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 ConditionsSimple 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’ 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 1 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.

Meteorological Conditions

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

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

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 !

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.

Surface Wind

2 7

6 0

TAKE OFFRunway 09

CROSSWIND COMPONENT13 KNOTS

HEADWIND15 KNOTS

SURFACE WIND130/20 KNOTS

THE VECTOR

40

Angle Off

50 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)

Wind Component

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

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 want to know the headwind

The Quick MethodThis 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 FogAs 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 FogThis slant visibility can be measured & if the runway visual range ( RVR ) is under 800 metres a safe landing is unlikely.

UNDER 800 METRES? ABORT!

PrecipitationThis is a fancy word for rain! Covers rain, sleet, snow, hail etcIt 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

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.

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 thewindscreen.

But you cannot heat the whole of the airframeSo 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

WING

Ice on leading edge

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 aerialsIt will also affect engine operation, so the best advice is to stay away from icing

Icing