2.2b aviation weather hazards

Types of Aviation Weather Hazards

* Thunderstorms
* Icing/Frost
* Fog
* Turbulence and Wind Shear

Thunderstorms

* Turbulence may cause **structural damage**
* Lighting can blind you and cause electrical problems

Necessary Components to create Thunderstorms

1. Unstable Air
2. High Humidity
3. Lifting Agent

Life Cycle of a Thunderstorm

1. **Cumulus Stage (3-5 mile height)**
2. **Mature Stage (5-10 mile height)**
3. **Dissipating Stage (5-7 mile height)**

**Cumulus Stage**

* Cloud is growing vertically
* May exceed 3,000 ft./min
* Droplets are increasing in size
* Towering Cumulus is the **end result** of this stage

**Mature Stage**

* Cloud droplets are very large
* Precipitation begins (“Nimbus”)
* Downdrafts begin
* Updrafts still exist!
* Lightning
* Friction of the air moving up and down causes it
* Icing

**Dissipating Stage**

* Storm loses its moisture content and its energy
* Starts to die
* Whole cycle takes 20 to 30 minutes

Types of Thunderstorms

* “Air Mass” thunderstorms
* “Frontal” thunderstorms

“Air Mass” thunderstorms

* Form as a result of daytime heating
* Typical in the afternoon
* Easiest to avoid - usually visible and not part of an organized line or formation

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“Frontal” Thunderstorms

* Associated with squall lines
* Form along the **leading edge** of a front
* **Cold front** → produce the largest, most active storms
* **Warm front** → will be “embedded” → **hard to see**
* Often form in **organized lines**
* Can be hundred of miles long
* Hard to avoid

Thunderstorm Avoidance

* Thunderstorm will exist as a threat for most of the summer (PROB30/40)
* **Visual avoidance** is the best
* 20 miles is the recommended distance

What if you can’t see the thunderstorm?

**Types**

* Night time
* Embedded storms blanketed by other clouds

**∴ Detection Equipment**

Thunderstorm Avoidance Equipment

* **On-board weather RADAR**
* Works well but
* Heavy and not possible on many aircraft

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* **Satellite Download**
* Can be viewed through many GPS devices
* Subscription required
* Not intended for thunderstorm avoidance
* Time lag

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* “Stormscope” + “Strikefinder”
* Both detect the presence of severe weather through lightning discharge
* Lightweight; can fit into any aircraft

Airframe Frost and Icing

* Not allowed to takeoff with snow, frost, etc. adhering to the critical surfaces of the airplane
* Adversely affects the **aerodynamic efficiency** of the airfoil
* Ice adds **weight**
* Significant but not the biggest factor

**Frost**

* Forms when water vapour sublimates directly onto your airplane
* **Must** be removed prior to flight
* Most common on cold, clear evenings

**Airframe Ice**

* Must have “super-cooled droplets”
* Found only in cloud or freezing precipitation

“Supercooled” Droplets

* Still in liquid form
* But temperature is at or below zero
* Becomes ice when disturbed by airplane
* Faster the airplane → More accumulation
* Tail ices up **faster** than wing
* Large droplets are worse than small

Where are large/small droplets found?

**Large droplets:**

* Cumulus Clouds
* Freezing Rain/Drizzle

**Small droplets:**

* Less accumulation
* “Stratus clouds”

Types of Icing

* Rime Ice
* Clear Ice
* Mixed Ice

**Rime Ice**

* Generally found in layer type cloud
* Small super-cooled droplets
* Freeze immediately; leaves numerous air spaces
* Ice is “milky” in appearance
* Tends to be more brittle

**Clear Ice**

* Generally found in Cumulus type clouds; FZRN or FZDZ
* Large super-cooled water droplets that spread before freezing; very few air bubbles
* Clear in appearance and is hard
* Tends to build rapidly

**Mixed Ice**

* Super-cooled water droplets of various sizes
* May appear as a combination of Rime and Clear Ice
* May build rapidly

Factors Affecting Ice Accumulation

1. Size and number of super-cooled droplets
2. Aircraft Speed
3. Surface Profile

Size and Number of Droplets

Ice will build rapidly when

* Super-cooled droplets are large and numerous
* Cumulus clouds are present
* Temperatures are just below freezing

Aircraft Speed

* The faster you go, the more ice you’ll catch
* Must balance desire to go slow with stall speed increasing

Surface Profile

* Ice builds faster on **thin profile surfaces** vs. thick ones
* Typically builds faster on tail than on main wings
* Can cause “**tail plane stall**”
* Tail plane holds nose up by creating **down force** on tail
* If down force removed, nose will pitch down
* Not controllable

Recognition and Recovery from Tail Plane Icing

**Occurrence**

* May notice buffeting similar to a normal stall
* May occur after flaps have been deployed
* May occur at normal operating speeds

**Recognition**

* Pushing forward to recover will **increase buffeting**
* Correct recovery requires gentle back pressure to stop buffeting
* Autopilot may mask tail plane icing

Winter Warm Front

* Common cause of airframe icing
* Allows for droplets to be within a **few degrees of freezing**
* Freezing Rain/Drizzle; problem to VFR pilots

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Warm Front Freezing Rain Risk

Flying from **cold to warm side**

* Snow
* Ice pellets
* Freezing rain
* Rain

Flying from **warm to cold side**

* Rain
* Freezing rain
* Ice pellets
* Snow

Turbulence/Wind Shear

* Problem: Aircraft is difficult to control and damage may occur
* Turbulence is rated according to table from AIM
* Wind shear is a problem for **take-off and landing**

What causes wind shear?

* Air moving in different directions and/or speeds

Common near the ground with

* Thunderstorms
* Strong surface inversions
* Strong surface winds
* Common near fronts

What happens in wind shear?

* May experience and “uncommanded” change in airspeed and/or aircraft
* Aircraft could stall or overshoot the runway

**Increased Performance Shear**

* Increase in airspeed
* Decrease in rate of descent

**Decreased Performance Shear**

* Decrease in airspeed
* Increase in rate of descent

Performance Shear

* Follows the headwind
* **“Headwind up”** → Performance up
* **“Headwind down” →** Performance down

Fog/Mist

* Restricts visibility
* Basically a cloud on the ground
* Fog is classified by the **process that contributes to its formation**
* **Fog →** ½ SM visibility or less
* Mist → More than ½ SM visibility
* Many processes which contribute to fog/mist formation

Types of Fog/Mist

* Radiation Fog
* Advection Fog
* Orographic (Upslope) Fog
* Arctic Sea Smoke
* Precipitation Induced Fog

Radiation Fog

* Air is cooled from below by contact with the ground
* Requires **narrow temp/dew point spread** but enough water vapour to be significant
* Wind → 5 kts or less
* Clear skies at night will allow ground to cool quickly and aids in formation

Advection Fog

* **Advection:** Horizontal movement of air
* Moist air moving over a cooling surface causes air to cool to dew point
* Cooling surface: Cold water currents or land
* Prevalent in coastal areas

Orographic (Upslope) Fog

* Requires a slope high enough to cause significant cooling
* Air is cooled adiabatically and is forced to rise until condensation occurs
* Prevalent where there are mountains but even a rise of a **few hundred feet** will work

Arctic Sea Smoke

* Cold air moves over open portions of ice pack
* Air is quickly saturated and condensation occurs rapidly
* Looks like “smoke” coming out of the holes in the ice

Precipitation Induced Fog

* Air becomes saturated by rain evaporating as it falls
* Must have constant rain of sufficient duration to increase moisture content of the air
* When dewpoint = temperature you have fog