Automatic Radar Plotting Aid – Officer of the Deck Phase 1 Practice Test 2026 - Free ARPA OOD Practice Questions and Study Guide

TCPA and CPA are both indicators of collision risk: TCPA is the time remaining until the vessels reach the closest point of approach, and CPA is the distance at that closest approach. When both are decreasing, you have less time to act and the closest distance between the vessels is getting smaller, so the danger of a collision is increasing. The appropriate action is to plan changes that increase both the time to closest point and the distance at closest point, such as altering course or speed to create more separation.

No-action would ignore the rising risk. If the target were moving away, TCPA would typically increase. If collision risk were decreasing, the indicators would not both be dropping.