In my perspective, Icing is one of the most incredible topics that could be discussed when talking about the impacts of environmental factors on performance during flight. However, for many people who are not aviation professionals, like passengers, icing accumulation on the wings or the tail does not seem like an issue, especially when a thin layer could barely be seen or a frost on the wings.
However, Icing can be highly hazardous to airplanes. Accumulation of ice over critical airplane areas, such as the wing's leading edge, ailerons, elevator, rudder, and engine cowls, can drastically affect aircraft's performance. Any accumulation of ice over one of these areas can cause interference with the airflow that goes over it and lead the aircraft into an abnormal situation that may end up eventually in an accident. For example, structural icing buildup over the wings would cause a change in the aerodynamic shape of the wing, a hazardous situation that causes the airplane to stall in higher airspeeds, decrease the coefficient of lift, and increase the coefficient of drag.
A good example of the danger of flying with structural ice on the wings is a fatal accident of a Beechcraft Challenger (N90AG) that departed from Birmingham, UK, to Bangor on January 4, 2002, and crashed a few seconds after liftoff. The AAIB (Air Accidents Investigation Branch) investigation concluded that the airplane took off with the wings contaminated with frost, which caused a change in the critical angle of attack of the wing that caused the airplane to stall.
One more essential system that can be affected by Icing is the pitot-static system that. Such a case can lead to false indications of vital primary flight instruments such as airspeed, altimeter, and vertical speed. In those cases, errors such as unreliable airspeed or false altitude reading can have a dangerous impact on the pilot's situational awareness and cause the crewmembers to recover in the wrong way.
In order to prevent hazardous situations related to Icing, modern airplanes that are flying into known icing conditions are equipped with ice protection systems. Those systems are divided into several anti-ice systems, such as wing anti-ice, engine anti-ice, pitot heat, and probes. Once a pilot is flying into icing conditions (Visible moisture and temperature colder than 10° C and warmer than -40° C), the anti-ice should be operating to warm up the surface and prevent and keep the surface clear of ice.
 
References
AC 91-74B, Pilot Guide: Flight In Icing Conditions, FAA (2015), Retrieved from
Report on the accident to Bombardier CL600-2B16 Series 604, N90AG, AAIB (2004), Retrieved from

 

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