Special Operational Procedures and Hazards — Page 152, Lesson 234

I want to walk you through the start of a critical chapter — Special Operational Procedures and Hazards — and we're diving straight into aircraft icing, which is one of the most serious threats you'll face as a professional pilot. Let's begin with two types of snow you need to distinguish. Dry snow occurs when the ambient temperature is below freezing. Wet snow occurs when the ambient temperature is near or above freezing. That distinction matters because wet snow has a higher moisture content and behaves differently on the airframe. Now, two fundamental processes. De-icing is the process of removing ice from an airframe. Anti-icing is the process where the formation of ice on the airframe is prevented. They are not the same thing — de-icing cleans off what's already there, anti-icing stops it from forming in the first place. The effects of icing are wide-ranging, unpredictable, and dependent on the individual aeroplane design. The magnitude of these effects depends on many variables, but the effects can be both significant and dangerous. Flight in known icing conditions is subject to the limitations laid down in OM Part B — that's your Operations Manual, Part B. Let me break down the main effects of icing into categories. First, aerodynamic effects. Ice alters the shape of the flying surface. Lift can be reduced by as much as 30%, and drag increased by up to 40%. That will significantly increase stall speed, reduce controllability, and alter flight characteristics. Those are not small numbers — a 30% loss of lift and a 40% increase in drag are dramatic. Second, weight effects. Ice adds considerably to the mass of the aeroplane and affects the centre of gravity. That changes how the aircraft performs and handles. Third, instrument effects. Ice forming on pitot tubes, static vents, or angle of attack vanes can cause errors in indications and give false attitude, airspeed, angle of attack, and engine power information for air data systems. In other words, your instruments can lie to you. Fourth, windscreen and canopy effects. Windscreens and canopies can become obscured. Interference can be caused by ice on aerials. Undercarriage operation may be affected by ice in the wheel wells. And ice films can cause skin friction. Fifth, ingestion hazard. Ice build-up on critical surfaces of the aeroplane may also break away during the take-off and be ingested into engines, possibly damaging fan and compressor blades. And here's a crucial point: the formation of ice on an aeroplane on the ground may have a completely different effect on aircraft flight characteristics than ice formation in the air. The text repeats this for emphasis — ice, frost, and snow formed on the critical surfaces on the ground can have a totally different effect than ice formed in flight. So you cannot assume that in-flight icing performance data applies to ground icing. Operators are required to establish procedures to be followed when ground de-icing and anti-icing is required. Also, operators are not to allow aeroplanes to fly in conditions where icing is expected unless the aeroplane is certificated accordingly. If an aeroplane is to be flown at night in icing conditions, a light is to be provided to illuminate the airframe so the crew can see if ice is accruing. That light must be positioned and of such intensity that it will not adversely affect the performance of any duty. Now, the Clean Aircraft Concept. This is a fundamental rule. During conditions conducive to aircraft icing, take-off shall not be attempted when ice, snow, slush, or frost is present, or adhering to the wings, propellers, control surfaces, engine inlets, or other critical surfaces. That is the clean aircraft concept — the aircraft must be clean before take-off. Finally, the variables that can affect the formation of ice are: - Ambient temperature. - Aeroplane skin temperature. - Precipitation rate and moisture content. - De-icing/anti-icing fluid type, concentration, and temperature. Each of these factors influences whether and how ice forms, and how effective your de-icing or anti-icing treatment will be.