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Ice and Rain Protection
Icing conditions are said to exist in flight if the TAT is below +10 deg C either with visible moisture in the
air or if the visibility is below 1500m.
Frost or Hoar Frost:
Most persistent on cold soaked aircraft on the ground in conditions of high relative humidity.
In flight frost or hoar frost forms when a cold airframe descends through warm air.
Runback Icing:
Temperature range is between +10 to 3 deg C. Water (warm rain) runs back (on cold aircraft) and
freezes as a clear glaze.
Glaze or Clear Ice:
Temperature range is between -3 to 8 deg C. Supercooled water droplets impact the surface, a small part
freezes while the remainder runs back and forms clear ice.
Rime Ice:
Temperature range is below -8 deg C. Super cooled rain droplets hit the airframe and freeze on impact
trapping the air. It has a milky white appearance.
Mixed Icing:
The aircraft is at an intermediate temperature between the glaze and rime thus a combination of both
causes mixed icing, making an opaque surface.
Ice Detection:
1) Vibrating Rod Systems: A rod is vibrated at 40 KHz. When ice forms (on it) it slows down. This
illuminates a warning light and activates a heater within the rod. After the ice melts the rod
returns to its 40 Khz frequency, extinguishing the warning light.
2) Pressure Operated Detectors: Its a tube protruding into the airflow with several small holes in the
leading edge and fewer smaller holes in the trailing edge. Ram air pressure holds a relay open. In
icing conditions the holes on the leading edge gets blocked by ice and thus pressure falls. The
relay closes and gives an icing warning. Through a heater element ice is melted to deactivate the
warning.
3) Serrated Rotor System.
4) Hot Rod system.
Thermal Anti-icing Systems:
Used by modern jets.
This uses large quantities of hot bleed air to heat the inner surface of the leading edge.
The air escapes through outflow holes under the wings or into the nacelles.
These systems will overheat without a flow of cooling air across the wings.
So if selected on the ground, a time delay of 12 seconds will elapse after the wheels lift off before the
system starts to operate.
Since they rely on engine bleeds, there is a performance penalty if they are used.
Pneumatic De-icing System:
Used by piston engined and turbo-prop aircraft.
The tubes are inflated to a pressure of 18 psi. At 8 psi a green light is on until the pressure drops again
below 8 psi at which point it goes out.
The boots should be operated when the ice is between 0.5 - 1.5cm thick.
If operated before 0.5 cm the ice can form a shell around the inflated profile of the boots andrender them
useless.
If operated after 1.5 cm the ice can be too strong to break off.
Thick wing sections use chord wise tubes and thin wing sections use span wise tubes.
If there is a single cycle (light aircraft) it typically takes 34 seconds.
For larger aircraft there are two speed settings:
Fast cycle - Heavy icing - 26 seconds interval.
Slow cycle - Light icing - 206 seconds interval.
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The "Weeping Wing" De-icing System:
It uses isopropyl alcohol to de-ice the aircraft in flight.
The fluid is pumped from a reservoir to porous metal strips inserted in the leading edges which sweat the
fluid out across the skin breaking the hold of the ice. allowing the airflow to remove it.
Maximum application time is 8 minutes. There is a warning when 30 minutes of fluid is left.
Electrical Propeller De-icing:
Ice is likely to form on the inner third of the back of the blade because of the slower rotational speed at
this radius and the lower air pressure.
Thus electrically heated boots are fitted around the blade's leading edge.
Sometimes a double boot is used to cover up to 50% of the blade. Beyond 50% the rotational speed
prevents ice adherence.
Electrical power is supplied via a slip ring and brushes.
To reduce the electrical load the power is cycled from the inner boots to the outer boots.
If the blades are in even pairs then power is applied to one pair at a time. If odd then all blades are heated
at the same time.
The spinner is normally electrically anti-iced rather than de-iced because ice tends to stay there once it is
formed.
Turbo-Prop Cockpit Indications:
Turbo-Prop systems operate at two possible speeds and the cycle can include all electrical anti-icing and
de-icing systems.
The fast cycle is for light icing conditions and slower one for heavy icing.
While the anti-icing systems are working a green light will be on continuously.
When both de-icing and anti-icing systems are operating it will flash.
