2.5.1.2 At the present time, there is no known operational procedure that… — Page 263, Lesson 232

I want to walk you through a critical section on operating in heavy precipitation and icing conditions. Let's start with paragraph 2.5.1.2. At the present time, there is no known operational procedure that can completely eliminate the possibility of engine damage or flame out during massive water ingestion. A flame out, as the name suggests, is when the combustion flame in a turbine engine goes out, causing a total loss of thrust. The exact mechanism of these water-induced engine stalls has not been determined, but it is believed that thrust changes may have an adverse effect on engine stall margins. In other words, if you change the thrust setting while the engine is gulping a huge amount of water, you could push the engine closer to an aerodynamic stall of the compressor blades, making a flame out more likely. Moving to 2.5.1.3 — to eliminate the risk of engine damage or flame out by heavy rain, it is essential to avoid severe storms. That's your primary defence. During an unavoidable encounter with extreme precipitation, the best-known recommendation is to follow the severe turbulence penetration procedure contained in the approved aircraft flight manual. That is your specific, approved procedure for your aircraft type. There is special emphasis on avoiding thrust changes unless excessive airspeed variations occur. So you set the recommended thrust and leave it alone, unless the airspeed swings dangerously outside limits. Flight research has revealed that water can exist in large quantities at high altitudes even where the ambient temperature is as low as -30°C. That means you cannot assume that just because it's very cold, the air is dry. Rain, sometimes heavy, may therefore be encountered and give rise to ice accretion and a possibility of the malfunctioning of pressure instruments. Ice can block pitot tubes or static ports, causing unreliable airspeed and altitude readings. Turbine engine igniters must be switched on. The igniters are the spark plugs inside the engine combustion chamber; turning them on continuously ensures that if the flame does go out, it can be relit immediately. Now paragraph 2.5.2 — heavy precipitation, which occurs in cumulonimbus clouds, may often be seen as shafts of rain below the cloud base. Where this precipitation does not reach the surface, the shafts are known as virga. Virga is rain that evaporates before hitting the ground. The evaporation cooling associated with virga may intensify existing downdraughts. As the rain evaporates, it cools the surrounding air, making it denser and accelerating its downward motion. This can create or strengthen a microburst. Let's move to section 2.6, Icing. Paragraph 2.6.1 states a firm operational rule: flight must not be initiated or continued into areas where the forecast icing conditions will exceed the icing limitations of the aircraft. Every aircraft has certified limits for how much ice it can tolerate; you must stay within those limits. Paragraph 2.6.2 — formation of ice on the airframe must always be considered likely when flight takes place through cloud or rain at a temperature below 0°C. That is the basic condition: visible moisture plus freezing temperatures equals ice. The temperature range favourable for ice accretion in thunderstorms is from 0°C down to -45°C, that is where water droplets can exist in a supercooled state. Supercooled water droplets are liquid water below freezing temperature; they freeze instantly on impact with the airframe. Below about -30°C, however, a large part of the free water content of the cloud has already frozen into ice crystals, so the risk of rapid ice accretion decreases. Let me show you a barogram from a thunderstorm passage that illustrates the pressure variations you might see on your altimeter during such an encounter. And here is a diagram showing microbursts, which are concentrated downdraughts up to 4 km in horizontal length with a limited lifetime. Finally, this figure shows the concentration of supercooled water droplets and how the shape of the aircraft affects ice accretion.