Rime ice is rough, milky, and opaque — Page 297, Lesson 286

Let’s start with the two fundamental types of ice you’ll encounter in flight: rime ice and clear ice. These are defined by the size of the supercooled water droplets that strike the aircraft and how quickly they freeze. Rime ice is rough, milky, and opaque. It forms when small supercooled water droplets hit the aircraft and freeze instantaneously. Because they freeze so fast, the droplets don’t have time to spread out or trap much air, but the result is a rough, white, frost-like deposit that disrupts airflow early. Clear ice is glossy, clear, or translucent. It forms when large supercooled water droplets freeze relatively slowly. The slower freezing allows the droplets to spread across the surface before solidifying, producing a smooth, transparent layer that can be harder to see and more aerodynamically deceptive. Now let’s move into piston engine induction icing — icing that affects the air intake system of piston engines. There are three distinct types here. First, impact icing. This is ice that forms in the intake areas caused by snow, a mix of snow and rain, or supercooled water droplets being drawn into the induction system. For turbocharged engines with fuel injection, this is actually the only icing hazard they face — they don’t have a carburettor, so they avoid the other types. Second, fuel icing. This is caused by water that’s present in the fuel itself freezing as it passes through bends in the induction piping. The water separates out and freezes when temperatures are low enough. Third, carburettor icing. This is the big one. It’s caused by two temperature drops happening together. One is the sudden temperature drop as latent heat is absorbed when fuel evaporates — that’s the fuel turning from liquid to vapour, which pulls heat out of the surrounding air. The other is the temperature drop due to the adiabatic expansion of the air as it passes through the venturi — the narrowing in the carburettor that speeds up the airflow, which also cools it. Carburettor icing is most dangerous within a temperature range of -10°C to +25°C, and it can occur in cloud, fog, or precipitation at any power setting. That wide range is why it’s such a persistent hazard. Now let’s look at jet engine icing. Ice may form on the intake lips or on the inlet guide vanes — those are the stationary vanes just inside the engine intake that straighten the airflow before it hits the compressor. If that ice breaks away and enters the engine, blade damage can occur. Some icing may also happen in the early inlet stages, particularly at high engine speeds and low aircraft forward speeds — for example, during the approach. In those conditions, there’s significant adiabatic cooling, and temperature reductions of 5°C or more can result inside the intake. This icing is particularly prevalent in freezing conditions associated with any form of precipitation. As a consequence, engine anti-icing must be selected ON when there is precipitation and the indicated outside air temperature is +10°C and below. That’s a hard rule — not a recommendation. There are other factors affecting jet engine operation in icing conditions. First, engine power indications may be in error if there is ice on the engine inlet P1 pressure probes — those probes measure total pressure at the engine face, and ice can block or distort the reading. Second, engine igniters should be used in potential icing conditions; otherwise, engine failure is possible. The igniters provide a continuous spark to relight the engine if a flameout occurs. Now, some broader points. Long flights at very low temperatures may cause fuel freezing, so the fuel freezing point specification for the aircraft type should be known — you need to know the temperature at which your fuel will start to form wax crystals. Clear ice can occur at ambient temperatures above zero when water droplets come into contact with an aircraft whose upper surfaces are at or below zero. That low skin temperature can be caused by very low fuel temperature conducting through the skin from the tanks. This icing can also occur on the ground in high humidity, rain, drizzle, or fog. It could then be snow-covered and difficult to detect. Break-up of this ice on take-off can be particularly hazardous to rear-engined aircraft, because chunks can be thrown back into the engines. Finally, operation of anti-icing or de-icing equipment usually implies a performance penalty — you lose some thrust or increase fuel burn to keep the ice off.