Definition
An uncommanded and rapid roll of the aircraft caused by airflow separation over the ailerons, typically resulting from ice accumulation forward of the aileron on the wing. The disturbed airflow can cause one aileron to lose effectiveness or deflect on its own, rolling the aircraft suddenly and sometimes beyond the pilot's ability to correct with normal control inputs.
Plain English
A sudden, unexpected rolling of the aircraft that the pilot did not command, caused by ice disrupting the airflow over the wings near the ailerons. The aircraft can snap into a bank without warning.
Context Anchor
Seen in icing discussions, especially when ice changes how the wings and ailerons behave in flight.
Derivation
Upset' here means a sudden, unintended departure from controlled flight — the aircraft's attitude is 'upset' from what the pilot wanted. Combined with 'roll,' it specifically describes that disturbance happening around the longitudinal axis.
Why Pilots Care
It can quickly lead to loss of control, especially when flying by instruments with limited visual reference.
Grounding Statement
Picture the airplane flying normally, then one wing suddenly drops because ice has changed how air flows over that wing.
Intuition Check
Upset does not mean the pilot is emotionally upset. Here it means the airplane has been disturbed from normal controlled flight. A roll upset is not just a small wing dip; it can be an uncommanded roll with weak or abnormal roll control.
Example Sentence 1
After encountering freezing rain, the crew felt the aircraft begin a roll upset and immediately requested a lower altitude to exit the icing layer.
Example Sentence 2
Prompt reduction of angle of attack helped stop the roll upset before it developed into a full departure from controlled flight.