For all helicopters, when must inflatable PFDs be worn?

Inflatable Personal Flotation Devices (PFDs) are essential safety gear designed to keep individuals buoyant in water. In aviation, particularly for helicopter operations, the requirement to wear PFDs focuses on enhancing safety during specific high-risk scenarios, particularly when there is a potential for water-related emergencies.

Wearing inflatable PFDs during operations over water is crucial because, in the event of a water landing or an emergency that leads to a forced ditching, these devices can significantly increase the chances of survival. Helicopters flying over bodies of water are exposed to risks of unintentional water landings due to mechanical issues, pilot error, or adverse weather, making the use of PFDs a necessary precaution in safeguarding personnel onboard.

The context of the other options clarifies further why they are not applicable as blanket requirements. For instance, routine maintenance does not necessitate the wearing of PFDs since the helicopter is not in operation. Similarly, limiting PFD use to specified phases of flight like landing overlooks the risks involved when the helicopter is operational over water. Therefore, the correct answer emphasizes the importance of preparedness and enhances safety measures during flight operations in challenging environments.