Percent Power Increase–a simple way to quantify an icing hazard

PERCENT POWER INCREASE - A SIMPLE WAY TO QUANTIFY AN ICING HAZARD

Abstract

Engineering studies on the icing hazard to aircraft have identified three meteorological variables that influence how supercooled water accumulates as ice on aircraft surfaces: (1) cloud liquid water content determines the amount of supercooled liquid water available for an aircraft to intercept; (2) effective droplet diameter determines the percentage of available cloud liquid water that impacts an airfoil as well as where ice accumulates; (3) air temperature determines the location where the water freezes.

Icing forecasts using these three variables can be computed with numerical model output. Weather forecasters must then communicate the threat to pilots in a manner that effectively details how it will affect aircraft performance. Current reporting and forecast terminology identifies the icing hazard as trace, light, moderate, or severe. Although these terms are based on a degradation to aircraft performance, they have been criticized as being vague and subjective.

Ice accumulation increases airfoil drag. This increased drag must be countered by increased thrust if the aircraft is to maintain speed and altitude. It stands to reason then that every aircraft has a quantitative icing dial onboard - its engine power gauge (SHP, RPM, manifold pressure, etc.). The increase required in engine power is a direct measurement of the icing hazard. If pilots were to report such an increase as a percentage, and meteorologists were to forecast icing hazards in the same manner, then much of the vagueness and subjectivity of the present reporting system could be reduced.

In addition to being a simple and potentially effective means for the pilot to report icing, the Percent Power Increase (PPI) can also be estimated from the model based icing forecast mentioned above. PPI estimates are based on empirical/physical relationships between the three meteorological variables and aircraft airfoil performance derived from wind tunnel data. PPI icing forecasts are experimentally produced and published on the Aviation Weather Center Homepage.