8.50. Assess Satellite and Radar Imagery, and Non Aviation -Specific Weather Information

With regard to VFR flight in a light aircraft over the Southern Alps, for typical weather scenarios, describe the meteorological considerations during flight planning and enroute including: 
a)  cloud base; 
b)  turbulence; 
c)  adverse and favourable winds; 
d)  visibility; 
e)  selection of tracks; 
f)  anticipated in-flight conditions; 
g)  the time frame of any weather change. Meteorological Services to Aviation 

When planning a VFR flight over the Alps, some of the conditions to consider are:

Weather Scenarios
Meterological Conditions
Flight Planning
En Route Information 

If it is found that the cloud base is above the mountain tops and visibility is good, the flight can go ahead safely, although turbulence will be inevitable on the lee side of the higher ground.  
If ridges and peaks are covered in cloud, then either continue the flight through clear or familiar valleys or return to base.

Turbulence is inevitable where the cloud base is above the mountain tops and ridges.
Winds are likely to come from various directions and differing levels of turbulence up / down draughts will be encountered. Turbulence is generally worse of the lee side of high ground. 

Unfavourable Winds

The wind from the northwest is associated with a pre-frontal situation on the Western Alps, and will be cloudy and unsuitable for VFR operations

Flight on the Eastern side is not recommended due to the turbulence, down draughts and rotor action

When the wind is Southeasterly; skies on the western side of the Alps will be clear but turbulence will be present as well as up and down draughts and rotor action will make flight difficult and unpleasant. On the Eastern side it will be cloudy, but for the most part smooth. 

When the wind is from the Southwest and associated with pre-frontal conditions, the weather is usually showery throughout the Alps; although the West Coast should be better except for passing showers

Flight on the Eastern side of the Alps should be clear but subject to turbulence and strong winds

Favourable Winds

Light winds produce satisfactory conditions for VFR flight
Often associated with anticyclonic or col conditions
Sometimes visibilty is reduced by haze
Early morning valley fog can be present
Snow clad surfaces will be covered in fog on windward side of the mountains when wind is light and air is sufficiently moist

White out conditions can be experienced when mountains and valleys are covered in snow

Reading the avaliable signs by flying on the lee side can give more information re increasing turbulence and up and down draughts
– scraggy lee edge cloud which evaporates as it slides down the lee side
– blowing snow or dust from from the ridges
– cloud on one side of a hill and none on the other
– tumbling cloud on the lee below ridge height

On the windward side these signs are:
– may not appear as good as the lee side but often turbulence can be a lot reduced
– depends on the amount of cloud, cloud base and visibility on the windward side

It is relatively easy to assess the weather once an over-flight of the Alps has been done
So long as peaks and ridges along the intended track remain clear of cloud the flight can continue

If numerous cloud build ups occur and especially if the tops are higher than the track – reconsider the flight
Descend – to see if the cloud base is above the mountain tops and visibility allows uninterrupted recognition of ground features – could be possible to continue but turbulence will be a problem

If the ridges and peaks are covered in cloud and the flight will be forced into unfamiliar valleys – return to departure point or find a better, clear area

Even though weather forecasts and reports can be satisfactory for the flight – it is important to have a contingency plan to divert – weather conditions can change abruptly

After departing, the weather may be clear on the West Coast but within a couple of hours it may have turned and become unsuitable for a VFR return – so considerations needs to be thought of and a diversion needs to take place 
Consider the amount of fuel if such a diversion occurs

In general terms, describe cloudiness, gustiness, visibility and turbulence at various locations within New Zealand during typical: 
a)  northwest wind regimes; 
b)  northeast wind regimes; 
c)  southwest wind regimes; 
d)  southeast wind regimes. 

Generally speaking, the local weather is dependent on the local topography. 

As wind is blown onto a mountain range, or higher ground, cloud is formed on the windward side of the high ground. 

Visibility is generally better on the lee side of the higher ground, due to having less cloud; whereas the windward side experiences a greater degree of cloudiness and precipitation. 

Turbulence will be more prevalent on the lee side.

 

a) Northwest wind

Cloudiness
West Coast will have more cloud (windward side)
East coast will have less cloud (lee side)

Gustiness
Winds through Cook Strait and Foveaux Strait can be gale force, due to a funneling effect 

Visibility
Better on East Coast
Worse or rain on West Coast

Turbulence
Generally less to none on the West Coast 

More turbulent on the East Coast

b) Northeast wind

Cloudiness
West Coast will have less cloud (lee side)
East coast will have more cloud (windward side)

Visibility
Better on West coast
Worse on East coast 

Turbulence
More on the West Coast

Less on the East Coast

c) Southwest wind

Cloudiness
Air is unstable and cumulus cloud will possibly form on both coasts
Strong likelihood of thunderstorm activity
More on West coast (windward side)
Less on East coast (lee side)

Visibility
Better on East coast

Less on West coast

Turbulence 

More on East coast

Less on West coast

d) Southeast wind

Cloudiness
More on East coast (windward side)

Less on West coast (lee side)

Gustiness
Smooth flying conditions

Visibility
Better on West coast

Less on East coast

Turbulence
More on West coast

Less on East coast

 For any area or location in NZ, determine the wind directions which expose that location to very poor flying conditions and the wind directions which result in sheltering;

Almost every location in New Zealand has at least one wind direction that results in that location being ‘exposed’ to
poor flying conditions. By ‘exposed’, we mean the flow is coming of the sea with little or no modification and/or
minimal sheltering from mountain ranges.
The table (figure 87) below lists the ‘exposed’ and ‘sheltered’ wind directions for each of the 17 Aviation Area Wind
Zones within New Zealand.
Note that the ‘Exposed to…’ column is the direction from which the worst weather can be expected, while the
‘Sheltered from…’ column lists the directions which will almost certainly deliver fine (if not turbulent) weather to the
area. Wind directions not mentioned may have weather ranging from fine to poor depending on the meteorological
situation, but these directions will not deliver the extremes of weather conditions.

Far North FN                    Exposed to NE                                         Sheltered from S to SE

Tamaki TA                         Exposed to N to NE                                  Sheltered from S to SE

Edgecumbe ED                Exposed to N to NE                                   Sheltered from SE to SSW

Te Kuiti TK                       Exposed to W                                            Sheltered from E to SE

Central Plateau CP          Exposed to NE                                           Sheltered from SSW

Mahia MH                        Exposed to E                                             Sheltered from W

Dannevirke DV                Exposed to E                                              Sheltered from NW

Sanson SA                      Exposed to W                                             Sheltered from NE

Straits ST                       Exposed to NW and S                                Sheltered from NE and SW

Tasman TN                    Exposed to N to NW                                   Sheltered from S                    

Kaikouras KA                Exposed to E                                               Sheltered from SW

Windward WW              Exposed to NW                                           Sheltered from SE

Plains PL                       Exposed to E                                              Sheltered from NW

Alpine AL                      Exposed to NW or E                                    Sheltered from E or NW

Clyde CL                       Exposed to S to SE                                     Sheltered from NE

Fiords FD                      Exposed to NW                                           Sheltered from E

Gore GE                        Exposed to E to S                                       Sheltered from NW

Identify “Westerly situations” and “Easterly situations” on a weather map and describe the impact of each situation on flying weather around NZ

Note: The conditions explained below for this objective are the ‘most likely’ conditions to exist in ‘westerly’ and
easterly’ situations. As with almost all weather scenarios in New Zealand, exceptions can and do apply, most often
due to small changes in water vapour content of the air.
Probably 85% of the weather we experience in New Zealand involves ‘westerly situations’ – winds from between NW
and SW across the country. To promote winds from these directions, the highs generally travel across or to the north
of New Zealand and the lows travel to the south of the country. 

In westerly situations, the highs and lows travel quite quickly from west to east which often results in a repeating 4 to
7-day weather pattern. West of the main divide and in the west of the North Island, we experience a day or so of
north westerlies ahead of a front, accompanied by low cloud and poor visibility in frontal precipitation. Following this,
we get a day or two of south westerlies bringing occasional showers before a ridge of high pressure delivers a few
days of settled weather. 

During summer, this pattern generally extends, with much longer fine spells, and shorter, less intense bad weather
spells.
In these westerly situations, the east of the country often experiences extended periods of fine weather which can  produce drought conditions. The only real problem for aviators in this scenario is that turbulence will frequently be in
the moderate to severe category. 

When the winds come from the east, the highs must be to the south of NZ and the lows to the north. When this
‘easterly situation’ occurs, the highs are often ‘anchored’ south of the Chatham Islands for extended periods of time.
This promotes east to north-east winds from Northland to south Canterbury, and because this wind direction is
stable, very poor flying conditions often persist for many days in the east of the country.

Meanwhile, in the west of the country, brilliant flying conditions are usually experienced (although it may be very
turbulent for aviation).

Describe how the following items govern the NZ climate:
a)  latitude; 
b)  oceanic surroundings; 
c)  topography. 

 New Zealand, located as it is between 34oS and 47oS, lies in the middle of the area defined as ‘Mid-Latitudes’. This
therefore defines the ‘Type of Weather Systems’ we will experience, typically mobile sequences of highs and lows.
(b) Oceanic surroundings;
Being surrounded by oceans means that our weather is ‘Maritime’ or ‘Moist’ in nature.
(c) Topography.
Our mountainous land mass, generally lying across the prevailing flow, ‘Modifies’ the weather resulting from (a) and
(b) above.
New Zealand lies in an area of eastward-traveling highs and lows with variable weather, high average water vapour
content, and mountains that produce strong orographic efects giving bigger contrasts between east and west than
between north and south.

a) Latitude

New Zealand, located as it is between 34 degS and 47 degS, lies in the middle of the area defined as ‘Mid-Latitudes’. This therefore defines the ‘Type of Weather Systems’ we will experience, typically mobile sequences of highs and lows.

b) Oceanic Surroundings

Being surrounded by oceans means that our weather is ‘Maritime’ or ‘Moist’ in nature.

c) Topography

Our mountainous land mass, generally lying across the prevailing flow, ‘modifies’ the weather resulting from (a) and (b) above. New Zealand lies in an area of Eastward-traveling highs and lows with variable weather, high average water vapour content, and mountains that produce strong orographic effects giving bigger contrasts between East and West than between North and South.

When high moisture air is added into the mix, regions in the west experience greater degree of cloudiness and precipitation. Thus; low ceilings and less visibility can be expected on the west coast, with more turbulence on the easterly sides. 

Describe the causes, factors involved and techniques commonly used to avoid or minimise the following;

Motions within the atmosphere can be broken down into waves, from the smallest waves: gusts with a wavelength of
just metres – to the largest: planetary waves with wavelengths of 10,000 km or so. All turbulence as experienced by
an aircraft is a function of the interaction of the plane in flight with one or more of these waves at an appropriate
wavelength.
When thinking about how an aircraft will be affected by wave motions, we need to consider the size of the aircraft
and its speed of travel. This is because severe turbulence is experienced when the wavelength and amplitude align
with the aircraft’s movement through the air. 

Figure 81 demonstrates how a light aircraft and a heavy aircraft may
experience completely opposite extremes of turbulence at different wavelengths.

Incidentally, in the severe turbulence generated downstream from a mountain range, these two wave lengths and
many others besides will be mixed together, meaning that regardless of size and/or speed, all aircraft are likely to
experience severe turbulence in these conditions. 

 (a) Convective (thermal) turbulence; 

Mature Cb’s contain very strong updrafts and downdrafts in juxtaposition to each other. These vertical winds can
reach speeds more than 5000 ft per minute in New Zealand, thus generating a very violent overturning motion,
sufficient at times to tear the wings of an aircraft. Since no VFR private pilot should be caught flying inside a Cb
cloud, the internal turbulence should not affect them.
Another source of turbulence associated with Cb’s is caused by Microbursts – columns of rapidly descending air
beneath the Cb cloud base. Despite common misconceptions, microbursts can and do develop in New Zealand and
extreme care should be exercised when considering taking of or landing with a Cb just of the end of the runway.
Almost all New Zealand’s microbursts will be ‘wet’ microbursts i.e. accompanied by rain, and therefore clearly visible.
If encountered, microbursts can force an aircraft down to the ground (figure 82).

Another source of convective turbulence is a by-product of the microburst. When a microburst hits the ground, it
spreads out horizontally, creating a phenomenon known as a First Gust or Gust Front (see figure
83). A First Gust or Gust Front is the boundary between the cold outflow air resulting from the microburst and the
warm inflow air feeding a Cb. The warmer air, being less dense, rises over the cold air, frequently creating what is
known as a roll cloud on the leading edge of an advancing Cb cell. In New Zealand, this gust front may precede the
Cb cell by up to 5km and the roll cloud may not be visible if there is insuficient moisture in the air.
It is not uncommon for the surface wind to change direction by 180o instantly with the passage of the gust front, and
for the wind to change from 10 knots or so ahead of it to gusts of 40 to 50 knots behind the gust front – a wind shear
of perhaps 50 or 60 knots.
Attempting to cut in front of an approaching Cb and gust front to land into a 10-knot head wind is fraught with
danger. If the gust front catches you before touchdown, the best you can hope for is a very hard landing. It just gets
worse after that.

(b) Mechanical turbulence – small scale and large scale; 

For the most part, large scale turbulence has been covered in chapter 8.32, Mountain Weather. However, there are a
few additional aspects of large scale turbulence that should be considered.
As most mountains have rugged terrain, and waves are associated with strong winds, the friction or boundary layer
will be deep and turbulent immediately above and in the lee of the ranges. The turbulent zone generated beneath
these waves may extend hundreds of kilometres out to sea (see figure 84).

In addition, strong updrafts and, more importantly, strong downdrafts are likely to exist. These vertical winds will
often exceed the performance of your aircraft, meaning that if caught in an updraft, the aircraft will soar like a glider
and gain height despite the pilot’s best efforts to lose height.
If the aircraft gets caught in a downdraft, the experience can be much more alarming, as even at full power and with
the aircraft set up for maximum rate of climb, it may well be descending at several thousand feet per minute. This
coupled with rising ground…well, you get the picture, and it’s not nice.
Low ground speed is another hazard, although it can be advantageous too. A low ground speed means you will be in
the danger zone for longer. Countering this is the fact that a low ground speed will give you a little more time to
make decisions about escaping or turning away from the ridge line.
A local low-level obstruction to the strong surface flow will create tumbling and turbulence downstream from the
object. Immediately downwind the air will be dumping toward the ground, creating a localised down-draught.
Helicopter pilots operating into pads immediately down-wind of a building or a row of trees should be very cautious
in strong wind scenarios.

(c) Wake turbulence;

Wake turbulence forms of aircraft wing-tips because of the high pressure under the wing being forced around the
end of the wing towards lower pressure above. 

Fig. 85 Wake Turbulence generated from Aircraft Wing-tips. 

The rotation generated slowly sinks and expands outward behind the generating aircraft (see Figure 85). It can be
disastrous if encountered at low-level, particularly during the take-off and landing phases of flight, when the induced
roll and yaw occurs with little height for recovery.
Wake turbulence only forms when the wings are loaded, so its generation ceases on touchdown and doesn’t develop
until the generating aircraft rotates on take-off. The turbulence generated will be worse if the generating aircraft is
heavy, slow and clean i.e. no flap and landing gear up. In calm conditions, the vortices generated of the wing-tips
during a landing or take-off will sink to the ground, and then spread out horizontally – away from the runway in opposite
directions. If, however, there is a slight cross-wind, one of the vortices may be pushed slowly toward and then over
the operational runway. Be very aware of this possibility.
There are several different options open to a pilot in terms of avoiding wake turbulence. The first two, dealing with
the landing and take-off phases of flight, are detailed in Figure 86. 

An approach above the approach path, and a
touchdown beyond the touchdown point of the generating aircraft will keep a light aircraft out of the unsafe zone.
Likewise, a rotation prior to the rotation, and a steeper climb rate than the generating aircraft will avoid the problem.
Where these options are not available, the next best option is to wait. There are recommended minimum time delays
which need to be applied between generating aircraft and following aircraft. If at a controlled airfield, ATC will advise
you of the hold time appropriate for the aircraft types involved.
Light aircraft can also generate wake turbulence which can be significant for following aircraft if close behind,
especially if conducting a streamed landing as part of a formation.

In the cruise, a light aircraft` should maintain a separtion of 5 NM behind a medium weight aircraft` and 6 NM
behind a heavy aircraft`.
And a final point of note: helicopters may also generate very dangerous wake turbulence, particularly large
helicopters like the RNZAF NH90’s.

 Describes the effects of low-level wind-shear on aircraft operations in the following;

(a) Take-off; 

There are a couple of ways in which your flight may be upset when encountering low-level wind-shear on take-off 

     (i) The first involves taking-of with a moderate to strong crosswind component (relative to the aircraft
type), and while this may not strictly involve wind-shear, it is non-the-less something you need to be
ready for as a pilot, otherwise, a runway excursion or a flight upset could result. A specific example
sometimes (though rarely) exists at Wellington Airport. As noted in objective 8.14.8 in the section on
Local Winds, Wellington is very much prone to terrain channelling. When the broad-scale flow over
central New Zealand is westerly, the surface wind at Wellington Airport will be from the north to NNW.
The dramatic change in wind direction from the top of the friction layer down to the surface is due to
terrain channelling and friction. The wind shear will be greatest when the air mass is stable. But if
conditions are unstable (e.g. with lots of convection), the shear is less, and the occasional strong westerly
gust may push down to the surface, creating a significant, but short-lived cross-wind on the Wellington
Airport runway.
It is not the purpose of this text to teach you how to fly the aircraft – that is the job of your flying
instructor. However, suffice-it-to-say, if a strong crosswind is present or encountered whilst taking off,
control corrections will be required to combat the natural tendency for the aircraft to weather-vane into
the wind, or to prevent a wing-drop, or worse. 

     (ii) The second happens if a major wind-shift is encountered during the take-off roll or soon after it. If, whilst
taking off into a headwind, you suddenly encounter a strong tailwind, the take-off roll distance will
increase markedly as your aircraft struggles to accelerate to the required airspeed for take-off.

If encountered shortly after take-off, you may encounter significant sink, and even if the aircraft doesn’t
sink, the rate of climb and the angle of climb will both be reduced. This type of shear also comes with a
fair amount of turbulence, so you will also have to deal with this whilst fighting to get the aircraft safely
away from the ground.
There is one bright light on the horizon in this situation however. It is very unlikely that you will ever
encounter such a major wind shift on take-off, without there being some significant visual indications to
warn you of the impending wind change. Most wind direction and speed changes of this nature at or near
ground level, are the result of a gust front associated with an approaching cumulonimbus cloud. Such
clouds, along with approaching heavy rain, a very dark horizon, and possible lightning and thunder
usually advertise their presence well before they arrive. If you suspect an approaching thunderstorm is
close by, delay the take-off.
In addition, be wary of high based Cb clouds – those likely to be found in Central Otago and perhaps
above the North Island Volcanic Plateau, especially if they have virga falling from their base. Stronger
wind-shears are often associated with these higher based clouds due to the cooling effect of the
evaporation accelerating the downdrafts.

(b) Approach and landing phases of flight; 

In the United States, the Federal Aviation Administration has conducted trials on low-level wind-shear and has
determined that a 35-knot wind-shear just above ground-level is enough to cause almost all pilots to lose control and
crash the aircraft. To put this into perspective, if a gust front associated with a travelling thunderstorm is approaching
an airfield and the wind changes from a northerly of 10 knots to a southerly of 25 knots with the gust front, a 35-knot
wind-shear has just occurred. Gust fronts in New Zealand often have gusts up around the 50-knot mark, so even
though our thunderstorms are relatively benign by world standards, they are still capable of causing significant low level wind-shear.
The forecast 2000 ft wind added to all domestic TAFs in New Zealand is there to help pilots anticipate the presence
of low-level wind shear. The greater the speed differential, and/or the greater the difference in the angle of the wind
between the surface and 2000 ft, the greater the chances of encountering low-level wind-shear.
There are many examples in the CAA files of aircraft in New Zealand encountering low-level wind shear which has
resulted in a heavy landing, or the aircraft landing short of the runway 

(Figure 80 below shows the flight profile of an aircraft on the approach to landing. In the top diagram, the aircraft
experiences ‘sink’ as it descends into the calm layer beneath the inversion. In this instance however, the pilot has
time to adjust to the wind-shear experienced.
In the second diagram, the sink occurs at a lower height. If the pilot applies too much power to overcome the sink,
the aircraft may end up above the ideal approach path, resulting in a touchdown further into the runway.
And in the bottom diagram, the shear-zone is encountered at very low-levels. If the pilot is slow to react to the sink,
the aircraft may touchdown short of the runway)

Define the term ‘wind shear’. 

Wind shear is defined as:

                  

                          A sudden change in wind speed and/or direction over a
short distance, either horizontally or vertically. 

Thus, wind shear creates tumbling motions within the atmosphere which are experienced by aircraft as turbulence.

Describe the cause(s), factors involved, dangers, and techniques commonly used to avoid or minimise: 
a)  thermal (convective) turbulence; 
b)  mechanical turbulence - small scale and large scale; 
c)  wake turbulence. 

a) Thermal (convective) Turbulence

Causes
– occurs when the surface warms and air particles near the surface warm and rise and the vertical convective currents produced cause turbulence
Dangers
– structural – loss of lift and increased angle of attack
– inadvertent stalling
– passenger discomfort
– loose articles
– disorientation
Avoidance
– avoid by flying above the turbulence cloud
– Select a route on windward side of mountains / ridges
– Slow down – this reduces the effects of turbulence
– Read the cloud – cumulus cloud is evidence of the presence of turbulence

b) Mechanical Turbulence

 Small Scale Mechanical Turbulence
 Obstructions to the windflow caused by buildings, shelterbelts and small hills = small scale turbulence
– in less than 15 kts windspeed turbulence occurs in the lee of obstructions
– standing eddies form at the front and rear of small hills
Dangers

– structural – loss of lift and increased angle of attack

– inadvertent stalling
– passenger discomfort
– loose articles
– disorientation
Avoidance
– avoid this by flying at high altitude
– tracking around the features

Large Scale Mechanical Turbulence

Similar to small scale turbulence but at a greater magnitude
Dangers
In strong winds and where Fohn winds and mountain waves can form there can be severe turbulence on the lee side of a mountain
Avoidance
Flying above cloud tops – but bearing in mind and not contravening rules / instructions

c) Wake Turbulence

 Formation
– a clockwise rotating vortex occurs at the Left wing tip and anticlockwise rotating vortex at the Right wing tip
– these rotating masses of air create wake turbulence
– are generated the instant the producing aircraft leaves the ground and can persist up to 5-6 minutes
Dangers
– structural – loss of lift and increased angle of attack
– inadvertent stalling
– passenger discomfort
– loose articles
– Disorientation
Avoidance
– abort take off or landing (go around)

– Climb up away from the vortex’s