What distinct symptom indicates the formation of carburettor ice in an engine with a CSU?

The formation of carburettor ice typically results in a specific symptom that can be observed during flight. In an engine with a constant speed unit (CSU), the distinct indication of carburettor ice is a dropping manifold pressure along with a decrease in indicated airspeed, while the engine RPM remains constant.

This phenomenon occurs because carburettor ice prevents the engine from drawing in enough air, which leads to a reduction in manifold pressure. The resulting lack of air affects the engine's performance and can cause a decrease in indicated airspeed, even though the RPM does not change due to the constant speed propeller governor maintaining the set RPM.

Other symptoms listed do not specifically correlate with the typical effects of carburettor ice in the context given. Therefore, the combination of decreasing manifold pressure and indicated airspeed is a clear and distinct indicator of the presence of carburettor ice. This understanding is crucial for pilots, as recognizing the signs early can lead to timely corrective measures to mitigate the problem and maintain engine performance.