What should the pilot in command report by radio as soon as possible when operating under IFR and not in radar contact?

When operating under Instrument Flight Rules (IFR) and not in radar contact, it is crucial for the pilot in command to maintain communication with air traffic control (ATC). Reporting passing each designated reporting point is essential because these points are specific locations along the flight route that help ATC track the aircraft's position and ensure safe separation from other traffic.

By reporting position at designated points, the pilot provides ATC with vital information about their flight progress and confirms adherence to the flight plan. This communication is especially important when radar services are unavailable, as it allows ATC to maintain effective oversight and manage air traffic safely.

While other parameters like altitude or turbulence might be significant in specific contexts, they do not directly fulfill the requirement to report positional information to ATC, which is critical when radar monitoring is not in effect. Such reports help maintain situational awareness and promote safety in the airspace system.