Ice and Rain Protection — Page 239, Lesson 302

Right, let's pick this up. We've just established the energy balance idea — that skin temperature of 0°C can occur over a wide speed range. Now I want to give you the crucial practical consequence of that, because it's a classic exam trap. Because that energy balance can happen at one end of the speed range with all droplets freezing, and at the other end with none freezing, the potential "catch rate" — sometimes called the "impingement rate" — and the actual icing rate are not simply related in this region. In plain terms: just because the aircraft is catching water droplets doesn't automatically mean ice is forming on it. The "no icing hazard" speed therefore depends not only on temperature and altitude, but also on the free water content of the atmosphere. So for severe conditions, that no-icing-hazard speed is about the maximum speed of subsonic aircraft. And here's the final influencing factor of note: icing does not occur above about 12 000 m — that's 40 000 ft — because the droplets are all frozen and in the form of ice crystals, and they will not adhere to the aircraft's surface. So above that altitude, you're safe from airframe icing, simply because the water is already solid and won't stick. Now let's move to the requirements and standards of protection. The aircraft must be cleared of ice, frost and snow prior to dispatch. That's a hard requirement. And CS-OPS — that's the European certification standard for operations — requires that public transport aircraft shall be provided with certain protective equipment for flights where the weather reports available at the time of departure indicate the probability that conditions predisposing to ice formation will be encountered. So the trigger is the departure weather report. But here's the important nuance: certain basic standards have to be met by all aircraft, whether or not they are required to be protected by CS-OPS. These are intended to provide reasonable protection if the aircraft is flown unintentionally for short periods in icing conditions. So even an aircraft not fitted with full anti-icing must still have a basic level of protection. The CS-OPS requirements cover such considerations as the stability and control balance characteristics, jamming of controls, and the ability of the engine to continue to function in icing conditions. Now, the two different approaches generally used. First, 'De-icing' — where ice is allowed to accumulate prior to being removed. Second, 'Anti-icing' — where the object is to prevent any ice accumulation. So de-icing is reactive, anti-icing is preventive. Keep those two straight. There are a number of avenues which need exploring, and these include detection and warning systems, and the methods used to protect the aircraft. The protection methods can be any or all of the following. Pneumatic — that's expanding rubber boots, which is mechanical. Thermal — which can be electrically heated, oil heated, or air heated. And liquid — which is freezing point depressant fluids, abbreviated FPD. Then we have the system functions. Ice detection is provided automatically by the provision of ice detectors which relay a warning to the flight crew. Anti-icing is the application of continuous heat or fluid. De-icing is the intermittent application of fluid, heat or mechanical effort. So note the contrast: anti-icing is continuous, de-icing is intermittent. These aspects will all be dealt with in detail later in the chapter. So for now, hold onto the key distinctions: de-icing versus anti-icing, the three protection methods — pneumatic, thermal, liquid — and the fact that icing stops above 40 000 ft.