Weather Hazards: Icing, Thunderstorms, Wind Shear, Visibility and Volcanic Ash
This domain is the operational hazard domain: aircraft icing, thunderstorm structure and by-products, low-level wind shear and microbursts, visibility reduction and volcanic ash. Questions typically ask for conditions, recognition or the correct avoiding action.
Aircraft icing
- Requires visible moisture and a temperature between 0°C and about -20°C, with the greatest risk between 0°C and -15°C.
- Clear (glaze) ice from large droplets in cumulus and nimbostratus, typically 0°C to -10°C: heaviest, fastest accretion.
- Rime ice from small droplets in stratiform cloud, typically -10°C to -20°C: rough, lighter, spreads rearward.
- Freezing rain means severe clear icing and immediate avoidance.
- Effects: lift loss, drag rise, stall speed increase, altered trim, blocked pitot and static sources.
Thunderstorms
- Requirements: sufficient moisture, conditional instability, and a lifting trigger (heating, front, orography, convergence).
- Life cycle: cumulus (updraughts only), mature (both updraught and downdraught, Cb, hail, lightning, gust front, microburst, possible tornado), dissipating (downdraughts, anvil, stratiform outflow).
- Hazards: severe turbulence, hail, icing, lightning, microburst, low-level wind shear, tornadoes. Avoid by at least 20 nm from severe cells and do not attempt to overfly a Cb.
Microbursts and low-level wind shear
- A downburst under a Cb, typically less than 4 km across, producing a headwind then a tailwind change of 15-45 kt near the surface.
- On approach the airspeed rises then falls sharply with sinking air; the correct response is full thrust, wings level, pitch to respect the stall warning, and do not change configuration until clear of the shear.
Visibility reduction
- Fog: radiation (clear, calm, long night), advection (warm moist air over a cold surface), frontal, upslope and steam fog. Mist is a visibility of 1,000-5,000 m; fog is below 1,000 m.
- Also haze, blowing dust or sand, snow, heavy rain and smoke.
Volcanic ash
- Not detectable by weather radar; gives engine damage, windscreen sandblasting, sulphur smell and St Elmo's fire.
- Advised by SIGMET and VA advisories; avoid the whole advisory area, do not descend through it.
Worked example
On final approach the airspeed rises by 15 kt, then falls 25 kt while the sink rate increases to 1,500 ft/min. This is a microburst. The go-around is initiated immediately with maximum thrust and the pitch attitude set to keep the aircraft out of the stall warning; configuration stays fixed until the shear is passed.
Traps that cost marks
- Assuming rain means icing: the droplets must be supercooled.
- Thinking rime ice is heavier than clear ice.
- Believing weather radar detects volcanic ash or CAT.
- Confusing mist and fog visibilities.