FDECGB Prop ammeter 14 -18A Manual Right manual override relay LH prop RH prop Left manual override relay Off Manual prop deice control switch Prop timer Auto prop deice 20A RH manual 20A Manual prop deice control 5A LH manual 20A Figure 15-37.  An American Airlines aircraft being deiced at  Syracuse Hancock International Airport.  Figure 15-36.  Propeller electrical deice system schematic.  15-25 Basically, all surfaces that have an aerodynamic, control,  sensing, movement, or measuring function must be clean.  These surfaces cannot necessarily be cleaned and protected  in the same conventional deicing/anti-icing manner as the  wings. Some areas require only a cleaning operation, while  others need protection against freezing. The procedure of  deicing may also vary according to aircraft limitations. The  use of hot air may be required when deicing (e.g., landing  gear or propellers). The deicing fluid must be accepted according to its  type for holdover times, aerodynamic performance, and  Frost deposits can be removed by placing the aircraft in a  warm hangar or by using a frost remover or deicing fluid.  These fluids normally contain ethylene glycol and isopropyl  alcohol and can be applied either by spray or by hand. It  should be applied within 2 hours of flight. Deicing fluids  may adversely affect windows or the exterior finish of the  aircraft, only the type of fluid recommended by the aircraft  manufacturer should be used. Transport category aircraft  are often deiced on the ramp or a dedicated deicing location  on the airport. Deicing trucks are used to spray the deicing  and/or anti-icing fluid on aircraft surfaces.  [Figure 15-37] Holdover Time (HOT) is the estimated time that deicing/ anti-icing fluid prevents the formation of frost or ice and the  accumulation of snow on the critical surfaces of an aircraft.  HOT begins when the final application of deicing/anti-icing  fluid commences and expires when the deicing/anti-icing  fluid loses its effectiveness.  Figure 15-38  shows a holdover  timetable for Type IV fluid.  Any deposits of ice, snow, or frost on the external surfaces of  an aircraft may drastically affect its performance. This may be  due to reduced aerodynamic lift and increased aerodynamic  drag resulting from the disturbed airflow over the airfoil  surfaces, or it may be due to the weight of the deposit over  the whole aircraft. The operation of an aircraft may also be  seriously affected by the freezing of moisture in controls,  hinges, valves, microswitches, or by the ingestion of ice into  the engine. When aircraft are hangared to melt snow or frost,  any melted snow or ice may freeze again if the aircraft is  subsequently moved into subzero temperatures. Any measures  taken to remove frozen deposits while the aircraft is on the  ground must also prevent the possible refreezing of the liquid. When aircraft surfaces are contaminated by frozen moisture,  they must be deiced prior to dispatch. When freezing  precipitation exists, and there is a risk of contamination of  the surface at the time of dispatch, aircraft surfaces must  be anti-iced. If both deicing and anti-icing are required,  the procedure may be performed in one or two steps. The  selection of a one- or two-step process depends upon weather  conditions, available equipment, available fluids, and the  holdover time to be achieved. material compatibility. The coloring of these fluids is also  standardized. In general, glycol is colorless, Type-I fluids  are orange, Type-II fluids are white/pale yellow, and Type- IV fluids are green. The color for Type-III fluid has not yet  been determined. Holdover Time (HOT) Frost Removal Critical Surfaces Deicing and Anti-icing of Transport Type Aircraft Deicing Fluid