Thunderstorms — Page 263, Lesson 231

Let’s talk about hail first, because it’s one of the most destructive things you can fly into inside a thunderstorm. I want you to understand that while hail encounters are usually short in duration, the damage to an aircraft can be severe. Hail can damage the leading edges of the wings, and that directly reduces the efficiency of the wing — meaning you lose lift and increase drag. Windscreens and other transparencies, like cockpit windows, may be shattered. The book gives a real-world example: in the Middle East, a VC-10 encountered a thunderstorm shortly after take-off, and hail severely damaged the airframe. The radome — that’s the nose cone housing the weather radar — was torn away. There was denting and damage to the skin in many areas. Notably, there was no evidence of a lightning strike in that case, so the damage was purely from hail. Now, here’s an important safety point: no fatal accidents to civil aircraft are known to have been attributable entirely to hail damage. But that does not mean hail is safe. Hail is a serious hazard at all altitudes at which civil aircraft operate. The evidence for this comes from a study of military aircraft accidents in the USA, where aircraft were damaged or destroyed by the combined effect of hail and turbulence. Additional evidence comes from the United States National Severe Storms Project, together with individual reports of encounters with hail in normal operations. So the risk is real, even if it hasn’t yet caused a fatal civil airliner crash on its own. Now let’s move to rain — specifically, water ingestion by turbine engines. Turbine engines have a limit on the amount of water they can ingest. That’s a hard design limit. In many thunderstorms, especially those in the developing stages, there are strong updraughts. If the updraught velocity in the thunderstorm approaches or exceeds the terminal velocity of the falling raindrops — that is, the speed at which a raindrop falls when gravity and air resistance balance — then very high concentrations of water may occur. The raindrops get suspended or even carried upward, piling up in one area. It is possible that these concentrations can be in excess of the quantity of water turbine engines are designed to ingest. If that happens, the result could be a flame out — that means the engine stops running — and/or structural failure of one or more engines. That’s about as serious as it gets in flight. And here’s the sobering bottom line from the book: at the present time, there is no known operational procedure that can completely eliminate the possibility of engine damage or flame out from water ingestion. That means your best defence is avoidance — stay out of those thunderstorms, especially the developing ones with strong updraughts.