Thunderstorms — Page 263, Lesson 235

I want to walk you through the icing hazards inside thunderstorms, starting from where the previous section left off. At the very low temperatures high in a thunderstorm, the atmosphere normally consists of ice particles, ice crystals, and snowflakes. Because there is almost no liquid water left at those temperatures, the chance of severe icing is greatly reduced. But here is the critical point: inside a thunderstorm cloud, the freezing level must be assumed to drop all the way down to the base of the cloud. Why? Because of the powerful downdraughts. Those downdraughts carry supercooled water droplets far below the normal freezing level. The practical consequence is that airframe icing can be expected everywhere inside a thunderstorm cloud — not just above the freezing level, but throughout the entire cloud. Now let's move to engine icing. For piston engines, loss of power can occur over a wide range of temperatures due to ice forming in the induction system. The proper use of carburettor heat, or any other induction anti-icing equipment, is essential to prevent or at least minimise that power loss. And here is an important extra point: even in clear air — not inside a cloud — if the humidity is high, specifically 60% or more, carburettor ice can easily form. Such high humidity often exists in areas of thunderstorm activity, so the risk is real even before you enter the cloud. For turbine engines — that is, jet engines — the danger you must recognise is a flameout whenever icing conditions are met. The procedure is clear: igniters must be switched on and must remain on, provided they are cleared for continuous operation by the manufacturer. In all circumstances, you must strictly follow the operator's or manufacturer's instructions to achieve maximum protection. Do not improvise. I also want to emphasise a combined hazard. When you are flying in a thunderstorm, anything more than very light ice accretion adds to the problems already caused by turbulence. There are three reasons for this. First, the increased weight of the aircraft. Second, the disturbance of the normal airflow over the wings and control surfaces. Third, the reduced effectiveness of the control surfaces themselves. So even a moderate ice load makes the turbulence problem significantly worse. Finally, the good news from experience. Provided you take the normal precautions — that is, using the anti-icing or de-icing equipment correctly — icing conditions need not be a grave hazard if you cannot avoid penetrating a thunderstorm area. However, the real danger comes from failure to recognise or anticipate icing conditions, and failure to use the equipment properly. Recognise the conditions, anticipate them, and use your equipment correctly.