When is a plane guard helicopter considered 'on station' during the day?

Answer: C — Within 20 nautical miles

A plane guard helicopter is considered 'on station' during the day when it is operated within a specified range of the primary aircraft, which is set at 20 nautical miles. This distance allows the helicopter to effectively monitor and assist with any emergencies that may arise, ensuring a timely response if needed. The choice of 20 nautical miles is based on operational procedures that balance the need for proximity to provide quick assistance while also accounting for the need to maintain safety and operational efficiency. This range is optimal for the helicopter to remain in contact with the aircraft and be capable of reacting swiftly if an emergency occurs, such as a ditching or an engine failure. By keeping the helicopter within this defined radius, air traffic control can ensure that critical support is available without compromising the operational integrity of the aircraft or violating safety protocols.