Free safety module 1 of 7Meteorology & Flight Planning12 min readLast reviewed: 2026-09-13
Build a pre-flight weather workflow that keeps VFR flights out of cloud: reading the Graphical Area Forecast (GAF), knowing when the rules require an alternate aerodrome, and setting a turn-back point before the weather sets it for you.
Scenario
Coastal VFR Passage Approaching Deteriorating Stratus. You are planning a day VFR flight from Moorabbin (YMMB) to Latrobe Valley (YLTV) via the coastal visual corridor. The GAF indicates broken stratocumulus at 2,500 ft with localised coastal drizzle lowering the cloud base to 1,000 ft AMSL. The forecast freezing level is 4,500 ft with moderate mountain waves over the Strzelecki Ranges.
Decisions and actions
Compare the GAF area by area against your personal minimums, and decide before departure the cloud base at which you will turn back.
Check whether an alternate is required (CASR Part 91 MOS 8.04; AIP ENR 1.1 para 10.7.2): for an aeroplane, if you will arrive during, or up to 30 minutes before, forecast cloud of more than SCT below 1,500 ft or visibility below 8 km at the destination, you must provide for a suitable alternate — unless flying day VFR within 50 NM of the departure point.
Know what CAVOK means (AIP GEN 3.5): visibility 10 km or more, no cloud below 5,000 ft or the highest minimum sector altitude (whichever is higher), no cumulonimbus or towering cumulus, and no significant weather.
Set a fixed turn-back point before terrain traps or coastal cloud banks, and brief it as a decision already made.
Safety traps
Plan-continuation bias ("get-there-itis") driving inadvertent VFR flight into cloud.
Severe turbulence and rotor zones on the lee side of mountain ranges during frontal passages.
Carburettor icing at descent power settings, even on a warm, humid day.
Check your understanding
1. Which conditions must all be met for CAVOK to be reported in an Australian METAR or SPECI?
Visibility 5,000 m or more and no cloud below 3,000 ft
Visibility 10 km or more, no cloud below 5,000 ft or the highest minimum sector altitude (whichever is higher), no CB or TCU, and no significant weather
Visibility 8 km or more, clear skies below 10,000 ft and no rain showers
Cloud base above 2,500 ft AGL and light, variable winds
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B. All four conditions are required: visibility of 10 km or more, no cloud below 5,000 ft or the highest minimum sector altitude, no cumulonimbus or towering cumulus at any height, and no significant weather.
2. When flying VFR near a mountain range with a strong wind blowing across the ridge, where is the highest risk of severe downdraughts and rotor turbulence?
On the upwind (windward) slope of the ridge
Directly above the peak of the ridge at high altitude
On the downwind (lee) side of the ridge, below crest level and in the rotor zone
At the base of the windward slope, well before the ridge
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C. Mountain-wave turbulence and rotor circulation develop downwind of the ridge, producing sink rates that can exceed the climb performance of a light aircraft.
Educational use only. This module does not replace current legislation, the AIP, ERSA, authorised weather briefings, the aircraft flight manual or instructor guidance. Verify operational information through official sources before flight.
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