Propeller Icing

Propeller icing will form on the airframe or propeller when flying in cloud or rain with ambient temperatures below 0 degrees Celsius (32 °F) down to temperatures around minus 40 degrees Celsius. Between 0 °C and minus 20 °C, icing will be most severe with glaze type of icing. From -20 °C to -40 °C rime ice will be more prevalent and below -40 °C icing is less likely to occur, but is still a possibility. Icing will form on those parts of the aircraft with relatively sharp or protruding items such as the wing’s leading edge, aerials, struts and of course, the leading edge of the propeller blades. The weight of the accumulated ice is less of a hazard than the adversely modified airflow over the prop blade. It only takes a small amount of ice to modify the shape of the blade’s leading edge and degrade performance thus causing a reduction in prop thrust and efficiency. The associated drag will reduce the rate of climb and cruise speed. In addition, if the prop has icing problems, then the wings are sure to be iced up as well. In theory, the aircraft structure should ice up before the prop blades. This is due to the blade tip’s high speed causing kinetic heating,

which increases the temperature of the blades: the heat rise being approximately proportional to the square of the speed (prop RPM). The blade’s inner portions will be rotating at a slower speed than the tips and therefore, will experience less of a temperature rise due to less kinetic heating. This accounts for the electric de-icing heater mats being positioned on the inner leading edge of the blade only and not extending to the tips

Uneven accumulation or shedding of the ice will put the blades out of balance causing severe vibrations, as opposed to a rough running engine. An ‘ice plate’ may be mounted on the side of the fuselage on twin-engine aircraft in the prop’s plane of rotation, for reinforcement of the fuselage skin against shedding ice strikes. In 1934, B. F. Goodrich pioneered the system of pulsating rubber de-ice boots on the wing’s leading edge Ice protection for the prop blades followed later in the form of electric heater mats, deice boots and a chemical system. If the blade’s leading edge is rebated to take the heating element, it is then known as a ‘rebated blade’. Propellers with the electro-thermal system installed are commonly referred to as ‘hot props’. The chemical system uses Ethylene Glycol, or similar fluid, which is also used in the cooling systems of liquid-cooled engines. The de-ice fluid is dispersed via a slinger ring mounted around the prop hub inside the spinner and centrifugal force carries the fluid along the blade via the ridges in the rubber boots.

As far as the electrical and chemical systems are concerned, there is no difference between anti-ice and de-ice systems. It is all a matter of timing, anti-ice prevents and de-ice cures. If icing is expected, prevention is better than a cure, so turn on the anti-ice system early before the ice has a chance to buildup to a dangerous level. If you have the misfortune to experience

prop icing with no anti-ice system on board, it maybe possible to remove the prop ice by flexing the blades using centrifugal force. This can be achieved by reducing the engine speed to around 2200 RPM with the propeller pitch control, then quickly move the prop control to fine/flat pitch. Several cycles maybe required to restore the prop to smooth running. After clearing the ice, or if icing is expected, run the engine at a higher RPM than normal to reduce the chance of ice forming. Finally, one last word on prop icing: keep the prop blades smooth and clean and apply a coating of silicone spray, it just makes it that little bit harder for the ice to cling to the blades.