AIRCRAFT ICE AND RAIN PROTECTION

NEED FOR ICE AND RAIN PROTECTION

The flight operation in present day is all-weather flying. To fly in all weather conditions, it is necessary:

To take protection against ICE build up that may affect safety and performance

To take protection against RAIN / MIST which may impair visibility?

Aircraft must be provided with:

·         Ice detection equipment

·         Ice protection equipment

 

 SOURCES OF ICE

Source of ice is the water in the atmosphere. Water may exist in the atmosphere in three forms:

• Invisible vapor
• Visible liquid particles or moisture
• Ice.

Invisible vapor and visible liquid particles in atmosphere may condense into ice on aircraft surfaces when they are at sub-zero temperatures. Ice already formed in the atmosphere may deposit on the aircraft surfaces.

 AREAS TO BE PROTECTED

Areas/Locations where ice is most likely to form and which are to be protected are:

• Wing leading edges
• Stabilizer leading edges
• Fin leading edges
• Wind shield
• Radome
• Stall warning probes/angle of attack sensors
• Pitot tubes
• Antennas
• Drain masts
• Engine air intake
• Propellers

 EFFECTS OF ICE FORMATION

Ice formation in the aircraft aerodynamic surfaces affects the performance and safety of the aircraft as a result of:

• Loss of lift due to change in wing section
• Increase in drag due to rough surface and friction over the wing upper camber
• Decrease in propeller efficiency due to change in blade profile
• Loss of control preventing control surface movement
• Increased load and wing loading Loss of inherent stability due to C.G changed because of weight of extensive deposition of ICE

 METHOD OF ICE DETECTION

  When an aircraft flies through icing weather condition, ice accumulates in ice detecting equipment. This equipment may be a probe/head for giving an electrical/electronic warning signal (light) in the cockpit or simply a visual indicator to be seen by a light from the cockpit when ice-accretion occurs on it.

 Pressure sensor method: These consist of a short stainless steel or chromium plated brass tube, which is closed at its outer end and mounted so that it projects vertically from a portion of the aircraft known to be susceptible to icing. Four small holes are drilled in the leading edge of this tube and in the trailing edge are two holes of less total area than those of the leading edge (see figure 4.1). A heater element is fitted to allow the detector head to be cleared of ice.

Figure 4.1: Pressure sensor method of ice detection.

Electro Mechanical Method: This consists of a serrated rotor, incorporating an integral drive shaft coupled to a small ac motor via a deduction gearbox, being rotated adjacent to a fixed knife-edge cutter (Figure 4.2). The motor causing is connected via a spring – tensioned toggle bar to a micro switch assembly.

The electrical motor continuously drives the serrated rotor on the detector head so that its periphery rotates within 0.050mm (0.0002 inch) of the leading edge of the knife-edge cutter. The torque therefore required to drive the rotor under non-icing conditions will be slight, since bearing friction only has to be overcome. Under icing conditions, however, ice will accrete on the rotor until the gap between the rotor and knife-edge is filled, where upon a cutting action by the knife edge will produce a substantial increase in the required torque causing the toggle bar to move against its spring mounting and so operate the micro-switch, to initiate a warning signal.

Hot Rod Method: This consists of an aluminum alloy oblong base (called the plinth): on which is mounted a steel tube detector mast of aerofoil section, angled back to approximately 30° from the vertical, mounted on the side of the fuselage, so that it can be seen from the flight compartment windows. The mast houses a heating element and in the plinth there is a built-in floodlight.

The heating element is normally off and when icing conditions are met ice accretes on the leading edge of the detector mat. The flight crew can then observe this. 

During night operations the built-in floodlight may be switched on to illuminate the mast. By manual selection of a switch to the heating element the formed ice is dispersed for further observance (see figure 4.3).

5 Ultra sonic vibrator method: This ice detector senses the pressure of Icing conditions and provides an indication in the flight compartment that such conditions exist. The system consists of a solid-state ice detector and advisory warning light. The ice detector is attached to the fuselage with its probe protruding through the skin Figure 4.4. The ice detector probe (exposed to the air-stream) is an ice-sensing element that ultrasonically vibrates in an axial mode of its own resonant