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 

Describe the techniques, and precautions that can be taken to manage the risks of VFR flight through fronts. 

Determine a minimum height and minimum visibility range below which not to continue the flight

If these minima are encountered – land at nearest available place and wait for the front to pass

When cold fronts are forecast – the West coasts are affected more than the East coasts

Passage of warm fronts would dictate that both coasts would be problematic

Follow the adage – “when in doubt, turn about” for safety

Describe the potential dangers to VFR flight through fronts.

To maintain visual reference below the cloud base can become difficult, therefore flying at a lower height is required

Navigation becomes more difficult especially if in unfamiliar terrain and can become difficult to maintain visual reference with the ground

Poor visibility due to precipitation and low cloud 

When flying along the coast there is a mixture of cloud precipitation and sea spray 

There is a risk of low level wind shear

Damage to aircraft windshield and skin – from hail 

Risk of icing if the freezing level is low and the combination of hail, snow and supercooled water are encountered

Limited or almost zero ability to manoeuvre the aircraft to a place where an unplanned landing can be made if circumstances dictate

Describe a cross section of the following occlusions and explain how each type develops: 
a)  cold occlusion; 
b)  warm occlusion. 

a) Cold Occlusion
Warm air always lies above cold air
When the air behind the occluded front is colder than the air ahead of it this is a cold occlusion

b) Warm Occlusion
When the air behind the occluded front is warmer than the air ahead of it this is a warm occlusion

State the events before, at, and after, an idealised warm front in terms of:
a)  pressure; 
b)  temperature; 
c)  wind velocity; 
d)  cloud; 
e)  precipitation; 
f)  visibility. 

Warm Front Events

a) Pressure
Before the front – Pressure falls
At the front – Pressure fall is arrested
After the front – the Pressure remains steady or slightly rises

b) Temperature
Before the front – Temperature can be steady or slightly decreased
At the front – there is a slight increase in temperature
After the front – little or no change in temperature

c) Wind Velocity
Before the front – the wind veers and slightly increases in strength
At the front – the wind backs
After the front – the wind becomes steady in speed and direction

d) Cloud
Cloud before a warm front: – CI, C, AS, NS, Cb in unstable air
At a front – ST, NS or possibly CU, SC or Cb
After a warm front – there is no high cloud but low level cloud persisting

e) Precipitation
Before the front – Precipitation is light rain becoming heavy and persistent
At the front the rain eases or stops or changes to drizzle
After the front – precipitation becomes occasional drizzle or rain

f) Visibility
Before the front – good but poor in rain
At the front – Very poor
After the front – Fair visibility but reduced in rain or drizzle

Describe a cross section of the typical warm front including cloud, temperature and freezing level changes, precipitation, and typical width. 

Cloud: – usually stratiform

Temperature: steady as the warm front approaches there is a usually gradual and subtle increase in temperature

Freezing level: changes ???????????

Precipitation: as the front approaches, precipitation becomes heavier and more persistent

Width: Often 1000kms

State the events before, at, and after, an idealised cold front in terms of: 
a)  pressure; 
b)  temperature; 
c)  wind velocity; 
d)  cloud; 
e)  precipitation; 
f)  visibility. 

a) Pressure

Pressure before the cold front usually falls
As the front passes through – the pressure drop ceases
Afterwards the pressure rises

b) Temperature

Temperature before the cold front is steady,
It abruptly decreases as the cold front passes then is steadily cold afterwards

c) Wind Velocity

Before the cold front the wind velocity veers slightly and increases in strength
During the passage – wind can back suddenly and may involve squalls
After the passage – wind becomes steady in direction and slowly decreases in strength

d) Cloud

Prior to the cold front the cloud is usually CS or AS
During the passage it can be CU Cb and NS depends on degree of stability
After the front has passed – a rapid clearance may be evident in  the isolated CU and Cb

e) Precipitation

Before the front – precipitation is unlikely and not common
At the front there may be heavy showers including hail
After the front the showers ease off quickly and become isolated

f) Visibility

Before the cold front – visibility is fair to good
At the front the visibility is very poor
After the front visibility improves again but can be reduced by showers