Special Operational Procedures and Hazards — Page 152, Lesson 236

I want to walk you through the start of Chapter 10, which covers Special Operational Procedures and Hazards. We're going to focus on ground-based de-icing and anti-icing, which is a critical safety topic. Let's begin with the conditions that matter. When an aircraft is contaminated by ice on the ground, there are three approved methods for de-icing. First, the application of de-icing fluids, which includes what are called 'taxi through' systems. Second, warming the airframe using hot air blowers. Third, manually sweeping the surface where frost and light ice have accumulated. Now, the key distinction here is between de-icing and anti-icing. De-icing is the removal of frozen deposits that are already there. Anti-icing is the protection of surfaces to prevent new deposits from forming. On the ground, these can be done as either a one-step or a two-step procedure. In a one-step procedure, de-icing and anti-icing are carried out at the same time. You use a combined de-icing and anti-icing fluid that both removes frozen deposits and protects the de-iced surfaces for a limited period of time. The two-step procedure involves a process of ice removal first, followed by a separate process of anti-icing. The fluids used in both processes have what are called 'holdover' times quoted against the nature of the ice to be removed or protected against, and a range of ambient temperatures. Let me define that term precisely. The Holdover Time, abbreviated HOT, is the estimated time during which the de-icing or anti-icing fluid will be effective. This is a critical operational limit. The aircraft must commence its take-off roll within the HOT. If it doesn't, the whole process — de-ice followed by anti-ice — must be repeated from the beginning. What determines the HOT? It depends on several factors: the Outside Air Temperature, which we abbreviate OAT, the temperature of the aircraft's skin, the type of precipitation, the wind, and the type and concentration of the fluid being used. An example of a HOT table is shown in Figure 10.1, which is a guideline for holdover times anticipated for ISO Type I fluid mixtures as a function of weather conditions and OAT. I want to be very clear — that figure is marked "EXAMPLE ONLY - NOT FOR OPERATIONAL USE." In real operations, pilots will be required to interpret the HOT. A similar table, or details of where to find a table, would be in Part A of the Operations Manual. Currently, there are three types of fluid in general use for turbojet aircraft: Type I, Type II, or Type IV. There is also a Type III, which can be diluted Type II or Type IV cleared for use on turboprop aircraft. Now, a very important procedural requirement: aircraft must be treated symmetrically. That means both wings, both tail surfaces, both sides — the treatment must be even and balanced. The pilot in command must ensure that the critical surfaces of the aeroplane are free of ice, snow, slush, or frost just prior to take-off. This check shall be accomplished as close to the time of take-off as possible, and it is normally made from within the aeroplane by visually checking the wings. So to summarise the key points: the Holdover Time is your window of protection, it depends on OAT, skin temperature, precipitation type, wind, and fluid type and concentration. If you exceed the HOT, you must repeat the entire de-ice and anti-ice process. And the pilot in command has the final responsibility to confirm the critical surfaces are clean just before take-off, usually by a visual check from inside the cockpit.