Weather Flashcards
Structural icing
- visible moisture
- aircraft surface at or below freezing
- outside air temperature 5 degrees (aircraft surface temperature will be cooler due to aerodynamic cooling)
Icing: is a large problem that can cause numerous problems for an aircraft in flight
- aerodynamically the aircraft is going to have a reduced ability to produce lift
- ice can build up and cause an increase of weight
- can cause problems with the pitot system
- ice can block air intake systems or carburetor ice can form stopping the engine altogether
- ice shedding
- dynamic rollover if skids get stuck
Types of structural icing
Clear ice:
0 to -10 degrees large super cooled water droplets. Accumulates quickly and is the most hazardous. Occurs when flying through cumulus clouds
Rime ice:
-15 to -20 degrees small super cooled water droplets. Freezes on contact and has a milky color due to air trapped in the ice. Found most often in stratus cloud layers.
Mixed ice:
-10 to -15 degrees mixed sizes of droplets, combo of clear and rime ice.
Frost:
Destroys lift just like rime but frost happens on the ground when temperature and dew point are below freezing. As frost accumulates on the air foil it can disrupt the smooth flow of air, which reduces the amount of lift produced ( increases profile drag)
Induction icing:
When ice forms in the carburetor and induction system of an engine causing power loss or engine failure.
- ice may form on the throttle valve (butterfly) or other surfaces in the carburetor, potentially causing the engine to stop running all together by choking off the air intake.
- carb ice can form between temperatures of -4 to 30 degrees
- temperature/dew point spreads of less that 15 degrees will hasten the formation of carb ice.
- the occurrence of carburetor ice is more likely as temperature decreases and humidity increases. Carburetor ice can occur at outside temperatures as high as 80 degrees and humidity at 50%
Causes and indications of carb icing
- Venturi effect (bernoulli’s principle latent heat of vaporization) as the fuel vaporizes it absorbs heat, reducing its temperature
- boyle’s gas law (low power setting) as the pressure of a gas lessens the temperature of that gas decreases (as found in the Venturi of a carburetor)
- carburetor icing is often indicated by a reduction in power available, by the engine running roughly or not responding to demands for more power. Application of carb hear results in a restoration of power.
Prevention of carb ice
- use carb heat to keep needle out of yellow area
- be aware of temperature and dew point spread at all times
- follow POH procedures and school procedures
- keep carb heat gauge out of yellow arc
- below 30 degrees with a temperature/dew point spread of less than 15, or if visible moisture is present, apply full carb heat when using less than 18” of MAP
Helicopter icing
Rotor blade ice: - decreases lift - increases weight - can shed into the tail rotor - can shed unevenly and unbalance the rotor Ice on other parts of the helicopter - skids (dynamic rollover) - antenna (loss of communication) - pitot tube (loss of ASI) - on the wind screen (loss of visibility)
Instrument icing: icing of the pitot-static system can adversely affect readings
- blocked pitot will cause ASI to act like the altimeter
blocked static tube: - airspeed indicator will indicate slow airspeed as you are climbing and faster airspeed as you are descending
- VSI will indicate no climb or descent
- Altimeter will show a constant altitude
Ice removal:
Anti-icing equipment; prevention, carburetor heat assist, heated pitot,
De-icing; removal, glycol, sun or you
Flying into icing conditions
- flying in known icing conditions is prohibited (POH limitations section)
- do not fly through clouds or rain when icing conditions are present
- look for ice forming on any protrusions of the aircraft; skids wind screen
- descend into warmer air, land or turn around if icing conditions are encountered
Have a plan: - check PIREP
- SIGMETS, AIRMETS, and TAFS
- know where to find ice free weather
Visibility:
defined as the greatest distance you can see and identify objects.
Inversions
- stable air is most susceptible to formations of temperature inversions
- ground cools nearby air by radiation, cooling the surrounding air more rapidly than the air above it
- since warm air rises, this lower level of cooler air stays stationary near the ground combination
- stable air = low or poor visibility
Reduced visibility: the factors affecting visibility in temperature inversions
- dust
- haze
- mist greater than 1/2 mile
- fog thicker than 1/2 mile
- clouds
Fog:
All types of fog are formed by air being cooled to its dew point and the air being saturated
Types of fog
- Advection fog: warm moist air blown over cool land. Mostly in coastal areas and can be very wide spread (100 miles) and it needs wind
- Steam fog: cool air moves over warm water. Can cause an icing hazard and turbulence
- Upslope: warm moist air being blown up a mountain also requires some wind
- Radiation: happens on calm, clear humid nights
- Frontal precipitation: warm rain falls through cool air, can be wide spread and not go away until precipitation stops or moves out of the area, most likely ahead of a warm front.
Components necessary for the formation of clouds
- Moisture
- Condensation nuclei
- Small temperature/dew point spread
Four families of clouds
- Low: from the ground to 6500’
- stratus family
- fog moderate icing
- turbulence - Middle: from 6500 to 20000’
- alto family
- moderate turbulence
- severe icing - High: from 20000’ and above
- cirrus family
- towering cumulous
- cumulous nimbus
Thunderstorms: produce the most dangerous aviation hazards such as hail, high winds, reduced visibility, tornadoes, wind shear
Conditions necessary for formation:
- high moisture content
- unstable air
- lifting action
3 stages of a thunderstorm
- Cumulus: primarily consists of updrafts (as the air rises it reduces heat, providing energy for more lifting and further condensation). Eventually the drops will become too big to be supported by updrafts and will fall out of the bottom of the growing cloud.
- Mature stage: start of the mature stage is signaled by the beginning of precipitation.
- updrafts and down drafts
- the water droplets are too large to be supported by the updrafts as they begin to fall they evaporate, this absorbs heat and they fall faster, creating down drafts.
- the mature stage is the most violent. This is when we experience all the dangerous weather phenomena associated with t-storms
- showery type precipitation
- turbulence and wind shear
- lightening - Dissipating stage:
- cloud has an anvil shape
- as the storm begins to run out of moisture there is nothing to fuel the updrafts, therefore the downdrafts begin to spread and take over the entire cell
- rain is steady and losing its strength
Squall line thunderstorms
- normally contains embedded thunderstorms
- often precedes fast moving cold fronts, not always frontal
- 100 to several miles long
- most sever weather destructive winds, heavy hail, and tornados
Virga
Rain evaporates before reaching the ground
- causes strong downdrafts
- can cause micro burst
Micro burst: very strong down draft
- found near thunderstorms or light rain with virga
- typically less than 1 mile in diameter
- downdrafts up to 6000fpm
- mostly dangerous during takeoff and landing
- lasts approximately 15 minutes
- signs in the desert are rings of blowing dust
Wind and air masses
Wind: is caused by the heating of the earth
- causes convection (rising of warm air, sinking of cool air)
- cause lows and highs
- can chart the wind with isobars (isobars are points of equal pressure)
1. Aloft parallel
2. Near the ground, perpendicular
Air masses:
- an air mass is a mass of air that has a consistent temperature and moisture content
- classifications
Polar vs tropical
- polar air masses originate in cooler climates
- tropical air masses originate in warm climates
Continental vs maritime:
- continental air masses originate over bodies of land
- maritime air masses originate over bodies of water
When two air masses meet you get a front
4 levels of turbulence
- light: slight changes in altitude
- Moderate: large changes in altitude and attitude, strain can be felt on the seat belts
- Severe: temporary moments of being out of control
- Extreme: out of control can cause aircraft damage
