Unacceptable risk. The risk that cannot be tolerated by the managing activity. It is a subset of identified risk that must be eliminated or controlled.

Unidentified risk. Risk not yet identified. Some unidentified risks are subsequently identified when a mishap occurs. Some risk is never known.

Undesirable risk. A tolerable risk that must be prepared for and mitigated as best as possible.

Visual flight rules (VFR). Code of Federal Regulations that govern the procedures for conducting flight under visual conditions.

Visual meteorological conditions (VMC). Meteorological conditions expressed in terms of visibility, distance from cloud, and ceiling meeting or exceeding the minimums specified for VFR.