Hazards Flashcards
What is wind shear defined as?
Change in wind velocity
What is a low-level jet?
A fast-moving stream of air at a low level
How are low-level jets formed?
Air circulating around a high hits a mountain range and is accelerated into a narrow stream along the range. A surface inversion must be present to shield the flow from the surface friction and allow speed to increase.
When are maximum wind speeds usually encountered with low-level jet streams?
Early morning (Pre-dawn) when the surface inversion is strongest
What is turbulence defined as?
Vertical wind shear
What is convective turbulence caused by?
Rising air currents such as found in and below cumulus and cumulonimbus cloud
Where is turbulence most severe in cumulonimbus cloud?
Where the boundary between up and down draughts produces strong vertical wind shear
How is mechanical turbulence generated?
Wind blowing over and around surface obstructions
What does the severity of mechanical turbulence depend on?
- Wind speed
- Height of obstructions
- Surface roughness
- Stability of atmosphere
In regards to mechanical turbulence; what will stable conditions result in?
Considerable lee-side turbulence
Lee turbulence may be less if….
There are unstable conditions as rising airlifts the turbulent air
Where is the worst turbulence in mountain waves?
In the rotor zone beneath the wave crests
What is frontal turbulence?
Wind shear at the boundary of two air masses of different temperature
What are the 3 types of airframe icing?
- Hoar frost
- Rime ice
- Clear ice
What is hoar frost?
A fine light crystalline deposit of ice that forms by deposition when the aircraft skin temperature falls below 0 degrees in high humidity conditions
When is a common time for hoar frost to form?
- Overnight on parked aircraft
- Aircraft with below 0 skin flys through warm moist air
What is hoar frosts biggest hazard?
It doesn’t add too much weight however it disrupts airflow over the aerofoil surface, reducing aerodynamic efficiency
What is Rime ice?
A white opaque rough deposit of ice formed by small supercooled water droplets freezing upon impact with aircraft surface
What is the most common temperature range for rime ice?
-10 degrees to -20 degrees
What is the cloud type generally associated with rime ice?
Stratiform
What is clear ice?
A transparent sheet of ice formed by slow freezing of large supercooled water droplets
What is the most common temperature range for clear ice?
0 degrees to -15 degrees
Why is clear ice the most dangerous?
It adds considerable weight and seriously reduces the aerodynamic efficiency
Where is clear ice encountered?
- The cumuliform cloud above the freezing level
- Also possible in thick altostratus and nimbostratus
- Orographic cloud is also likely to produce clear ice due to the large droplet size
When is carburetor ice possible?
Over a wide range of outside air temperatures but humidity must be relatively high
What is oblique visibility?
Visibility on a slant
What are microbursts?
Strongly concentrated downbursts of air from the base of a convective cloud
What are microbursts often associated with?
Squall lines, thunderstorm activity, and virga
What is the typical life cycle of a microburst?
15 minutes
What happens to an aircraft in a microburst?
The aircraft will initially experience an increasing headwind that will improve performance, followed by a decreasing headwind the down draught and sink followed by an increasing tailwind
What is the main concern with a microburst?
The velocity of the downburst will exceed the climb potential of the aircraft
