1. AIR NAVIGATION
Part 5
Weather

2. 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

3. 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

4. Weather

5. 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

6. 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.

7. Meteorological Conditions
Consequently, it is necessary to define the weather conditions in which beginners may fly.
These are called

8. Visual Met Conditions
a simplified version of the rules are set out in the following table
VMC

9. ABOVE 3000’ BELOW 3000’
Visibility - 8 KM
Visibility - 5 KM
distance FROM cloud:
1000’ vertically
1500m horizontally
distance FROM cloud:
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

10. 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.

11. This weather is known as Instrument Met Conditions
Only aircraft with suitable
equipment and
pilots with suitable instrument ratings may fly in IMC
IMC

12. 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

13. 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)

14. THE VISUAL CIRCUIT 2
2 6
5 KM VISIBILITY
1500’ CLOUDBASE

15. On the airfield we must also note the effect of 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.

16. 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 !

17. It is very rare to find the wind blowing exactly along the runway
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.

18. THE VECTOR Angle Off 90°- Angle Off = 6 0 TAKE OFF HEAD 40 WIND
15 KNOTS 40
SURFACE WIND
130/20 KNOTS 50
90°- Angle Off =
2 7
CROSSWIND
COMPONENT
13 KNOTS

19. This is a standard table to enable you to work out the wind component.
Angle between wind direction & runway heading for crosswind
component
THE TABLE 10 20 30 40 50 60 70 80 90 W i n d s p e K o t
This is a standard table to enable you to work out the wind component
Note: these angles
are from the vector
triangle shown
minus angle off
For headwind component - Angle between wind direction and
runway heading

20. To use the table you need the angle between the runway heading & the wind direction (angle off)

21. 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.

22. This gives the cross wind component as 13 knots 4010 40 50 60 70 80 90 20 30
Angle between wind direction & runway heading for crosswind
component W i n d s p e K o t
For headwind component - Angle between wind direction and
runway heading
Note: these angles
are from the vector
triangle shown
minus angle off
This gives the cross wind component as 13 knots 40 20
You use the bottom angles if you know the headwind

23. This is somewhat easier & and definitely quicker
The Quick Method
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
This is somewhat easier & and definitely quicker

24. 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

25. 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!

26. It causes the following problems:
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

27. Apart from thunderstorms, the main hazard is ice
Airborne Hazards
Apart from thunderstorms, the main hazard is ice
Even in VMC icing can form on an airframe at certain temperatures.

28. This can be fatal, but why ?
TEMPERATURE
ICE !
This can be fatal, but why ?

29. 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.

30. On the wings this means the shape of the wing changes & will eventually cease to be an aerofoil
Ice on leading edge
WING

31. 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

32. 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