Highveld Thunderstorms - Know the Dangers

By Garth Calitz

Flying light aircraft in or near thunderstorms poses a considerable risk anywhere in the world, but this danger is especially pronounced in South Africa. The country's geography and climate are particularly favourable for severe convective weather, particularly in the summer. For general aviation pilots navigating around South Africa, especially the Highveld region, it is crucial to understand the distinctive features of South African thunderstorms for safe flight planning and decision-making.

The Highveld, stretching across Gauteng, Mpumalanga and parts of the Free State, is especially prone to intense afternoon and evening thunderstorms between October and March. Operating at elevations well above sea level, light aircraft already suffer reduced engine and aerodynamic performance due to high density altitude. When thunderstorms develop, powerful updrafts and downdrafts far exceed the climb or descent capability of most light aircraft. Encountering such vertical air movement can result in sudden altitude deviations, loss of control or even structural damage.

Very few South African general aviation airfields possess sophisticated wind shear detection systems. Thunderstorms in the region frequently produce strong gust fronts and microbursts that can occur with little warning. Pilots departing or approaching smaller airfields, often surrounded by open terrain, are particularly vulnerable. A sudden tailwind or loss of headwind close to the ground can result in an unrecoverable sink rate, especially in lightly powered aircraft operating near maximum take-off weight.

South African thunderstorms are well known for producing large hail. Hail damage is common on the Highveld, with windscreens, leading edges and propellers usually taking the brunt of the damage. Heavy rainfall can rapidly reduce visibility to near zero, making continued VFR flight impossible. In addition, heavy rain can disrupt pitot-static systems, causing unreliable airspeed and altitude indications, an especially dangerous situation for pilots without extensive instrument flying experience.

Lightning activity in South African thunderstorms is among the highest in the world. While most aircraft can tolerate a lightning strike, light aircraft, especially older types common in the local general aviation fleet, may suffer avionics damage or electrical failures. A lightning strike can also cause temporary night blindness or disorientation, a serious risk when flying near mountainous terrain or in busy controlled airspace around major centres such as the Gauteng area.

Although icing is often associated with winter weather, severe icing can occur inside South African summer thunderstorms due to supercooled water droplets at altitude. Not many light aircraft in South Africa are certified for flight into known icing and few are equipped with effective anti-icing systems. Rapid ice accumulation can degrade performance and controllability in a matter of minutes, particularly during climbs through cloud layers.

Carburettor icing is a frequently underestimated hazard in South African conditions and is particularly dangerous when operating in and around thunderstorms. High humidity, moderate ambient temperatures and reduced engine power settings, common during descents, approaches or manoeuvring to avoid weather, create ideal conditions for ice to form inside the carburettor. Even on warm Highveld summer days, the temperature drop caused by fuel vaporisation and airflow through the carburettor venturi can result in rapid ice formation.

In thunderstorm conditions, heavy rain and rapidly changing air temperatures further increase the risk. Carburettor ice can develop quickly and often without obvious warning, leading to a gradual or sudden loss of engine power. At low altitude, such as on approach or while attempting to divert around weather, this power loss can be catastrophic. Prompt and correct use of carb heat is essential, yet many accidents occur when carb icing is either not recognised in time or misdiagnosed as another engine problem.

Outside major airports, pilots may have limited access to real-time weather information. Datalink weather products, where available, can be delayed and should not be used for close-in storm avoidance. Visual cues can also be misleading, as thunderstorms may be embedded in cloud layers or obscured by haze, a common feature of South African summer conditions.

Avoidance remains the only safe strategy, "When there is doubt, there is no doubt". Local aviation experience and guidance are essential, so please don't hesitate to ask the locals for advice, who on most occasions will advise maintaining a wide berth, at least 20 nautical miles from active thunderstorms. Extra caution should be exercised on the downwind side of storms, where gust fronts, wind shear and hail can extend far beyond the visible cloud. When thunderstorms are forecast along a route, delaying departure, planning early diversions or cancelling the flight altogether is often the safest decision.

South Africa's weather can really dish out some most intense thunderstorms that pilots will experience anywhere in the world. When you mix that with high-density altitude, not-so-great facilities at smaller airfields and the sneaky danger icing, you've got a pretty serious challenge on your hands. To fly safely in South Africa, it's crucial to plan around the weather, manage your energy carefully and be ready to steer clear of trouble.

Stay safe out there, and remember flying should always be fun. To quote aviation legend Jim Davis, "There is no shame in joining the Live Cowards Club". https://livecowards.substack.com/p/jims-famous-live-cowards-club