Chapter 4 - Preventative Measures and Deicing Procedures

Deicing and Anti-Icing the Airframe

27.  Operational procedures employed in aircraft ground deicing and anti-icing vary, depending on the type of accumulation on the surface of the aircraft and the type of aircraft. The general procedures used by aircraft operators are similar and are based on the procedures recommended by the aircraft manufacturer, which, in turn, may be based upon procedures recommended by the fluid manufacturer, engine manufacturer, and the SAE HOT guidelines provide guidance based upon SAE recommendations for the application of SAE Types I, II, III and IV fluids as a function of outside air temperature (OAT).

28.  An aircraft may be deiced by any suitable manual method. Parking the aircraft in a heated hangar for an appropriate amount of time to melt all contamination is a common deicing procedure for a smaller aircraft. Using wing covers or other temporary shelters will often reduce the amount of contamination and the time required for deicing and anti-icing aircraft, especially when the aircraft must be stored outside. Some types of contamination such as light, dry snow can be removed with a shop broom, or very light frost can be rubbed off using a rope sawed across the contaminated area.

29.  Deicing is normally accomplished using heated water or solutions of heated water and FPD fluids, often followed by anti-icing using cold, Type II, III, or IV that have a longer HOT. Each fluid has very unique characteristics and handling requirements.

30.  One of the more common deicing procedures in commercial operations involves using water, FPD fluids, or solutions of FPD fluids and water. High pressure spraying equipment is often used in large operations to add physical energy to the thermal energy of deicing fluids. Heating these fluids increases their deicing effectiveness; however, in the anti-icing process unheated fluids are more effective because the thickness of the fluids is greater.

31.  Deicing and anti-icing with FPD fluids may be performed as a one-step or two-step process, depending on predetermined practices, prevailing weather conditions, concentration of the FPD used and available deicing and anti-icing equipment and facilities.

32.  The one-step method is accomplished using a heated FPD mixture. In this process, the residual FPD fluid film provides a very limited anti-icing protection.

33.  The two-step procedure involves both deicing and anti-icing. Deicing is accomplished with hot water or a hot mixture of FPD and water. The ambient weather conditions and the type of accumulation to be removed from the aircraft must be considered when determining which deicing fluid to use. The second (anti-icing) step involves applying a mixture of SAE Type II, III or IV and water to the critical surfaces of the aircraft.

Caution:

Anti-icing fluid should typically be applied within 3 minutes of deicing with a heated deicing fluid.

The effectiveness of Types II, III and IV fluids can be seriously diminished if proper procedures are not followed when applying it over Type I fluid. Consult the fluid manufacturer for further information.

Ensure Type IV fluids are applied evenly and thoroughly and that an adequate thickness has been applied in accordance with the fluid manufacturer's recommendations.

Under no circumstances should SAE Type II, III or IV fluids, be applied directly to the following areas of an aircraft:

• Pitot heads, static ports and angle-of-attack sensors;
• Control surface cavities;
• Cockpit windows and the nose of fuselage;
• Lower side of the radome underneath the nose;
• Air inlets and intakes; and
• Engines.

34.  Figure 1 demonstrates how an aircraft must be systematically and symmetrically de-iced and anti-iced in weather conditions conducive to icing. Each aircraft surface requires a specific cleaning technique.

35.  Generally, the fuselage should be de-iced and anti-iced from the top down. Clearing the top of the fuselage manually instead of by spraying requires that personnel use caution not to damage protruding equipment (e.g., antennae) while deicing. Spraying the upper section with heated FPD fluid first allows the fluid to flow down, warming the sides of fuselage and removing accumulations. This is also effective when deicing the windows and cockpit windshield of the aircraft. Direct spraying of these surfaces can cause thermal shock, resulting in cracking or crazing of the windows. Deicing the top of the fuselage is especially important on aircraft with an aft-mounted centreline engines. The ingestion of ice or snow can result in compressor stalls or engine damage.

36.  The radome or nose of the aircraft should be de-iced to eliminate snow or ice accumulations from being projected into the crew's field of vision during take-off. The nose also contains navigation and guidance equipment; therefore, it must be cleared of accumulations to ensure proper operation of the sensors.

37.  The cargo and passenger doors must also be de-iced and anti-iced to ensure proper operation. All hinges and tracks should be inspected to ensure that they are free of accumulation. Although accumulation may not impair operation on the ground, it may freeze at flight altitude and prevent normal operation at the aircraft's destination. Frozen accumulation may also cause damage and leakage on cargo and passenger door latches and seals.

38.  Sensor orifices and probes along the fuselage (e.g., static ports, pitot tubes, air intakes or temperature sensors) require caution during the application of FPD fluid. Direct spraying into these openings can damage the equipment, or residues could result in faulty readings.

39.  The wings are the main lifting surfaces of the aircraft and must be free of contamination to operate efficiently. An accumulation of frost, ice or snow on the wing changes the airflow characteristics, reducing its lifting capabilities, increasing drag, increasing stall speed and changing pitching moments. The weight increase is slight and its effects are secondary to those caused by surface roughness.

40.  On many aircraft, deicing of the wing begins at the leading edge wing tip, sweeping in the aft and inboard direction. This procedure avoids increasing the snow load on outboard wing sections, which under some very heavy snow conditions could produce excessive wing stresses. This method also reduces the possibility of flushing ice or snow deposits into the balance bays and cavities.

41.  For aerodynamic reasons, ensure that the deicing and anti-icing procedures are conducted in a symmetrical fashion.

FIGURE 1.  SYSTEMATIC AND SYMMETRICAL DEICING OF AIRCRAFT

42.  If ice accumulation is present in areas such as flap tracks and control cavities, it may be necessary to spray from the trailing edge forward. Also, under some weather or ramp conditions, it is necessary to spray from trailing edge. Consult the aircraft manufacturer for specific details.

43.  It is important for operators to consider the configuration of their aircraft during deicing. Manufacturers may indicate that their aircraft need to be in a specific configuration during the deicing and anti-icing process. However, if an aircraft is in a clean configuration, that is with all high lift devices retracted, during deicing the operator needs to consider what untreated areas of the wing are subsequently exposed to freezing precipitation once the devices are extended/deployed. The areas under a leading edge flap or slat, if not protected by anti-icing fluids, have the potential of becoming a contaminated critical surface prior to take-off. Air operators need to consider this scenario and may need to develop additional procedures to ensure that the aircraft is taking off in an uncontaminated condition.

Two possible options include: delaying slat/flap deployment until just prior to take-off; and deploying the devices prior to de/anti-icing so that the surfaces under these devices are treated.

Caution:

Taxiing in wet/slush conditions, even after de/anti icing, may contaminate flap/slat and landing gear door/sensor surfaces and may cause takeoff and/or after takeoff problems. Most Manufacturers recommend that flap/slat devices be deployed just prior to takeoff and taxi speed reduced to minimize splashed contaminants from freezing to landing gear door/sensor surfaces.

44.  The tail surfaces require the same caution afforded the wing during the deicing procedure. It is important that both sides of the vertical stabilizer and rudder be de-iced because it is possible for directional control problems to develop on certain aeroplanes if the contamination is removed from one side only. The balance bay area between moveable and stationary tail surfaces should be closely inspected. For some aircraft, positioning the horizontal stabilizer in leading-edge-down position allows the FPD fluid and contaminants to run off rather than accumulate in balance bays, while others may require the horizontal stabilizer in the leading-edge-up position. Consult your aircraft manuals for complete information.

45.  Balance bays, control cavities and gap seals should be inspected to ensure cleanliness and proper drainage. When contaminants do collect in the surface juncture, they must be removed to prevent the seals from freezing and impeding the movement of the control surface.