IFR METEOROLOGY

帅哥

IFR METEOROLOGY The 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