What is the most adverse effect on an aircraft caused by rain?

The most adverse effect on an aircraft caused by rain is the creation of a water film that is roughened by the impact of raindrops. This phenomenon can significantly influence the aerodynamics of the aircraft. When raindrops strike the aircraft's surface, they can disturb the smooth flow of air, leading to an increase in drag and a potential loss of lift performance.

This roughening of the airflow can lessen the efficiency of the wings and jeopardize the aircraft's stability during flight, particularly at lower speeds or when approaching landing. The turbulence created by this roughened water film can make it more challenging for pilots to maintain control and may lead to an increase in fuel consumption due to the higher drag.

While reduced visibility and increased turbulence are indeed concerns during rain, they do not directly impact the interaction of the aircraft's structure with airflow as significantly as the roughening of the water film does. Loss of engine power and equipment failure can occur under certain conditions, but they are not direct effects of rain itself. Thus, the way rain affects the aerodynamics through this roughening process makes it the most adverse effect.