Chapter 5 - Ground Crew Supplement

Two-step Deicing/Anti-icing

67.  Generally is used when the aircraft is contaminated and when precipitation is active. If a two step procedure is used, the first step is typically performed using a deicing fluid; however, alternate deicing technology or mechanical methods may be used depending upon circumstances.

68.  The selection of fluid type and concentration depends on the ambient temperature, the weather conditions and the desired holdover time. When performing a two-step process, the freezing point of a fluid used for the first step must not be less than 3°C above ambient temperature. The freezing point of a SAE Type I fluid used for one-step or as the second step of the two-step operation must be at least 10°C below the ambient temperature. The second step must be completed as quickly as possible following first step fluid application (not more than 3 minutes). The two step process may need to be performed area by area.

69.  When deicing fluid is used in step 1, the application of the second step fluid will flush away the first step fluid and leave a film of anti-icing fluid which is designed to be of adequate thickness. If freezing of the deicing fluid has occurred, step 1 must be repeated. Refer to SAE ARP 4737 document for additional details.

70.  SAE Type I fluids have limited effectiveness as an anti-icing fluid due to their short holdover time. SAE Type II or IV fluids used as deicing/anti-icing agents may have a temperature application limit of -25°C. The application limit may be lower provided a 7°C buffer between the freezing point and the ambient temperature is maintained and the fluid has been demonstrated to be aerodynamically acceptable at this ambient temperature.

First Step:

• Apply heated ADF until all of the frozen contaminants have been removed from the aircraft's critical surfaces. The ADF fluid is typically heated so that it will arrive at the application nozzle at around 60-82°C (140-180°F).
   
• No frozen contaminants shall remain after application of an ADF, including under the fluid.
   
• Aircraft surfaces should be treated symmetrically for aerodynamic reasons.

Second Step:

• Apply the AAF to aircraft surfaces before any freezing of the ADF occurs. Typically the application of AAF should occur within 3 minutes of deicing with an heated ADF.

Caution:

• 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.

71.  Figure 2 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.

72.  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 engine. The ingestion of ice or snow can result in compressor stalls or engine damage.

73.  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.

74.  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.

75.  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.

76.  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.

77.  On many aircraft, de-icing 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.

78.  The various aircraft types that will be de-iced at a station need to be identified. The operators must be very familiar with any unique deicing considerations based upon aircraft type.

FIGURE 2.  SYSTEMATIC AND SYMMETRICAL DEICING OF AIRCRAFT

79.  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.

80.  It is important for operators to consider the configuration of their aircraft during de-icing. Manufacturers may indicate that their aircraft need to be in a specific configuration during the de-icing and anti-icing process. However, if an aircraft is in a clean configuration, that is with all high lift devices retracted, during de-icing 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.

81.  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.

82.  The tail surfaces require the same caution afforded the wing during the de-icing 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. For some aircraft, the horizontal stabilizer must be in the leading-edge-up position. Consult your manuals for complete information.

83.  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 impending the movement of the control surface.