飛行課程 Stage 2 - Weather Hazards
Content:
Thunderstorms
Turbulence
Wind Shear
Icing
Restrictions to Visibility
Volcanic Ash
1. Thunderstorms
Thunderstorm Formation
Three conditions before a thunderstorm can develop:
Unstable air
High moisture content
A lifting force
Weather can change rapidly; always get a weather briefing before you fly, and obtain updates enroute
Conditions that Initiate Lifting:
Orographic lifting - occurs when air is forced over a geological barrier
Frontal systems - occur when a warm airmass moves across a cold airmass
Convection - occurs when the heating of the earth's surface creates thermals
Categories of Thunderstones:
Airmass thunderstorms
Relatively short-lived
Seldom produce hail or strong surface winds
Typically caused by convection
Severe thunderstorms
Last at least two hours
Generate wind gusts of 50 knots or greater
Produce hail at least 3/4 inch in diameter
Spawn tornadoes
Thunderstorm Characteristics:
Characterized by the number of cells they contain:
Single-cell thunderstorm: Contain one cell and are typically airmass thunderstorms
Multicell thunderstorm: Contain clustered cells that interact with each other and lengthen the lifespan of the storm
Supercell thunderstorm: Contain strong winds, rotating updrafts, and possibly tornadoes
Squall Lines: Multiple types of thunderstorms that gather into a single band of thunderstorms. They produce the most intense weather hazards for aircraft
Frontal Thunderstorms form along any type of front. The type of front determines the characterstics of these storms
Along a fast-moving cold front - cumulonimbus clouds are often in a parallel line ahead of the frontal surface
Along a warm front - existing stratiform clouds often obscure the storms
Along an occluded front - any kind of weather is possible
Thunderstorm Lifecycle
Three stages:
The Cumulus stage?
Cumulus clouds
Rising, cooling air that condenses into water
Strong, continuous updrafts
The Mature stage
Cumulonimbus clouds and falling precipitation
The most violent weather of a thunderstorm
The Dissipating stage
Downdrafts
Decreasing precipitation and cloud break-up
A stratiform appearance of the storm cloud
Thunderstorm Hazards
A thunderstorm 20 miles away can produce hazards to your flight
Lightning
Severe turbulence, which often develops in an around cumulonimbus clouds
Strong areas of turbulence occur in the area between the updraft and downdrafts
Hail, which can cause significant change to aircraft, especially if it exceeds a diameter of 3/4 inches
Funnel clouds, which contain winds that can exceed speeds of 200 knots
2. Turbulence
Turbulence Categories:
Low-level turbulence (LLT) - results from air encountering obstacles, convection, or updrafts
Occurs within a few thousand feet of the ground
Originates from surface heating or friction?
Types of LLT
Mechanical turbulence occurs when:
Obstacles deflect the irection of airflow
Speed difference between obstructed wind and the smooth wind cause rotational currents
Strong winds flow nearly perpendicular to steep hills or maountain ridges
Convective turbulence occurs during the day when the land is warm and weather is fiar; it forms under these conditions:
The warm surface of the earth unevenly heats the air creating rising thermals
When cold air moves horizontally over a warmer surface
Frontal turbulence occurs in the narrow zone ahead of fronts:
Fronts moving over flat ground at 30 knots or more produce moderate turbulence
Fronts moving over rough terrain produce moderate or greater turbulence
Frontal turbulence is more predictable than mechanical and convective turbulence because it occurs near weather fronts
Clear air turbulence (CAT)
Caused by erratic vertical movements between high-level airmasses and the resulting wind shear
Typically occurs at more than 15,000 ft. AGL
Develops in thin layers that can extend for many miles and can exist in non-convectiveclouds
Displays no visual indications of its presence
Can develop when adjacent layers of air move at different speeds
Often develops in or near the jet stream
Occurs suddenly
Mountain wave turbulence?
Results from smooth, stable air crossing a mountain barrier at a speed of 40 knots or more
Occurs when a mountain displaces a layer of air upward, causing the stable air to rise over the peaks and descend on the leeward side, creating wave turbulence
Characteristics of mountain wave turbulence include:
Wave pattern extending 100 miles or more downwind
Crests extending above the highest peaks
Rotor cloud development
Turbulence typically occurring along the lee slopes
Cloud formation, including cap clouds, roll clouds, and lenticular clouds, which may appear stationary even though they may contain winds of 50 knots or more
Managing Turbulence
To prepare for turbulence:
Slow the aircraft to recommended maneuvering speed and attempt to minimize abrupt attitude changes
Change the aircraft's altitude if necessary
Alert passengers to the upcoming turbulence
Tighten your seat belt
Secure loose objects
Ensure the cabin is adequately ventilated
Considering flying the approach slightly above the normal airspeed
Consider discontinuing the approach in especially strong winds
To prepare for turbulence near mountaous terrain:
Climb to altitudes between 3,000 and 5,000 ft. above the peaks
Begin your climb before reaching the crest of the mountain range
Approach the ridge from a 45-degree angle
Considering scheduling a mountain checkout before you fly
Managing Severe Turbulence
Tighten your seatbelt, put on the shoulder harness, and make sure that your massengers do the same
Secure all loose objects
Plan and hold the course that gets you out of the turbulence in the shortest time
Set the power for the turbulence peneration airspeed recommended in the POH
Disengage autopilot Altitude Fold and Speed Hold features
Try to maintain a level flight altitude; allow the altitude and airspeed to vary
If you are near a thunderstorm, turn up the cockpit lights to their highest intensity and keep your eyes on the flight instrumnets
If inside a thunderstorm, do not turn back
3. Wind Shear
Wind Shear Conditions
One of the most dangerous flight hazards, especially during takeoff and landing
A sudden, drastic shift in wind direction or velocity - or both - over a short distance
Can occur at any altitude when adjacent winds are moving in different directions
Causes:
Convective precipitation in thunderstorms - rain cools the air and causes downdrafts
Downdrafts create a shear zone that separates the surrounding air and the cooler air of the downdraft
Microbursts are one of the most dangerous forms of wind shear
Frontal systems - the shear zone occurs where airmasses meet
Other conditions associated with wind shear include:
Low-level temperature inversions
Clear air turbulence
The jet stream
Wind Shear Hazards
Creates sudden updrafts, downdrafts, and extreme shifts in wind direction and velocity:
Inability to maintain vertical speeds, altitudes, and flight paths
Difficulty in clearning obstacles after takeoff and during the approach to landing
Overshooting or undershooting the runway
Deviating from the runway centerline on takeoff and landing
Indications of wind shear:
Variations in air speed that exceed 15 knots
Decreasing head winds, increasing tail winds, or shifts from head wind to tail wind
Deviation from normal vertical speed by 500 ft. per minite or more
Deviation from normal pitch attitude by 5 degrees or more
Visual indicators such as rain shafts or virga
Characteristics of Microbursts
Concentrated downdrafts of cool, dense air that are extremely hazardous to flight
Are associated with thunderstorms and virga
Result from precipitation-induced down drafts that emerge from the base of convective clouds
Produce damaging winds that spread laterally from the base of the downburst in a vortex ring
Last about 15 minutes, with wind speeds from 25 to 150 knots
Produce an average headwind or tailwind change of 45 knots
Microburst Hazards
Flight hazards caused by microbursts include:
Downdrafts as strong as 6,000 ft. per minute
Precipitation
Gusty horizontal winds near ground level
Strong horizontal wind shear from side to side
Turbulence in vortext rings
Can extend as far as one nautical mile horizontally and 1,000 ft. vertically
Downdrafts and wind shifts are especially dnagerous near the ground during takeoff and landing
4. Icing?
Structural Ice Hazards
Icing:
Requires two conditions: visible moisture and temperatures of 0°C or less
Accumulates at the highest rate in creas of freezing rain
Can build up to as much as three inches on the leading edge or an airfoil in as little as five munites
As the structural ice accumulates, it affects aircraft performance:
Drag and weight increase
Lift decreases
Thrust decreases
Types of Ice:
Rime ice (In cumulus?
Forms when tiny, supercooled water droplets strike the aircraft surface and freeze instantly
Has a milky appearance
Changes the shape of the airfoil and affects lift, posing serious hazards
Forms between -15°C and -20°C
Clear ice
Forms when large supercooled water droplets strike and aircraft surface and flow over it before they freeze
Appears clear and forms a hard glaze
Is hazardous because it adheres tenaciously to the aircraft and is difficult to remove
Forms between 0°C and -10°C
Mixed ice
Forms when large and small supercooled water droplets freeze to each other or to snow or ice
Appears as a hard, rough-edged mass on the leading edges of the airfoils with ice particles embedded within the clear ice
Can pose the hazards of both rime ice and clear ice
Forms between -10°C and -15°C
5.?Restrictions to Visibility
Haze
Consists of a concentration of fine, dry particles
Occurs in stable atmospheric conditions with light winds
Extends from a few thousand feet AGL to 15,000 ft AGL
Obscures visibility, but visibility above the haze is usually good
Discolors objects viewed through it
Smoke:
Consists of a concentration of combusted particles
Affects visibility depending on the amount of smoke, the wind velocity, turbulence, and the smoke's proximity to its source
Gives the sky a reddish or orange tint
Diffuses and gives the sky a grayish or bluish appearance similar to haze
Smog:?
Is a combination of fog and smoke
Builds up in areas where topographical barriers trap stable air
Dust:
Can blow for hundreds of miles in windy conditions
Gives distant objects a tan or gray tint
6. Volcanic Ash
Consists of gases, dust, and ash that affects visibility
Causes abrasions and pits that damage aircraft surfaces and external equipment
Can clog systems and damage engines
Is often indistinguishable from ordinary engines
Can extend to great heights
