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 a simplified version of the rules are set out in the following table VMC

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

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 Only aircraft with suitable equipment and pilots with suitable instrument ratings may fly in IMC 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 2 2 6 5 KM VISIBILITY 1500’ CLOUDBASE

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.

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 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.

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

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 5 1 2 2 3 4 4 4 5 5 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 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

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 40 10 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 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 0 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

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

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

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!

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

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.

This can be fatal, but why ? 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 Ice on leading edge WING

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