What defines severe icing?

Severe icing is characterized by an accumulation of ice that poses significant threats to the control, performance, and safety of an aircraft. An accumulation of over 3 inches per hour indicates very serious icing conditions, as this level of ice buildup can overwhelm the aircraft's de-icing and anti-icing systems, create excessive drag, and drastically reduce lift. This can lead to loss of control and other critical operational challenges.

The distinctions made in the other options refer to varying severity levels of icing that, while hazardous, do not reach the critical danger level indicated by more than 3 inches of ice accumulation per hour. The quantification of ice is important for pilots and aviation personnel to assess risk and make informed decisions regarding flight safety.