Investigation into a crash that occurred in Ireland has revealed a previously unknown phenomenon caused by the speed of the propeller (RPM) on the formation of ice on those parts of the aircraft which are in the prop wash (slipstream).

Comprehensive research has shown that the speed of the propeller may affect the type of ice formation that will occur in the prop wash on the surface of the wing.

Many propeller manufacturers recommend that the speed of the propeller is increased in icing conditions to improve the performance of the de-icing system of the propeller and to reduce the vibrations resulting from uneven distribution of ice crystals and when ice is ejected from it.

It is clear from the above study, that other effects are also possible when the speed of the propeller is not increased in icing conditions, at least when engine and propeller are positioned in certain ways.

The formation of ice on the propeller blades can cause a very turbulent air current in the prop wash of a propeller rotating at normal speed. It has also been confirmed in the study, that in extreme icing conditions this turbulence stimulates the formation of a thin and rough ice layer on a cold wing, over 100% of the chord line in the region of the prop wash. This type of icing causes penetration of the wing boundary layer and severe impairment of the lift factor - far more than ordinary ice formation causes.

It has also been observed that the turbulence in the prop wash can be reduced by increasing the speed of the propeller, which is also normal to increase the impact of the de-icing system of the propeller blades. The reduced turbulence then reduces the formation of ice on the nearby wing parts and airfoils.

It is therefore very important for the flight crew to ensure exact compliance with the procedures as described in the aircraft flight manual and/or operations manuals of flights in icing conditions or conditions in which such conditions can be expected, especially instructions on conditions where propeller speed should be increased.

AIC hereby cancelled:

B 004 / 2004

NOTAM incorporated in this AIC:

NIL

END