosswindcomponent
Windspeed
in
Knots
For headwind component - Angle between wind direction andrunway heading
5 1 2 2 3 4 4 4 5 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
80 70 60 50 40 30 20 10 0Note: these anglesare from the vectortriangle shownminus angle off
40
20
You use the bottom angles if you know the
headwind
The Quick Method
This is somewhat easier & and definitely quicker
ANGLE BETWEEN WIND DIRECTION AND RUNWAY HEADING FOR CROSS WIND COMPONENT
DEGREES0-1515-3030-4545-6060-90
ZERO1/4 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 light 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
Floods runways
If it is frozen it can stick to the airframe and cause takeoff
problems
Once a fluid has frozen on the airframe it must be removed with de icing fluid
Airborne Hazards
Apart from thunderstorms, the main hazard is ice
Even in VMC icing can form on an airframe at certain temperatures.
TEMPERATURE
ICE !
This can be fatal, but why ?
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.
On the wings this means the shape of the wing changes & will eventually cease to be
an aerofoil
WING
Ice on leading edge
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
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