IFR METEOROLOGY
IFR METEOROLOGYThe following is a summary sheet of the knowledge required above PPL MET standard for the IFR theory examination. It is assumed that students have a complete understanding of the PPL MET topics.
Icing
Icing falls into several main categories;
• Carby Icing
• Hoar Frost
• Airframe Icing
-
Rime Ice
-
Clear Ice
Hoar Frost
Forms by direct deposition of crystalline ice from water vapour out of clear air. It will most likely form under the following conditions:
1.
Formation on an aircraft parked overnight with conditions similar to those which would create dew, but under sub-zero temperatures.
2.
Formation onto an aircraft's sub-zero surface as it descend into moist, warmer air.
Hoar frost will result in increased induced drag, reduced acceleration on take-off, increased stalling speed and reduced control effectiveness at slow speeds.
CAR 244 requires the removal of all frost and ice from an aircraft before flight.
Airframe Icing
For airframe icing to occur the aircraft must be flying in visible moisture at or below zero temperatures.
If the droplets freeze rapidly, air can be trapped within causing the ice to appear opaque and brittle, this is known as RIME ICE.
It is relatively easy to remove by descent or conventional de-icing
equipment.
If the droplets are large, they freeze slowly, forming a large sheet of CLEAR ICE. This type of ice is hard to remove and is very dense covering a large portion of the airframe.
Two major factors determine whether and what type of icing is likeiy to occur:
1.
Outside air temperature
2.
The size of the visible water droplets
Icing occurs at or below the "freezing level", but sometimes can occur above the freezing level if the aircraft is exposed to freezing rain.
Details
Rime Ice
Clear Ice
Appearance
White, brittle, opaque due to trapped air mainly on leading edges
Clear, hard, smooth surface
No trapped air.
Spreads over surface.
Types of supercooled water droplets
Small
Large
Rate of freezing
Rapid
Slow
Temperature range
0°C to -30°C
especially -10°C
0oC to –10oC (-15oC)
Cloud types / precipitation
Stratiform cloud, tops of Cu cloud
Rain clouds: Nimbus
Ease of removal
Easy
Very difficult
Hazards
Disturbs aerofoil shape Increases drag
Increased stall speed Coats aerials and windscreen
Increased weight
Control freezing
Unless an aircraft is equipped with de-icing equipment, the aircraft cannot fly into "known icing conditions". This term relates to an area of very high icing probability as dictated by the forecast. For IFR, the freezing level in and outside of clouds must be able to be calculated by the PIC.
Eg:
Surface temp. = +18oC
SALR = 1.5o/1000ft
Cloud base
3000ft
AGL
DALR = 3o/1000ft
Calculate the difference in freezing levels inside and outside the cloud.
Thunderstorms
Requirements:
• Moisture
• Atmospheric instability
• A trigger mechanism:
Convection (inland Australia)
Orographic
Cold stream effect (moist air flow advancing equatorwards)
Nocturnal equatorial
Convergence of air
Fast moving cold fronts
Stages of a thunderstorm:
1.
Cumulus (Developing stage):
• Rapid growth of a towering Cu cloud under favourable conditions
• Strong updraughts
• Clear ice within the cloud above the freezing level, rime above
• No precipitation or downdraughts
2.
Mature Stage:
• Strong up and downdraughts with intense precipitation
• Severe windshear
• Severe turbulence in the lower to middle levels of the cloud
• Rain, hail, thunder, lightning
• Airframe icing
3.
Dissipating Stage:
• Downdraughts only
• Cloud cools and spreads out forming nimbostratus
• Airframe icing
• Precipitation but not hail
ADF's tuned near frequency 250 kHz will be most strongly affected by thunderstorm activity.
Mountain Waves
Criteria for Formation:
-
A
strong
wind
blowing
at
right
angles
to
a
mountain
range
-
Windspeed increasing with height, at least 25kt at the ridge height
-
A stable layer at ridge height with unstable layers above and below
Features:
• Always up and down draughts
• Lenticular clouds are the best indication of mountain waves present
and tend to sit above the wave crests, however they are not always
present with mountain waves.
• Rotors and rotor clouds may also develop under the wave crests on
the lee side of the mountain
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